ALBERT

Notes: The formation of C2H− is observed in two broad resonance bands when C2H2 is irradiated with vuv light. The higher-energy band has partially resolved structure, approximately linear pressure dependence, and a threshold at 16.335±0.021 eV. It is attributed to photoion-pair formation (C2H−+H+) consequent upon predissociation of one or more Rydberg states. This threshold, together with IP(H) and EA(C2H), gives D0(HCC–H)≤5.706±0.022 eV≡131.6±0.5 kcal/mol, or ΔH0f0 (C2H)≤134.5±0.5 kcal/mol. The lower-energy band has an approximately quadratic pressure dependence and curved step-like structure. It is attributed to photoelectron-induced dissociative attachment mediated by a πg shape resonance. The threshold, at 878.5±2.0 A(ring), corresponds to a photoelectron energy of 2.715±0.032 eV. This threshold combined with EA(C2H)=2.969±0.010 eV, yields D0(HCC–H)≤5.684±0.033 eV≡131.1±0.7 kcal/mol, or ΔH0f0 (C2H)=134.0±0.7 kcal/mol.

Notes: The time-dependent form of the Lippmann–Schwinger integral equation is used as the basis for a novel wave-packet propagation scheme. The method has the advantage over a previous integral equation treatment in that it does not require extensive matrix inversions involving the potential. This feature will be important when applications are made to systems where in some degrees of freedom the potential is expressed in a basis expansion. As was the case for the previous treatment, noniterated and iterated versions of the equations are given; the iterated equations, which are much simpler in the present new scheme than in the old, eliminate a matrix inversion that is required for solving the earlier noniterated equations. In the present noniterated equations, the matrix to be inverted is a function of the kinetic energy operator and thus is diagonal in a Bessel function basis set (or a sine basis set, if the centrifugal potential operator is incorporated into an effective potential). Transition amplitudes for various orbital angular momentum quantum numbers can be obtained from: (1) Fourier transform of the amplitude density from the time to the energy domain, and (2) direct analysis of the scattered wave packet. The approach is illustrated by an application to a standard potential scattering model problem.

Notes: Collisional removal rate constants for the OH radical in v=12 of the ground electronic state are measured for the colliders CO2, O2, N2, H2, He, and Ar. OH molecules, generated in v=8 by the reaction of hydrogen atoms with ozone, are excited to v=12 by direct overtone excitation with pulsed infrared laser light. The temporal evolution of the v=12 radicals is probed as a function of collider gas pressure by a time-delayed pulsed ultraviolet probe laser. The probe laser is used to excite the molecules via the B 2Σ+–X 2Πi(0,12) electronic transition, and the resulting B 2Σ+–A 2Σ+ fluorescence is detected. We measure rate constants for CO2:(5.6±1.5)×10−11; O2:(1.6±0.2)×10−11; He:(3.6±0.6)×10−12; H2:(3.0±0.8)×10−12; Ar:(2.6±0.5)×10−12; N2:(2.5±0.7)×10−12 (all in units of cm3 s−1). These rate constants are over fifty times faster in all cases than the vibrational relaxation rate constants for the lower levels (v=1 and v=2) of the ground state.

Notes: The electronic and dynamic properties of all-trans polyacetylene have been calculated on the basis of oligomer calculations on H–(CH)10–H and H–(CH)22–H at the ab initio 6-31G level. The calculated ir and Raman intensities are in good agreement with the experimental relative intensities.

Notes: We explore a mechanism for the remarkable charge isolation of the localized, or trapped, electrons found in the crystalline electrides Cs(18C6)2 and Cs(15C5)2. 133Cs NMR measurements show only ≈ 0.05% atomic character of the spin density at the Cs nucleus, consistent with many features of the structure and measured properties which indicate that the localized electron distribution is centered at the anion vacancies. The optical absorption data suggest that the localized electrons, which give rise to the Curie-law spin susceptibility, must penetrate appreciably into the crown ethers, (18C6) and (15C5), which encapsulate the Cs. We suggest that the large reduction of the spin density at the Cs nucleus is due to a Coulomb barrier resulting from negative charge on the oxygens. A crude model, one electron moving in two spherical charged shells surrounding the Cs core, illustrates the mechanism and accounts accurately for the ratio of spin densities at the Cs nucleus found in the 18C6 and the 15C5 electrides. Hartree–Fock calculations for an idealized model of an isolated Cs(18C6)2 molecule, namely Li(9C3)2, tend to support the mechanism.

Notes: The macroscopic properties of two-phase random heterogeneous media depend upon an infinite sequence of n-point functions S(i)n(x1,x2,...,xn) giving the joint probability of finding n points with positions x1,x2,...,xn all in phase i. This paper reports the first study and calculation of the two-point probability function S(i)2 for distributions of oriented, hard spheroids with eccentricity ε in a matrix. This is a useful model of statistically anisotropic two-phase media, enabling one to examine the special limiting cases of oriented disks (ε=0), spheres (ε=1), and oriented needles (ε=∞).

Notes: A modified dynamic reaction coordinate algorithm for tracing reaction paths is implemented in the framework of ab initio molecular orbital calculations. This method requires fewer energy and gradient evaluations than the traditional intrinsic reaction coordinate methodology and produces reaction pathways of acceptable accuracy. The approach is applied to the 1,5 hexadiene Cope rearrangement for which we trace the pathways passing through the chair and boat transition states. Analysis of the lowest energy pathway indicates that the rearrangement is concerted and synchronous.

Notes: Dynamic percolation theory is used to obtain the tracer diffusion coefficient in binary mixtures of "noninteracting'' lattice gas (with only the blocking interactions, i.e., double occupancy of a lattice site is forbidden) within the effective medium approximation (EMA). Our approach is based on regarding the background particles as a changing random environment. The result is expressed in terms of two fluctuation time parameters which we attempt to determine self-consistently. We compare two possible choices for these parameters which are consistent with our former results for the single component system. The resulting tracer diffusion coefficient for both choices compares well with numerical simulations whenever single bond EMA is expected to be reliable. Comparison is also made with the theoretical results of Sato and Kikuchi [Phys. Rev. B 28, 648 (1983)] and discrepancies between both theories are discussed.

Notes: The orientational properties of crystalline A–TCNB are calculated using a Monte Carlo procedure, based upon a previously reported site–site expression for the interactions between molecules. The nearly static TCNB complexes are orientationally quiescent. The A molecules respond to an orientational double well potential provided by the TCNB, and by their mutual interactions. As a consequence, the dominant part of the orientational Hamiltonian depends on only a single degree of freedom, from which the results are determined. Properties calculated include the orientational structures, internal energy, specific heat, and the phase transition temperature. Various order parameters are calculated to understand the microscopic nature of the transition, which seems to show features of both an order–disorder and displacive character, much like experimental interpretations.

Notes: The Monte Carlo method is applied to the study of disordered conformations of polymethylene chains confined in cylindrical mean-field potentials. It is assumed that the molecule, which is composed of 30 united atoms (methylene groups), has fixed bond length and bond angle, and makes quasicontinuous bond rotations. Various statistical properties of the molecule, such as dihedral angle distributions, dihedral angle pair correlations, transverse fluctuations, etc., are calculated vs strength of the mean-field potential. The dihedral angle distributions calculated exhibit the marked reduction of the gauche peaks with increasing potential; it implies the increasing inaccuracy of the usual rotational isomeric model. The dihedral angle pair correlations reveal novel characteristics of the dihedral angle fluctuation: the fluctuation has approximate period of four bonds with marked tendency for the next nearest bonds to counter-rotate. The characteristics are more conspicuous under weaker potential constraint. There are large transverse fluctuations of the chain, the average linear form of the chain being still maintained. These characteristic dihedral angle fluctuation and the transverse deviation of the chain are found to be well understood by a small scale kink model.

Notes: We report two-dimensional Monte Carlo, mean-field, and exact one-dimensional studies of a statistical-mechanical model for the ripple (Pβ') phase of hydrated phosphatidylcholine lipid bilayers. The model is a p-state chiral clock model coupled to an Ising model through the chiral field. The microscopic parameters of the model are fixed by independently obtained values for pairwise molecular interactions. The one-dimensional model possesses a "floating fluid phase'' characterized by exponentially decaying, spatially modulated correlations. Solutions of the mean-field equations for the model include modulated phases but these are found to be metastable states, and the mean-field phase diagram is dominated by phases characteristic of the p-state chiral clock model. Monte Carlo simulations also reveal clock model phases. However, when unphysical effects due to mod p counting in the Hamiltonian are eliminated, the Monte Carlo simulations reveal a modulated phase, intermediate in temperature between a high-temperature disordered phase and a low-temperature ordered phase which is identified with the chain tilt (Lβ') phase.

Notes: By scanning the frequency of a single-mode dye laser crossed to a molecular beam of Cs2, we measured the excitation spectra of the C 1∏u–X 1∑+g transition by detecting selectively the molecular fluorescence and the emission from the dissociated Cs(62P3/2) atoms. Dependence of the predissociation on each vibrational and rotational level C 1∏u (v,J) is studied by comparing the intensities of molecular fluorescence and atomic emission. This predissociation is found to depend strongly on the vibrational quantum number v but weakly on the rotational quantum number J, and to occur most strongly around the v=3 level. The Franck–Condon factors between the RKR potential curve of the C 1∏u state and several repulsive potential curves are calculated, and are compared with the observed predissociation rates. The potential curve of the (2)3∑+u state, which is expected to be repulsive and to cause the predissociation through the spin–orbit interaction, is estimated to cross the potential curve of the C 1∏u state between the left turning points of v=1 and v=0 levels.

Notes: The presence of dipole-forbidden ("hidden'') excited electronic states in centrosymmetric chromophores can in principle be inferred from preresonance Raman excitation profiles (REPs). As the excitation radiation is tuned through the appropriate energy range, vibronic coupling between the state of interest and nearby "allowed'' electronic states will produce interference effects in the ground state scattering intensity. We have used the Kramers–Heisenberg dispersion relation to establish a one-to-one correspondence between the vibrational modes of the 2 1Ag excited state and the interference features in the experimental preresonance REP of all-trans diphenyldecapentaene (DPDP). The parameters utilized to predict the REP of DPDP were obtained without adjustment from absorption and fluorescence excitation spectra. The satisfactory representation and interpretation of the structure in the experimental spectrum establishes preresonance Raman excitation as a viable technique for characterizing such hidden states in non-fluorescing molecules.

Notes: Nonphotochemical hole burning in glassy host/guest matrices generates a change of the site energy distribution of the guest molecules. This distribution of shifted energies has been determined by deconvoluting the difference of white-light excited fluorescence spectra before and after hole burning. For this aim, a special numeric algorithm has been developed. For the host/guest-system tetracene in ethanol, the observed shifts are within the inhomogeneously broadened line, but a shift to higher energies is predominant and amounts to about 20 wave numbers.

Notes: The paper presents and extends an earlier model for the relaxation dynamics within solids and desorption of adsorbates on surfaces. The model retains the discrete nature of the adsorbates while adopting a continuum representation for the solid. Extensive one-dimensional model calculations are carried out along with sample calculations in the two-dimensional case. Specifically, in the one-dimensional case, the formulation is for a diatom with harmonic intramolecular bond and a Morse potential between the diatom and the remaining solid as well as an atomic inclusion in a two-dimensional bulk solid. The excitation mechanisms are initial disturbances that take the system away from its equilibrium configuration and acoustic pulses incident on the molecule. An extensive parametric study is carried out by varying the width, amplitude, and frequency content of the pulse along with the strength of the Morse bond. These calculations permit an understanding of the roles of the various parameters in determining the relaxation rate associated with energy transfer and desorption processes for the case of the diatom bound to the solid surface as well as an atom as a guest in the solid. The observed physical phenomena span the linear to the highly nonlinear regime.

Notes: A quantitative analysis of the temperature dependence of the motional broadening of the Raman bands associated with the guest alkane molecules accommodated in the hexagonal phase of urea inclusion adducts of even-numbered n-alkanes of n–C14H30 through n–C22H46 was performed according to the site-hopping theory. The profiles of a certain polarization component of the following five bands; the CH2 antisymmetric stretch νa(CH2) (XY), the CH2 scissoring δ(CH2)(ZZ) and the CH2 twist t(CH2)(XZ), and the symmetric and antisymmetric CC stretches νs(CC)(ZZ) and νa(CC)(XZ)[Z(parallel)c,X,Y⊥c], were found to be reproducible by a single Lorentzian function in the whole temperature range covering both the orthorhombic and hexagonal phases. Difference in the broadening behavior among the five Raman bands was interpreted quantitatively in terms of the equation derived previously by the authors. For each adduct, the value of the potential barrier height E* to the rotational motion of the alkane molecules was obtained in a good constancy from the temperature dependence of the half-width of the bands which belonged to different symmetry species and exhibited different broadening behavior. The value of E* for the C-16 adduct agreed well with the barrier height calculated on the basis of van der Waals intermolecular potential functions and the crystal structure. The E* value was found to increase monotonically with an increase in the chain length of the guest n-alkane molecule, except for an anomalous increase at C-20, as has been observed in the chain-length dependence of the transition temperature between the orthorhomic and hexagonal phases. The end-gauche content of the guest molecules in the hexagonal phase was evaluated as about 5 mol% irrespective of the chain length. For the alkane vibrations, bandshifts with variation in temperature were measured and analyzed according to the libration–torsion theory presented by Wood et al.

Notes: The ground state of the SiC3 molecule is found to be a closed-shell cyclic C2v symmetry structure which can be described as a four-membered ring with a transannular (cross ring) carbon–carbon bond, r(C–C)=1.469 A(ring). Theoretical studies with a triple-zeta plus double-polarization function (TZ2P) basis set in conjunction with the configuration-interaction technique at the TZ2P self-consistent-field optimized geometries predict this rhomboidal structure to be 4.1 kcal/mol more stable than the linear triplet Si–C–C–C isomer. A second closed-shell rhomboidal C2v symmetry structure with carbon–silicon transannular bonding, r(Si–C)=1.880 A(ring), was located and characterized as a local minimum lying 4.3 kcal/mol above the ground-state rhomboidal structure at this level of theory. Higher-level theoretical methods, including contributions from triple excitations, with larger basis sets will be required to obtain a more definitive set of relative energies.

Notes: The reference interaction site model (RISM)-polaron model of excess electrons in liquids is extended to treat the competition between attractive and repulsive branches of electron-liquid atom pseudopotentials. For certain choices of parameters, a delicate cancellation occurs leading to unusually large values of electron mobility over a narrow range of liquid densities. This behavior has also been observed in experiment. The RISM-polaron theory is used to interpret these experiments. It is shown that because of the topological disorder of a liquid, attractive interactions alone lead to electron localization and no anomalously large mobilities. Similarly, repulsions by themselves do not produce the anomaly. The cancellation that can occur is a manifestation of the quantum nature of electronic states in an annealed random system. Comparison of the present results with those that might follow from a classical percolation model suggests that the latter does not provide a correct description of electron mobilities in liquids. The cancellation is different but analogous to the Ramsauer–Townsend effect.

Notes: We report evidence of modification of solvent reorientation induced by the presence of polymer in polymer/solvent systems. Depolarized Rayleigh spectroscopy (DRS) has been used to probe the rotational motion of neat Aroclor and polystyrene (PS) solutions with polymer concentration 0.05, 0.086, and 0.146 g/cm3. The depolarized (IVH(ω)) spectra were recorded in the temperature range 40 to 140 °C using Fabry-Perot interferometry. Two Lorentzian lines were found to fit well the experimental IVH(ω) of PS/Aroclor above 60 °C revealing two populations of solvent. The fast relaxation time τf is virtually insensitive to variations of PS content in contrast to a considerable slowing down effect on the slow reorientation time τs obtained more precisely from photon correlation and oscillatory electric birefrigence measurements at low temperatures (〈0 °C). The intensity Is of the narrow Lorentzian component, which was found to increase rapidly below a characteristic temperature at given PS concentration, is associated with the slow orientation of the solvent in the PS environment. Surprisingly, the onset of PS induced modification occurs at rather high temperatures (∼Tg+110 K). The values of the intensity ratio Is/If are rationalized in the framework of the restricted rotational diffusion model. Alternatively, the study of the single Lorentzian IVH(ω) spectra of the neat solvent suggests a significant amount of orientational pair correlations and is consistent with clustering in liquid Aroclor.

Notes: We investigate the free energy curve as a function of the reaction coordinate of the electron transfer by the cumulant expansion method. The deviation from the linear response which gives rise to a harmonic form of the free energy curve is expressed in terms of the cumulants. The validity condition of the linear response is investigated using the l-particle correlation length λl. It is a central result that the linear response applies only when the conditions λ2 (very-much-greater-than) λl(l ≥ 3) and λ2(very-much-greater-than)a hold, where a is the sum of solute and solvent radii.

Notes: The x-ray diffraction measurements of methanol adsorbed on graphite have been taken over the temperature range 32–150 K and the coverage range 0.02–0.17 molecule A(ring)−2. The diffraction data are analyzed to yield the configuration of the molecule on the surface by means of the least-squares fitting of the whole pattern. The data show clearly that zigzag chains of alternating weak and strong hydrogen bonds, as evidenced by the long and short 0⋅⋅⋅0 separations, are formed in the crystalline monolayer of methanol on graphite. The crystalline solid film near the complete monolayer melts around 142 K in comparison to bulk methanol which melts at 175.4 K.

Notes: The domain structure self-assembled under a steady Couette flow was investigated on a semidilute solution of polymer mixture (polymer A + polymer B + solvent) at a composition near the critical one by use of the in situ light scattering method. This method permits a quantitative analysis of the scattering profile I(qy) in the plane perpendicular to the shear flow as a function of shear rate S at a given quench depth ΔT(0)=Tc(0)−T. Here, qy is the component of the scattering vector q in the plane concerned, Tc(0) the critical temperature at S=0, and T[〈Tc(0)] the temperature of the experiment. Effects of shear rate on the self-assembled structure were pronounced, and they were classified into five regimes A to E. At the lowest S (regime A), I(qy) was expressed by a linear combination of the Porod scattering and the Ornstein–Zernike (OZ) scattering, the former being due to the domain structure and the latter to critical composition fluctuations inside the domains. At higher S (regime B), I(qy) was complex. However, with increasing S further (regimes C and D), it was universally represented by the squared Lorentzian form {1+[qy(ξ⊥)d]2}−2, with a shear-rate-dependent length parameter (ξ⊥)d which depends on S−n with n=1/4 to 1/3. This fact indicates the self-assembling of a domain structure which we call oriented random two-phase structure. At the highest S (regime E), I(qy) followed the OZ scattering, thus suggesting the system to change to the shear-induced homogenized state.

Notes: Excitation of crystalline KBr, KCl, and LiF with electrons of energy 60–1000 eV produces weak emission in the 360–420 nm region, identified as the B→ X electronic transition in the CN radical. The emission is attributed to CN molecules that leave the surface of the crystal in their electronically excited B 2Σ+ state. The resulting spectra, which are rotationally unresolved, are analyzed by novel nonlinear fitting procedures to yield information about the vibrational and rotational population distributions. For CN(B) produced via electron-stimulated desorption from LiF, the vibrational populations approximate a temperature of 1500 K, while the rotational abundances can be represented as a sum of two Boltzmann distributions having temperatures of 660 K (81%) and 90 K (19%). For ESD from KBr, the rotational distribution is adequately represented as a single Boltzmann at ∼ 590 K.

Notes: The local structure of fluids composed of chain-like molecules (modeled as a pearl necklace of freely jointed hard spheres) is investigated via an integral equation approach. A Yukawa closure for the direct correlation function is used in the framework of polymer-RISM theory. This introduces two free parameters which are chosen by matching the theory's predictions for the compressibility and the contact value of the site–site distribution function, to an equation of state and simulation data, respectively. The theory shows good agreement when compared to Monte Carlo simulations of tangent diatomics, and freely jointed 4-mers, and 8-mers. Methods for estimating the contact value of the site–site distribution function are discussed.

Notes: The ultraviolet-photochemistry of molecularly adsorbed oxygen on Pd(111) has been studied using pulsed laser light with 6.4 eV photon energy. Three processes occur upon irradiation: desorption of molecular oxygen, conversion between adsorption states, and dissociation to form adsorbed atomic oxygen. By using time-of-flight spectroscopy to detect the desorbing molecular oxygen and post-irradiation thermal desorption spectroscopy (TDS) to characterize the adsorbate state, a detailed picture of the photochemical processes is obtained. The data indicate that the O2 molecules desorbing with low translational energies from the saturated surface as well as the conversion of adsorbed molecules between binding states are induced by the photoinduced build-up of atomic oxygen on the surface. Analysis of a proposed reaction model reproduces the observed data and yields detailed rates. Polarization analysis indicates that the photochemical processes are initiated by electronic excitations of the substrate.

Notes: Experiments on benzene have established that its lowest triplet state (3B1u) is conformationally unstable owing to vibronic coupling with the next higher state (3E1u). This instability was found to be critically dependent on the influence of a crystal field. An analogous vibronic coupling is to be expected in the singlet manifold, but here no direct evidence is available for a conformational instability. The distortion behavior of benzene is of importance for the interpretation of its photophysical and photochemical properties. We have therefore determined the potential-energy surfaces of the 1,3B1u and 1,3E1u states along the two-dimensional distortion coordinate S8(ρ,cursive-phi) using ab initio multireference single and double excitation-configuration-interaction calculations. The results show that for both B1u states the hexagonal conformation is unstable and lies 800 cm−1 above a wide, virtually cylindrical trough. A calculation of the vibrational spacing in the 3B1u state yields good agreement with the experimentally observed frequency. The calculation of intensities in the absorption and emission spectrum for this state qualitatively agrees with the experiment. An estimate is made of the interaction of the excited molecule with neighboring molecules in a crystal, which indicates that the crystal-field induced energy variations in the trough should be of the order of 10 cm−1. Combination of our calculations with experimental data shows that the vibronic coupling in the B1u states of benzene should not be looked upon as a static coupling in which the molecule is permanently distorted to one conformation but as a dynamic one in which the molecule makes excursions over the entire potential-energy surface.

Notes: The configuration-interaction (CI) study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from (i) the photoelectron detachment spectra of their anions and from (ii) the photodepletion spectra of the neutral species, reproduces observed excitation energies, intensities for allowed transitions, and permits an assignment of cluster structures. For Na−2–4 the linear anionic geometries are responsible for the photoelectron detachment spectra. In the case of Na−5, both planar and linear anionic isomers seem to contribute to the recorded spectrum. The calculation of optically allowed states for Na3(C2v) and Na4(D2h) structures and oscillator strengths yield rich spectra which have been fully assigned to the observed ones. In the case of Na8, the Td and the related D2d forms give rise to an intense transition located at ∼495 nm and the weak fine structure shifted to the red in full agreement with the measured spectrum. A molecular versus collective excitation interpretation of absorption spectra is discussed.

Notes: The zone-center magnon modes of LuFeO3 have been studied by first-order Raman scattering. At room temperature, the two magnon modes (M1 and M2) associated with the canted, antiferromagnetically ordered Fe3+ moments are seen at 18.5 and 22 cm−1, respectively. Their corresponding symmetries, determined through polarization measurements on single crystals, are consistent with the magnetic point group (m'm'm) of the crystal. The distinct scattering observed from phonons is also discussed in the context of the dependence of the phonon spectra on the mass of the rare-earth atom.

Notes: The NaLi 3 1Σ+(C)→2 1Σ+(A) electronic transition has been observed in the infrared region after laser excitation of the 1 1Π(B) electronic state and subsequent collisional energy transfer between the 1 1Π(B) and 3 1Σ+(C) electronic states. The spectra were recorded at high resolution by Fourier-transform spectroscopy. Thirteen vibrational bands were analyzed, providing detailed information for the 2 1Σ+(A) (v=0,...,4) and 3 1Σ+(C) (v=5,...,13) vibrational levels. Rotational perturbations have been observed in the spectra. The nearly 1200 observed lines belonging to 1 1Π(B)→2 1Σ+(A) and 3 1Σ+(C)→2 1Σ+(A) transitions have been assigned and reduced to molecular constants in a linear least-squares fit. Perturbations observed in the upper electronic states have been reduced using a nonlinear least-squares fit to a 1Σ∼1Π effective Hamiltonian matrix model. Deperturbed molecular constants and perturbation parameters are obtained for the 1 1Π(B) electronic state (v=0,...,6) and the 3 1Σ+(C) electronic state (v=5,...,13) levels. Propensity rules concerning the energy gaps and the conservation of angular momentum, during the energy transfer, are inferred from the intensity distributions of anomalous lines.

Notes: High resolution HeI (584 A(ring)) photoelectron spectra have been obtained for the tetrameric clusters of the group V elements: P4, As4, and Sb4. The spectra establish that the ground 2E states of tetrahedral P+4, As+4, and Sb+4 are unstable with respect to distortion in the ν2(e) vibrational coordinate. The E⊗e Jahn–Teller problem has been treated in detail, yielding simulated spectra to compare with experimental ones. Vibronic calculations, extended to second order (quadratic coupling) for P+4, account for vibrational structure which is partially resolved in its photoelectron spectrum. A Jahn–Teller stabilization energy of 0.65 eV is derived for P+4, which can be characterized in its ground vibronic state as being highly distorted, and highly fluxional. Linear-only Jahn–Teller coupling calculations performed for As+4 and Sb+4, show good qualitative agreement with experimental spectra, yielding stabilization energies of 0.84 and 1.4 eV, respectively.

Notes: This paper presents the results of a temperature dependence study of time resolved fluorescence depletion (TRFD) measurements of intramolecular vibrational energy redistribution in the molecule p-cyclohexylaniline. TRFD scans of five vibrational bands of p-cyclohexylaniline were taken at several molecular beam conditions corresponding to rotational temperatures in the range of 8–110 K. The results are attributed to two possible coupling mechanisms and are shown to be consistent with previous work. Although rotational effects are probably dominant, our data also indicate that excitation of low (40 cm−1) vibrations may contribute to enhanced relaxation.

Notes: Methods employing high resolution HeI (584 A(ring)) photoelectron spectroscopy have been applied to the tetrameric clusters of the group V elements, to resolve details of vibronic and spin–orbit structure in the first three electronic states of P+4, As+4, and Sb+4. Measured spacings of distinct vibrational progressions in the ν1 mode for the 2A1 states of P+4 and As+4, yield vibrational frequencies of 577 (5) cm−1 for P+4 and 350 (6) cm−1 for As+4. Franck–Condon factor calculations suggest bond length changes for the ions in the 2A1 states of 0.054 (3) A(ring) for P+4 and 0.060 (3) A(ring) for As+4. Strong Jahn–Teller distortions in the ν2(e) vibrational mode dominate the structure of the 2E ground states of the tetrameric ions. Both Jahn–Teller and spin–orbit effects appear in the spectra of the 2T2 states of the tetrameric ions, with the spin–orbit effect being dominant in Sb+4 and the Jahn–Teller effect dominant in P+4. Vibrational structure is resolved in the P+4 spectrum, and the ν3(t2) mode is found to be the one principally active in the Jahn–Teller coupling. A classical metal-droplet model is found to fit well with trends in the IPs of the clusters as a function of size.

Notes: In femtosecond laser-pulse experiments the pump pulse, with duration comparable or shorter than a typical period of intra- or intermolecular vibrations, creates a nonstationary wave packet. In this paper we use the density-matrix method to analyze creation of space–time coherences by the pump pulse and their effect on the probe pulse. Expressions for the density-matrix jumps, induced by the probe pulse, have been obtained in a general case. The material equations, determining propagation of the probe pulse, have been derived.

Notes: We report the results of theoretical and experimental studies of the rotationally resolved photoelectron spectra of O2 at low temperature leading to the v+=0, 1, and 2 levels of the X 2Πg state of O+2. A delayed, pulsed field ionization technique is used in conjunction with a coherent VUV radiation source to obtain high resolution spectra near threshold. The data are compared with theoretical results obtained using static-exchange photoelectron orbitals and a full description of the mixed Hund's case (a)–(b) ionic ground state. Agreement with experiment is good, especially for the v+=1 and v+=2 levels. Analysis of the rotational branch intensities yields detailed information on the angular momentum composition of the shape resonance near threshold. We also show that the dependence of the electronic transition moment on internuclear distance caused by the shape resonance leads to a significant dependence of the rotational branch intensity on ion vibrational level.

Notes: During quantum reactive scattering calculations for the title reaction a pronounced resonance structure became apparent in the energy dependence of state−to−state differentialscattering calculations. This resonance structure is explained.(AIP)

Notes: The photoelectron spectrum of the FH−2 anion is reported. The spectrum provides a probe of the transition state region for the F+H2 reaction. The experimental spectrum is compared to the recent simulation by Zhang and Miller which assumes the T5a potential energy surface for the F+H2 reaction. The experimental spectrum is substantially broader. While this may be due to inaccuracies in the T5a surface, the possibility of additional transitions to low-lying excited electronic surfaces not included in the simulation must also be considered.

Notes: The emission bands of helium hydride near 5500 and 6400 A(ring) were analyzed for 4HeH, 3HeD, and 4HeD. They are assigned to the emission of the coupled states D 2Σ+, (3d, L=2) and (for the deuterides) C 2Σ+, v=3 to the A 2Σ+ (5500 A(ring)) and B 2Π (6400 A(ring)) states. The 3d, L=2 state is treated as pure Hund's case (d). The coupling of the electronic states is homogeneous and described by constant matrix elements. Only in the case of 4HeH, strong predissociation of the D state was observed for N'≥3. The emission spectra were observed after neutralization of a fast (15 keV) mass-selected HeH+ beam in potassium vapor.

Notes: The emission bands of helium hydride near 5200, 5300, and 6000 A(ring) were analyzed for 4HeH and 3HeD. They are assigned to the emission of nine (HeD) or eight (HeH) coupled electronic states to the A 2Σ+ (5200 and 5300 A(ring)) and B 2Π (6000 A(ring)) states. Because of the high rotational temperature of 2500 – 3500 K and several perturbations, a very complex rotational structure was observed. The strongest band near 6000 A(ring) is emitted by the five components of the 3d states, which show strong uncoupling of the 3d electron from the symmetry axis. The D 2Σ+ and 3pE 2Π states interact by Λ-type doubling interaction and with the 3d states since interactions with Δl=±1 are allowed in a strongly polar molecule as HeH. The line intensities are affected by interference effects due to the interactions of the electronic states, by different lifetimes of the upper states and by predissociation, which is only strong for the A' parity component of 4HeH.

Notes: Collinear molecular beam photodepletion was used to obtain particle specific electronic absorption information for Na3, Na4, and Na8 in a wavelength range from 370–835 nm. We critically discuss the experimental method used and the deconvolution procedure applied to the resulting data to yield absolute absorption cross sections. The spectra contain much information on the cluster-size–dependent transition from molecular to bulk-like optical response and are interpreted in terms of various computational approaches ranging from classical electrostatic to ab initio large scale configuration interaction.

Notes: This paper concludes a theoretical study of vibrational dynamics in the bifluoride ion FHF−, which exhibits strongly anharmonic and coupled motions. Two previous papers have described an extended model potential surface for the system, developed a scheme for analysis based on a zero-order adiabatic separation of the proton bending and stretching motions (ν2,ν3) from the slower F–F symmetric-stretch motion (ν1), and presented results of accurate calculations of the adiabatic protonic eigenstates. Here the ν1 motion has been treated, in adiabatic approximation and also including nonadiabatic couplings in close-coupled calculations with up to three protonic states (channels). States of the system involving more than one quantum of protonic excitation (e.g., 2ν2, 2ν3 σg states; 3ν2, ν2+2ν3 πu states; ν3+2ν2, 3ν3 σu states) exhibit strong mixing at avoided crossings of protonic levels, and these effects are discussed in detail.Dipole matrix elements and relative intensities for vibrational transitions have been computed with an electronic dipole moment function based on ab initio calculations for an extended range of geometries. Frequencies, relative IR intensities and other properties of interest are compared with high resolution spectroscopic data for the gas-phase free ion and with the IR absorption spectra of KHF2(s) and NaHF2(s). Errors in the ab initio potential surface yield fundamental frequencies ν2 and ν3 100–250 cm−1 higher than those observed in either the free ion or the crystalline solids, but these differences are consistent and an unambiguous assignment of essentially all transitions in the IR spectrum of KHF2 is made. Calculated relative intensities for stretching mode (ν3, σu symmetry) transitions agree well with those observed in both KHF2 [e.g., bands (ν3+nν1), (ν3+2ν2), (3ν3), etc.] and the free ion (ν3,ν3+ν1). Calculated intensities for bending mode (ν2, πu symmetry) transitions agree well with experiment for the ν2 fundamental in the free ion and KHF2(s), and for a πu transition in KHF2 which we assign to ν2+2ν3, but are far too small to explain the prominence of progression bands (ν2+nν1) and especially the strong overtone 3ν2 in the spectrum of KHF2(s). Intensity of the progression bands (ν2+nν1) in KHF2 can be explained by hydrogen bonding between adjacent FHF− ions; in NaHF2(s) where such interaction is absent, the band (ν2+ν1) is 50–100 times weaker, in agreement with calculations.The relatively high intensity of the 3ν2 band, which also appears strongly in NaHF2(s), remains the major unexplained feature of the bifluoride spectrum in these solids. Suggestions are made for further experiments on the FHF− and FDF− systems which could test predictions of this dynamical analysis.

Notes: A general method is developed for determination of cylindrically symmetric velocity distributions from Doppler profile measurements. This method applies Kinsey's Fourier transform Doppler spectroscopy [J. L. Kinsey, J. Chem. Phys. 66, 2560 (1977)] to distributions arising from photodissociation and uses an orthogonal polynomial expansion to perform the integral transforms analytically. This method is shown to offer an improvement in stability over direct numerical solution of the integral equation and to have applicability to distributions which are not "separable,'' that is, which cannot be separated into a product of speed- and angle-dependent factors. The method is applied to experimental measurements of the collisional relaxation of a fast anisotropic distribution of I[2P1/2] atoms in a thermal bath (preceding paper). It is shown that the nascent distribution is separable, but the distribution does not remain separable throughout the relaxation process.

Notes: The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.

Notes: The vibrational dephasing of nitrate ions was studied in a molecular dynamics simulation of molten LiNO3, which included all degrees of freedom of vibrating nitrate ions. For the interionic interaction, a Coulomb pair potential with a Born-type repulsion was adopted as a standard potential, and the effect on vibrational dephasing of a potential well of varying depth between Li+ and O of NO−3 was studied. Vibrational correlation functions 〈Qi(0)Qi(t)〉 for the ν1 and ν2 modes of NO−3 were calculated and the vibrational spectra were obtained from their Fourier transforms. It was found that the vibrational correlation functions for the two modes decayed rapidly and the vibrational linewidths increased considerably as the well depth increased. Two simulations for the harmonic and the anharmonic intraionic potentials for NO stretching suggested that pure interionic interaction induced broadening dominated the band width of the ν1 mode in this melt, while vibrational anharmonicity coupled to the forces due to the environment did not play any important role. Results of the simulation were compared with the infrared and isotropic Raman band shapes in molten LiNO3. The assumed interionic potentials in the present simulation were found to result in two slow vibrational dephasing of the ν1 mode and too fast dephasing of the ν2 mode as compared with the spectroscopic results. The effect of vibration–rotation coupling on the vibrational spectra was found to be small in this system.

Notes: The ground and several l=0 breathing mode vibrational excited state wave functions of HeN clusters are determined for N=3–5, 20, 70, and 240, using the variational Monte Carlo method. These wave functions incorporate one-, two-, and three-particle correlation effects and give binding energies, density profiles, and vibrational excitation energies accurately. The larger clusters have liquid-like structure, characterized by a pair distribution function showing approximately two coordination shells. The smallest clusters (N=3, 4) have extensively delocalized structures, which on average are equilateral triangular and tetrahedral, respectively. The N=5 cluster has a totally symmetric average structure, which can only be described in terms of a quantum liquid. No molecular structure, whether rigid or floppy can be assigned in this case. The relative importance of various correlation effects in clusters of different sizes is analyzed and discussed. These wave functions are completely analytical and are convenient as importance functions in diffusion and Green's function Monte Carlo calculations.

Notes: A method for the calculation of quantized rate constants of chemical reactions in condensed phases is presented. The method focuses on the evaluation of nuclear tunneling corrections for classical activation free energies of diabatic and adiabatic reactions. The diabatic problem is treated by the quasiharmonic dispersed polaron model, using both a second order quantum mechanical rate constant, which is exact for quasiharmonic systems, and a semiclassical approximation based on an analytical density matrix formulation. The adiabatic free energy functionals are obtained by using the corresponding diabatic system as a reference state. A path integral formulation is also used both as a guide for the derivation of the adiabatic correction and as an alternative method for problems of limited dimensionality. The close relationship between the free energy functionals of the present approach and those developed in our earlier studies of electron transfer and proton transfer reactions is pointed out. The method is illustrated by studying several simple one and two dimensional problems and by stimulating deuterium isotope effects in proton transfer reactions in solutions. The calculations of the isotope effects reproduce the corresponding experimental trend.

Notes: We present an approximate quantal model to study the double continuum problem arising in the complete fragmentation of X⋅⋅⋅BC(v)⋅⋅⋅Y van der Waals(vdW) complexes, where BC is a conventional diatomic molecule vibrationally excited and X and Y are rare gas atoms, through vibrational predissociation (VP). Assuming a near equilibrium geometry of the complex and using an adiabatic approximation for describing the oscillation in the angle formed by the BC⋅⋅⋅X and BC⋅⋅⋅Y weak bonds, the rates for complete fragmentation are expressed in the frame of Fermi's "Golden Rule''. Double continuum wave functions may be obtained by a perturbative treatment that allows one to take properly into account the symmetry of the problem in the particular and very frequent case X≡Y.

Notes: Using the polarized laser photofragment photoionization technique, measurements have been made of the degree of orientation of CH3I in ||JKMJ〉=||111〉 and ||222〉 parent rotational states under the influence of homogeneous electric fields (E) from 0 to 1.0 kV cm−1. From a series of experiments on hexapole-oriented molecules in weak fields, it has been found that the original degree of orientation of the symmetric-top molecules can be recovered after they pass through a homogeneous weak-field region provided that the field strength therein exceeds some minimum value (E(approximately-greater-than)0.3 V cm−1), sufficient to maintain an orientation axis. Even though the ||JKMJ〉 parent states have "relaxed'' via hyperfine coupling to an ensemble of ||FJKIMF〉 states, when the molecules later pass into a strong E field, the ensemble is restored to its original degree of orientation. Another set of experiments in moderate to strong fields provided "saturation curves,'' i.e., the dependence of the degree of orientation upon E. The results show that quite high field strengths (〉0.5 kV cm−1) are required to ensure total decoupling of J from I and thus recovery of the full orientation. From these experiments it is now clear that one can control the degree of molecular orientation by placing the state-selected molecules in a know E field.

Notes: The H+N3→NH(X 3∑−,a 1Δ, b 1∑+)+N2 reaction has been studied in a molecular beam-gas scattering arrangement in order to determine the nascent product state distribution. The NH product in specific rovibronic/fine-structure states has been detected by laser fluorescence excitation. The relative cross sections for formation of various vibrational levels in the a 1Δ electronic state were determined to equal 1:1.0±0.3:1.4±0.3:≤1.5 for v=0 through 3, inclusive, while the v=0 to v=1 population ratio in the X 3∑− state was found to be 1:0.015±0.003. The rotational distributions in all vibronic levels were found to be characterized by temperatures near 300 K, suggestive of relaxation of the nascent rotational distributions. By comparison of the populations of a specific pair of X 3∑− and a 1Δ state levels and with summation over the derived rovibrational distributions, an electronic state branching ratio of 3.2±1.3 was obtained for the X 3∑− to a 1Δ electronic state branching ratio. An upper limit of ≤0.02 was also derived for the ratio of the b 1∑+ v=0 to a 1Δ v=0 populations. These results are compared with NH fragment distributions observed in the photodissociation of HN3(X˜ 1A') and with our expectations based on our fragmentary knowledge of HN3 potential energy surfaces.

Notes: Ab initio SCF and semiempirical calculations have been performed on some geometrical configurations of the system H3O+(H2O)6, with either three or four water molecules in the first solvation shell. The dispersion energy has been evaluated from perturbation theory. It is shown that the pair approximation overestimates the stability of the second category of structures. However, the present work confirms that geometries with either three or four water molecules in the first solvation shell are close in energy. Comparison is done with results obtained from several semiempirical expressions and parameters available in the literature.

Notes: Complete active-space multiconfiguration self–consistent field followed by multireference configuration-interaction calculations are carried out on low-lying electronic states of YNH and NYH. We find the X 2Σ+ linear state of Y–N–H to be 55 kcal/mol more stable than the bent NYH and 59 kcal/mol more stable than the linear N–Y–H. Our calculations confirm the recent assignment of the first observed spectra generated by laser vaporization of Y metal + He/NH3. The theoretical dipole moment of the Y–N–H molecule (3.06 D) is in excellent agreement with an experimental value of 3.06 D obtained by Simard et al. The theoretical Y–N and N–H bond lengths are also in good agreement with the experimental results.

Notes: Ab initio averaged relativistic effective core potentials (AREP) and spin–orbit (SO) operators are reported for the elements Cs through Rn. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definitions thereby permitting the treatment of core–valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets for the lowest energy state of each atom are tabulated. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic excitation energies and SO splitting energies with all-electron relativistic values. Calculated atomic excitation energies are found to agree to 0.12 eV and SO energies to 3.4%.

Notes: The desorption of CH4 physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision-induced desorption of CH4 in the low coverage limit of an isolated CH4 molecule and from a saturated CH4 monolayer is measured as a function of the kinetic energy and incident angle of the Ar beam. The dominant mechanism for collision-induced desorption is determined to involve the direct collision of the incident Ar with the physisorbed CH4. Indirect, surface mediated desorption processes and multiple desorptions are found to be unimportant. Three-dimensional, classical molecular dynamics simulations based upon a hard sphere/hard cube model of the direct collision mechanism show that the complicated dependence of the desorption cross section at low CH4 coverage on the Ar energy and incident angle is the result of two competing dynamical effects: the increase in the geometrical collision cross section and the decrease in the Ar kinetic energy that can be transferred to CH4 motion normal to the surface as the Ar incident angle increases. Multiple Ar–CH4 collisions and mirror collisions are found to make relatively minor contributions to the cross section for collision induced desorption. Normal energy accommodation during the CH4-surface collision plays a significant role in determining the threshold energy for desorption. At high CH4 coverage, the obstruction of small impact parameter, head-on Ar–CH4 collisions by neighboring CH4 molecules at large angles of incidence is the origin of the difference in the cross section observed for low and high CH4 coverage.

Notes: Classical trajectories have been employed in a study of the intramolecular dynamics and unimolecular decomposition of the 2-chloroethyl radical. A potential-energy surface was constructed by using the available experimental data and theoretical results. The following reaction channels were included in the study: ⋅CH2CH2Cl→CH2=CH2+⋅Cl, ⋅CH2CH2Cl→CH2=CHCl+⋅H. Mode-specific behavior was investigated by computing ensembles of trajectories for initial conditions (1) in which the normal-mode vibrations of the radical were assigned zero-point energies and a single C–H local stretch on the radical end of the system was excited, and (2) in which the normal modes were all excited so as to distribute the total energy uniformly throughout the radical. First-order rate coefficients were calculated both for the disappearance of the reactant and for the two chemically distinct reaction channels. The results do not indicate significant, if any, mode-specific effects. Energy transfer from and into local C–H stretching modes was studied. Relaxation of an initially excited C–H bond is observed to be irreversible and complete within about 0.6 ps.

Notes: Rotationally symmetric groups containing nuclei with nonzero spin often exhibit uniquely quantum mechanical properties. Symmetry requirements on the total wave function restrict the allowable combinations of electronic, vibrational, rotational, and nuclear spin wave functions. NMR relaxation depends on operators that act on both the nuclear spin and rotational wave functions. The interdependence of the wave functions produces quantum statistical weights in the rotational correlation functions that can differ from semiclassical or classical probability distributions. The quantum statistical weights for the reorientational correlation functions in NMR relaxation have been derived for A3 and AX3 systems of spin 1/2 nuclei and for deuterium relaxation in a deuterated methyl or equivalent group. It is found that classical weights apply to deuterium relaxation. The A3 system has distinctly nonclassical weights. And the AX3 system has eight sets of weights, with different weights applying to different types of correlation functions. One of the eight is the classical weight, and two of the sets can lead to terms that do not arise in the usual, semiclassical theory. It is shown that, under certain circumstances, the predictions of this quantum mechanical theory differ from those of the semiclassical model. The present theory also predicts that changing the symmetry of the electronic/vibrational states can alter the NMR relaxation behavior. Calculations with a stochastic quantum dynamical model indicate that these differences may be experimentally important for, among other things, relaxation involving a methyl group in a biomolecule at physiological temperatures.

Notes: The importance of noncompact (nonspherical) cavities in determining the size of polymer molecules in random media is studied by means of generalized Flory–Lifschitz arguments and computer simulations. The simulations are performed for a freely jointed chain in one and two dimensions using a novel Monte Carlo algorithm that effectively eliminates the effects of the finite size of the random medium. For the one-dimensional case, the simulation result for the exponent ν (=0.31±0.02), characterizing the scaling of the mean-square end-to-end distance of the chain R with the number of monomers, is in excellent agreement with the ν (=0.33) predicted by the previously developed Flory–Lifschitz theory based on the notion of compact cavities. A generalized version of the theory that accounts for noncompact (for d〉1) "tube''-like cavities with L(αR2 ) being the length of the tube, and D being the diameter in d-1 transverse directions, predicts that ν=1/(2d+4), or 1/6, depending on the nature of the tube for d〉1. This result is consistent with simulation results for the Gaussian chain in two dimensions. The theory also predicts that when one end of the chain is anchored and self-avoidance is included ν=2/3, which suggests a certain similarity between this problem and that of the directed walk in a random environment.

Notes: We have performed Monte Carlo simulations of a Langmuir–Hinshelwood reaction between two species A and B adsorbed on a square lattice, with the goal of determining how spatial correlations between the species vary with reaction rate. Adsorption of each species occurs when a gas-phase molecule, either A or B, impinges upon a vacant lattice site. The probability that a molecule impinges upon and adsorbs successfully into a vacant lattice site per unit time is pa/2 for both species. Desorption is not allowed and the surface reaction is allowed to occur only between nearest-neighbor AB pairs. For each nearest-neighbor AB pair, the probability of reaction per unit time is pr. A novel feature of this investigation is that we explicitly simulate the diffusion of the particles on the lattice. The particles are allowed to migrate by hopping to vacant nearest-neighbor sites, where the probability of a hop per unit time is pm. In all these simulations we have set pm to be unity, and varied pr from 0.01 to unity. We have also set pa=pr/5 for all the simulations in order to maintain moderately low fractional surface coverages. "Islanding'' of each type of particle occurs even for the lowest value of pr used, although the entire surface is never poisoned. For range of values of pr used, the "islands'' grow to a finite steady-state size. We also found that the islands that are formed are consistent with a dimension of two. A nearest-neighbor correlation function φ is defined to describe the process of islanding, and the dependence of φ upon pm/pr is studied. By studying this simple model we show that quite large inhomogeneities can be reasonably expected to occur in catalytic systems even when reaction probabilities are small compared to diffusion rates, and that these inhomogeneities affect total reaction rates.

Notes: The time-dependent Hartree grid (TDHG) method is extended into an ab initio algorithm for obtaining exact quantum wave packet dynamics. The new algorithm employs a superposition of orthogonal zeroth order time-dependent basis functions generated from a single TDHG wave packet trajectory. The superposition coefficients are themselves time-dependent, and are responsible for mixing the basis functions in such a way as to represent exact solutions of the time-dependent Schrodinger equation. Evolution of the superposition coefficients is governed by a set of first-order linearly coupled ordinary differential equations. The couplings between coefficients are given by matrix elements of a naturally identified interaction potential taken between members of the zeroth order basis. In numerical tests involving computation of S-matrix elements for collinear inelastic atom–Morse oscillator scattering the method proves accurate, flexible and efficient, and appears to be easily extendable to more complicated systems.

Notes: Quasidegenerate perturbation theory using a Bloch wave operator formalism is applied to the calculation of the rich resonance spectrum of a model polyatomic molecule undergoing photodissociation. The sharp structures (peaks and dips patterns) of the total dissociation rate as well as the partial probabilities to obtain the fragments at given internal states for various incident photon energies, are interpreted in terms of tunneling, shape or Feshbach resonances.

Notes: A fully spin-adapted approach to many-electron density matrices is developed in the context of the unitary group approach to many-electron systems. An explicit expression for the single-electron spin-density operator, as a polynomial of degree two in the orbital U(n) generators, is derived for the case of spin-independent systems. Extensions to spin-dependent systems are also considered, leading to the appearance of total-spin transition densities, whose general properties are investigated. A corresponding formalism for the two-electron density matrix, which is capable of further generalization, is also developed. The results of this paper, together with recent developments on the matrix elements of the U(2n) generators in the electronic Gel'fand basis, afford a versatile method for the direct calculation of one- and two-body density matrices in the unitary group approach framework.

Notes: The nondynamical correlation error in first row transition metal complexes is studied through calculations on the permanganate ion. The source of the error is the well-known Hartree–Fock failure in the weak-interaction limit, which is shown to exist for both the metal–ligand and the ligand–ligand bonds: the metal–ligand and the ligand–ligand distances are large compared to the size of the metal 3d and ligand 2p atomic orbitals (AO's). Pauli repulsion between ligand orbitals and 3s/3p orbitals prevent the metal–ligand and ligand–ligand distances to become small enough for efficient overlap and bonding. In multiply bonded systems the Hartree–Fock error does not show up in excessive electron repulsion, but leads to localization of the bonding orbitals (which sometimes requires symmetry breaking), resulting in a loss of covalent character. It is shown how, in the MnO−4 ion, the bonding electrons of E symmetry are localized on the oxygens while the T2 electrons are localized on the metal. The mechanism behind this (unphysical) localization is studied in detail, making use of a simple model system. The covalent character is reintroduced in configuration interaction or multiconfiguration self-consistent-field calculations: density is transferred from the ligand to the metal in the E bonds and vice versa in the T2 bonds. The total metal 3d occupation, however, remains unchanged. Several configuration selection schemes in the space of bonding, nonbonding, and antibonding orbitals are tested with the purpose to recover a large fraction of the nondynamical correlation error but still retain a manageable wave function. It is shown that the "nonbonding'' O2p orbitals play an important role in the correlation process and cannot be excluded (kept closed) in a correlated calculation if quantatively correct results are required.

Notes: The ground and ligand field excited states of CrF3−6 have been studied, using different Gaussian basis sets of atomic natural orbitals. Each state was first optimized separately in a complete active space self-consistent field (CASSCF) calculation, including three active electrons in the 2t2g and 4eg shells. Further correlation was then added by using either a singles and doubles configuration interaction approach (SDCI) or by the average coupled pair functional method (ACPF) with the CASSCF configuration space as the reference space. Thereby the number of correlated electrons was raised up to 15. It is shown that the quartet–quartet transitions, corresponding to a 2t2g→4eg excitation, are described already very accurately at the CASSCF level. Further improvement of the 4A2g→4T2g transition was obtained by extending the CI treatment so as to include the F 2p electrons from the 1t2g,3eg, and finally also from the 6a1g shell. For the intraconfigurational t32g quartet–doublet transitions on the other hand, the best results were obtained by an 11 electron CI treatment, including the Cr(III) 3s and 3p electrons.

Notes: We apply the variational Monte Carlo method to the atoms He through Ne. Our trial wave function is of the form introduced by Boys and Handy. We use the Monte Carlo method to calculate the first and second derivatives of an unreweighted variance and apply Newton's method to minimize this variance. We motivate the form of the correlation function using the local current conservation arguments of Feynman and Cohen. Using a self-consistent field wave function multiplied by a Boys and Handy correlation function, we recover a large fraction of the correlation energy of these atoms. We give the value of all variational parameters necessary to reproduce our wave functions. The method can be extended easily to other atoms and to molecules.

Notes: Guided ion-beam techniques are used to measure the cross sections as a function of kinetic energy for reaction of SiH4 with He+, Ne+, Ar+, Kr+, and Xe+. State-specific data for the 2P3/2 ground spin–orbit states of Kr+ and Xe+ are also obtained. The products observed in the He, Ar, and Kr systems are SiH+x for x=0–3. For the Ne system, formation of SiH+x x = 0–2, is seen, while in the Xe system only SiH+3 and SiH+2 are observed. Reactions of He+, Ne+, Kr+, and Xe+ show little dependence on kinetic energy, but for the case of Ar+, the reaction probability and the product distribution are highly sensitive to the kinetic energy of the system. Thermal reaction rates for all of the reactions are derived and compared with previous measurements. The results for these reactions are explained in terms of vertical ionization from the 1t2 and 3a1 bands of SiH4. The relationships of these reactions to plasma deposition and etching are also discussed.

Notes: Rate coefficients for the ion–ion recombination process Xe++F−+He→XeF*+He have been determined in ambient helium gas in the density range from 0.2 to 0.7 NL (Loschmidt's number NL=2.687×1019 cm−3). The experimental methods consisted of observing the conductivity decay during the afterglow of a photoionized plasma, in conjunction with mass spectrometry of plasma ions and optical spectroscopy of the XeF* excimer radiation. The measured rate coefficients agree well with theoretical results by Bates and Mendaš and Monte Carlo simulations by Morgan et al.

Notes: We have measured the infrared photodissociation spectra of argon clusters containing SiF4, as a function of the cluster size n (for n≤ 103) using molecular beam laser spectroscopy. The clusters were produced by both the conventional seeded expansion of a dilute mixture and by a "pickup'' method where, upon colliding with it, the chromophore sticks to the surface of a cluster made in a neat solvent expansion. Furthermore, the spectra of small SF6/Arn clusters (n≤50) have been remeasured with the improved resolution resulting from the use of two line and tunable isotopic CO2 lasers. These data, together with previously published data on SF6/Ar, indicate a remarkably different behavior for these two solute–solvent combinations. The preferred "site'' for SiF4 is at the surface of Ar clusters of all sizes, regardless of how the molecule is introduced to the cluster, while appreciable amounts of SF6 reside at the surface only when the cluster is large and the impurity is deposited onto the cluster surface. The behavior of SiF4 and SF6, together with the analogous behavior of other polyatomic chromophores, the IR spectra of which have been measured and reported previously [D. J. Levandier, M. Mengel, R. Pursel, J. McCombie, and G. Scoles, Z. Phys. D 10, 337 (1988); D. J. Levandier, S. Goyal, J. McCombie, B. Pate, and G. Scoles, J. Chem. Soc. Faraday Trans. 86, 2361 (1990)], can be rationalized in terms of molecular dynamics simulations of similar systems which are presented in the paper by Perera and Amar [L. Perera and F. G. Amar, J. Chem. Phys. 93, 4884 (1990)]. The combination of the theoretical and experimental results confirm the usefulness of infrared photodissociation spectroscopy for the study of the structure of clusters and suggest that assuming any particular location for an impurity in a cluster in the absence of experimental evidence or, at least, a dynamics calculation, can easily lead to wrong conclusions.

Notes: A molecular potential energy surface may be approximately described in terms of the energy, energy gradients, and second derivatives at a discrete set of points along a reaction path. The accuracy of this approach is limited, in part, by the level of ab initio theory that can be employed to obtain energy gradients and second derivatives. A method is derived and demonstrated whereby the energy surface can be scaled to achieve a greater degree of accuracy, using a set of energies calculated at a higher level of ab initio theory.

Notes: Large-scale calculations using atomic natural orbital (ANO) basis sets have been performed on Cu5O to establish the effects of correlation of the 3d shell on oxygen chemisorption. The largest calculation performed correlated 51 electrons in a basis set containing 205 ANO's. Correlation of the 3d shell is found to increase the chemisorption energy by 13(16±2) kcal/mol and decrease the height above the surface of the oxygen by 0.23(0.38) a0 with CI superposition error corrections included. The 2E state of the bare cluster is found to be stabilized by 10(8) kcal/mol relative to the 4A2 state as a result of 3d correlation. The values in parentheses were obtained using an approximate core–valence correlation operator which thus gives results in very good agreement with the core correlated calculations.

Notes: Stretching potential energy and dipole moment functions have been calculated for the ground electronic states of the HNC+ (X 2Σ+) and HCN+ (X 2Π) radical ions by singles and doubles configuration interaction (CI-SD) with a basis set of 74 contracted Gaussian-type orbitals. By comparison to analogous calculations on the CN radical, the ground state rotational constants B0 of HNC+ and HCN+ are predicted to be 47, 122±70 MHz and 40, 185±70 MHz, respectively, by CI-SD with the addition of a size consistency correction, designated CI-SD(s). Stretching band origins and intensities have been obtained from the analytical potential energy and dipole moment functions using variational methods. The stretching fundamental band origins, ν1 and ν3, and their intensities are predicted to be 3464 cm−1/2507 cm−2 atm−1 and 2212 cm−1/2.3 cm−2 atm−1, respectively, for HNC+ and 3099 cm−1/1189 cm−2 atm−1 and 1806 cm−1/70.3 cm−2 atm−1 for HCN+. The equilibrium dipole moment of HNC+ calculated by CI-SD is just 0.66 D, while that of HCN+ is 3.63 D. In agreement with earlier studies the HNC+ isomer is predicted to be lower in energy than HCN+, and our calculated energy of isomerization, 18.91 kcal/mol, is in good agreement with this previous work. The Renner–Teller effect in the HCN+ isomer is predicted within the harmonic approximation by CI-SD(s) to be small, with the Renner parameter ε and the Renner–Teller constant εω2 being −0.056 and −39.2 cm−1, respectively (ω2=696 cm−1). Quadrupole coupling constants have also been computed for HNC+ and HCN+, as well as for the neutrals HCN and HNC. The proton affinity at zero temperature PA0 of CN is 150.6 kcal/mol by CI-SD(s), and the adiabatic ionization potentials of HNC and HCN are 273.3 and 307.7 kcal/mol.

Notes: The performance of atomic natural orbital (ANO) basis sets has been studied by comparing self-consistant field (SCF) and full configuration interaction (CI) results obtained for the first row atoms and hydrides. The ANO results have been compared with those obtained using a segmented basis set containing the same number of contracted basis functions. The total energies obtained with the ANO basis sets are always lower than the one obtained by using the segmented one. However, for the hydrides, differential electronic correlation energy obtained with the ANO basis set may be smaller than the one recovered with the segmented set. We relate this poorer differential correlation energy for the ANO basis set to the fact that only one contracted d function is used for the ANO and segmented basis sets.

Notes: We report singlet and triplet state splittings (ΔEST) for fluorine-substituted methylenes and silylenes using dissociation-consistent configuration interaction (CI) (based on generalized valence bond wave functions). These relatively simple CI calculations emphasize correlation consistency between the singlet and triplet states. Values of ΔEST for CH2, CF2, SiH2, and SiF2 are in excellent agreement with available experimental results, and we expect the predictions for the other cases CHF (14.5) and SiHF (41.3) to be equally accurate. This result strongly suggests that the correct choice among the experimental values for ΔEST of CHF is 14.7±0.2 kcal/mol.

Notes: The quantum mechanics of energy flow in many-dimensional Fermi resonant systems has several connections to the theory of Anderson localization in disordered solids. We argue that in high dimensional and highly quantum mechanical systems the energy flow can be modeled as coherent transport on a locally but weakly correlated random energy surface. This model exhibits a sharp but continuous transition from local to global energy flow characterized by critical exponents. Dephasing smears the transition and an interesting nonmonotonic dependence of energy flow rate on environmental coupling is predicted to occur near the transition.

Notes: Three-dimensional potential energy and dipole moment surfaces have been calculated for the 25 electron radicals NF2 and O−3 in their 2B1 ground electronic states by the complete active space self-consistent field (CASSCF) method with basis sets of 87 (NF2) and 99 (O−3) contracted Gaussian-type orbitals. Spectroscopic constants have been calculated from the analytical potential energy functions for each species, and the results for NF2 are compared to the available experimental data. Predictions of the rotational and rotational–vibrational spectra of O−3 have been made by comparison to the NF2 results. Vibrational band origins have been calculated by perturbation theory and also variationally in a basis of distributed Gaussian functions. Rotationless dipole moment matrix elements and vibrational band intensities have been determined from the CASSCF dipole moment functions. The fundamental vibrational band origins and intensities of O−3 are predicted to be 979 cm−1/0.87 cm−2 atm−1 (ν1), 565 cm−1/17.8 cm−2 atm−1 (ν2), and 739 cm−1/2620 cm−2 atm−1 (ν3). Smaller basis set calculations of the first three excited electronic states of O−3 have also been carried out, and the results are compared to previous photodissociation experiments.

Notes: Potential energy and dipole moment functions have been calculated for the first two excited electronic states of CO by several ab initio methods using large Gaussian basis sets. Similar calculations on the ground state have also been performed to provide a basis for comparison. The types of calculations on the a 3Π state included complete active space self-consistent field (CASSCF), single reference single and doubles configuration interaction (CI-SD) including a size consistency correction [CI-SD(s)], quadratic CI-SD (QCI-SD), and quadratic CI-SD including the effects of triple excitations [QCI-SD(T)], and version 1 of the coupled electron pair approximation (CEPA-1).The best calculated dipole moment functions for the a 3Π state have been found to be consistent with earlier theoretical results with respect to shape (slope, μ'e), and also more accurate in predicting the equilibrium value μe. Theoretical dipole moment functions for the a' 3Σ+state are presented for the first time, at the CI-SD, CI-SD(s), and CASSCF levels of approximation. The first two of these yield estimates of μe that are in very good agreement with that derived from the analysis of perturbations in the molecular beam electric resonance (MBER) Stark effect of the a 3Π state. The (a–a') electronic transition moment has also been calculated at the CASSCF and CI-SD levels. Our CASSCF value of 0.23 D (v‘=4, v'=0) compares well with values derived from the MBER Stark effect.

Notes: In the Onsager theory for the phase transition from the isotropic fluid to the nematic liquid crystal phase, the Helmholtz free energy of a fluid of hard convex bodies (HCBs) is expressed as the sum of an entropy of a mixing-like term and an energy-like term (from the interaction of the HCBs). Whereas the Onsager theory expresses the interaction term in a virial expansion and determines the consequences of B2 alone, here we extend that treatment to incorporate B3 (with its attendant dependence on the mutual orientation of three HCBs). For HCBs (and specifically for D∞h ellipsoids) with large aspect ratios (5:1 or greater), the incorporation of B2 and B3 suffices to predict the variation of the order parameter 〈P2[cos(θ)]〉 with density in accord with the Monte Carlo (MC) results of Allen and Wilson. As the aspect ratio decreases (from 5:1) to more spherical molecules (say 3:1), virial coefficients of higher order than B3 contribute to the interaction term and their effect is represented in part by the y-expansion (or resummation) theory proposed by Barboy and Gelbart. In this y-expansion–third virial-Onsager theory, the predicted transition densities are in accord with the MC values of Frenkel and Mulder for prolate ellipsoids. Neither the y expansion nor the direct B2 and B3 theories find the phase diagram (i.e., transition density and order parameter regarded as a function of aspect ratio) to be symmetric for prolate and oblate ellipsoids. The dependence of B3 on the mutual orientation of the ellipsoids is also discussed and previous work is also addressed.

Notes: The structure of NH3⋅H2S has been determined from microwave and radiofrequency spectroscopy of this complex and its deuterated isotopomers, using molecular beam electric resonance techniques. Rotational constants, electric dipole moments and nitrogen quadrupole coupling constants were obtained from the spectra. The molecule was found to have a linear, hydrogen bonded structure with the ammonia as the proton acceptor. The N⋅⋅⋅S distance is 3.639 A(ring), the root-mean-square angular deviation of the NH3 axis from the N–S axis is 24.6° and the H2S C2 axis is 40.5° from the N–S axis. Although the molecule is an asymmetric rotor, first-order Stark effects were observed for K=1 rotational levels. These Stark effects are caused by torsional oscillations which are essentially ammonia monomer free internal rotation. Similar effects were observed for NH3⋅H2O and are reported here.

Notes: In this paper, we report on two dimensional (2D) 31P cross polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments on the coupled two-spin systems, sodium pyrophosphate decahydrate, Na4P2O7, 1OH2O (SP) and tetraphenyldiphosphine-1-oxide, (C6H5)2PP(O) (C6H5)2 (TPPO), including antiecho (COSY), double-quantum NMR, and zero-quantum NMR experiments. These experiments are generalizations of the absolute mode 2D Fourier transform antiecho COSY performed under MAS condition by Kentgens, de Boer, and Veeman [J. Chem. Phys. 87, 6859 (1987)]. The 2D sideband intensities for these experiments on polycrystalline samples are shown theoretically to be real. There are two mechanisms of coherence transfer; homonuclear J coupling and dipolar coupling. Theory shows that the zero-quantum signal for the coupled two spins can not be observed by using a (CPx−τ−(π)x−τ−(π/2)x−t1−(π/2)−t2 pulse sequence, when the coherence transfer is due to J coupling. When, however, the coherence transfer is induced by the flip-flop term of the dipolar coupling Hamiltonian, the zero-quantum signal can be observed by that pulse sequence. The preparation time dependences of the double-quantum and the zero-quantum sideband patterns, are expected, when the coherence transfer is induced by dipolar coupling. The zero-quantum signal was very weak for TPPO, while it was strong for SP. The apparent preparation time dependence of the zero-quantum sideband pattern was observed for SP. These results suggest that the coherence transfer is mainly due to J coupling in TPPO, where the two 31P nuclei have different isotropic chemical shifts. While, on the other hand, the dipolar coupling is more important in SP, where the two 31P nuclei have the same isotropic chemical shifts but different orientations of the chemical shift tensors. The 2D sideband intensities of the antiecho COSY spectrum of TPPO were calculated, and the relative orientation of the two chemical shift tensors was determined.

Notes: Results of a line shape and resonance light scattering study of the S1←S0 and S2←S0 electronic transitions of azulene in isopentane and cyclohexane are reported. The results are analyzed using two different non-Markovian master equations that make different assumptions about the statistical properties of the bath. For both these origin transitions we find that the solution dynamics fall in the so-called intermediate modulation regime. If exponential decay is assumed for the bath correlation function we obtain a correlation time of the bath of 25 fs for the S1←S0 transition and of 13 fs for the S2←S0 transition at room temperature. From the frequency dependence of the ratio of fluorescence to Raman yields of the S1←S0 transition we calculate an excited state lifetime of 1.4±0.2 ps using the parameters of the bath derived from the line shape analysis, and irrespective of which master equation is used.

Notes: A theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner by solving the time-dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density-matrix formalism. Explicit eigenstate-free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density-matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three-mode model of the vibronic coupling of the S1(nπ*) and S2(ππ*) states of pyrazine. The effect of excited-state vibronic coupling and ultrafast S2→S1 internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.

Notes: The polarized reflection spectra of single crystals of PDA-CPDO [poly-1-(N-carbazolyl)penta-1, 3-diyn-5-ol], which are π conjugated between the side groups and the main chain, have been measured in the photon energy region from 1.38 to 32 eV for the first time with a polarized synchrotron radiation source. Absorption spectra have been calculated using the Kramers–Kronig relation. Transitions in the visible absorption spectrum, which have been previously attributed to an interband transition in a similar polydiacetylene, have been found to be highly dichroic with respect to the direction along the polymer backbone. The absorption spectrum in this region reveals two broad (ΔE(approximately-greater-than)0.4 eV) peaks at 1.9 and 2.7 eV with an almost equal absorption coefficient of 7.5×104 cm−1. The spectral features in the range from 3 to 8 eV result from electronic transitions of the carbazolyl side groups. A single broad (ΔE∼10 eV) absorption band observed at 18 eV is due to either transitions of σ electrons to higher σ* or π* states of the carbazolyl group, or to ionization processes.

Notes: A molecular beam rf–optical double resonance experiment was performed on 87SrF in its naturally occurring abundance ratio. The natural occurring abundances of the strontium isotopes are 86Sr (9.8%), 87Sr(7.02%) and 88Sr (82.5%). Numerous magnetic dipole allowed transitions between ρ-doublets in the X 2Σ+ state were measured to an accuracy of 3 kHz. The observed spectra were analyzed in terms of an effective Hamiltonian which includes the magnetic hyperfine and electric quadrupole interactions arising from the 87 Sr (I=9/2) and 19F (I=1/2) nuclei. The extracted spectroscopic hyperfine parameters were interpreted in terms of a simple molecular orbital picture for the electronic nature of the X 2Σ+ state. A comparison is made to previous results for the more abundant 86SrF and 88SrF isotopic forms.

Notes: A quantum mechanical treatment of a double minimum system interacting with a heat bath is presented for the purpose of interpreting experimental data on transfer kinetics in condensed hydrogen-bonded systems. The model describes the transfer motion in one or two dimensions. The heat bath is represented by a set of harmonic oscillators and the interaction by a term linear in the system coordinates and in the bath coordinates. Extending an earlier random field approach, the present treatment consistently accounts for the quantum nature of the total system. With crystalline benzoic acid dimer used as an example, the master equation for the populations of the energy levels of the hydrogen transfer motion is derived. Transition probabilities consistent with the principle of detailed balance are obtained, based on a representation with explicit off-diagonal tunnel interactions for pairs of states localized on different sides of the barrier and with diagonal terms describing the rearrangement of the heat bath as a consequence of the tunneling motion. The activation of the double minimum transfer process with increasing temperature is related to the excitation of the local vibrations in the two potential wells.

Notes: Predissociation linewidths of the (3,0)–(11,0) Schumman–Runge bands of 18O2 and 16O 18O in the wavelength region 180–196 nm have been obtained from the published measurements of the absolute absorption cross sections of Yoshino et al. [Planet. Space Sci. 36, 1201 (1988); 37, 419 (1989)] and spectroscopic constants of these molecules of Cheung et al. [J. Mol. Spectrosc. 131, 96 (1988); 134, 362 (1989)]. The linewidths are determined as parameters in the nonlinear least-squares fitting of calculated to measured cross sections. Predissociation maxima are found at upper vibrational levels with v'=4, 7, and 10 for 18O2 and for 16O 18O. Our predissociation linewidths are mostly greater than previous experimental values for both isotopic molecules.

Notes: A quantum calculation has been performed using the centrifugal-sudden distorted-wave (CSDW) method for the three-dimensional Cl+DCl→ClD+Cl reaction. Three potential energy surfaces have been employed: two extended London–Eyring–Polanyi–Sato surfaces [denoted Bondi–Connor–Manz–Römelt (BCMR) and Persky–Kornweitz 3 (PK3)] and a scaled and fitted ab initio one (denoted sf-POLCI). Quantities calculated include: cumulative reaction probabilities, integral cross sections, rotational product distributions, and rate coefficients. Differential cross reactions are also reported for the PK3 surface, which are compared with the results from a simple semiclassical optical model (close agreement is found). We also compare the Cl+DCl results with earlier CSDW calculations for Cl+HCl→ClH+Cl. The rotational distributions are strongly perturbed by isotope substitution and are sensitive to variations in the potential surface. In contrast, the H and D rate coefficients for all three surfaces agree with experimental values, except for Cl+DCl on PK3.

Notes: A method for calculating and symmetry analyzing total molecular photoionization cross sections is presented. The technique is based on the LCGTO–Xα method and employs Stieltjes imaging. It allows applications to molecular systems comparable in size to those treated so far with the continuum multiple-scattering Xα method, but avoids the pitfalls of the muffin-tin approximation to the electronic potential. Photo cross sections for valence ionization of CO are found in good agreement with experiment and with previous calculations. From a final state symmetry analysis for the 1π level, the absence of kσ* shape resonance which appears in the 4σ and the 5σ ionization channels is attributed to small transition moments. This is in contrast to a previous treatment where this difference has been rationalized as caused by a channel dependent final state potential. The photoionization cross sections for the four highest valence orbitals of benzene were calculated in better agreement with experiment than found in a previous Xα–SW treatment. The improvement is especially significant for the 1e1g(π) highest occupied molecular orbital where at least part of the experimentally observed structures are attributed to shape resonances. Some of the observed resonance features in the valence orbital photo cross sections of benzene were identified with resonances found in carbon K-shell ionization.

Notes: Classical trajectory calculations of transport and relaxation properties have been performed for Ar–N2 mixtures using the potential energy surface (PES) recently determined by Bowers et al. [J. Chem. Phys. 88, 5465 (1988)]. Generalized cross sections have been computed in the temperature range 77.3–1000 K. Extensive comparisons have been carried out with available measurements and with other calculations. The present system exhibits greater efficiency for rotational energy transfer (RET) processes and its interaction shows a deeper potential well than that of previously computed surfaces. A larger number of trajectories (up to 28 500 at the lowest total energy examined) has therefore been required to obtain converged results. The PES employed here shows impressive agreement with the available measurements for a wide variety of properties of the system and appears to be the most reliable currently available for Ar–N2 gaseous mixtures.

Notes: A nonadiabatic theory of electron transfer in solutions is proposed in which the spatial dispersion of both intercenter interaction and activation energy is taken into account as well as the mutual diffusion of particles and diffusion along the reaction coordinate to the intersection point of potential energy terms. When spatial diffusion is considered to be the slowest step, the transition to the conventional theory is demonstrated and the generalized local probability of the transition is introduced to describe correctly the activationless electron transfer. It has been found that in the opposite case when diffusion along the reaction coordinate is hampered, the theory is actually nonlocal but allows the calculation of the rate constant which may be kinetic, pseudodiffusive or superdiffusive depending on the reaction heat.

Notes: A generalization of the well-known Flory and Flory–Huggins mean-field approximations to the equation of state is derived for a three-dimensional lattice model in which a monomer occupies an entire unit cell, and many bond lengths and bond angles are possible. By measuring the probability for particles to be in contact with the walls of the system, the pressure is determined via computer simulation over the full density range from dilute solution to dense melt. The results are used to test the mean-field predictions. Comparing the equation of state of the present model to those of conventional lattice models and of hard-sphere chains in continuous space, it is seen that our method approximates the continuum limit far better than single site lattice models. Also the large n des Cloizeaux scaling behavior is approached more rapidly.

Notes: We perform numerical calculations on a simple cubic lattice for a standard diagonally disordered tight-binding Hamiltonian, whose random site energies are chosen from a Gaussian distribution with variance ∑2. From phenomenological renormalization group studies of the localization length, we determine that the critical disorder is σc≡∑c/J=6.00±0.17, which is in good agreement with previous results (J is the nearest neighbor transfer matrix element). From our calculations we can also determine the mobility edge trajectory, which appears to be analytic at the band center. Defining an order parameter exponent β, which determines how the fraction of extended states vanishes as the critical point is approached from below, this implies that β=1/2, in agreement with a previous study. From a finite-size scaling analysis we find that π2/ν=1.43±0.10, where π2 and ν are the inverse participation ratio and localization length critical exponents, respectively. This ratio of exponents can also be interpreted as the fractal dimension (also called the correlation dimension) D2 of the critical wave functions. Generalizations of the inverse participation ratio lead to a whole set of critical exponents πk, and corresponding generalized fractal dimensions Dk=πk/ν(k−1). From finite-size scaling results we find that D3=1.08±0.10 and D4=0.87±0.09. The inequality of the three dimensions D2, D3, and D4 shows that the critical wave functions have a multifractal structure. Using a generalized phenomenological renormalization technique on the participation ratios, we find that ν=0.99±0.04. This result is in agreement with experiments on compensated or amorphous doped semiconductors.

Notes: The orientational relaxation of molecules in a supercooled liquid is known to exhibit interesting dynamical behavior. As the temperature of the liquid is lowered towards its glass transition temperature, there is a bifurcation of the relaxation dynamics into primary (α) and secondary (β) processes; the former is associated with the collective motion responsible for the glass transition, while the latter is associated with single particle motion. In this paper we present a theory of orientational relaxation in a supercooled liquid. This theory provides both qualitative and quantitative descriptions of the (αβ) bifurcation phenomenon at a molecular level. The theory exploits the properties of a time dependent free energy functional which explicitly includes the effects of the collective motions in the liquid on the orientational motion of a solute (or a tagged) molecule. In the overdamped limit, this analysis leads to two coupled Smoluchowski equations for the orientation distribution function. These equations, when solved, reveal the essential features of the (αβ) bifurcation phenomenon. Explicit calculations are presented for orientational relaxation in a liquid of dipolar hard spheres, a liquid of nonpolar ellipsoids, and an orientationally disordered solid. Our calculations demonstrate the ubiquity of the (αβ) bifurcation phenomenon and they reveal many of its aspects. The relevance of the present work to current theories of glass transition is discussed briefly.

Notes: A microscopic model is presented for anharmonic vibrations of ethylidyne, 3/4 CCH3, chemisorbed on the Pt(111) surface. The model includes 24 vibrational modes of the adsorbate and of the solid. A quantum-mechanical calculation based on second-order perturbation theory is used to interpret experimental data on vibrations of 3/4 CCH3/Pt(111) and 3/4 CCD3/Pt(111). The measured temperature dependence of the CC infrared fundamental and of the umbrella mode fundamental can be accounted for by anharmonic coupling between the CC stretch and the three PtPt stretch coordinates at the base of the adsorbate. Line shapes calculated using classical molecular dynamics disagree significantly with quantum-mechanical results, the apparent reason being overestimation of vibrational energy transfer in the classical calculation. A semiclassical approximation is suggested, in which all the high frequency adsorbate modes except the infrared absorbing mode are frozen; the remaining modes are treated by classical mechanics. The semiclassical calculation agrees much better with the quantum-mechanical results, and can be extended to higher dimension in a straightforward fashion.

Notes: We have investigated the diffusion of clusters on a triangular lattice using Monte Carlo simulations. A cluster is defined as a two-dimensional collection of particles which are connected to each other, either directly or indirectly through other particles in the cluster, by nearest-neighbor bonds. Each particle is allowed to hop, with probability αδb/2/(α−δb/2+αδb/2), to a vacant nearest-neighbor site with the constraint that the hop does not break the cluster. The change in the number of bonds is given by δb. The equilibrium clusters are correlated animals with structure controlled by the parameter α. We show that the diffusion coefficient of a cluster can be decomposed into two factors. One is a measure of the weighted length of the "active'' perimeter and the other is a measure of the correlation between pairs of steps taken by the cluster during its walk. The perimeter measure is asymptotically proportional to cluster size N, as anticipated for ramified animals, but it crosses over to N1/2 dependence for smaller compact clusters with α〉1. Our focus is on the accurate determination of the size and structure dependence of the correlation factor, which is more sensitive to statistical fluctuations. As a result, we describe the scaling of the cluster diffusion coefficient with cluster size.

Notes: The behavior of the density and the refractive index of a microemulsion of sodium dodecyl sulfate, water, n-pentanol, and n-dodecane is investigated near its critical end point. Measurements are made simultaneously and both are to an accuracy of a few ppm. In contrast to the density data which can be fitted to a regular, linear behavior in the whole temperature range (28.3–30.5 °C), the refractive index exhibits a pronounced anomaly near the critical point. The temperature dependence of such an anomaly cannot be described by a power law behavior. However, it can be understood as a chemical change connected to morphological modifications in the dispersed phase, a phenomenon connected to the close vicinity of sponge-like phases. This approach accounts well for the data.

Notes: Explicit analytical expressions are obtained for the rate of nucleation over different paths in a binary system. It is shown that anisotropy in reaction rates and anisotropy in the free energy surface can cause nucleation to occur bypassing the saddle point. Homomolecular nucleation is demonstrated to be the natural limit of binary nucleation as the concentration of one component goes to zero. Explicit expressions are also obtained for the time lag of binary nucleation by using the singular perturbation approach. It is shown that the time lag associated with different paths of nucleation is essential in determining the relative importance of different nucleation pathways.

Notes: The zeroth and second spectral moments for the far infrared spectral intensity of a polar liquid are analyzed from first principles, taking into account the first-order dipole–induced dipole contribution. The results, based on evaluating the equilibrium averages of appropriate correlation functions, are simplified by considering that the liquid pair distribution function can be approximated by a truncated expansion in terms of Wigner rotation functions. Averages involving three- and four-molecule distribution functions are not considered in the present calculation. The calculated zeroth moment for acetonitrile, M(0)=17.5 D2, compares favorably with the experimental value of 20.7 D2. The difference probably reflects the neglect of three- and four-molecule terms as well as higher order induction effects.

Notes: The spectroscopy of the B˜ 2A'–X˜ 2A' system of the formyl radical has been studied by laser-induced fluorescence. HCO was generated by photolysis of acetaldehyde, and a tunable laser operated near 245 nm excited eight bands of B˜–X˜. The (0,0,2)–(0,0,0) band has been rotationally analyzed, yielding A'=14.46 cm−1 and (B'+C')/2=1.13 cm−1 for this slightly asymmetric top; asymmetry splitting and spin doubling are observed. The intense branches have ΔK=0 but there also are weaker perpendicular components with the transition moment near the b axis. Vibronic transition energies agree with those from matrix absorption but with a 130 cm−1 blueshift. Resolved fluorescence spectra to X˜ levels as high as 15 000 cm−1 furnish vibrational constants for the ground state.

Notes: Raman spectra of zero pressure formed N2O4 solid layers are reported. Sample composition is extremely dependent upon deposition conditions. For ordered and pure solid N2O4(D2h), produced by slow NO2 deposition, temperature cycling over the range in which the solid is stable shows no significant spectral changes and does not result in autoionization, as argued in a previous Raman study. Fast and low temperature deposited layers are amorphous and multicomponent, showing bands of disordered and isomeric molecular N2O4 and of ionic NO+NO−3, nitrosonium nitrate. For nitrosonium nitrate, three solid modifications (two crystalline, one amorphous) can be characterized spectroscopically. In the amorphous phase, a light induced, temperature dependent, reversible transition between molecular and ionic nitrogen tetroxide is observed below 150 K. The paths leading to nitrosonium nitrate formation are examined.

Notes: The reduced specific viscosity of fully sulfonated sodium neutralized polystyrene under "free salt'' conditions was investigated as a function of temperature. It was found that the position of the maxima is strongly dependent on temperature. Small temperature changes (ΔT=20 K) introduce shifts of two orders of magnitude in the polyelectrolyte concentration at which the maximum appear. At a given temperature Cpmax is linearly dependent on molecular weight, the slope of the linear plot is temperature dependent increasing with temperature. The value of Cp at the maximum increases linearly with concentrations of externally added salt at all temperatures and molecular weights. At a given molecular weight, the logarithm of Cpmax is inversely dependent on temperature. The activation energy was calculated and found to be independent of the molecular weight of the polyelectrolyte. The dependence of the reduced specific viscosity on normalized polyelectrolyte concentration (Cp/Cpmax) resulted in one "master curve'' for all temperatures at a given molecular weight. Below the maximum, at lower polyelectrolyte concentration, a linear dependence of ηsp/Cp on Cp was obtained even for salt-free solutions. The apparent intrinsic viscosity and the Huggins coefficient were calculated, as it is done for noncharged polymers in the linear regime. High values of apparent intrinsic viscosity and the Huggins coefficient were obtained. The high measured values cannot be explained by hydrodynamic contribution of fully stretched molecules, indicating that even at extremely high dilutions the main contribution is the one of long range interactions (Culombic or others). The dependence of the apparent intrinsic viscosity on molecular weight was established. These measurements could be performed thanks to the availability of the apparatus developed by us which makes possible accurate measurements of the shear viscosity of low ionic strength, dilute polyelectrolyte solutions, down to polymer concentrations below one part per million.

Notes: The relative yield of photoinduced desorption from NO-exposed Si(111)7×7 has been measured as a function of photon power, wavelength, polarization, incident angle, and coverage of coadsorbed potassium. The results are analyzed in terms of two possible mechanisms: direct photoelectronic excitation of the NO-surface complex and interaction of hot carriers photogenerated in the substrate with the NO-surface complex. The substrate-mediated mechanism is found to be principally responsible for the photoreactions.

Notes: The light scattering by an initially monodisperse population of Gaussian random coils in a theta solvent is described by the well known Debye function: P(θ)=D[u(θ)] =(2/u2)(e−u−1+u), where u=u0≡〈S2〉q2. S is the radius of gyration, and q=(4πñ/λ)sin(θ/2) is the magnitude of the scattering vector, λ being the vacuum wavelength of the incident light, ñ the index of refraction, and θ the scattering observation angle. If the molecules undergo random scission, P(θ)=D(u0+r), where r is the average number of scissions per original molecule. In a good solvent, one should include the effect of the second virial coefficient A2 on the light scattering. In the single contact approximation this can be done by using Kc/R=1/MP(θ)+2A2c for an originally monodisperse solution. K is a constant, R the absolute Rayleigh scattering ratio, and c the concentration. The above equation is generalized to originally polydisperse solutions and branched random coils without loops. We discuss its mathematical limits and range of validity, and how to apply it to experimental situations. It is speculated that it may work fairly well when the penetration function ψ is less than about 0.1. We also discuss possible methods of extending its range of validity.