ALBERT

Notes: The B 1Σ+u←a 3Πg transition of Cd2 (natural abundance) and 114Cd2 has been observed by laser excitation spectroscopy and analyzed. By exciting the Cd2 B←a transition in the visible (560≤λ≤730 nm) while monitoring B 1Σ+u→X 1Σ+g (bound→free) emission in the ultraviolet (∼270–310 nm), more than 40 red-degraded vibrational bands were recorded. Analysis of the spectrum has yielded vibrational constants for both the a 3Πg and B 1Σ+u states: ωe″=153.6±4.0 cm−1, ωe″xe″=0.52±0.06 cm−1, ωe′=105.3±1.0 cm−1, and ωe′xe′=0.44±0.03 cm−1. In addition, ΔRe≡ReB−Rea was determined to be 0.31±0.03 A(ring). Rotational structure has been partially resolved for 114Cd2 bands in the 620–655 nm and 719–723 nm regions, and the spontaneous emission lifetime of the a 3Πg state and the rate constant for quenching of Cd2 (a 3Πg) by collisions with background Cd atoms have been determined to be 8.6±2.5 μs and (2.2±0.3)×10−13 cm3 s−1, respectively. Also, analysis of the B→X emission (Condon internal diffraction) spectra produced when specific B 1Σ+u vibrational levels are populated has yielded ReX−ReB=0.95±0.02 A(ring). © 1996 American Institute of Physics.

Notes: A novel modification of the flux operator formalism is introduced that combines the merits of the flux operator approach with those of complex absorbing potentials. The method is used to determine initial-state selected reaction probabilities for a broad energy range from a single appropriately chosen time-dependent wave packet. The propagation may be performed solely in the coordinates of the reagents arrangement channel. State-to-state transition probabilities can also be obtained when appropriate projectors are included. In contrast to similar methods the present one does not require the calculation of derivatives with respect to the reaction coordinate. More importantly, it avoids the need to (E,t)-Fourier transform the wave packet at every grid point on a dividing surface. The proposed formula, though completely general, is especially well suited to handle multiconfiguration time-dependent Hartree wave functions. As a check of the reliability initial-state selected reaction probabilities for the collinear H+H2→H2+H reaction are calculated and compared with (numerically) exact results. We also show that the initial wave packet may be placed close to the interaction region when its energy distribution is corrected for the mean potential energy. © 1996 American Institute of Physics.

Notes: Twelve potential energy surfaces that have been proposed for the CO2–Ar interaction have been considered in detail. The anisotropies of these surfaces are compared and their ability to predict the interaction second virial coefficient as a function of temperature has been examined. Intermolecular bending and stretching quadratic force constants predicted by each and the mean square torque calculated for each are compared with the experimental values. Quantum diffusion Monte Carlo simulations provide the average rotational constants and geometry for the ground vibrational state as well as the dissociation energy in each case. These are compared with the experimental values. Classical trajectory calculations were carried out to obtain 45 types of thermal average cross sections for six of these surfaces. Various thermophysical properties such as mixture viscosity, mixture thermal conductivity, and diffusion coefficient, calculated from these cross sections and the NMR relaxation cross sections, are compared with experimental data. It is found that the spectroscopic constants define the depth and shape of the well at the global minimum, whereas the NMR cross sections and mean square torque probe the anisotropy in a broader sense. The thermophysical properties (viscosity, diffusion coefficient, and thermal conductivity) are not strongly discriminating between the surfaces, whereas the temperature dependence of the second virial coefficient detects the weaknesses in the low and upper repulsive walls of those surfaces that were modified specifically to improve greatly the shape of the well so as to reproduce the spectroscopic constants. © 1996 American Institute of Physics.

Notes: The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780–17 090 cm−1 (596–585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A2′ ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm−1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm−1 is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1×104 at 16 780 cm−1 to 6×107 s−1 at 16 880 cm−1, and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice–Ramsperger–Kassel–Marcus (RRKM) rate. The top of the NO+O2 barrier is estimated to lie at 16 900±15 cm−1. Translational energy measurements of specific NO (X 2ΠΩ,v,J) levels showed that O2 is highly excited, with a population inversion extending to energies above the a 1Δg threshold, in agreement with previous work. It is possible that the main O2 product is X 3∑g−, though some participation of a 1Δg cannot be ruled out. Within the experimental uncertainty, b 1∑+g is not produced. © 1996 American Institute of Physics.

Notes: A theoretical framework for the solute electronic structure description under nonequilibrium solvation is developed via multipole expansions of a quantum dielectric continuum solvent formulation of Kim and Hynes [J. Chem. Phys. 96, 5088 (1992)]. By employing a spherical cavity for the solute and invoking a Born–Oppenheimer description for the solvent electronic polarization P(vector)el, the cavity boundary effects on the solute electric and solvent polarization fields are taken into account exactly. The solute–solvent electronic correlation effects are also included within the dielectric continuum context in the fast P(vector)el limit. Another novel feature of the theory includes the cavity size variation with the solute electronic charge distribution and its thermal fluctuations. This effectively accounts for, e.g., electrostriction, largely ignored in many solution-phase quantum chemistry calculations based on the reaction field methods. By employing a coherent state description for P(vector)el, we obtain electronically adiabatic free energies as a function of the cavity radius variable that measures the fluctuating cavity size and the solvent coordinates that gauge the nonequilibrium solvent orientational polarization P(vector)or. These define multidimensional electronic free energy surfaces, upon which nuclear dynamics occur. Their local structure near equilibrium, along with the solute polarizability effects on the force constant matrix, is analyzed. With a polaron description for the P(vector)or kinetic energy, it is found that the frequency relevant for ultrafast inertial solvation dynamics decreases as the Pvec;or multipole character increases. This is in qualitative agreement with recent molecular solvation theory predictions. As for the cavity, the frequency associated with its symmetric breathing mode is examined by analyzing our previous molecular dynamics simulation results via the equipartition principle. It is found that the cavity frequency is comparable to that of P(vector)or. The variation of the equilibrium cavity size with the solute charge distribution and its influence on free energetics are also studied. Model calculations in water show that the cavity size decreases with the increasing solute dipole moment. This results in a significant reduction of equilibrium free energy, compared to that obtained with the neglect of the electrostriction effect. © 1996 American Institute of Physics.

Notes: The theoretical formulation developed in the preceding article [H. J. Kim, J. Chem. Phys. 105, 6818 (1996)] is analyzed via a second-order perturbation method and applied to the static electronic spectra of polarizable solutes in solution. In the Born–Oppenheimer (BO) framework of the solvent electronic polarization P(vector)el, the solute electronic wave functions, together with their (free) energy levels and associated Franck–Condon (FC) energies, are examined in the presence of a spherical cavity of arbitrary size and a nonequilibrium solvent orientational polarization configuration P(vector)or. It is found that the solute electronic structure and its free energetics vary strongly with both P(vector)or and the cavity size. The solute dipole enhancement due to solvation decreases with increasing cavity size. Comparison with the self-consistent (SC) reaction field theory predictions shows that classical P(vector)el is more effective in polarizing the solute than quantum P(vector)el couched in the BO description. This is due to the dispersion stabilization mechanism present in the latter. The static electronic spectroscopy is studied to linear order in the solute polarizability and in the cavity size difference between the lower and upper electronic states involved in the FC transition. In the case of the vanishing cavity size difference, our analytic results for the solvent spectral and Stokes shifts are compared with various existing theories and the sources of the discrepancies are briefly discussed. The effects of the cavity size variation on the electronic spectra are illustrated by using a simple two-state model description for the solute. It is found that even in a nonpolar solvent, there can be a significant Stokes shift arising from the cavity size relaxation subsequent to the FC transition. Also the cavity size fluctuations can make a non-negligible contribution to the spectral line broadening. © 1996 American Institute of Physics.

Notes: The dynamics of clusters (H2O)nH+ (n=1,2,3,4) interacting with an NH3 molecule has been studied by first-principles Born–Oppenheimer molecular dynamics (BOMD) simulations. These small clusters are chosen as prototype systems for studying the mechanisms of proton transfer at atomistic level. We focus on the fundamental steps of proton motion in molecular clusters, the dynamical consequences of proton affinities, and the interplay between proton motion and proton affinity in these systems. A characteristic feature of the motion, the forming and breaking of O–H bonds in H3O+ is analyzed in detail. The transfer process is found to be consecutive along a quasi-one-dimensional channel. The umbrella mode in NH3 can easily be excited to direct the lone pair of the ammonia molecule to the water clusters. The hydronium ion, however, reorients mainly via rotation. When NH3 reaches one terminal water molecule of a protonated water cluster, the system undergoes a series of intermediate states in which the mobile protons travel within the water clusters, H3O+ transients are formed as protons approach individual water molecules. The lifetime of the H3O+ transient is 8–20 fs, or 1–3 vibrational periods of the O–H stretch mode. Proton transfer is observed for n=1, 2, 3, although for n=3 NH+4(H2O)3 is in existence with NH3(H2O)2H+. For n=4, NH3(H2O)4H+ is the dominant statistical configuration. Vibrational spectrum of NH3(H2O)4H+ is analyzed in detail. The features of the spectrum can be used, in principle, to probe the proton motion in the transition state region reactions. In these calculations, the electronic charge distribution is calculated concurrently with the nuclear dynamics. An analysis of isocharge density surfaces gives qualitative and quantitative descriptions of the dynamics of electronic redistribution. The BOMD is performed in the framework of density functional theory with local spin density and generalized gradient approximations. © 1996 American Institute of Physics.

Notes: This article presents a derivation of the rate of reaction in the quantum activated rate problem. In this problem, one studies the rate of a chemical reaction when the reaction is placed in a dissipative bath. Our derivation defines the rate in terms of the flux autocorrelation function and proceeds via the recently developed interaction representation for nonadiabatic corrections to adiabatic evolution operators. This methodology is an infinite order resummation of nonadiabatic corrections to evolution operators. The approach produces an analytic expression which yields accurate results over a range of temperatures, viscosities and system parameters through the Kramers turnover region. © 1996 American Institute of Physics.

Notes: The deviations of the second-order state-specific effective Hamiltonian method from the strict size consistency are analyzed. Provided that complete or separable model spaces are used, these deviations can be suppressed by a proper choice of nonuniform shifts of energy denominators. © 1996 American Institute of Physics.

Notes: The ν5 antisymmetric stretching vibration of 1Σ+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079–2081 cm−1 were assigned to this band. A combined least squares fit of these transitions with previously reported ν6 transitions yielded the following molecular constants for the ν5 band: ν0=2 079.673 58(17) cm−1, B″=0.014 321 4(10) cm−1, and B′=0.014 288 9(10) cm−1. The IR intensity of the ν5 band relative to ν6 was found to be 0.108±0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD). © 1996 American Institute of Physics.

Notes: This paper emphasizes the complementarity between optical and magneto-optical measurements on f elements. The magnetic circular dichroism, circular dichroism, and polarized absorption spectra of Na3Nd(ODA)3⋅2NaClO4⋅6H2O (NdODA), have been recorded at 4.2 K between 5000 and 30 000 cm−1. The electric dipole contributions to the f–f transitions are analyzed using a parametric model which contains 12 parameters which are determined by fitting the calculated to the empirical dipole strengths of 50 α and 16 π polarized absorption lines. The reliability of these intensity parameters is tested by simulating the more sensitive magneto-optical (magnetic circular dichroism) and chiroptical (circular dichroism) spectra. © 1996 American Institute of Physics.

Notes: The effect of interaction potentials on the rate coefficients of irreversible diffusion-influenced bimolecular reactions is studied. If the region of configurational space where reaction can occur is small, the rate coefficient k in the presence of an interaction potential U is shown to be simply given by k0, the rate coefficient in the absence of the interaction potential, scaled by 〈exp(−βU)〉, the average Boltzmann factor in the reaction region. For a finite size of the reactive region, this relation is generally more accurate for potentials with smaller values of 〈exp(−βU)〉 and, at a given value of 〈exp(−βU)〉, more accurate for potentials that vary less significantly around the reactive region. © 1996 American Institute of Physics.

Notes: Infrared molecular beam depletion spectroscopy has been employed to study the vibrational spectroscopy of water molecules and small water polymers [(H2O)n, n=2,3,4] embedded in large liquid helium clusters (HeN, 100≤〈N〉≤5000). The spectral region between 3500 and 3800 cm−1 was covered with an injection-seeded optical parametric oscillator. The following vibrational bands could be located and investigated: for the monomer the ν3 asymmetric stretch, for the dimer the asymmetric stretch of the proton acceptor molecule and the free and bonded O–H stretches of the donor molecule, for the trimer both free and bonded O–H stretches, and for the tetramer the free O–H stretch. By comparison with the data on free gas phase water complexes, it was found that the helium host clusters induce only minor perturbations in form of small frequency redshifts and that they constitute an ideal nano-matrix. Furthermore, it was found that both the water molecule and the dimer rotate almost freely in the host cluster and that the internal-rotationlike tunneling motion of the water dimer is not quenched. Due to the extremely low internal temperatures, a splitting of the trimer band associated with the O–H ring vibration could be resolved for the first time. This splitting indicates that the trimer structure is asymmetric, as has been predicted by theoretical calculations. © 1996 American Institute of Physics.

Notes: Recently the anti-Stokes spectroscopy after strong vibrational excitation via resonant absorption was rediscovered, for the study of population relaxation dynamics. A closer theoretical inspection of this experimental approach reveals that properly chosen polarization conditions allow the study of molecular reorientational motion. Furthermore the sign of a/γ (the invariants of the Raman polarizability tensor) can be determined. Experimental data for chlorinated alkanes will be presented. © 1996 American Institute of Physics.

Notes: The polarization dependence of the (2+1) and (3+1) resonance enhanced multiphoton ionization of OCS have been investigated in the 70 500–74 500 cm−1 energy region. This region contains a complex system of bands arising from the excitation of the 4p Rydberg states. The symmetry of most of the observed bands have been unambiguously determined based on the intensity changes of the two and three photon resonant spectra using both linearly and circularly polarized light. These results generally confirm the assignments suggested in a previous study. New bands have also been observed and some new assignments are proposed. The vibrational frequencies ν1, ν2, and ν3 of the Rydberg states in that energy region are determined. © 1996 American Institute of Physics.

Notes: The imaginary part of the magnetic dipole–dipole cross correlation spectral density in spin 1/2 systems generates a novel type of three-spin Hamiltonian with terms IizIjzIkz, IizIjxIkx, and IizIjyIky and permutations in the spin labels i, j, k. These terms cause asymmetric positions of the spin multiplet components, which could not be explained by scalar J-coupling and chemical shift terms. © 1996 American Institute of Physics.

Notes: The laser excitation spectrum of tungsten methylidyne, WCH, has been recorded in the 12 000–15 400 cm−1 region. A total of 14 vibronic bands of WCH and 16 bands of WCD have been observed in this region. Rotational analysis shows that the ground state is X˜3/2(2Δ), with the substitution structure r0(W≡C)=1.73665 A(ring) and r0(C–H)=1.0765 A(ring). The excited vibronic levels have been assigned, on the basis of their WCH/WCD isotope shifts and their wavelength resolved fluorescence patterns, to three electronic states, A˜3/2(2Δ), B˜1/2(2Π), and C˜5/2(2Φ), at 12 090, 13 392, and 14 110 cm−1, respectively. The wavelength resolved fluorescence spectra have also established the low-lying vibrational levels of the ground state. The ground state bending fundamental lies at 660 cm−1, while the W–C stretching frequency is 1006 cm−1; the corresponding frequencies in WCD are 501 cm−1 and 953 cm−1, respectively. No evidence for the C–H stretching frequency has been found. Emission has also been observed to a low-lying electronic state, 813 cm−1 above the X˜3/2 state. The pattern of rotationally resolved emission to this state clearly indicates that it is a 4Σ1/2 state. Its bending frequency is 612 cm−1, and its W–C stretching frequency is 971 cm−1, indicating a slightly longer bond length than in the X˜3/2 state. High resolution cw laser spectra of the (0,0) bands of the two lowest excited electronic states [A˜3/2(2Δ) and B˜1/2(2Π)] reveal a small splitting of the four principal tungsten isotopes (182W, 183W, 184W, and 186W) which serves to confirm the presence of tungsten in the carrier. Hyperfine splitting associated with the 183W nucleus (I=1/2) has been observed for the (0,0) band of the A˜3/2−X˜3/2 system, allowing the electron configuration of the ground state to be elucidated. © 1996 American Institute of Physics.

Notes: A tunable color-center laser and a molecular-beam electric-resonance optothermal spectrometer have been used to record the infrared spectra of the C–H stretching vibrations of HCCH–NH3, HCCD–NH3, NCH–NH3, and HCCCCH–NH3. The hydrogen-bonded C–H stretching vibrations of NCH–NH3, HCCH–NH3, and HCCCCH–NH3 are redshifted by 200.88126(30), 75.1042(38), and 127.4(1) cm−1 from the respective free monomer modes. The non-hydrogen-bonded C–H stretches are less perturbed by complexation, being blueshifted by 0.2992(3) cm−1 in HCCCCH–NH3 and redshifted by 1.179(1) cm−1 in HCCD–NH3. Consistent with the much larger perturbation of the monomer vibration for the bonded C–H stretch, the B rotational constants increase by 1%–2% for the bonded C–H stretch excited NCH–NH3 and HCCH–NH3 complexes, but change by less than 0.1% for the nonbonded C–H stretches in HCCCCH–NH3 and HCCD–NH3. The decoupling of the two C–H stretches in HCCH–NH3 is not sufficient to allow the observation of the nonbonded C–H stretch in the complex, which correlates to the Raman-active symmetric C–H stretch of acetylene. Also, no spectra were observed for the weaker N–H stretching vibrations of the complexes, consistent with the very weak intensities of these modes in the monomer. The homogeneous linewidths of the transitions, assumed to be a measure of the vibrational predissociation rate, are approximately two orders of magnitude larger for the bonded C–H stretches than for the nonbonded C–H stretches. The similarity in homogeneous widths for the nonbonded C–H stretches in HCCD–NH3 and HCCCCH–NH3, of 7–12 MHz, suggests that the rate of vibrational energy flow along acetylene chains is only weakly dependent on chain length.

Notes: The hyperfine structure of the b 3Πu, 2 3Πg, and 3 3Πg states of 7Li2 has been studied by continuous wave perturbation facilitated optical–optical double resonance fluorescence excitation spectroscopy. The b 3Πu state has case bβJ coupling scheme. The hyperfine splittings of the two perturbed b 3Πu v=19, N=10, J=11e, s, F1 and v=19, N=5, J=4e, a, F3 levels were resolved and hyperfine constants determined to be +9.90 MHz and −20.94 MHz, respectively. Fermi contact is the main source of the hyperfine interaction and bF(b 3Πu)=+107 MHz has been obtained. The coupling schemes of different 2 3Πg levels are different: N=4, 6 levels observed from the b 3Πu v′=19, N′=5, J′=4e, a, F3 intermediate level and the N=10 levels observed via the b 3Πu v′=19, N′=10, J′=11e, s, F1 intermediate level have case bβJ coupling; the N=5 levels observed via the b 3Πu v′=19, N′=5, J′=4e, a, F3 intermediate level and the N=9, 11 levels observed from the b 3Πu v′=19, N′=10, J′=11e, s, F1 intermediate level are closer to case bβS coupling. Hyperfine constants have been calculated and the Fermi contact constant, bF(2 3Πg)=60 MHz is obtained. The 3 3Πg state is a Rydberg state and has case bβS coupling with bF(3 3Πg)≈100 MHz. © 1996 American Institute of Physics.

Notes: The first of a new class of doubly excited valence states of van der Waals molecules, Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−), has been discovered and characterized by resonance two-photon ionization spectroscopy. This state has been found to be quite strongly bound (D0=2850±100 cm−1 and to have a very short bond length (R0=2.41±0.02 A(ring)). The analogous singly excited state, Mg(3s3pπ 3PJ)⋅Ar(3Π0−), in stark contrast, has a very much smaller bond strength (D0=160±40 cm−1) and a longer bond length (R0=3.66±0.02 A(ring)). In fact, the new Mg(3pπ3pπ 3PJ)⋅Ar state is more than twice as bound than even the Mg(3s 2S1/2)⋅Ar+ ground-state ion (D0=1240±40 cm−1, R0=2.82 A(ring)). The strong bonding is due both to the transverse alignment of the Mg(3pπ) electrons and particularly to the lack of a Mg(3s) electron. The resulting loss of Mg(3sσ)/Ar(3pσ)2 repulsive forces facilitates close approach of the Ar atom. This is turn allows attractive Mg(3pπ)2/Ar dispersive forces and Mg2+/Ar ion/induced-dipole type forces to continue to much smaller internuclear distances, creating a strong, short van der Waals bond. Possible mechanisms for the parity-dependent predissociation and the apparent "spin–spin'' type splittings observed for the Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−) state are also discussed. © 1996 American Institute of Physics.

Notes: We present a theoretical study of the induced transient birefringence of a low density homogeneous molecular gas in a resonant pump–probe experiment. The molecular coherent state induced by the resonant pump field is described by second-order perturbation theory. The induced birefringence can be detected by a delayed probe pulse propagating through the molecular medium after illumination by the pump pulse. In the case of a nonresonant probe, the birefringence is linearly proportional to the mean value of the electronic polarizability of the molecular gas. The birefringence signal is composed of distinct components due to population change and those of rotational, vibrational, and mixed vibrational–rotational origins. This is demonstrated by numerical simulations on Li2 gas. Moreover, the quantum beats contained in the birefringence, as a function of the time delay between the pump and probe pulses, is dominated by the pure rotational motion. Finally, the birefringence is sensitive to the shape of the applied pump pulse and dependent on the spectral phase of the pump pulse. © 1996 American Institute of Physics.

Notes: Thermal rate constants for capture of permanent and induced dipoles by ions are calculated by the statistical adiabatic channel model (SACM) using analytical representations of adiabatic channel eigenvalues. Linear, symmetric, and asymmetric top dipoles with closed and open electronic shells are considered. Analytical representations of the capture rate constants over broad temperature ranges are presented for all systems. Numbers of open channels W(E,J) are also investigated. The present analytical SACM treatment provides an economical, transparent, and accurate approach to the considered type of capture processes. At the same time, the implementation to ion fragmentation processes is elucidated. © 1996 American Institute of Physics.

Notes: This paper in the series gives our full account of the preliminary results reported in a communication [Cheng, Zhong, and Zewail, J. Chem. Phys. 103, 5153 (1995)] on real-time femtosecond (fs) studies of the transition state of charge-transfer (CT) reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidate the microscopic elementary dynamics and the structure of the transition states for the isolated, bimolecular reaction of benzenes (electron donor) with iodine (electron acceptor). The transition state is directly reached by fs excitation into the CT state of the complex Bz⋅I2, and the dynamics is followed by monitoring the product build up or the initial transition-state decay. We further employed the fs resolution in combination with the kinetic-energy resolved time-of-flight and recoil anisotropy techniques to separate different reaction pathways and to determine the impact geometry. Specifically, we have studied: (1) the temporal evolution of the transition state (τ

Notes: Classical trajectory calculations of ion–permanent quadrupole+induced dipole capture processes are performed over wide ranges of conditions. The results are expressed in two-parametric analytical form, and analyzed with respect to the transition from sudden to adiabatic dynamics. The results are compared with the statistical adiabatic channel model (SACM). For this purpose, adiabatic channel potential curves are expressed in approximate analytical form. The agreement between classical trajectory and SACM results is excellent in the range of adiabatic quasiclassical motion of the collision partners. Analytical expressions can well describe the temperature variation of the capture rate constant in the transition range between the quantum and classical limits. © 1996 American Institute of Physics.

Notes: Classical trajectory calculations of ion–permanent+induced dipole capture processes are performed over very wide ranges of conditions. The results are represented in a simple, two-parametric analytical form of high precision. The transition from adiabatic to nonadiabatic dynamics is expressed in terms of the Massey parameter. In the adiabatic range, perfect agreement (better than 0.4%) of the derived thermal capture rate constants from classical trajectories and results from accurate statistical adiabatic channel (SACM) calculations is obtained. © 1996 American Institute of Physics.

Notes: Reflection and transmission UV-Vis spectroscopy have been applied to study the molecular orientation and surface density of rhodamine-6G molecules physisorbed on optically flat quartz (SiO2) substrates. Our results have shown that for the s-polarized excitation, the submonolayer of physisorbed rhodamine-6G dye molecules causes enhanced reflection in the wavelength region from 400 nm to 600 nm where the electronic transition takes place. For the p-polarized excitation, the reflection is enhanced when the angle of incidence is smaller than Brewster's angle of quartz at 55.6°, and the reflection is reduced when the angle of incidence is larger than Brewster's angle of quartz. An independent method has been established in this paper by which the molecular orientation can be determined accurately by carrying out optical measurements in both the reflection and transmission directions. © 1996 American Institute of Physics.

Notes: Frequency-shifted excitation by phase-incremented pulses of arbitrary shape can be solved by introducing a second rotating frame. The results show that a phase-incremented pulse can be decomposed into an infinite number of unsymmetrically amplitude-scaled and phase-shifted effective radio frequency (rf) fields, which are responsible for the unsymmetrical and phase-shifted centerband and sideband excitations. In addition to a universal phase shift associated with each band of excitation, a phase inversion occurs when the scaling factor of the effective rf field becomes negative. Also if the total phase increment of the pulse is not equal to 2kπ (k integer) an additional phase shift equal to the total phase increment will be introduced. By properly choosing the total phase increment the phase shift of the centerband can be compensated. A computer program based on the Bloch equations is developed to calculate directly the excitation profiles of phase-incremented pulses, which agrees well with the theoretical predications. © 1996 American Institute of Physics.

Notes: Electronically excited states of ozone have been studied by electron-energy-loss spectroscopy. Two broad bands without visible vibrational structure are observed at 1.8±0.2 eV and 2.05±0.05 eV under scattering conditions favoring singlet excitation, that is a scattering angle of cursive-theta=10° and residual energy Er=20 eV. The lower is assigned as 1A2, the higher as 1B1 (Chappuis band). Bands with rich vibrational structure are observed under scattering conditions favoring triplet excitation, cursive-theta=30°–135° and residual energy Er=1–3 eV. At least two vibrational progressions can be discerned. The first has an origin at 1.30 eV, the origin of the second cannot be determined unambiguously, it is either at 1.53 or 1.45 eV. The well-known Hartley band and a number of other singlet and triplet excited states are observed at higher energy losses. Excitation functions and angular distributions of the triplet band at 1.30 eV and of the Hartley band are presented. The absolute value of the differential cross section for excitation of the Hartley band is given. © 1996 American Institute of Physics.

Notes: The 12C16O2−, 13C16O2−, 12C17O2−, and 12C16,17O2− radical anions have been generated by four independent methods and isolated in neon and argon matrices for detailed ESR (electron spin resonance) investigations. Included with these experimental measurements for the various magnetic parameters of CO−2 are high level ab initio calculations (MR SD-CI and others) of the 13C and 17O hyperfine A tensors. Some of the calculations included the effects of a 42-atom neon cage on the electronic structure of CO−2. Previous ESR studies of CO−2 have been conducted in more perturbing environments, such as ionic crystals, where the close proximity of the counter cation can alter the anion's properties. A comparison of the earlier measurements in more interactive materials with these theoretical and rare gas matrix results reveals a significantly different distribution of the spin density. The neon magnetic parameters (MHz) for CO−2 are gx=2.0018, gy=1.9964, gz=2.0010; for 13C, Ax=320.4, Ay=296.1, Az=394.5; for 17O, Ax=−81.6, Ay=−74.9 and Az=−151.8. The argon results are similar to these neon values; isotropic spectra in argon were also observed at elevated temperatures that yielded giso and Aiso parameters consistent with the low temperature (4 K) anisotropic spectra. © 1996 American Institute of Physics.

Notes: The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°–26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics.

Notes: Level anticrossings and quantum beats have been observed for the fluorescence from single rotational levels of the vibrationless S1 pyrimidine in a supersonic free jet. The density of triplet levels (ρ) and the coupling matrix elements (v) deduced from the data obtained for R(0) transition confirm that the singlet–triplet coupling belongs to the weak coupling limit (vρ(very-much-less-than)1) of interstate interaction, in which electronic relaxation from the prepared singlet state to the triplet manifold does not occur in the absence of collisions. © 1996 American Institute of Physics.

Notes: The complex semiclassical formalism of Robert and Bonamy is used to calculate both half widths and line shifts for water vapor in a bath of nitrogen. The assumed intermolecular potential is a combination of electrostatic, Lennard-Jones 6-12 atom–atom, induction, and dispersion terms. The complex valued resonance functions have been previously evaluated when the assumed potential was electrostatic only. In this work these functions are evaluated when the potential is extended to include the atom–atom terms. Calculations made in the 3ν1+ν3 vibrational band of H2O are in good agreement with experimental results for both the half width and line shifts. It is shown that the imaginary parts are important for both the line shift and half width calculations. © 1996 American Institute of Physics.

Notes: The intensity anomalies in the spin–orbit and rotational branching ratios in the zero kinetic energy pulsed-field ionization (ZEKE-PFI) spectra via the F 1Δ2, D 1Π1, and f 3Δ2 Rydberg states of HCl have been studied. In general, the branching ratios are observed to depend on three parameters employed in the pulsed field ionization experiment: (i) the delay time between excitation and ionization; (ii) the magnitude of the bias electric field; and (iii) the magnitude of the applied pulsed electric field. The results can be rationalized on the basis of the increasing number of autoionization decay channels that become available to the high-n Rydberg states as each ionization threshold is surpassed. The delay dependence of the ZEKE-PFI spectra via the F 1Δ2 state has been analyzed in more detail by ab initio calculations. These calculations show that the observed spin–orbit branching ratios can be reproduced thereby giving evidence for a nonexponential decay of the high-n Rydberg states (n≈100). © 1996 American Institute of Physics.

Notes: Ethylthio (C2H5S) radicals were formed on laser photolysis at 248 nm of diethyl disulfide (C2H5SSC2H5) or ethyl mercaptan (C2H5SH) in a free-jet expansion. The fluorescence excitation spectrum was recorded in the spectral region 398–432 nm. The origin lies at 23 519.6 cm−1, approximately 799 cm−1 greater than previously reported. Two main progressions with spacings near 420.5 cm−1 (C–S stretch) and 256.0 cm−1 (CCS bend) are dominant. Additional active fundamental vibrational modes of the B˜ state are at 718.4, 862.8, 1054.6, 1158.9, and 1203.3 cm−1. Observation of hot bands enables accurate determination of four low-lying vibrational modes of the ground state at 271.9, 296.0, 478.3, and 672.4 cm−1. The dispersed fluorescence was recorded in the spectral region 415–525 nm. We identified several additional vibrational modes of the X˜ state at 890, 957, 1075, 1257, 1290, 1470, 2950, and 3050 cm−1. Theoretical calculations at the MP2 level were performed to predict vibrational frequencies of both B˜ and X˜ states, and for the latter state were also with the B3-LYP density functional theory; the results agree satisfactorily with experimental observations. © 1996 American Institute of Physics.

Notes: Ab initio quantum mechanical methods were employed to study the spectroscopic constants and potential energy surfaces of H2SiO, HSiOH, the HSiOH dimer, and the Si2H radical. Consideration of the spectroscopic constants of silanone, cis- and trans-HSiOH and Si2H began with the TZ2P SCF level of theory. We predict a strongly bonded cis-HSiOH dimer. The structure of the cis-HSiOH dimer was optimized at the DZP SCF, DZP CISD, DZP+diff CISD and DZP MP2 levels. The hydrogen bond energy of the dimer is 14.8 kcal/mol at the DZP MP2 level and 12.0 kcal/mol at the DZP CCSD/DZP CISD level. The vibrational frequency of one Si–O bond stretch in the HSiOH dimer is 967 cm−1 at the DZP MP2 level, close to the 951 cm−1 and 986 cm−1 fundamentals observed experimentally for HxSiyOz aggregates. Therefore, it is possible that the HSiOH dimer has been observed in matrices. The potential energy surface of the Si2H radical was studied initially at the DZP CISD level. We found a bent Cs 2A″ Si2H structure which is 10.8 kcal/mol higher in energy than the C2v 2B1 structure. The C2v Si2H structures were optimized at the TZ2P (f,d) CCSD level. The 2B1 state is predicted to lie ΔE0=1.6 kcal/mol lower in energy than the 2A1 state of Si2H radical. © 1996 American Institute of Physics.

Notes: We have studied the temperature dependence of dielectric relaxation times in terms of the peak frequency fmax(T) of dielectric loss ε″(ω) and the dc-conductivity σdc(T) of several glass-forming liquids, covering 12 decades in the peak frequency fmax and 9 decades in σdc. Although dc-conductivity samples the mobility of ionic tracers, its variation with temperature is similar to that of fmax(T). The fmax(T) and σdc(T) are analyzed using the temperature-derivative method and compared to the viscosity data η−1(T). While most liquids reveal a common Vogel–Fulcher–Tammann (VFT) behavior for fmax, σdc and η−1 in an extended temperature range T≥Tm, some liquids deviate from this behavior by displaying a crossover at T=TA to an Arrhenius regime. In these cases the quantity fmax(T) decouples from the common curves for σdc(T) and η−1(T) and attains activation energies in excess (∼40% for alcohols) of those related to translational processes. For many samples a departure from the VFT behavior occurs at lower temperatures TB〈Tm which tends to retard the glass transition. The onset of this qualitative change in the temperature dependence at TB turns out to be a characteristic temperature also in other experiments. © 1996 American Institute of Physics.

Notes: The critical adsorption ellipsometric measurements of five solutions of polystyrene in cyclohexane for different polystyrene molecular weights collapse to a single universal curve when scaled as a function of nBξ/λ, where nB is the polymer solution refractive index, ξ=ξ0Nnt−ν is the correlation length, and λ is the wavelength of incident light in vacuum. From this universal feature we deduce the value of the polymerization critical exponent n=0.258±0.017. We consider both the volume fraction order parameter (cursive-phi) and a symmetrized order parameter (ψs) together with both the renormalization group (RG) and Monte Carlo (MC) simulation forms for the surface scaling function P+(x). The symmetrized order parameter gives significantly better agreement with experiment than the volume fraction order parameter. The combination of RG and ψs provides better agreement with experiment than does the combination of MC and ψs. © 1996 American Institute of Physics.

Notes: The order–disorder transition (ODT) and the ordering kinetics have been studied in two symmetric diblock copolymers of styrene and isoprene and their binary mixtures with rheology. The binary mixtures formed a single microdomain (with lamellar morphology) composed of short and long chains as confirmed by small angle x-ray scattering (SAXS). The order–disorder transition temperature (TODT), obtained from the discontinuity in the storage modulus G′, varies linearly with the number average degree of polymerization. For shallow quenches, the ordering kinetics proceed via nucleation and growth. The characteristic time of this process scales with N¯−1/3, where N¯ is the Ginzburg parameter, in agreement with the theoretical predictions [G. H. Fredrickson and K. Binder, J. Chem. Phys. 91, 7265 (1989)]. Our results indicate that the ordering kinetics of symmetric diblock copolymers near the ODT are fluctuation controlled. © 1996 American Institute of Physics.

Notes: We describe simulations of a polymer melt sample containing 640 chains, each chain being composed of 100 methylene groups. Using these large scale simulations radii of gyration have been calculated with a statistical error of ∼2%. At this level of precision it is shown that even for this modest chain length configurational equilibrium is not achieved in less than 4 ns at 500 K. Values of the square radii of gyration, square end-to-end distances and proportion of conformers at equilibrium show good agreement with predictions of the Flory model of alkane chains in which all long range interactions along the chains are ignored. These simulations were performed using a Fujitsu AP1000 massively parallel processing machine with 1024 processors in conjunction with a recently described domain decomposition molecular dynamics algorithm [D. Brown, J. H. R. Clarke, M. Okuda, and T. Yamazaki, Comput. Phys. Commun. 83, 1 (1994)] capable of handling systems containing rigid bond constraints and threea are made with previous accurate data obtained for smaller samples containing just 10 n=100-site chains using the alternative cloning method of parallel processing [D. Brown, J. H. R. Clarke, M. Okuda, and T. Yamazaki, J. Chem. Phys. 100, 1684 (1994)]. © 1996 American Institute of Physics.

Notes: We report high resolution electron microscope (HREM) observations and atomistic simulations of the bending of single and multi-walled carbon nanotubes under mechanical duress. Single and multiple kinks are observed at high bending angles. Their occurrence is quantitatively explained by the simulations, which use a realistic many-body potential for the carbon atoms. We show that the bending is fully reversible up to very large bending angles, despite the occurrence of kinks and highly strained tube regions. This is due to the remarkable flexibility of the hexagonal network, which resists bond breaking and bond switching up to very high strain values. © 1996 American Institute of Physics.

Notes: The intermediate dynamic light scattering function has been computed for a single A–B diblock copolymer chain in dilute and many-chain solutions, assuming a nonrefractive solvent (i.e., setting opposite contrast factors for the A and B units). Theoretical calculations, based in the Pecora scheme for the Rouse model, have been compared with the results obtained with a dynamic Monte Carlo algorithm for chains with nonoverlapping units placed in a cubic lattice. These numerical functions are also compared with the prediction of a single exponential described by the first cumulant method, which is consistent with the application of the linear response theory. The homopolymer case is also discussed. © 1996 American Institute of Physics.

Notes: We report the results of molecular dynamics simulations of Langmuir monolayers of F(CF2)20F using both a united atom model and a recently developed anisotropic united atom model of the molecular force field. A comparison of our simulation results to experimental measures of the structure of Langmuir monolayers and lamellar crystals of F(CF2)20F indicates that the anisotropic united atom model provides the better representation. Simulations using the new model at 275 K generate an azimuthally disordered rotator phase monolayer, while simulations at 150 K generate a monolayer phase with azimuthal order. The relative intensities of peaks in the in-plane structure function can be used to distinguish between the azimuthally ordered and disordered phases. Our simulations also predict, in agreement with experiment, that at large average areas per molecule the monolayer breaks up into islands that have a close-packed structure at the center but are disordered at the periphery. © 1996 American Institute of Physics.

Notes: We report the results of molecular dynamics simulations of close-packed Langmuir monolayers of F(CF2)11COOH using both a united atom model and a new anisotropic united atom model of the molecular force field. The stimulus for this investigation was the interpretation of the data from a recent grazing incidence x-ray diffraction study of this system [M. Goldmann et al., J. Phys. II France 4, 773 (1994)] to imply the existence of some degree of azimuthal ordering of the molecules in the close packed monolayer. With our new potential field, simulations at 275 K generate a monolayer in an azimuthally disordered rotator phase, while simulations at 150 K generate a monolayer phase with weak azimuthal order. We find that the relative intensities of peaks in the in-plane structure function can be used to distinguish between azimuthally ordered and disordered phases of the monolayer. Our simulation results are consistent with the available experimental data except for the temperature at which azimuthal ordering is detected, which difference is likely due to the inaccuracy of the intermolecular potential used in the simulations. © 1996 American Institute of Physics.

Notes: We apply the optimal control technique (OCT) to design an optical pulse pair that controls the population transfer in a medium of three-level atoms. The absorption and reshaping of the controlling pulses by the medium are taken into account. The efficiency of the population transfer is improved significantly compared with designs that do not incorporate pulse absorption and reshaping. © 1996 American Institute of Physics.

Notes: Rotational spectra for endohedral Li+@C60 and Na+@C60 are calculated at different temperatures. Most of the features in these spectra are related with the degree of anisotropy in the atom–cage interaction. While the low anisotropy for Na+@C60 results in rather simple spectra with the 2B oscillation typical of a diatomic molecule, the more eccentric and anisotropic Li+@C60 produces complex spectra with rotational and librational bands. Some interesting effects are induced by the cage rotation, which has been incorporated through a semiclassical formalism. © 1996 American Institute of Physics.

Notes: Accurate quantum mechanical calculations are carried out to test the sensitivity of the spectroscopy and dynamics of the B state of ArCl2 to the steepness of the Morse term, α, of an atom–atom potential. It is discovered that the predissociation dynamics for this molecule are very complicated even in the Δv=−1 regime due to resonances in the continuum manifold of states. In both the Δv=−1 regime and the Δv=−2 regime the rate of vibrational predissociation and the product rotational distribution are extremely sensitive to the value chosen for α, but not in a regular way. For the Δv=−2 regime the variations can be attributed to spacings between resonances and the overlaps of the bright state wave functions with nearby dark states as expected from the intramolecular vibrational relaxation model. In the Δv=−1 regime, the variations are shown to originate from resonances in the v−1 continuum set of states. Although this makes it difficult to determine the value for α, a value of 1.8 A(ring)−1 is probably close to the true value. The most useful new data to determine the potential would be measurements of the lifetimes for as many vibrational levels as possible and rotational distributions for excitation to low vibrational levels of the B state. © 1996 American Institute of Physics.

Notes: The nonradiative transition rates from the single vibronic levels of the first singlet excited state to the ground state were estimated using a time-dependent method based on Fermi's golden rule. In the present method, the initial wave packet is constructed with the use of the nonadiabatic coupling matrix elements calculated by ab initio molecular orbital method. The wave packet dynamics calculation is carried out using the reaction path Hamiltonian. The vibrational relaxation on the ground state surface is treated by introducing the effective Hamiltonian. The parameters required to construct these Hamiltonians were obtained with the complete active space self-consistent field wave function and the electronic matrix elements of nonadiabatic coupling between the ground and first singlet excited states were calculated with the state-averaged complete active space self-consistent field wave function analytically. The calculated rate constants were in good agreement with the experimental ones. It is found that vibrational relaxation in the ground electronic state is an important factor in obtaining the nonradiative transition rate constants. © 1996 American Institute of Physics.

Notes: A Green function filter operator (H−E.1)−1 is used within a wave operator iterative method for the computation of interior eigenvalues of large Hamiltonian matrices. Test calculations on a coupled oscillator system show a significant improvement in the convergence of the wave operator iterative method when the filter is used with a one-dimensional active space. Less convincing results are obtained with multidimensional active spaces involving several states in near resonance; other filter functions such as (H−E.1)−2 seem to be more suitable for this case. © 1996 American Institute of Physics.

Notes: (2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of ArXe have been recorded between ≈78 000 and 80 110.0 cm−1. Single isotopomer data was obtained using a time-of-flight (TOF) mass spectrometer. Vibrational analyses for several transitions involving ArXe excited states that dissociate to Ar(1S0)+Xe*(6p) are presented, in some instances, for the first time. In addition to vibrational numbering and constants, excited state symmetries were deduced from separate REMPI/TOF spectra recorded with linearly and circularly polarized light, while excited state bond lengths were derived from Franck–Condon factor calculations. Some of the excited states were found to have potential humps and/or unusual vibrational band intensity distributions. Where possible, the nature of the perturbations is discussed. © 1996 American Institute of Physics.

Notes: A physical picture of a conjugated chain as a collection of almost planar segments, separated by large angular breaks arises from a microscopic model which includes conjugation and steric interactions. The conjugation part of the standard phenomenological Hamiltonian for torsional motion is also derived from the model. We obtain a probability distribution of the length of segments between those breaks as the relevant factor for the behavior of the chain. We also perform numerical simulations of the structure and properties of these chains; the results of this are in agreement with our analytic predictions. In explaining experimental data for optical properties, such as the second hyperpolarizability, γ, our theory provides improved agreement over previous models. © 1996 American Institute of Physics.

Notes: Raman intensity calculations have been performed for nine small main-group molecules using the Kohn–Sham density functional method. A combination of numerical and analytic derivation techniques was used as implemented in the program package DEMON. The effect of the applied functional, the basis set augmentation, and the numerical fitting of the exchange-correlation potential have been investigated along with other aspects of the computations. The results obtained at the local level using valence triple-zeta plus 2 polarization functions (VTZP+) basis sets compare well with experiment and with the results obtained from the Hartree–Fock and correlation methods using large basis sets, whereas nonlocal corrections did not yield improvements in the predicted local Raman intensities. Systematic analysis proved the sensitivity of the results obtained with the gradient corrected nonlocal functional to the numerical fitting applied in the calculations of the exchange-correlation terms. We demonstrated that omitting the fitting procedure from nonlocal calculations improves the quality of the Raman intensities while the grid used for fitting does not have an influence on the Raman intensities. Effects of the reference geometry, step size for evaluating the numerical derivatives and the threshold of energy convergence were also tested. © 1996 American Institute of Physics.

Notes: This article investigates a new approach to the optical generation of large-amplitude coherent molecular vibrations in condensed media. On the basis of analytical results using pulse propagators in the classical Franck approximation, we are led to investigate the efficacy of driving vibrational motion in the electronic ground state by impulsive stimulated Raman scattering with a timed sequence of electronically preresonant femtosecond laser pulses. Numerically exact computations are performed on a model system of dilute molecular Iodine in a low-temperature host crystal. Vibrational relaxation is incorporated via Redfield theory. The results indicate that under a variety of conditions, chemically significant (greater than 0.1 A(ring)) displacements can be produced in a Raman active mode with a fair measure of control over wave packet spreading, and without substantial population loss due to electronic absorption. © 1996 American Institute of Physics.

Notes: The nonreactive catalytic system made up of the reduction of NO by CO on a square lattice of sites is studied by means of dynamic equations in the pair approximation, showing some characteristics and limitations of the model in systems with a nontrivial behavior. © 1996 American Institute of Physics.

Notes: Measurements of the coexistence curves for phase-separated polymer solutions of polymethylmethacrylate in 3-octanone (PMMA/3-OCT) for six different molecular weights of PMMA have been carried out. We have tested the scaling behavior of the order parameter Δφ (the difference between polymer volume fractions in the two coexisting phases) for its dependence on the degree of polymerization N of the polymer chains and the reduced temperature ε=(Tc−T)/Tc. The experiments reveal that, in the asymptotic regime, the measured order parameter has power-law dependence on both N and ε, i.e., Δφ=3.8N−0.21ε0.34. Furthermore, when the symmerization procedure of Sanchez [J. Appl. Phys. 58, 2871 (1985)] is used to analyze the data, it suggests that εN0.46 may be used as a scaling variable for phase separated polymer solutions. © 1996 American Institute of Physics.

Notes: We have studied electron stimulated desorption (ESD) of negative ions from PF3 molecules adsorbed on a Pt substrate over a wide electron energy range (0–175 eV). ESD from adsorbed PF3 gives rise to several negative ion fragments: F− (predominantly), F−2, P−, and PF−. The F− yield produced in the electron energy range 0–15 eV proceeds via dissociative electron attachment (DEA); the F− yield exhibits a peak around 11.5 eV with an onset around 7.5 eV. At electron energies above ∼15 eV, F− ions are produced via dipolar dissociation (DD). We have found that the F− ions produced from a 1 ML PF3/Pt surface via a DEA process with 11.5 eV electron impact desorb with a peak kinetic energy of ∼0.7 eV, while the F− ions generated via DD by 175 eV electron impact desorb with a peak kinetic energy of ∼1.2 eV. The F−2 yield curve also shows a peak at ∼11.5 eV; the onset of the F−2 yield from adsorbed PF3 is ∼9 eV. The F−2 yield in the electron range 9–15 eV is initiated via DEA. The P− signal from PF3 adsorbed on Pt has an onset at ∼16 eV. We have identified some possible DEA and DD processes leading to desorption of negative ions from adsorbed PF3. We suggest that Rydberg core-excited (1-hole, 2-electron) transient anion states of PF3, formed by capture of low energy electrons, dissociate to produce the F− and F−2 ions for E(e)〈15 eV. © 1996 American Institute of Physics.

Notes: Molecular dynamics simulations have been used to study friction in Langmuir–Blodgett monolayers of perfluorocarboxylic acid and hydrocarboxylic acid on SiO2. The frictional force of perfluorocarboxylic acid is found to be about three times as large as that of hydrocarboxylic acid. The qualitative aspects of this simulation results are consistent with known experimental results. In order to interpret the difference in the frictional force, a series of simulations have been carried out by changing molecular potential parameters. The simulation results suggest that the 1–4 van der Waals interaction is the main cause of the larger frictional force for perfluorocarboxylic acid than that for hydrocarboxylic acid. The results also show that frictional force is roughly proportional to the excess root mean square fluctuation of the potential energy under shear from the equilibrium. The relation between the frictional force and the energy needed for molecular deformation under shear condition is also discussed. © 1996 American Institute of Physics.

Notes: The 220 nm photolysis of the hydroperoxyl radical, HO2, is investigated by probing the ejected OH fragments using Doppler and polarization spectroscopy. Analysis of the measured line profiles reveals that the OH fragments are predominately (84%) formed with the partner oxygen atom in its electronically excited 1D state with a smaller component (16%) being associated with oxygen atoms in the 3P ground electronic state. Measurement of OH fragment internal state distribution indicates that the 23 200 cm−1 of available energy is primarily released as electronic excitation of the oxygen atom (fel=0.57) and to a lesser extent as relative translation of the products (ftr=0.41). The internal degrees-of-freedom of the OH fragment receive very little of the available energy and are found to be fairly cold (fvib〈0.004 and frot=0.014). For the primary O(1D) dissociation channel the measured 〈μ⋅v〉 correlation is strongly positive (βμv=0.61) indicating a preference for parallel alignment of the electronic transition moment and the recoil velocity vector in HO2, consistent with the excited state being of A″ symmetry. All other bipolar moments are close to zero for this pathway (βμJ=−0.10, βvJ=−0.04, βμvJ=−0.06) independent of the probed rotational quantum state of the OH fragment. For the minor O(3P) pathway a comparable set of bipolar moments is obtained. An investigation into the source of OH fragment rotation reveals that the combined contributions from out-of-plane rotation, generated by initial parent thermal motion about A-inertial axis, and in-plane rotation, generated by the combination of bending mode zero-point energy and final state interaction on the excited potential energy surface, result in negligible 〈v⋅J〉 correlation in the photodissociation of a thermally distributed sample of HO2 at 300 K. © 1996 American Institute of Physics.

Notes: We present a close coupled wave packet method to treat reactive collisions in hyperspherical coordinates. Within this approach, the information needed for the time-dependent propagation is the same as that required for a time-independent close coupling treatment. The method is tested on the simple collinear D+H2 reaction. We compare the time-dependent technique with the more established time-independent close coupling treatment. Finally, we show that time-dependent properties are useful to gain insight into the detailed mechanism of the reaction. © 1996 American Institute of Physics.

Notes: This paper presents a new approach to operator resummation corrections to adiabatic evolution operators. It is shown that an infinite order correction produces an operator that is equivalent to a propagator in the interaction representation. For a problem in which the adiabatic approximation assumes that certain degrees of freedom are held fixed, the interaction representation correction is just the interaction propagator of the coupling for these degrees of freedom. This formulation allows simple physical interpretation and simple mathematical evaluation of the full correction. No power series or cumulant methods are needed. Application to double well splitting when coupled to a bath oscillator shows the approach to be highly accurate. © 1996 American Institute of Physics.

Notes: The dynamics of very high molecular Rydberg states, with special attention to the extreme long time behavior, is discussed using a quantum mechanical multichannel scattering theoretic formulation. Detailed computational results are provided for an electron revolving about a dipolar core in the presence of an external dc field. Two distinct effects are highlighted, trapping and dilution where the former is an origin of long time stability when there are very many bound states but much fewer states that are directly coupled to the continuum. Both trapping and dilution act to elongate the intermediate time decay of the high Rydberg states. The extent of dilution can be varied by changing the magnitude of the external dc electrical field. The formalism and specific results are discussed also towards the implications to other types of unimolecular processes. In particular it is argued that the study of molecular Rydberg states does suggest a possible route to mode selective chemistry. © 1996 American Institute of Physics.

Notes: We show that a comparison of the depletion spectra of Na+n (n=2–9,11,21) clusters recorded at low temperature and optically allowed transitions determined for the stable structures using ab initio methods accounting for electron correlation allows the assignment of the cluster geometry to the measured features. Due to the large mobility of atoms in alkali metal clusters, the influence of temperature on structural and electronic properties is significant. The lowering of temperature reveals new spectroscopic features which are structure dependent. Optical response properties of small cationic Na+n clusters are characterized by rich molecularlike spectroscopic patterns, also with increasing size, and differ substantially from those found for neutral clusters. It has been clearly demonstrated that not only the number of valence electrons but its mutual interplay with the geometric properties determine optical response features. © 1996 American Institute of Physics.

Notes: Ten basis sets of double and triple-zeta quality augmented by polarization and diffusion function in conjunction with density functional theory (DFT, with the Becke–Lee–Yang–Parr exchange-correlation potential), Møller–Plesset MP2 to MP4 (SDTQ), coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitation [CCSD(T)] levels of theory were applied in studies of the molecular geometry and stability of the H2CO...HCl complex. Interaction energy (corrected for the basis set superposition error and zero-point vibrational energy contributions) predicted at three highest levels used; the MP4(SDTQ)/6-311++G(2df,2pd)//MP2/6-311++G(2df,2pd), CCSD(T)/6-311G(2d,2p)//CCSD/6-311G(2d,2p), and MP2/aug-cc-pVTZ (augmented correlation consistent polarized valence triple-zeta)//MP2/6-311+G(2df,2pd) amounts to −2.65, −2.61, and −3.88 kcal/mol, respectively, while the DFT/6-311++G(2df,2pd) level gives −2.86 kcal/mol. It appears that within a given computational method (e.g., MP2, DFT) interaction energy slightly depends on the chosen reference geometry (e.g., optimized at HF, MP2, CCSD, or DFT levels). A significant influence of the BSSE on the interaction energy and molecular geometry of the complex is analyzed. Reliable MP2/6-311++G(2df,2pd) and CCSD/6-311G(2d,2p) levels of harmonic frequencies, infrared intensities and potential energy distribution analysis are presented for the complex and its components and compared to the available experimental data. © 1996 American Institute of Physics.

Notes: Potential energy surfaces for the low-lying electronic states of the Pd2CO complex are studied using the second order perturbation theory of Møller and Plesset (MP2) and the complete active space multiconfiguration self-consistent-field method (CASSCF) followed by large scale multireference singles+doubles configuration interaction (MRSDCI) computations that included up to 3.95 million configuration spin functions (CSFs). The bridged structure was found to be more stable by 1.3–1.5 eV compared with the linear structure depending on the level of theory. The calculated MP2 vibrational frequencies for Pd2CO are compared with the experimentally determined values for different chemisorptive sites. The properties of Pd2CO are compared with the corresponding properties of Pt2CO. © 1996 American Institute of Physics.

Notes: We extend the density functional approach to the statistical mechanics of inhomogeneous fluids to calculate the rate of heterogeneous nucleation of the gas-to-liquid transition by a planar solid substrate. Comparison with classical nucleation theory (extended to incorporate the line tension that results from three-phase contact) reveals the inadequacy of the latter approaches as the spinodal is approached. Wetting and drying transitions have a large effect on the usefulness of classical theory. Free energies of formation for critical heterogeneous nuclei and their shapes and density profiles are calculated from density functional theory. © 1996 American Institute of Physics.

Notes: In this paper we investigated the single particle dynamics in a colloidal crystal by means of video microscopy and digital imaging methods. To obtain the self term of the van Hove function Gs(x,τ) with a reasonable good statistical accuracy we analyzed up to two million particle displacements per time channel τ of Gs(x,τ), which corresponds to the analysis of 3000 video frames, each with approximately 1000 particles. It was found that the Gaussian approximation almost perfectly accounts for the experimental results, so that the time evolution of Gs(x,τ) can solely be described by the mean square displacement 〈x2(τ)〉. The measured mean square displacements were compared with the model of a Brownian particle bound in a harmonic potential [Physica A 157, 705 (1989)]. The experimental curves approach the plateau value 〈x2(τ→∞)〉 of the mean square displacement much slower than is predicted by the model. From the plateau value of the mean square displacement the Lindemann parameter was determined for various samples. © 1996 American Institute of Physics.

Notes: Analytical approximations for the pair direct correlation function (DCF) of molecular fluids and their mixtures are derived within the frame of a new formalism based on weighted density functional methods which represents a generalization of Rosenfeld theory for hard spheres mixtures [J. Chem. Phys. 89, 4271 (1988)]. These approximations rest upon the geometrical properties of individual molecules such as the volume, the surface, and the mean radius. They are Percus–Yevick (PY) like in nature and reduce to the analytical PY solution for DCF in the hard sphere case. By construction the approximations incorporate several interesting features: They yield the Mayer function in the low density limit as expected, and they are anisotropic at zero separation as well as at contact. In addition they predict an orientational instability of the isotropic phase with respect to the nematic phase, a feature that is absent from the Percus–Yevick theory. Comparisons are made with the Percus–Yevick numerical results for the DCF for various convex hard bodies such as hard ellipsoids of revolutions (prolate and oblate), prolate spherocylinders, cutspheres, and generally the agreement is very good for a large range of liquid densities. Analytical expressions for the virial and compressibility routes for the pressures are also given. The results obtained for a large varieties of convex bodies are in very good agreement with corresponding numerical Percus–Yevick results. These approximations can be generalized to inhomogeneous systems in a straightforward manner. © 1996 American Institute of Physics.

Notes: The effect of pressure on the electron mobility in mixtures of n-hexane and 2,2-dimethylbutane was studied over the whole concentration range. The variation of mobility with pressure is discussed in terms of a two-state model, and provides information on the volume changes occurring on localization. The observed volume changes are interpreted as the difference between the cavity volume and the electrostriction volume of the localized electron. © 1996 American Institute of Physics.

Notes: A new theory is presented for calculating the Helmholtz free energy based on the potential energy distribution function. The usual expressions of free energy, internal energy and entropy involving the partition function are rephrased in terms of the potential energy distribution function, which must be a near Gaussian one, according to the central limit theorem. We obtained expressions for the free energy and entropy with respect to the ideal gas, in terms of the potential energy moments. These can be linked to the average potential energy and its derivatives in temperature. Using thermodynamical relationships we also produce a general differential equation for the free energy as a function of temperature at fixed volume. In this paper we investigate possible exact and approximated solutions. The method was tested on a theoretical model for a solid (classical harmonic solid) and some experimental liquids. The harmonic solid has an energy distribution, which can be derived exactly from the theory. Experimental free energies of water and methanol could be reproduced very well over a temperature range of more than 300 K. For water, where the appropriate experimental data were available, also the energy and heat capacity could be reproduced very well. © 1996 American Institute of Physics.

Notes: The electron-spin-resonance spectra were measured for free 6Li7 clusters by using techniques of molecular beam magnetic resonance. The spectra show a large hyperfine constant a1 from two equivalent nuclei and a much smaller hyperfine constant a2 from an additional five equivalent nuclei. The cluster geometry consistent with the spectra is a regular pentagonal bipyramid. It has been found that the septemer has two isomers which give the same value for a1 to a high accuracy and slightly different values a2a and a2b for a2. There is a relationship between a1 and a¯2=(a2a+a2b)/2 of the form a1/5a¯2=1.03(approximately-equal-to)1. The a1 and a2 indicate that the septemer is fluctuating in structure between two geometries, the regular pentagonal bipyramid and a deformed one of it. The former corresponds to an oblate ellipsoid of revolution in the ellipsoidal shell model, while the latter means a superposition of five equivalent deformations from the oblate ellipsoid to prolate ellipsoids. The deformation must be symmetrized by the superposition, because the ground electronic state of Li7 has no orbital degeneracy in spite of the fivefold axial symmetry of the structure. The symmetrized shape oscillation keeps the vibrational motions of the septemer constant, because the five equivalent deformations mutually cancel all restoring forces. The two ellipsoidal configurations, oblate and prolate, are brought into resonance by a Jahn–Teller effect which takes place on the prolate side. The presence of the two like isomers provides evidence of the Jahn–Teller effect, because there are two mutually independent modes of the Jahn–Teller distortion which divide the septemers into two symmetry types. The simple relation a1(approximately-equal-to)5a¯2 further confirms that the electronic states in the two ellipsoidal configurations are in resonance. © 1996 American Institute of Physics.

Notes: Electronic properties of silicon–fluorine cluster anions (SinF−m; n=1–11, m=1–3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon rod in an He carrier gas mixed with a small amount of SiF4 or F2 gas. The highly abundant clusters are SiF−m (m=3 and 5) and SinF− (n=6, 7, and 10) in their mass spectra. In the photoelectron spectra of SiF−m (m=1–5), the clusters having odd m have higher electron affinity (EA) than those having even m, indicating that the even/odd alternation in EA is attributed to their electronic structures of a closed/open valence shell. Comparison between photoelectron spectra of SinF− and Si−n (n=4–11) gives the insight that the doped F atom can remove one electron from the corresponding Si−n cluster without any serious rearrangement of Sin framework, because only the first peak of Si−n, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping. In some clusters, furthermore, the vibrational structures could be resolved to determine a vibrational frequency and to presume the geometry with ab initio molecular orbital calculations. © 1996 American Institute of Physics.

Notes: Quasiclassical trajectory and quantal calculations are carried out for the Penning ionization system N2+He*(23S)→N+2(2Σg,2Πu,2Σu)+He+e− based on an ab initio resonance potential and energy widths which were obtained in previous work. Total and partial ionization cross sections are evaluated for the collision energy range of 0.1–1.0 eV. For the trajectory calculation, the collisional energy dependence of the cross section is in better agreement with a recent experiment on state resolved Penning ionization than calculations using the classical sudden approximation. The result in the high frequency rotation limit is significantly different from that for the sudden approximation, which is in contrast to the H2–He* system. The results for quantal calculations using the sudden and spherical-potential approximations confirm the reliability of classical treatments. The results obtained suggest that analyses with the widely used atomic-target models lose their validity for significantly anisotropic systems in which targets have large moments of inertia. © 1996 American Institute of Physics.

Notes: State-to-state differential cross sections have been measured for scattering of NO(2Π1/2,j=0.5) by Ar at kinetic energies of 117, 149, 442, and 1694 cm−1. The differential cross sections at each collision energy are presented as a function of final state (Ω′,j′) at constant center-of-mass scattering angle. Center-of-mass angular distributions are also given for final rotational states (2Π1/2, j′=1.5, 2.5, 8.5, 12.5, and 14.5) at a kinetic energy of 442 cm−1, and for j′=18.5 at a kinetic energy of 1694 cm−1. Rotational rainbow structure is seen in both types of data. The results are generally in good agreement with quantum scattering calculations carried out by Alexander [J. Chem. Phys. 99, 7725 (1993)] using newly calculated ab initio potential energy surfaces, and thus may serve as a new benchmark for the microscopic dynamics of molecular energy transfer in open-shell molecules. © 1996 American Institute of Physics.

Notes: In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of n¯=6 to n¯=90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by n¯=38.4(Γ*/1000)1.64, where Γ* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size. © 1996 American Institute of Physics.

Notes: On the assumption of external bath equilibrium, a set of simultaneous linear generalized Langevin equations (GLE) for a microscopic Hamiltonian is derived, whose potential function includes cubic (i.e., nonlinear) coupling terms, which are linear in internal coordinates but quadratic in external bath coordinates. Furthermore, on the linear GLE treatment, a closed expression of time-dependent friction coefficient and a rate constant in the Grote–Hynes theory (GHT) are derived microscopically, reflecting the reactant and solvent structures. By comparing the rate constant of GHT with that of the multidimensional transition-state theory (TST) for the whole solution system, we conclude that these rate expressions are different from each other and the deviation is due to the dynamic effect via the nonlinear coupling among the reaction, internal, and external normal coordinates. Moreover, the friction coefficient depends on temperature and the deviation becomes larger with temperature increasing. By the second-order perturbation theory, we have estimated the deviation which is approximately equal to a transmission coefficient κ, for a real cluster reaction system: the formic acid–water–water system. We have obtained κ of 0.92, which is smaller than unity. A mode analysis shows that two hindered translational motions of the solvent with low frequencies prevent the reaction from proceeding. Besides, we have investigated the isotope effect of a medium water molecule and found that the dynamic isotope effect for the reaction is quite large, i.e., κ for heavy water is much smaller than that for light water. Not the change of the reactive frequency on the free energy surface but that of the frictional effect in the deuterium substitution mainly contributes to the isotope effect. Further, the temperature dependence of κ for the reaction has been estimated and it is found that κ becomes smaller with temperature increasing and the change of the frictional effect in temperature contributes to the temperature dependence of κ more largely than that of the reactive frequency on the free energy surface. © 1996 American Institute of Physics.

Notes: Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all 12 samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experiment. The GCMC results are compared with a strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–Ar system are compared. © 1996 American Institute of Physics.

Notes: The vibrational relaxation and the diffusion of diatomic molecules in the zeolite silicalite have been studied through molecular dynamics simulations in the microcanonical statistical ensemble. The adopted model accounts for the vibrations of the framework and sorbed atoms using a harmonic potential for the silicalite and a Morse potential for the diatomic molecule. The results show that the framework favors the relaxation of diatomics oscillating at frequencies near to its characteristic vibrational frequencies, leading in such cases to lower relaxation times and to an increasing in the energy exchanged per collision. The diffusion of a two-site oscillating molecule representing ethane has been also investigated; the diffusion coefficient and the heat of adsorption agree very well with the experimental data. Arrhenius parameters for the diffusion have been calculated, and some insights into the diffusion mechanism have been obtained from log–log plots and by inspection of the distribution of the ethane molecules in the silicalite channels. Therefore the simplified model adopted seems to adequately describe the diffusive motion and the guest–host energy exchanges, and it could be useful in order to study simple bimolecular reactions in zeolites. © 1996 American Institute of Physics.

Notes: In order to examine the effect of fluctuating (nonpreaveraged) hydrodynamic interaction on the hydrodynamic-radius expansion factor αH and also the viscosity-radius one αη, a Monte Carlo simulation study is made on the basis of the polymethylenelike rotational isomeric state chain in the Zimm rigid-body ensemble approximation. All the results obtained are consistent with the recent finding (by Yamakawa's group) that the experimental values of αH are appreciably smaller than the values predicted by any available theory and rather agree with the theoretical and experimental values of αη, indicating that the effect is significant on αH but is negligibly small on αη if any. The present results along with the recent experimental ones also confirm the validity of the quasi-two-parameter scheme for αH and αη as well as for the gyration-radius expansion factor αS; i.e., these expansion factors are functions only of the scaled excluded-volume parameter z˜ defined in the Yamakawa–Stockmayer–Shimada theory of the excluded-volume effect in the Kratky–Porod wormlike chain and the helical wormlike chain. © 1996 American Institute of Physics.

Notes: Experimentally motivated parameters from a base-pair-level discrete DNA model are averaged to yield parameters for a continuum elastic rod with a curved unstressed shape reflecting the local DNA geometry. The continuum model permits computations with discretization lengths longer than the intrinsic discretization of the base-pair model, and, for this and other reasons, yields an efficient computational formulation. Obtaining continuum stiffnesses is straightforward, but obtaining a continuum unstressed shape is hindered by the "noisy'' small-scale structure and rapid helix twist of the discrete unstressed shape. Filtering of the discrete data and an analytic transformation from the true normal-vector field to a natural (untwisted) frame allows a stable continuum fit. Equilibrium energies of closed rings predicted by the continuum model are found to match the energies of the underlying discrete model to within 0.5%. The model is applied to a set of 11 short DNA molecules (≈ 150 bp) and properly distinguishes their cyclization probabilities (J factors) when compared both to experimental cyclization rates and to Monte Carlo simulations. The continuum model does not include entropic contributions to the free energy. However, because of its rapid and accurate computation of internal energy, the continuum model should, when combined with further work on entropic effects, be a useful method for computing experimental DNA free energies. © 1996 American Institute of Physics.

Notes: Resonant multiphoton detachment spectroscopy has been used to obtain vibrationally resolved spectra of the C 2Π←X 2Π electronic transitions in C−4, C−6, and C−8. Transitions due to vibrational excitations in the totally symmetric stretching modes as well as the bending modes are observed. The electron detachment dynamics subsequent to multiphoton absorption are studied by measuring the electron emission time profiles and electron kinetic energy distributions. The observation of delayed electron emission combined with the form of the electron kinetic energy distributions indicates that these species undergo the cluster equivalent of thermionic emission. This interpretation is supported by comparing the experimental results to a microcanonical model for cluster thermionic emission. © 1996 American Institute of Physics.

Notes: Rotational spectral lines of the H2CSi molecule are observed in the millimeter-wave and submillimeter-wave regions. The molecule is produced in a glow-discharge plasma of a gaseous mixture of SiH4 and CO. Rotational constants and centrifugal distortion constants have precisely been determined from observed frequencies of 32 a-type transitions with J=5–4 to J=12–11 and Ka=0 to Ka=4. From the observed inertial defect, the vibrational frequency of the CH2 rocking mode is estimated to be as low as 331 cm−1, which is consistent with the large quartic centrifugal distortion constants, DJK and d2. Higher-order centrifugal distortion constants up to the octic terms are necessary to obtain a good fit between the observed and calculated frequencies within experimental uncertainties. The low vibrational frequency and the necessity of the higher-order centrifugal distortion terms indicate a floppy nature of the CH2 rocking mode. A preliminary radioastronomical search for H2CSi in space has been carried out toward a few astronomical sources without success. © 1996 American Institute of Physics.

Notes: A classical-quantum correspondence study of a saddle-node bifurcation in a realistic molecular system is presented. The relevant classical structures (periodic orbits and manifolds) and its origin are examined in detail. The most important conclusion of this study is that, below the bifurcation point, there exists an infinite sequence of precursor orbits, which mimic for a significant period of time the (future) saddle-node orbits. These structures have a profound influence in the quantum mechanics of the molecule and several vibrational wave functions of the system present a strong localization along the saddle-node periodic orbits. A striking result is that this scarring effect also takes place well below the bifurcation energy, which constitutes a manifestation of the so-called "ghost'' orbits in configuration and phase space. This localization effect has been further investigated using wave packet dynamics. © 1996 American Institute of Physics.

Notes: The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphous ices (LDA and HDA), and a predicted fragile–strong transition are accounted for in a unified manner by reconciling an idea due to Stanley and co-workers introducing a second critical point separating LDA and HDA ices with a conjecture proposed by Speedy that LDA is a different phase from a normal water, called water II. The reconciliation is made on the basis of results from extensive molecular dynamics simulations at constant pressure and temperature. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural, and dynamic properties at atmospheric pressure, suggesting liquid–liquid phase transition. This transition is identified with an extension of the experimentally observed LDA–HDA transition in high pressure to atmospheric pressure. Thus, we propose a new phase diagram where the locus of the second critical point is moved into negative pressure region. With this simple modification, it becomes possible to account for the divergence of the thermodynamic response functions at atmospheric pressure in terms of the critical point and the spinodal-like instability of HDA. The unstable HDA undergoes a transition to LDA phase in lower temperature. The transition is also observed in high pressure region such as 200 MPa while it disappears at negative pressure, −200 MPa. This reinforces our proposed phase diagram in which there is no continuous path from a supercooled state to LDA at atmospheric pressure. It is argued that the HDA–LDA transition is accompanied by a fragile–strong transition. A possible mechanism of avoiding crystallization of aqueous solutions is also discussed in terms of a difference in hydrogen bond number distribution between LDA and HDA. © 1996 American Institute of Physics.

Notes: The observation and analysis of the high resolution spectrum of the CH2 rocking fundamental of the ethyl radical, C2H5, at wavelengths close to 18.9 μm is reported. The band origin is found to be at 528.1 cm−1. The spectrum shows evidence for a very low barrier to internal rotation, or torsion, in this species. A simple model Hamiltonian, based on an assumed structure with G12 symmetry, qualitatively reproduces the observations and implies a torsional barrier of approximately 20 cm−1 in both the zero point and excited vibrational states. The Hamiltonian cannot reproduce the observations to the level of the experimental accuracy and the importance of several neglected terms is tested and discussed. The observed torsional splittings imply that, within the confines of the model, the a-inertial and internal rotation axes in the molecule are coincident to within 1°. In addition to their intrinsic interest, the spectra will be useful for future state resolved studies of the kinetics of chemical reactions involving the radical. © 1996 American Institute of Physics.

Notes: We present a comparative study on the ultrafast nonlinear optical response of a novel conjugated zinc porphyrin system. The linear optical absorption spectra of these molecules all show the Q-band and B-band transitions of the basic porphyrin unit. We have taken spectrally resolved ultrafast pump–probe measurements on monomer, dimer, and polymer solutions, which allows us to compare their excited state dynamics and relate these to their linear optical absorption. The spectra show several common features, but these features have markedly different decay dynamics. The bleaching is preferential in the Q band for the polymer and the B band for the monomer. The polymer Q-band bleaching shows a two-component decay, of approximately 700 fs and 170±50 ps time constants in a biexponential fit, which we attribute to both exciton–exciton annihilation and exciton diffusion to recombination centers on the polymer chain. The Q band of the dimer also has a two-component decay with 13±5 and 1250±70 ps time constants which we attribute to rotational diffusion of the excited molecule in solution, and decay to the ground state, respectively. The B-band bleaching in the monomer is long lived and has a decay constant of approximately 3.5±0.5 ns; from the absorption recovery of the B band we estimate a triplet yield of 0.8. All molecules exhibit broad π*−π* absorptions in the visible spectral region (between the Q band and the B band). In particular, we show that the monomer has potential as a broadband optical limiter in the visible region from 455 (2.72 eV) to 620 nm (2.00 eV); we estimate that its excited state absorption cross section is 8.5 times that of its ground state cross section at 532 nm (2.33 eV). There is also clear evidence of triplet transitions in the dimer and monomer; the triplet absorption feature ranging from 940 (1.32 eV) to 1000 nm (1.24 eV) rises 10 ps after excitation in the dimer which suggests a short intersystem-crossing time. © 1996 American Institute of Physics.

Notes: A new technique for constructing representations of point groups, called symmetrized boson representation is introduced. This method is particularly useful when describing vibrations of large molecules and for high overtones. The method is illustrated by applying it to stretching overtones of octahedral molecules, SF6, WF6, and UF6 with group Oh. © 1996 American Institute of Physics.

Notes: The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment. © 1996 American Institute of Physics.

Notes: Spinodal decomposition of colloids in the initial and intermediate stages is described on the basis of the Smoluchowski equation. Hydrodynamic interaction is treated in an approximate way. In the intermediate stage, where a dominant length scale exists, the static structure factor is found to exhibit universal scaling behavior. The corresponding dynamic scaling function is derived from the nonlinear equation of motion for the static structure factor. This scaling function is a much more sharply peaked function of the wave vector than the well-known empirical Furukawa scaling function, which applies to the transition stage where sharp interfaces contribute considerably, giving rise to a Porod tail at larger wave vectors. The wave vector where the static structure factor exhibits a maximum is found to vary with time—in the intermediate stage—like ∼t−α, where the exponent α varies between 0.2 and 1.1, depending on the relative importance of hydrodynamic interaction. Experiments on a microemulsion and binary polymer melt confirm the predicted scaling behavior. © 1996 American Institute of Physics.

Notes: Cluster formation of Lennard-Jones particles (65 536 atoms in a unit cell with an overall number density equal to 0.0149) was simulated by molecular dynamics. The temperature was set to decrease linearly with time by various thermostats, starting from a gas state temperature and ending at zero temperature. With the Nosé–Hoover thermostat, it was found that the translational temperature of the clusters suddenly decreased almost to zero when the cluster formation drastically increased around a reduced temperature (T*) of 0.5, while the internal temperature decreased linearly. Using the Andersen thermostat, which could simulate the aggregation of particles in an inert gas, both the internal and translational temperatures decreased almost linearly with time. When these thermostats were used, cluster–cluster and cluster–atom collisions did not give any magic number peaks in the size distribution up to 250 atoms/cluster at any temperature. Careful tracing of the cluster growth of 13-atom clusters showed no difference in reactivity between icosahedral and nonicosahedral clusters. To simulate cooling in a supersonic jet, a thermostat which controlled only the translational temperature was introduced. After the clusters were formed by cooling the system with this thermostat, their internal temperature stayed at T*≈0.5, while the translational temperature decreased linearly to zero with time as it was controlled. A long-time evaporation from these high-temperature clusters gave peaks at 13 and 19 (and less significantly at 23 and 26) which are magic number sizes corresponding to single, double, triple, and quadruple icosahedra, respectively. The internal temperatures of 13- and 19-atom clusters were higher than those of other size clusters. Higher evaporation energy was observed for the clusters of 13, 19, 23, and 26 atoms than for other size clusters after the long-time evaporation, but only the 13-atom clusters had the higher evaporation energy after cooling by the Andersen thermostat. These results suggest that magic number clusters were formed by evaporation to be trapped at the magic number sizes, and not by either cluster–atom or cluster–cluster collisions. Analyses of the radial distribution functions and the overall shapes of the generated solidlike clusters consisting of many isomers revealed the following characteristic features: The clusters around 13 and 26 atoms were close to being spherical, and the clusters around 19 atoms were oblate. Clusters around 13 atoms had an icosahedron-based structure. The clusters around 55 atoms formed by the Nosé–Hoover and the Andersen thermostats were close to spherical and had an ordered structure. Clusters from 30 to 50 atoms had a disordered structure or a mixture of the different series of structures. © 1996 American Institute of Physics.

Notes: It is shown in this paper that the wetting behavior of n-alkanes on a water surface can be predicted using the wetting theory of Cahn, together with experimental results for n-alkane vapor adsorption on water which are readily available in the literature. The evolution of the wetting behavior with the carbon number of linear alkanes and with temperature is determined and wetting temperatures are calculated. The results agree with experimental observations at room temperature, except for pentane which appears to be a limiting case. The determination of the wetting temperature demonstrates that in order to account for the wetting behavior, additional effects of long-range forces have to be taken into account. © 1996 American Institute of Physics.

Notes: The adsorbed state of thiophene on Si(100)-(2×1) surface at 300 K has been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). (2×1) LEED pattern at 300 K is sustained after the saturated exposure of thiophene, and the saturation coverage is estimated to be ∼0.6 by AES, suggesting that thiophene molecule is chemisorbed molecularly on the Si(100) surface most likely by σ bonds between C and Si atoms. UPS spectrum for the chemisorbed thiophene shows not only the π orbital shift but also the σ orbital shift. Semiempirical PM3 calculations based on the cluster model propose that the thiophene molecule adsorbs on the Si(100)-(2×1) surface by forming di-σ bonds between C atoms of thiophene and Si atoms of the surface. © 1996 American Institute of Physics.

Notes: We have extended two previously introduced models of n-alkanes to numerically investigate the liquid-state dynamics of branched alkanes. We compare our results with new experimental measurements of diffusion and 13C-NMR T1 relaxation. Significant systematic and quantitative agreement is found between simulated and experimental values. We demonstrate the role of branching in controlling the temperature dependence of diffusion and point out the effect of global single-chain relaxation processes on the local intramolecular dynamics probed by the 13C-NMR experiment. © 1996 American Institute of Physics.

Notes: We use flow light scattering to study the phase transitions in semidilute solutions of polystyrene and polybutadiene in dioctylphthalate under simple shear flow. The phase-separated solution was brought into a single-phase state by increasing the shear rate, γ(overdot), above a critical shear rate, γ(overdot)c,i. The solution was then brought back into a two-phase state by lowering γ(overdot) below a critical shear rate, γ(overdot)c,d. We previously reported a large hysteresis effect in solutions with off-critical compositions; γ(overdot)c,i is always higher than γ(overdot)c,d. Shear-drop experiments were conducted to illuminate the origin of this hysteresis effect. The experimental results showed that at temperatures close to the cloud point temperature the formation of phase separated structures did not occur until up to 22 h after lowering the shear rate below γ(overdot)c,i. Thus the hysteresis effect was found to be due to a surprisingly slow ordering process at γ(overdot) close to γ(overdot)c,i. The ordering induced by the shear drop needs a much longer time than the homogenization induced by increasing shear. If the time scale of observation is sufficiently long, the hysteresis effect disappears, yielding the drop of cloud point temperature with shear, ΔTc(γ(overdot)), given by ΔTc(γ(overdot))∝γ(overdot)1.0±0.1 for the off-critical mixtures, rather than ΔTc(γ(overdot))∝γ(overdot)0.5±0.1 found previously for the near critical mixtures. Finally, the incubation time was found to initially increase with γ(overdot) and then decrease with a further increase of γ(overdot), suggesting that the ordering mechanisms are different in the low and high shear-rate regimes. © 1996 American Institute of Physics.

Notes: We examine the dissociative adsorption of hydrogen on metal surfaces by employing a reduced dimensionality quantum scattering method. Using hyperspherical coordinates, we show how the six-dimensional (6D) H2/metal surface dissociation problem may be reduced to two dimensions by treating explicitly the bonds breaking and forming, while adding zero-point energy corrections to the effective potential for all other degrees of freedom. We present calculations of reaction probabilities for dissociation of H2 on Ni(100) in three dimensions and on Cu(111) in four dimensions using this approach, and show their substantial agreement with those obtained using far more expensive exact methods. Five-dimensional and 6D results for H2/Cu(111) are also presented, and compared to the lower-dimensional results. © 1996 American Institute of Physics.

Notes: The dynamic specific heat Cp(ν) and thermal conductivity, κ, of the simple glass-forming liquid di-n-butylphthalate are measured over a wide frequency range (4 mHz–8 kHz) by a nonadiabatic ac technique due to Birge and Nagel. The relaxation frequency obtained from the specific heat is found to be non-Arrhenius down to the lowest temperatures attained in this measurement. The temperature dependence of the relaxation frequency is in close agreement with that obtained from shear modulus and dielectric susceptibility measurements indicating that all motions in the supercooled liquid are strongly coupled to one another. The shape of the relaxation spectrum changes with temperature. The width of the relaxation is similar to that of the dynamic specific heat of other supercooled liquids. The thermal conductivity is frequency independent through the calorimetric glass transition. The static specific heat is reported over a wide range in temperature extending up to 100 K above the melting point. No evidence is found for thermodynamic anomalies accompanying the proposed formation of spatial inhomogeneities in the liquid. The high frequency relaxation shape exhibits a power law which crosses over to a weaker frequency dependence as seen in the dielectric response. © 1996 American Institute of Physics.

Notes: Semiempirical molecular orbital calculations are performed to study the physisorption of carbon monoxide molecules at the MgO (100) surface. This surface is simulated by clusters of different surface and layer sizes. Besides the submonolayer coverage four different types of monolayers are investigated. The stability and structure of the adsorbate system are determined. An adsorption energy per unit cell of the substrate is defined for a comparison of the stability of different monolayers with varying coverages. Agreement is found between the theoretical and experimental overlayer structure. © 1996 American Institute of Physics.

Notes: The photofragment excitation spectra for O(1D) production from the 317–327 nm photolysis of ozone under supersonic free-jet and low-temperature flow conditions show structure superimposed on an underlying continuum. Doppler profiles of the nascent O(1D) photofragments confirm that the O(1D) formed by photolysis at the wavelengths of the peaks in the photofragment excitation spectrum arises from the hitherto unobserved spin-forbidden predissociation to O(1D)+O2(X 3Σg−) products. © 1996 American Institute of Physics.

Notes: The absorption spectrum of pyrazine to the S2 electronic state can be usefully described by a 4-mode system interacting with a 20-mode bath. In this paper wave packet propagation techniques, using the multiconfiguration time-dependent Hartree approach, are used to study this problem. The investigation was made in stages so as to study the nature of the wave function needed to correctly describe various properties of this multimode problem: the absorption spectrum; the energy exchange between the system and the bath; and the rate of inter-state crossing. It was found that, despite the relatively weak system–bath coupling, a multiconfigurational wave function was necessary to describe the interaction between the two parts of the problem. While it was not possible to treat the full 24-mode problem with such a wave function, the spectrum for a 14-mode system, which includes all the important bath modes, has been calculated in this way. The results, in agreement with the path integral calculations of Krempl et al. [J. Chem. Phys. 100, 926 (1994)], show that the effect of a model bath linearly coupled to the system is to reduce the vibrational structure of the spectrum, so as to produce a broad envelope analogous to that observed experimentally. The details of the spectrum are however different for the two methods. The effect of introducing anharmonicity to the bath was also studied, with the result that this leads to a yet broader spectrum.

Notes: We report here on the first high resolution infrared absorption spectra of the semistable nitrile oxide, NCCNO. All of the fundamental modes of vibration (except for the lowest-frequency bending mode) and several combination bands have been measured with a Fourier transform spectrometer at a resolution of 0.005 cm−1. In this paper, we present analyses of υ4, the C–C stretching mode at 714.753 94(6) cm−1, υ6, the NCC bending mode at 403.925 97(6) cm−1, υ6+υ7 at 490.123 62(6) cm−1, and the tentatively assigned υ5+υ6 combination band at 826.291 86(8) cm−1. A simultaneous least squares fit of these four bands gives ground state rotational constants of B0=0.077 085 54(34) cm−1 and D0=4.570(30)×10−9 cm−1. © 1996 American Institute of Physics.