ALBERT

Notes: We have carried out a normal mode analysis of the molecular vibrations of pyridine adsorbed on silver surfaces or coordinated to various molecules and ions. There are two types of bonding that result in distinct shift patterns of the normal mode frequencies upon adsorption or coordination. "Metal ion-type bonding'' is characterized by a strong pyridine–substrate bond which results in large frequency shifts, while "halogen-type bonding'' causes comparatively smaller shifts due to weaker bonding. Adsorption on silver surfaces corresponds to weak halogen-type bonding, and the major cause of the observed frequency shift is the electron transfer (back donation) into the aromatic ring upon adsorption.

Notes: The 69GaP+ and 71GaP+ molecular ions have been generated by the combined methods of photoionization/laser vaporization for trapping in neon matrices at 4 K for electron spin resonance (ESR) investigation. The ground electronic state of GaP+ has been determined as X 4∑ and its electronic structure based on nuclear hyperfine properties is compared with the isovalent ion, GaAs+. The magnetic parameters (MHz) for Ga31P+ are: g⊥ =1.9836(5); A⊥ (69Ga)=188.6(1); A⊥ (71Ga)=239.4(1); A(parallel) (69Ga)=260(30); A(parallel) (71Ga)=300(10); A⊥ (31P)=108.0(1); and A(parallel) (31P)≈200(150). The g and A values observed for 31P(4S1/2) atoms in these neon matrices show closer agreement with the gas phase parameters compared to previous values reported for the heavier rare gases.

Notes: In highly compressed liquid nitrogen at 295 K we observe a sharp increase in the vibrational dephasing rate T−12 with increasing density, indicating that at high fluid densities (2.3 to 2.9×1022 cm−3) the modulation of the vibrational frequency shows a stronger density dependence than the bath correlation time due to the action of short range repulsive forces. The observed behavior shows good agreement with results of previously published molecular dynamics simulations and a fit to a binary interaction model. When the phase transition from the liquid to the solid state takes place we observe a sudden drop in T–12 by a factor of 4. We also present the variation of the Raman line shift of liquid and solid nitrogen with pressure over a region of 0.5 to 4.6 GPa at 295 K.

Notes: We report measurement of product state distributions for the rotationally and/or vibrationally excited HX formed in collisions of translationally hot H atoms with HX (X=Cl, Br, and I) at 1.6 eV collision energy. The product state distributions are probed after only one collision of the fast H atom, using coherent anti-Stokes Raman scattering spectroscopy. Whether proceeding by inelastic collisions or reactive exchange, the transfer of translational energy to vibrational and rotational energy is quite inefficient in H+HX collisions at 1.6 eV. For all three hydrogen halides only 2–3% of the initial translational energy appears as HX vibration. For H+HCl only 6% of the initial energy is converted to HCl rotational energy, while for H+HBr and H+HI, this percentage is twice as large, 11–12%, but still small. The indistinguishability of the two H atoms involved makes it impossible to distinguish reactive exchange from inelastic energy transfer in these H+HX collisions. However, the difference in rotational energy partitioning for H+HBr and H+HI as compared with H+HCl, suggests that reactive exchange is dominant in the former and inelastic energy transfer dominates in the latter. The total cross sections for the combined energy transfer/reactive exchange do not change much with the identity of X, being 13±3, 11±2, and 11±2 A(ring)2, for H+HCl, H+HBr, and H+HI, respectively.

Notes: Rotational transition in atom–diatom chemical reaction is theoretically studied. A new approximate theory (which we call IOS-DW approximation) is proposed on the basis of the physical idea that rotational transition in reaction is induced by the following two different mechanisms: rotationally inelastic half collision in both initial and final arrangement channels, and coordinate transformation in the reaction zone. This theory gives a fairy compact expression for the state-to-state transition probability. Introducing the additional physically reasonable assumption that reaction (particle rearrangement) takes place in a spatially localized region, we have reduced this expression into a simpler analytical form which can explicitly give overall rotational state distribution in reaction. Numerical application was made to the H+H2 reaction and demonstrated its effectiveness for the simplicity. A further simplified most naive approximation, i.e., independent events approximation was also proposed and demonstrated to work well in the test calculation of H+H2. The overall rotational state distribution is expressed simply by a product sum of the transition probabilities for the three consecutive processes in reaction: inelastic transition in the initial half collision, transition due to particle rearrangement, and inelastic transition in the final half collision.

Notes: Deexcitation (Penning ionization) cross sections of He(2 1P) by Kr and Xe have been obtained in a region of mean collisional energy between 18–38 meV by a pulse radiolysis method. Fairly large cross sections of above 100 A(ring)2 and their collisional energy dependence are interpreted by the Penning ionization cross sections based on a long-range dipole–dipole interaction. Validity of the theoretical formula for the Penning ionization cross section (the Watanabe–Katsuura formula) is discussed. Two kinds of the cross sections have been further calculated by means of the impact parameter method with experimentally simulated classical trajectories; in one procedure, the polarization axis of the p-state helium has been assumed to rotate in order to keep collinear or perpendicular configuration with respect to the interatomic axis, in the other procedure, the polarization axis is fixed in a certain direction. The classical motion of the particles have been shown to cause considerable influence on the absolute values and the collisional energy dependence of the cross sections. The influence has increased accordingly to the attractive force of the interatomic potential, i.e., in order of Ar〈Kr〈Xe. Modified form of the dipole–dipole autoionization width with the electron exchange interaction is also discussed. It has also been suggested that rotation of the p-state atomic polarization depends strongly on the van der Waals interaction with the target atoms. The effect of the rotation has been shown to be most prominent for Xe but small for Ar.

Notes: The electron attachment rate constants ka for SF6 have been measured in dilute mixtures of SF6 in high pressure (〉1 atm) N2, Ar, and Xe buffer gases at room temperature (T≈300 K) over a wide E/N range (electric field strength to gas number density ratio), corresponding to mean electron energies 〈ε〉 from near thermal electron energies (≈0.04 eV) to 〈ε〉≈4.3 eV. Particular attention has been paid to the effects of space charge distortion, molecular impurities, and changes in the electron energy distribution function on the measured electron attachment rate constant values at the lower E/N values in these mixtures. The present measured thermal electron attachment rate constants in SF6/N2 and SF6/Xe gas mixtures are in excellent agreement with recent accurate measurements of these parameters in several SF6/buffer gas mixtures. At higher 〈ε〉 values, the present SF6/N2 measurements are in fair agreement with previous measurements, while no previous measurements using Ar and Xe buffer gases have been published. These measurements have been used in numerical two term, spherical harmonic Boltzmann equation analyses of the electron motion in these gas mixtures to obtain the low energy (〈10 eV) nondissociative and dissociative electron attachment cross sections for SF6. The present derived electron attachment cross sections are compared with recently measured and derived nondissociative and dissociative electron attachment cross sections for SF6. The primary value of the present results is in the large and overlapping 〈ε〉 ranges of the present ka measurements for the three buffer gases compared with that for SF6/N2 gas mixtures alone, which in turn, makes these measurements ideal for testing cross-section sets in SF6 for use in many applied studies.

Notes: A simple electrostatic polarization model is applied to the low lying electronic states A 2 Π, B 2 ∑+, and A' 2 Δ of the alkaline earth monohalides which correlate to the electronic d state of the free metal ion. The number of fit parameters can be greatly reduced using relations which are derived from the well known angular part of the free ion wave function. The model predicts energies and electric dipole moments for all Ca, Sr, and Ba monohalides in good agreement with experimental data. The model can also be applied to the C state confirming the highly ionic character of this state.

Notes: A newly derive set of quantum fluid dynamical equations appropriate for the description of mixed state dynamics is presented. Based essentially on moments of the Wigner function, the theory presented here uses an expansion of the pseudodistribution function in conjunction with a renormalization technique to obtain a semiclassical approximation for the dynamics of the probability, probability current, and energy densities. Particularly simple equations, ideal fluid dynamics with a nonlocal potential, result from using a local Maxwellian ansatz for the Wigner function. Further, the transformed potential may be easily computed from the fields being convected. Analogies with classical kinetic theory and fluid dynamics are exploited whenever possible.

Notes: Chemical equilibrium calculations are reported on the dissociation of shock-compressed liquid nitrogen to infer the monatomic nitrogen–nitrogen potential in the condensed dissociated phase at high temperature. The analysis suggests that two types of interatomic potentials with very different physical characteristics can about equally describe experimental shock-wave data. The two differ in the interatomic potential parameter α, characterizing the stiffness of the exponential repulsion, and the parameter ε which scales the repulsive and attractive interactions. The first type, with a large α and a small ε, produces a continuous shock dissociation without thermodynamic phase change, and the second, with a small α and a large ε, predicts that the shock dissociation can accompany a first-order phase change. The predicted first-order phase change is new and similar to a theoretically predicted plasma phase transition. The calculation suggests that the transition in nitrogen at high compression may be associated with a physical change at the atomistic level from a van der Waals-type to a much stronger (metallic, semimetallic, or covalentlike) interaction. The implications in interpreting shock-wave data and possible future experiments to distinguish the two potentials are also discussed.

Notes: We have carried out a series of ion dynamics simulations on binary silicate systems ranging from MnO–SiO2, which shows a large liquid–liquid miscibility gap, to K2O–SiO2, which does not unmix even below the stable liquidus. We have used Coulomb attractions plus Born–Mayer–Huggins repulsive pairwise-additive potentials, evaluating the energies with a full Ewald summation. Energies of the systems were determined at a constant pressure using the Andersen algorithm. In the simulated systems (which are too small to permit phase separation) results show that in the MnO–SiO2 system the heats of mixing are indeed positive, and greater than any conceivable T⋅ΔS value, in the composition range 0.5〈XSiO2 〈1.0 in which phase separation occurs in practice. In K2O–SiO2, by contrast, ΔHm is always negative. Analysis of angular correlations as a function of temperature using new "circular distribution functions,'' allows the origin of the unfavorable mixing energy to be determined in terms of structural/geometrical incompatibilities. At high silica contents the incorporation of smaller divalent cations, which seek four coordination, can only be realized by distorting the network from its lowest energy topology to provide the requisite nonbridging oxygens. On cooling, the divalent cations are seen to move out of these unfavorable sites as a first step towards phase separation on a macroscopic scale. In the case of large low-charge-density cations, a greater degree of bridging is retained by forming large gaps which can easily host the cations and even allow them to diffuse freely.

Notes: The clustering and percolation of particles in binary mixtures of randomly centered spheres are examined based on a selective particle connectivity criterion in which only particles of different species are allowed to form directly connected bonds. This problem is different from the usually studied "simple'' percolation problem in which pairs of particles form directly connected bonds as long as they are separated by a distance σ or less. The percolation threshold and pair-connectedness function of the binary mixture are determined based on the connectivity Ornstein–Zernike integral equation in the Percus–Yevick (PY) approximation. It is shown that, within the PY closure, the present system can be mapped into the Widom–Rowlinson model in the theory of liquid state. The percolation thresholds and the pair–connectedness functions of the particles are numerically computed for a wide range of particle densities and number fractions. It is found that their percolation densities differ considerably from those found in the simple percolation problem for a binary mixture of randomly centered spheres. To our knowledge, this is the first study of selective particle clustering and percolation in multicomponent mixtures of particles.

Notes: The properties of the continuum Potts model are used to derive integral equations for the properties of correlated percolation. Specifically, extended Born–Greeen–Yvon (BGY) equations are derived for the two-point connectedness function in continuum percolation systems. Two different types of correlations among the percolating elements are considered: those due to a two-body potential and those due to impenetrable inclusions occupying a fraction of the system volume. In both cases, the superposition approximation is derived and solved numerically. The variation of the percolation threshold with increasing correlation is calculated and discussed. Finally, higher order corrections to the superposition approximation are obtained by analyzing the relevant Mayer series.

Notes: The interaction of oxygen with Pd(110) has been studied by thermal desorption spectroscopy (TDS), work function change (Δφ), and Auger electron spectroscopy (AES) at 100〈T〈900 K. Thermal desorption spectra reveal three major peaks, a low temperature one (α) being from molecular adsorbates and the other two (β1 and β2) from atomic oxygen. The β1 state is believed to be associated with subsurface oxygen. At 100 K, the population of molecular oxygen increases with increasing oxygen coverage. Above 200 K, all adsorbates are present as atomic oxygen. The rate of population of β1 decreases as T increases. This is explained as a kinetic effect due to the formation of the c(2×4) structure which hinders population of the β1 state. The oxygen induced work function change is found to depend strongly on both oxygen coverage and T. At a given T, Δφ is controlled by surface oxygen coverage, surface structural changes, as well as the formation of subsurface oxygen. A decrease of the intensity ratio of certain spectral features (I0 /IPd ) in the AE spectra is also observed, which might be associated with migration of surface oxygen to the subsurface region. The migration of oxygen to subsurface sites at low temperature is facilitated by the lateral displacements of surface Pd atoms which occurs on oxygen exposure at low T. A physical picture of the interaction of oxygen with a Pd(110) surface is tentatively proposed.

Notes: The nematic behavior exhibited by a variety of amphiphiles is driven by the coupling between the growth and alignment of elongated aggregates. We have previously studied these systems using a model derived from lattice combinatorics, with attendant restriction of particle orientations to the three mutually orthogonal lattice axes. In this work we present a generalization of the above model for rod-like aggregates with a continuum of orientations. Overall, the calculated phase diagrams are similar to those obtained earlier for discrete orientations. This includes the second order isotropic-to-nematic transition in the weak aggregation regime. However, the nematic-to-nematic transition, obtained previously for intermediate aggregation strength, is not reproduced. We also consider the consequences of solvent filled spaces in the aggregates. Increasing the effective excluded volume of the solute by this mechanism causes the isotropic-to-nematic transition to shift to lower concentrations. This response of the phase behavior to the incorporation of solvent in the aggregate is important for the interpretation of experimental data.

Notes: Monte Carlo simulations based on a partition of the conformational energy into short-range and long-range interactions are performed in order to estimate the free energy and entropy of short polypeptide chains. Values of these quantities are obtained by using an "umbrella-sampling''method particularly adapted to the partition of energy. Reference systems are completely determined by considering short-range interactions alone. In this case, the chain model allows us to calculate the average energy, the free energy and entropy. Moreover, the configuration of minimum energy is determined by an algorithm proposed for chain models with only nearest neighbor interacting units. When long-range interactions are also considered, the relative effects of the two types of interactions on the chain configuration can be appreciated by estimates of entropy variations. Such variations are related to the folding of the chain which can be modulated by using different dielectric constants in order to take into account solvent effects. The different methods of calculation are applied on models of the polypeptide hormones enkephalin and β-casomorphin.

Notes: The electronic properties of compensated InP crystals can be used to sensitively monitor gas–surface interactions. When a gas is adsorbed on these low carrier density semiconductors both the conductance and minority carrier lifetime exhibit large changes which we interpret in terms of band bending. Changes of greater than 50% in the bulk-averaged conductance of Fe-compensated semi-insulating InP crystals have been measured for adsorption of ∼0.5 monolayers of Cl2. Using modulated NO2 molecular beams the conductance changes are demonstrated to be fast (〈1 ms to steady state) so as to be capable of yielding quantitative rate information about the gas–surface interaction.

Notes: The Raman scattering due to resonance with a perpendicularly polarized electronic transition of a symmetric top molecule in the gas phase is described in a sum-over-all-states approach. The derived intensity and depolarization ratio expressions in an irreducible two-photon tensor basis are applied to the analysis of the ν2 (a1 ) and ν6 (e) bands of CH3 I derived from resonance with the predissociated X→B absorption system. A lifetime of 0.5±0.1 ps is determined for the electronic origin and several K-specific rovibronic levels of the v'6 =1 band of the resonant excited state. These results are contrasted with recent dynamical interpretations of the corresponding jet-cooled CH3 I absorption spectra.

Notes: The Raman ν4 symmetric C=O stretching mode of N,N-dimethylacetamide (DMA) is reported as a function of pressure from 1 to 4000 bar within the temperature range 40 to 100 °C. As in acetone, the isotropic line width ω1/2 (ISO) decreases with increasing density, in contrast to the opposite trend usually found in other molecular liquids. The noncoincidence effect, δω, which is the frequency difference between the peak maxima of the isotropic and the anisotropic Raman bands, increases with increasing density. These trends indicate the presence of strong intermolecular dipolar interactions between DMA molecules. Two theories of the noncoincidence effect due to McHale and Logan are used to interpret the density dependence of δω for DMA. In addition, we also used these two theoretical models to calculate δω for acetone using the results obtained earlier in our laboratory. McHale's theory can predict the correct density dependence for the noncoincidence effect when the dielectric screening factor is ignored. Logan's theory can account for the density behavior of the noncoincidence effect in DMA and in acetone.

Notes: The chemiluminescence of IF produced by the I2+F2 reaction has been the subject of several works. In this paper, we present a study of this reaction under crossed beam conditions. The chemiluminescence spectrum has been recorded at several collision energies and the rovibrational distribution of the IF molecules has been deduced from the spectra by a linear least square fit. This distribution presents a strong vibrational inversion and extends to very high J values. It is very different from all the previously reported rovibrational distributions observed in the IF chemiluminescence. This result confirms that at least two different mechanisms contribute to populate the IF B state in the I2+F2 reaction, and our experiment is the first one to give the spectrum of the I2+F2 chemiluminescent reaction occurring in a single collision regime.

Notes: A measurement of the N(2D0) and N(2P0) yields in predissociation of the e 1Πu, e' 1Σ+u, and b' 1Σ+u states of molecular nitrogen is reported using photofragment spectroscopy. These states are prepared by laser excitation from the long-lived a‘ 1Σ+g (v=0) level in a fast molecular beam. A position- and time-sensitive detector is used to monitor the two correlated fragments produced by single molecular dissociation events. The translational energy imparted in the fragments establishes the electronic energy of the resulting nitrogen atoms and hence the branching among the available dissociation limits of nitrogen. Extremely pronounced rotational dependence in the branching yields is observed for the e and e' members. The dependence mimics, in part, known molecular perturbation of the Rydberg states, and it reveals a complex competition among continuum states for the population in the Rydbergs.

Notes: The dynamical formulation of electron transfer rate theory, including inner sphere and outer sphere reorganization terms, electronic structure polarization, and an ab initio treatment of the electrons on donor, acceptor, and bridge molecules, is presented and applied to electron transfer betwen benzene/anion radical and pyridine. The formulation involves the use of mean field (Ehrenfest) relations to obtain the time evolution of electron and vibration operators. This formulation yields an effective density matrix for the time evolution of the electronic system; the elements of this density matrix depend on averages over electronic and vibrational motions. For the electron transfer system actually studied, the rates are strongly dependent upon relative geometry of donor and acceptor, and maximize sharply at geometries such that electronic levels on donor and acceptor become degenerate—the so-called "coincidence event'' geometry.

Notes: A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 A(ring)2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative producttranslational energy distributions which are required to independently test the applicability of the statistical dissociation model.

Notes: Photodissociation of NO has been investigated in the energy region between 8.89 eV, the energy required to produce N(2D)+O(3 P), and 9.26 eV, the ionization limit. Using the technique of resonance-enhanced multiphoton ionization (REMPI), we have demonstrated that there are areas in this energy region in which there are large yields of N(2D), and others where the yields are small. Localized concentrations, from 2-hν dissociation of NO at 270 nm, are an order of magnitude greater than can be obtained from a discharge in N2.

Notes: Large-scale CEPA calculations have been carried out for the astrophysically important molecules C3O and HC3O+. C3O has a linear equilibrium geometry with short terminal CC and CO bond lengths of 1.273 and 1.149 A(ring) and a central CC equilibrium bond length of 1.300 A(ring). Upon protonation the terminal CC and CO bond lengths are shortened by 0.061 and 0.026 A(ring), respectively, while the central CC bond length experiences an elongation by 0.052 A(ring). C3O has a noticeably large equilibrium dipole moment of 2.535 D with the positive end at the oxygen site. The IR intensity of the ν1 band at 2229 cm−1 is extremely large (72 756 cm2 mol−1). Although the intensity of the corresponding ν2 band of HC3O+ at 2318 cm−1 is only about one half as large this band appears to be most promising for future IR diode laser investigation. The proton affinity of C3O at 298 K is 885±5 kJ mol−1.

Notes: Completely theoretical calculations for the structure, anharmonic force field up to partial sixth order, and vibrational frequencies of C3H+3, C3H2D+, C3HD+2, and C3D+3 have been carried out. The procedure included ab initio calculation of the vibrational energy surface at the 6-311 G4* + local MP4(SDQ) level and anharmonic vibrational configuration-interaction (CI) calculation using a variational expansion in a large, well-selected harmonic oscillator basis. The geometric parameters of C3H+3 are optimized as 1.3632 A(ring) for the C–C bond length and 1.0795 A(ring) for the C–H bond length. The fundamental vibrational frequencies of C3H+3 are 3193.5, 1622.1, 1015.5, 3148.8, 1297.5, 916.2, 756.8, and 1002.7 cm−1; those of C3H2D+, (3190.6, 3144.8), 2405.3, 1580.3, 1274.7, 913.4, 1001.2, 3146.3, 1295.2, 995.4, 717.8, 916.7 and 663.6 cm−1; those of C3HD+2, are 3164.7, 2434.0 (1541.6, 1506.8), 1276.0, 672.1, 824.3, 2361.6, 1261.3, 966.7, 762.3, 960.3, and 603.9 cm−1; and those of C3D+3 are (2536.7, 2451.8), 1487.4, 837.5, 2358.1, 1256.9, 671.9, 562.6, and 823.5 cm−1. The frequencies enclosed in parentheses are for Fermi resonance pairs, the prediction of which is handled automatically by this procedure. The theoretical vibrational frequencies agree with the experimental data available from both Raman and infrared spectra with 0.5% relative mean deviation. This agreement is as good as the experimental data allow, since the experiments were carried out in condensed phases and in the presence of counterions. The first overtone vibrational frequencies of C3H+3 are also predicted. The general applicability and present limitations of the method are discussed.

Notes: It is shown that a recently proposed quadratic configuration interaction (QCI) method, when limited to single and double substitutions (QCISD), represents a special case of the single reference coupled cluster approach. When applied to higher levels of substitutions (QCISDT) the method ceases to be size extensive. The relationship of QCISD method with existing coupled cluster approaches is shown in detail.

Notes: Numerical multiconfiguration self-consistent-field (MCSCF) procedure is employed to calculate the hyperfine interaction energy for the 3He4He+ cation at different internuclear separations. A conventional vibrational averaging of the energy allows the calculation of hyperfine splitting in the IR spectrum. This is done for several of the lowest vibrational states. We predict that the hyperfine splitting will get larger with the increasing vibrational excitation. Various different MCSCF wave functions are used in the study to verify the convergence of the hyperfine parameters and to determine the importance of the electronic correlation.

Notes: A general statistical mechanical theory is developed to describe structural and thermodynamic properties of surfactant monolayer films at the interface between water and a hydrophobic solvent. It is assumed that the surfactants are comprised of a single head group and one or more flexible hydrocarbon tails, and that the head group serves only to constrain one end of the molecule to the aqueous interfacial plane. Each chain is characterized by the profile of volume it occupies perpendicular to the interfacial plane. Since the position of the maximum in the volume profile varies with conformation, the lateral excluded area of each conformation is approximated as an average over all pairs of conformations. By assuming "ideal'' two-dimensional mixing of solvent with the chains, and of chains with each other, the entropy of the monolayer is then determined. For purposes of determining interaction energies, the surfactant chains are also characterized by the position and orientation of their surface area available for nearest-neighbor contact. The orientational distribution of chain surface may be highly anisotropic, particularly at high molecular surface densities when the chains are largely aligned, so the total area of intermolecular contact cannot be determined from the chain segment profile alone, as in regular solution theory. Interaction energies among chain, solvent, and water are reduced to two parameters, one related to the chain–solvent interfacial tension, and the other to the difference of chain–water and solvent–water interfacial tensions. The equilibrium chain probability distribution is obtained by minimizing the free energy with respect to the distribution, from which all structural and thermodynamic properties can be predicted. In the subsequent paper (part II), pressure-area isotherms are predicted using a modified cubic lattice model for the chains, and shown to be characterized by two first-order phase transitions.

Notes: Standard microcanonical rate-constant theory can be cast in a form which closely resembles and, in the case of a sufficiently large medium, reduces to that of the canonical ensemble. This relation permits a new understanding of the role of an activation energy in a chemical reaction. A facile methodology is also proposed for the extraction of thermodynamic data from kinetic studies of small media.

Notes: In this report, we describe some general features of the monotonic Lagrangian grid (MLG) that are important in molecular dynamics simulations. The monotonic Lagrangian grid is a highly efficient and general algorithm for tracking particles and computing interactions in simulations of systems consisting of large numbers of particles. Further, the MLG algorithm is highly adaptable to vector or parallel processing. The mode of implementing the MLG algorithm depends on the nature of the particle simulation. We present results of simulations for calculating the dimer mole fraction as a function of temperature in a three-dimensional N2 gas system. The results are consistent with experimental and theoretical results and demonstrate the effectiveness of the MLG algorithm in simulating gas systems and monitoring dynamically rare processes.

Notes: We study the asymptotic behavior of the position-orientation profile ρ(1) for a dipolar hard sphere fluid in contact with a neutral hard wall. First, by a virial expansion we show that ρ(1) is not totally determined by the classical image potential, i.e., by a dielectric continuum model. The exact expression of ρ(1) far from the wall is derived by using a renormalized cluster expansion. As predicted by the dielectric continuum model, ρ(1) exhibits an orientational structure and a long tail which decreases as the inverse cubic power of the distance to the wall. We first examine the density profile ρ0(1) which corresponds to the isotropic part of ρ(1). We show that ρ0(1) contains a part reminiscent of the image potential but also some contributions which depends on the pair correlation function and the triplet direct correlation function in bulk phase. When this last function is neglected, ρ0(1) can be considered as the result of a force balance in the interface. In the general result, the triplet direct correlation allows us to obtain a very compact expression for ρ0(1). It is shown that the asymptotic behavior of ρ0(1) reveals the same microscopic properties as the classical electrostriction phenomena which is observed in bulk phase and in presence of an external electric field. Thus, ρ0(1) can be considered as the result of a natural electrostriction induced in the interfacial region by the image potential. The term of lowest symmetry in the orientational structure describes the alignment of a molecule relative to the normal to the wall. Far from the wall, the orientational profile is proportional to the dipolar contribution of the Kerr constant. Thus, the asymptotic behavior of the profile and the Kerr effect are determined by the same function characterizing the alignment of dipoles. This alignment is observed via an external field in the case of the usual Kerr effect and it is naturally induced by the image potential in the interfacial region. The triplet direct correlation function gives rise also to some additional orientational structures which are ignored in the dielectric continuum model. The exact results derived in this paper include some bridge diagrams and consequently they are beyond the wall–particle hypernetted chainapproximation.

Notes: Below 72 K ice doped with KOH transforms to an ordered phase known as ice XI. A powdered sample of such doped ice annealed for several days below 72 K was studied by neutron diffraction and found to be a two-phase mixture of ice Ih and ice XI. The ice XI has an orthorhombic unit cell, and the diffraction profile is consistent with the predicted Cmc21 structure.

Notes: For a special class of polydisperse fluid models we have developed methods for obtaining (numerically) exact solutions to the equilibrium conditions. The models considered are special in the sense that the functional dependence of thermodynamic properties on the mole–fraction distribution density F(I) and microscopic parameter functions is restricted. For example, the pressure may depend only on a single moment of F(I). For our special class of models, we show how the dew/bubble conditions can be given a simple two-dimensional geometric representation, and the dew/bubble conditions are reduced to solving two equations in two unknowns. The solution of the full equilibrium conditions can be obtained by an iterative procedure given the solutions of the dew/bubble equations. Three specific models, based on the van der Waals and Soave–Redlich–Kwong equations of state, are examined in detail. We also show how to implement these new solution techniques when the mixture contains a finite number of chemical species.

Notes: Below a temperature TB, the rotational relaxation of small molecules in supercooled liquids appears to bifurcate into slow α and faster β relaxations. The characteristic time for the former diverges in Vogel–Fulcher fashion at the glass transition Tg and that of the latter follows Arrhenius behavior from above TB to below Tg. We have examined models for α- and β-rotational relaxation. Above TB, it appears that only β relaxation occurs and that τ1/τ2≈3, where τl is the relaxation time for the lth order spherical harmonic, while below TB it seems that for α relaxation this ratio is closer to unity and for β relaxation it is unknown. The value of three for the ratio is characteristic of rotational diffusion, the smaller ratio of large angular jumps and of restricted diffusion. We have ascribed β relaxation to rapid angular diffusion within a long-lived torsional potential well and the α relaxation to random restructuring of the torsional potential itself. Thus, the appearance of the α relaxation signals the emergence of a longer length scale, which is consistent with the observation that the τ corresponding to the α relaxation appears to diverge at the glass transition.

Notes: This paper describes the application of the numerical solution of the Percus–Yevick approximation for a multicomponent system of sticky hard spheres to small angle x-ray scattering experiments. The effect of polydispersity on the pair correlation function is quantitatively described, thus enabling the analysis of experimental data. In this way quantitative estimates of the size distribution width, interaction radii and the measure of attraction between particles can be obtained. The attractive interaction in a system of sticky spheres is known to give rise to a transition analogous to a gas–liquid phase transition. The effect of polydispersity on this transition is discussed. Finally it is shown how the model can be used to describe small angle x-ray scattering from an AOT microemulsion system.

Notes: A constant pressure Monte Carlo formalism, lattice dynamics, and classical perturbation theory are used to calculate the thermal expansion, pressure–volume relation at room temperature, the temperature dependence of the zone center libron frequencies, and the pressure dependence of the three vibron modes of vibration, in solid CO2 at pressures 0≤p≤16 GPa and temperatures 0≤T≤300 K. The agreement with experiment is good. At room temperature the observed Pa3 phase is predicted for P≤11 GPa, above which a transition occurs into an orthorhombic Cmca structure, with a volume change upon transition of ΔV=0.4 cm3 /mol. Calculated physical quantities in this phase are consistent with experiment.

Notes: We extend previous work [G. J. Vroege and T. Odijk, Macromolecules 21, 2848 (1988)] on the formation of nematic liquid crystals from uncharged semiflexible polymers to the charged case. The polyelectrolytes are modeled as long slender worm-like cylinders interacting via both hard-core and electrostatic repulsions in the second virial approximation. Analogously to rod-like polyelectrolytes it is possible to describe the result of charge by introducing an effective diameter and a twisting effect. We then calculate numerically the orientational distribution function as a function of concentration. We determine the effect of charge on the phase transition and—in the nematic phase—its influence on the order parameter and the global persistence length (directly connected to the splay elastic modulus). Although the phase transition concentrations are clearly dependent on charge, the properties of the nematic phase at the transition are not—in contrast to the result for rod-like polyelectrolytes.

Notes: We present a replica calculation and obtain analytically the size of a polymer chain with excluded volume interactions in quenched random media. We show that the size of the polymer is shrunk as the density of the impurities constituting the random medium is increased. We demonstrate that this is a general phenomenon. There are three regimes depending on the strength of the impurity density v. In the weak impurity density regime the polymer obeys the self-avoiding statistics with its gyration radius R depending on its typical length L according to R∼L(D+2)/(d+2), where D is the dimension of the polymer and d is the space dimension. In the intermediate regime the polymer is in the unperturbed state where R∼L(2−D)/2. In the third regime of localization occurring for sufficiently large v, R∼v−(2−D)/[4D−(2−D)d] in the absence of three-body interactions and R∼v−1/d LD/d in the presence of strong three-body effects. We examine the effect of long-range interactions of the type w||r||−α between the segments of the polymer separated by r on the localization and find that the polymer collapse is suppressed for realistic impurity densities [v〈wR(d−α)/2].

Notes: Two problems are addressed in the present study: the degree of copper 3d covalency involved in the chemisorption of oxygen at the fourfold hollow site of Cu(100) and the separability of the correlation energy into contributions from the 3d shells on copper and from the valence sp band. The investigation was carried out at the all-electron level using a Cu5 cluster as a model of the Cu(100) surface. The analysis shows that the 3d covalency is of practically no importance in the system considered, contributing only 1–3 kcal/mol to the total chemisorption energy of 89 kcal/mol. The correlation energy was found to be separable to within 5 kcal/mol. A configuration-interaction calculation on the Cu5O system using the one-electron effective core potential developed previously yielded a correlation energy in close agreement with the all-electron results.

Notes: Rotational energy transfer processes into the A and E symmetry species of the symmetric top molecule 13CH3F have been studied. In this time-resolved double resonance experiment a tunable millimeter/submillimeter wave spectrometer was used to monitor the change in strength of rotational transitions in the ν3 vibrational state after a Q-switched CO2 laser pumped the K=3, J=5 level in ν3. A simple numerical simulation of rotational energy transfer allowed the 13CH3F system to be modeled and collisional energy transfer rates to be obtained from the data. Two important processes were studied. The first, a process that obeys the spin statistic selection rule ΔK=3n has a rate of 29±6 ms−1 mTorr−1. The second, a vibrational quantum number swapping collision that effectively transfers population between the A and E symmetry species and thereby transcends the spin statistic selection rule, has a rate of 6.6±0.7 ms−1 mTorr−1, about 1.4 gas kinetic collisions. The numerical simulations and these rates, along with previous measurements of the ΔJ=±1 rate and vibrational decay rates, provide an accurate characterization for a large body of varied experimental data.

Notes: The decay of excited stretching overtones of some local modes (C–H, C 3/4 C,C(Double Bond)C, C(Double Bond)O) attached to a hybrocarbon chain is theoretically investigated. The assumptions used in a previous paper [A. Lami and G. Villani, J. Chem. Phys. 88, 5186 (1988)] are critically examined and the model is improved by allowing the transfer of up to three quanta from the local mode (simulated by a Morse oscillator) to the harmonic bath of the chain modes. Use is made of a Lanczos tridiagonalization procedure. It is shown that the population transfer from the local mode to an aliphatic carbon chain is rapid but not complete due to the fact that the local mode frequency lies above the top of the band of chain modes and to the particular structure of the Hamiltonian for kinetically coupled stretching modes. It is argued that the complete decay should involve other low frequency modes and should require a longer time.

Notes: A dynamically biased statistical theory is used to calculate state specific cross sections for the nearly thermoneutral ion–molecule reaction N+(3PI)+H2( j)→NH++H. Conservation of parity and nuclear spin, and the anisotropy of the long-range interaction potential are taken into account explicitly. The theory is used to calculate cross sections for all three 3PI states of the N+ ion and for rotationally excited hydrogen ( j=0–6). ln addition, the calculation has been performed for different electronic degeneracy factors (gs =1/3, 1/2, and 1), and for reaction endothermicities ΔH varying between 15 and 20 meV. The integral cross sections are used to determine numerically exact thermal rate coefficients. Comparison with published experimental results allows to determine gs and ΔH. The room-temperature data lead to gs =1/2, while rate coefficients measured below 50 K are very sensitive to the threshold behavior and are best fitted with ΔH=17 meV. Some new experimental results, measured with a liquid nitrogen cooled RF (radio frequency) ring electrode ion trap are also included. For H2 they are in good agreement with both theory and former experiments. Some discrepancies occurring for D2 as target will be discussed.

Notes: Ab initio molecular quantum mechanics has been applied to the unimolecular dissociation of H2CO. Basis sets as large as triple zeta plus double polarization (TZ+2P) were used in conjunction with complete optimization of all stationary point geometries. The classical barrier height is predicted with the TZ+2P basis set to be 101.9 (SCF), 95.0 (CISD), 90.4 (CCSD), and 86.8 kcal/mol (CCSDT-1). With correction for zero-point vibrational energies, the activation energy is predicted to be 81.4 kcal/mol, in good agreement with experimental estimates.

Notes: This paper reports results of an experiment involving two-laser resonance-enhanced photoionization of benzene. The excitation sources were two frequency-doubled dye lasers. The first laser pumped the molecule to a selected vibronic level of its first excited singlet state (1B2u), from where it was ionized by a time-delayed pulse of the second laser. The ion yield depends on the intermediate vibronic state as well as on the wavelength of the ionizing laser. From the structures and intensities of the measured ion spectra we derived vibrational frequencies and molecular parameters of the ground electronic state of the ion to a remarkable accuracy. The contributions of autoionizing Rydberg levels to the ionization cross section can clearly be distinguished from direct ionization. Several resonance peaks were assigned to transitions to vibrational modes within these Rydberg states.

Notes: Fluorescence excitation and emission spectra have been measured for solid solutions of 2,2':5',2‘-terthiophene in n-decane at 77, 10, and 4.2 K. At 4.2 K the spectra exhibit full vibrational resolution (origin inhomogeneous FWHM approximately 5 cm−1). At 4.2 K narrow band excitation and detection establish that there are four independent but nearly identical excitation/emission pairs with origins at 24 806, 24 827, 24 835, and 24 879 cm−1. Whether this multiplicity is caused by the presence of different isomers or comes from a single isomer that can occupy in the n-decane lattice in four different ways is not known with certainty. However, the similarity of the vibronic development and energetic considerations suggest that the spectra arise from a single isomer present in four rather similar n-decane sites or from four slightly different conformations of a single isomer. The overlap of excitation and emission origins and the vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (probably reasonably described as the 1 1A1 to 1 1B2 transition where the excited state is derived from the ground state by the promotion of one electron from the HOMO to LUMO).

Notes: We report laser correlated angular distributions of K atoms scattered off an HF nozzle beam which has been partially vibrationally excited to the v=1 state by modulated infrared laser radiation. Measurements have been performed at mean relative translational energies ranging from 0.25 to 0.82 eV. As a consequence of the preparation technique the distributions directly reflect differences between the nonreactive scattering off HF in the excited state and in the ground state (v=0). The data exhibit well resolved uncommon features which are rationalized assuming spherically symmetric potentials with different well depths ε0 and ε1 for K+HF (v=0) and K+HF (v=1), respectively. Adopting the value ε0=117 meV we find ε1 =151 meV. Employing a vibrationally adiabatic model, this finding is traced back qualitatively to the difference between the vibrational energies of HF as a free molecule and in the proximity of a K atom (at well distance). Furthermore, we report angular distributions of K scattered off unprepared HF molecules (v=0) measured at mean translational energies ranging from 0.13 to 0.64 eV. In the range 0.17 to 0.38 eV the data show clearly resolved rainbow structures from which the well depth ε0 of a spherically symmetric potential is deduced. The obtained value (ε0=117 meV) is roughly a factor of 5 larger than expected from the well depths of homologous systems. However, at the lowest translational energy achievable (0.13 eV) we find another faint rainbow which we associate with a shallow well of 26 meV depth. To rationalize these results we propose a double minimum potential for K+HF and attribute the shallow and deep well to interactions of K with the H and F side of the molecule, respectively.

Notes: Quenching cross sections σQ have been measured for several rotational levels N' in the A 3∏i, v'=0 state of NH, for a variety of collision partners. Ground state NH was generated in a room temperature discharge flow and excited with a pulsed laser, and the time decay of fluorescence was measured. The radiative lifetime for the levels N'=1 to 5 is 418 ± 8 ns. The σQ's are generally large, up to 90 A(ring)2, and decrease with increasing N'. This indicates the influence of an anisotropic, attractive interaction in most but not all cases. The present values of σQ are compared with those of other experiments; because σQ depends both on N' and collision energy, experimental conditions must be carefully specified to yield results which are readily comparable.

Notes: The pressure broadening and line coupling cross sections in the Fano–Ben Reuven theory of line shapes are calculated for bending bands of CO2 in a bath of He atoms. Molecular collision dynamics are simplified by invoking the infinite order sudden (IOS) approximation for molecular rotational and vibrational angular momentum in a manner similar to but not identical with the method developed by Clary and shown to be accurate for CO2–He. Numerical values are obtained using a pairwise additive interaction potential developed by Clary. Predictions are in good accord with data for various infrared bands and pure rotational Raman spectra. It is found that all the pressure broadening and state-to-state cross sections depend on only a few dynamical factors (generalized IOS cross sections) and are therefore closely interrelated. Results are used to assess models developed previously to analyze line shapes in this and similar systems.

Notes: We use the capillary wave model of a liquid–vapor interface as a working example to show how to construct the free energy density functional from studies of the direct correlation functions of a reference system, and vise versa.

Notes: Molecular beam techniques have been used to probe the dynamics of the trapping and trapping–desorption of Ar at a hydrogen-saturated W(100) surface. Trapping probabilities have been measured as a function of incidence energy Ei, and angle θi for a surface temperature Ts of 85 K. We find that this probability scales approximately with Ei cos θi, rather than Ei or the so-called "normal energy'' Ei cos2 θi. Trapping probabilities approach unity for low energies, falling to 0.5 and 0.05 for Ei cos θi ∼30 and 100 meV, respectively. The time-of-flight distributions of scattered Ar are clearly bimodal in many cases, having both direct–inelastic and trapping–desorption components. The latter component has been characterized over a wide range of conditions to provide information on the desorption of Ar from this surface. We find that desorbing species emerge with a near-cosine angular distribution for Ts (approximately-equal-to)85 K. However, these distributions become increasingly noncosine as Ts is raised, becoming substantially broader than cosine. In addition, at the lowest temperature employed (∼85 K), the velocity distributions of the desorbing atoms are well described by Maxwell–Boltzmann distributions characteristic of the surface temperature. At higher temperatures, these distributions are still approximately Boltzmann, but the characteristic temperature falls below Ts. The "lag'' between this effective temperature and Ts increases with Ts and is most pronounced for atoms desorbing at angles close to the normal. We show that the desorption results are very close to the predictions of a model in which angular and velocity distributions for desorption are synthesized by applying detailed balance arguments to the trapping data. Similarly the trapping results are close to trapping curves extracted from the desorption data.

Notes: The orientational order parameters 〈P2〉 and 〈P4〉, of cadmium stearate Langmuir–Blodgett multilayers have been calculated from steady state fluorescence anisotropy experiments. It has been shown that it is valid to model the polarization components using the assumptions that the fluorescent probes are axially symmetric, the film is azimuthally symmetric within the plane, and that the rotational motion is slow enough to be neglected. Although the data do not preclude a dependence of anisotropy on thickness, within the sample-to-sample variations, there is no significant effect of thickness on orientational order. The order parameters for newly deposited films are 〈P2〉=0.33 and 〈P4〉=0.02. The orientational order decreases in the first 600 h after deposition, and then remains stable for at least several months.

Notes: This paper forms the third in a series on the optical properties of a system consisting of a uniform, radially uniaxial coating of molecules on a small isotropic sphere. In this paper, the induction of Raman activity in a coated metal sphere by the fields generated by the coupled surface phonon modes of the coating is considered. Thus, the surface phonon modes induce surface plasmons in the metal that result in its polarizability being modulated. The phenomenon, which requires dipole active rather than Raman active molecular modes to be operative, we have termed surface phonon induced Raman scattering (SPIRS). Scattering intensities predicted via SPIRS and surface enhanced Raman scattering (SERS) have been compared for CO on a spherical silver particle. That for SPIRS is found to be of the order of or greater than that for SERS, depending on exactly how the surface region of the metal sphere is modeled. Some predictions based on SPIRS are reminiscent of data previously assigned to SERS but not explained by current models.

Notes: Picosecond time-resolved measurements of vibrational energy relaxation from ν=1 to ν=0 for C–H stretching modes of the terminal methyl group in 9 Cd stearate Langmuir–Blodgett monolayer on an evaporated silver film were made. The experiments used infrared–visible sum spectroscopy to dynamically probe vibrational level populations. To the authors' knowledge, this is the first direct measurement of vibrational energy relaxation for molecular adsorbates at a bulk metal surface. Multicomponent decay processes with lifetimes of 3 ps to 〉1 ns indicate complex intramolecular vibrational energy transfer processes in these ordered monolayer films, which may be different than for similar molecules in liquids.

Notes: We point out a relationship between stretched-exponential relaxation, φ(t)=φ0 exp[−(t/τ)β], and the Vogel–Fulcher temperature dependence, τ=τ0 exp[A/(T−T0)], seen in many glasses near the glass transition. Recent experiments have shown that, in some glasses, the exponent β decreases as the temperature is lowered towards T0. Under rather nonrestrictive assumptions, this temperature dependence in φ(t) leads directly to Vogel–Fulcher behavior.

Notes: The fluorescence of Eu3+ doped lanthanum–calcium–zirconium–silicon borate ceramic was studied at 77 and 300 K using laser excited site selective spectroscopy. The fluorescence spectrum excited at 514.532 nm reveals the presence of three distinct sites for Eu3+. The sites were assigned to Eu3+ substituting for lanthanum in a ninefold coordination site in LaBO3 and Eu3+ substituting for Ca2+ in six- and sevenfold coordination sites in Ca2B2O5. To assign the spectra to the definite sites a phenomenological crystal field analysis was conducted, using C2v symmetry. The crystal field for Eu3+ in Ca2B2O5 was found to be dominated by electrostatic effects. The large 7F1 level splitting and the value of the B20 parameter support the concept of strong ionic bonding between europium and oxygen in Ca2B2O5.

Notes: This paper presents the results of the time-resolved study of intramolecular vibrational relaxation in the molecule fluorene. The results represent the first extensive study of a molecule using the technique which we have developed known as the time-resolved fluorescence depletion technique. Fluorescence depletion decays and dispersed fluorescence spectra of 19 vibronic features of fluorene are presented. The decays show a progression of dynamic behavior including stationary behavior at low densities of states, quantum beating at intermediate densities, and fast decay of the initially prepared state at high state densities. The data allow us to assign IVR lifetimes to several vibronic levels of fluorene from 27 ps at 1425 cm−1 of excess vibrational energy to ≤10 ps at ∼2000 cm−1. The degree of spectral congestion in the associated dispersed fluorescence spectra is shown to be related to the dynamic behavior of the vibronic features. In addition, the effects of intermolecular rotational coherences on the time-resolved fluorescence depletion decays of fluorene are explored. With these results we are able to confirm the assignments of the excited state rotational constants which we have made using rotational band contour simulations.

Notes: We report the characterization of the first excited singlet states of CdNe, CdAr, and CdKr, which correlate with Cd(5s5p 1P1) and the ground-state rare-gas atoms. The van der Waals molecules were created in a free jet supersonic expansion and studied by laser-induced fluorescence, dispersed fluorescence, laser pump/probe action spectra, and spectral simulations. The C 1Π1 states are found to be more strongly bound than their triplet counterparts: 116Cd20Ne (De=89 cm−1, ωe=23.36 cm−1, ωexe=1.80 cm−1, re =3.61±0.05 A(ring)); 116Cd40Ar (De=544 cm−1, ωe=47.97 cm−1, ωexe=1.11 cm−1, re=3.28±0.05 A(ring)); 114Cd84Kr (De=1036 cm−1, ωe=56.72 cm−1, ωexe=0.81 cm−1, Δre(C 1Π–X 1Σ+) =1.16 A(ring)). This is attributed to spatial differences between the atomic p orbital of the singlet vs the triplet excited state of the Cd atom. The D 1Σ0+ states of CdAr and CdKr were found to be repulsive for Franck–Condon accessible internuclear distances. No production of Cd(5s5p 2PJ) states from predissociation of any C 1Π1 molecular state was observed.

Notes: The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 A(ring) and Ne2 center of mass to Cl2 center of mass distance of 3.12 A(ring). This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ'=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive "half-collisions'' which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.

Notes: We describe a one-laser optical–optical double resonance (OODR) scheme whereby sub-Doppler Zeeman splittings of individual rovibronic transitions are presented in a uniquely simple and diagnostically powerful format. Each rovibronic transition yields at most two Zeeman resonance features, each associated with the approximately M-independent (J,M+1)–(J,M − 1) Zeeman interval in either the upper or the lower state. Two laser beams, ωl and an acousto-optically shifted sideband at ωl + ωA0 , copropagate through the sample. A sideband-OODR-Zeeman (SOODRZ) resonance results when a (J,M+1)–(J,M − 1) interval is Zeeman tuned through resonance with ωA0 . The SOODRZ effect, detected against a dark background in a magnetic rotation spectroscopy configuration, is illustrated for several lines of the NiH B 2 Δ5/2–X 2 Δ5/2 (1,0) band.

Notes: Xenon enriched with nonmagnetic isotopes, in which the content of magnetic isotopes is less than 0.4%, has been used as the low temperature matrix for the observation of electronic spin resonance (ESR) spectra of SiH3 radicals and H atoms. The SiH3 radicals were generated by the reaction of SiH4 with H atoms produced by the photolysis of HI. Well resolved proton hyperfine structures, which were smeared out into a broad singlet spectrum when natural xenon was used as the matrix, were observed. The ESR parameters were determined by numerical deconvolution and compared with those obtained for the radicals trapped in other matrices. Simple doublet spectrum of H atoms without superhyperfine structures was also observed after the photolysis of HI at 4.2 K.

Notes: Mass-resolved, resonant multiphoton ionization (MPI) spectroscopy has been used to identify and characterize transient species produced in a pulsed, supersonic glow discharge source. Vibrationally hot (up to v‘=9), but rotationally cold nitric oxide is characterized by (1+1) MPI via the A 2Σ+ state and (2+1) MPI through the C 2Πr state. Nine A←X and six C←X hot bands are observed; only four of these have been previously characterized. Accidentally overlapping C and A state hot bands can be separately studied by using different order MPI schemes. Implications for several previous studies are discussed. Additionally, the 3P0,2 metastable rare gas atoms are readily formed and detected by MPI as are metal atoms sputtered from the electrodes.

Notes: It is now well established that the unsubstituted linear polyene 1,3,5,7-octatetraene efficiently undergoes cis–trans photoisomerization even when substituted in an n-alkane matrix cooled to liquid-helium temperatures. The fact that this photochemical reaction takes place under these conditions opens the possibility of using the techniques of photochemical hole burning to uncover details of the microscopic mechanism of this isomerization. In this paper we report the demonstration of photochemical hole burning for all-trans-octatetraene in n-hexane, show that the hole width for the zero-phonon component of the 0–0 band in the limit of zero temperature and zero hole depth is within experimental error equal to the value predicted from the measured fluorescence lifetime, analyze the dependence of hole width on temperature, and show that the relative quantum yield for hole burning increases by approximately a factor of 35 as vibrational energy in the excited state is increased beyond a threshold of approximately 950 cm−1. This threshold agrees well with the previously determined barrier to trans, cis isomerization.

Notes: A simple model for the coupling of two chemical oscillators is proposed. The model consists of twotwo-variable subsystems, each of which gives rise to a "cross-shaped phase diagram'' containing two different steady states plus regions of bistability and of oscillation. The subsystems are coupled by diffusion of both species. Extensive numerical simulations reveal that when one subsystem is in the bistable and the other in the oscillatory region of the parameter space, coupling can result in birhythmicity, period doubling, and chaos. When both subsystems are in the oscillatory region, the coupled behavior is even richer, including quasiperiodicity, entrainment, chaos, and phase death (cessation of oscillations), with various hysteresis phenomena between these modes of behavior. The relation of these observations to behavior found experimentally in coupled chemical oscillators is discussed.

Notes: High resolution fast beam photofragment spectroscopy on the a 3Σ+u has been used to measure level positions and widths of the c 3Σ+g in the positive energy region near the top of its barrier. Tunneling through the barrier produces energetic neutral fragments that diverge from the beam and are detected. The information is used to determine the shape and height of the barrier to spectroscopic accuracy.

Notes: We present two new analytic potential-energy surfaces suitable for studying the competition between the abstraction reaction H+DCl→HD+Cl and the exchange reaction H+DCl→HCl+D. In the abstraction channel the surfaces are only slightly different from the Stern–Persky–Klein GSW surface, but the exchange barrier on both surfaces is raised by inclusion of a three-center term fitted to ab initio extended-basis-set multireference configuration interaction calculations with scaled external correlation. The two surfaces differ significantly only for the steepness of H–Cl–H bend potential. The exchange and abstraction saddle points are characterized by harmonic analysis for H2Cl, HDCl, and D2Cl, and we also compute vibrationally adiabatic barrier heights including anharmonicity. We also report thermal rate constants and activation energies for both reactions mentioned above.

Notes: We report infrared cross sections for the photodetachment of the molecular anion NO− in the energy range of 1100–1500 cm−1. The measurements are made with a coaxial ion beam–laser beam aparatus. We observe a vibrational autodetachment resonance centered at 1284±10 cm−1. The resonance has a full width at half-maximum of 170 cm−1 corresponding to a lifetime of 0.35×10−12 s for NO−(v=1). The present value for the vibrational frequency of NO− is significantly lower than previous estimates obtained from photoelectron spectroscopy of NO− and from electron scattering resonances in NO. We discuss the implications of our results with regard to previous measurements of infrared photodetachment cross sections in the region 3000–4100 cm−1.

Notes: Positron annihilation lifetime and Doppler-broadening measurements were performed on n-dotriacontane (n-C32H66) and n-tritriacontane (n-C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid–solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values determined by other techniques. In the case of n-C32H66 additional phase transitions not reported previously in the literature were observed. The lifetime vs temperature curves could be reasonably well interpreted within the framework of the free volume model taking into account the effect of special types of molecular motions and structural defects. On the basis of our measurements, linear dependence of ortho-positronium (o-Ps) lifetime on the size of the cavity around o-Ps could be expected. The proposed empirical relationship proved to fit well to lifetime data for molecular crystals as a function of their molecular volume. The o-Ps yield showed an abrupt decrease with increasing temperature at 190 and 165 K in the case of n-C32H66 and n-C33H68, respectively. This effect was attributed to the onset of rotation of the methyl end groups of the paraffin chains.

Notes: We have studied the phase separation behavior of polymer solutions in two-dimensional elongational flows that are sufficiently weak to avoid the coil–stretch transition. On the basis of a simple model that neglects hydrodynamic interactions, we find no shifts in the infinite molecular weight θ point, or coexistence curve. For finite chain lengths, there are flow-induced shifts of both quantities. These shifts are calculated to leading order in a loop expansion and are found to be order (Sζ)2N7/2, where S is the extension rate, ζ is the monomer friction coefficient, and N is the degree of polymerization. Our results are in qualitative agreement with experimental observations that find demixing promoted by flow.

Notes: We report exact results for two and three dimensional directed models of polymer chain adsorption–desorption transition in the presence of attracting substrate. A number of properties of this transition are evaluated explicitly. These include the phase diagram, the adsorption fraction and the monomer-density profile. Our results are obtained by a novel application of the transfer matrix method particularly suitable for studies of polymer chain conformations.

Notes: Electron slowing-down processes in molecular oxygen gas in the subexcitation domain (below the ionization threshold) are studied by using the Spencer–Fano (SF) equation and its simplification, the continuous-slowing-down approximation (CSDA), both in time-dependent and time-independent representations. Compared to the previously studied cases of N2 and CO2, O2 has the special features in its inelastic cross sections of (i) strong delta-function-like peaks in the vibrational excitation cross section below 1.3 eV and (ii) very low energy thresholds of electronic excitation channels. These features provide a stringent test for the CSDA. Indeed, our results clearly show for the first time that the CSDA fails even qualitatively to reproduce the electron degradation spectrum given by the exact SF method over the whole energy regime studied.

Notes: The least-action semiclassical algorithm for multidimensional tunneling probabilities [B. C. Garrett and D. G. Truhlar, J. Chem. Phys. 79, 4931 (1983)] has usually been employed by interpolating tunneling paths between two limits, a least-motion limit appropriate for large reaction-path curvature and a minimum-energy limit appropriate for small reaction-path curvature. In the present study we test whether, when the reaction-path curvature is small, more accurate results might be obtained by using a general small-curvature reference path. Least-action algorithms with both types of reference paths are compared to each other, to five other semiclassical approximations, and to accurate quantal dynamical rate constants for one three-dimensional and two collinear reactions with the mass combination L+H L'→L H+L' where L and L' denote light atoms (H or D) and H denotes a heavy atom (Br). We find, perhaps surprisingly, that the usual least-action method works best. This is encouraging because the minimum-energy reference is easier than the small-curvature reference to extend to polyatomic reactions.

Notes: Bare silicon cluster ions are observed to undergo exothermic sequential clustering reactions with SiD4 at room temperature. Si+1–7 and Si−1–7 are created by laser evaporation and trapped in the ion cell of a Fourier transform mass spectrometer in the presence of SiD4. Clustering reactions are observed only for Si+1–3 and Si+5. Si+4,6,7 and the negatively charged silicon clusters do not react exothermically with SiD4. All of the reactive silicon clusters encounter chemical constraints to rapid growth of increasingly larger SixD+y species. Ab initio electronic structure calculations are used in concert with phase space theory calculations to deduce the structures of the products of the clustering reactions. These structures are found to be closely related to the lowest energy structures of the bare clusters if the degree of deuterium saturation is low. The inertness of unreactive clusters with 2–5 silicon atoms is correlated to unusually stable structures. Larger unreactive clusters with six or more silicon atoms appear to lack the divalent silicon center required to activate the Si–D bonds of SiD4. These findings are related to the phenomenon of hydrogenated silicon particle formation in silane plasmas.

Notes: Fourier transform–electron spin resonance (FT–ESR) was utilized to measure the photoinduced charge transfer in the system Zn tetraphenylporphyrin/duroquinone with a time resolution of 10 ns. The separate observation of the intensity/time profile of different hyperfine lines in the spectrum, which was made possible by application of FT–ESR, enabled the independent determination of transient acceptor radical polarization originating from the triplet- and radical-pair mechanism. The observed temperature dependence and the absolute value of the electron transfer rate lead to the conclusion that the process is diffusion limited with no indication of an activation barrier.

Notes: We present in this paper an application of the Lanczos algorithm to the fitting of a potential energy surface from the experimental spectrum. This method presents two definite improvements on the conventional approach: (i) the Lanczos algorithm allows to treat ultra-large basis sets (120 000 states in this calculation) and (ii) it is possible to tune selectively the calculation to specific components of the spectrum (e.g., a given combination band nνi+mνj) thus reducing considerably the complexity of the fit. This method has been applied to the CD3H molecule, considering all the vibrational degrees of freedom. Converged line positions have been obtained for high overtones of the C–H stretching mode (up to 16 000 cm−1). The accuracy of the fitted surface is demonstrated by direct comparison of experimental and calculated spectroscopical parameters.

Notes: A perturbation–trajectory method for determining the dynamics of gas–surface collision processes is described. The method is based upon the assumption that the motions of Q-zone atoms are unaffected by the collision process at the lattice surface. This assumption leads to a P-zone Hamiltonian that incorporates the effects of Q-zone motion in terms of time-varying P-zone–Q-zone interactions. The collision dynamics of the P zone are determined from an ensemble of stoichastic trajectories using this coupled Hamiltonian. The method is applied to three systems: (1) collinear inelastic atomic collisions with a ten-atom chain, (2) the inelastic scattering and absorption of NO on a Ag(111) surface, and (3) the collision and subsequent surface reactions of SiH2 on a Si(111) surface. Comparison of the perturbation results with those obtained using the full system Hamiltonian shows that under certain conditions the perturbation procedure yields very accurate results with a significant reduction in computational requirements. In general, the accuracy of the perturbation calculations increases as the incident-to-lattice-atom mass ratio decreases. A decrease in the strength of the interaction between the incident molecule and the Q zone, the incident translational energy, or the lattice temperature also improves the accuracy of the perturbation treatment. The method is therefore best suited to the study of inelastic, light-molecule collisions with heavy-atom surfaces at low temperature. Comparisons with previously reported gas–surface studies that employ a Langevin approximation are also given.

Notes: A nuclear magnetic resonance (NMR) 1D flow measurement technique is described and demonstrated experimentally. The technique uses a "meander coil'' in the NMR probe for the radio frequency (rf) excitation and detection. The meander coil gives rise to highly inhomogeneous B1 field, whose direction varies linearly as a function of spatial coordinates. The detected NMR signal acquires a frequency shift which is proportional to the speed of the nuclear spin. In a sense, this frequency shift is similar to the Doppler shift. The flow speed can be measured by one free induction decay (FID) without applying B0 field gradient.

Notes: We report the formation of the anion–atom complexes ArO− and ArCl− by electron stimulated desorption from multilayer argon films containing O2, N2O, and Cl2 molecules.

Notes: The total electron attachment rate constant ka(〈ε〉,T) for SO2F2 has been measured, in a buffer gas of N2, as a function of the mean electron energy 〈ε〉 (0.046–0.911 eV) and temperature T (300–700 K) using an electron swarm technique. From the measured ka(〈ε〉,T), the total electron attachment cross sections σa(ε,T) were determined. At 300 K the σa(ε,T) exhibits a maximum at ∼0.22 eV which is due to dissociative electron attachment and an increase below ∼0.1 eV which is due to the formation of parent negative ions SO2F−2 at near zero energy. At T=400 K, σa(ε,T) has only one main peak at ∼0.13 eV which is due only to dissociative electron attachment reflecting the depletion of the parent anions and the prevalence of the fragment negative ions as T increases. The main peak of σa(ε,T) shifts to lower electron energies with increasing T so that at 700 K the peak is located at ∼0.03 eV. The value, σda(εmax), of the total electron attachment cross section at the peak energy εmax increases by a factor of ∼32 as T increases from 300 to 700 K. The analysis of these results—and similar earlier work—leads to the conclusion that the increase in σda(ε,T) for the dissociative electron attachment processes in molecules, with increasing T results mainly from an increase with T of the internal energy (principally vibrational) of the molecule.

Notes: A theory is developed of the dependence of the steady-state electron–ion pair recombination rate constant on the electron mean free path. The problem, in classical mechanics, is reduced to the stochastic dynamics of the electron in the one-dimensional effective potential V˜(R)=−kTRc/R −2kT ln R. Rc is the Onsager length Ze2/4πεkT. For a large mean free path λ, the recombination rate is determined by energy relaxation of electrons which cross the transition state of V˜(R) at RT=Rc/2, whereas for small λ the Debye result for spatial diffusion-controlled recombination is obtained. The theory gives the dependence of the rate in the crossover regime where λ is comparable to Rc. The results are in good agreement with experiment and Monte Carlo simulations.

Notes: Almost-degenerate perturbation theory is used to derive an effective Hamiltonian describing the vibrational states of H2CO. Eigenvalues have been determined for energies up to 8600 cm−1 above the zero-point energy. Both curvilinear and rectilinear representations of the vibrational dynamics are presented and explored. Although differences are observed between the two effective Hamiltonian matrix elements, their eigenvalues generally agree to better than a wave number for the energies studied. Using the Watson Hamiltonian, the mechanism of rotationally induced vibrationally mixing is investigated as a function of K, the projection of the total angular momentum onto the body-fixed a axis. The combination of a-axis Coriolis coupling and Fermi couplings leads to extensive vibrational mixing between the rotational–vibrational states in this energy regime.

Notes: In this paper we consider the effects of two-electron, one-center interactions when added to the one-electron, two-center molecular orbital model. There are, therefore, two parameters considered: The standard Hückel β (two-center, one-electron) term and the Hubbard one-center, two-electron term U. It is shown how the change in the ground state as one changes the U/β ratio is highly dependent on the presence or absence of odd member rings.

Notes: The structure of hard sphere fluid confined to cylindrical capillaries in equilibrium with the bulk fluid phase is considered. Simple analytical approaches are examined and compared with the results of simultaneous grand canonical ensemble Monte Carlo simulations. The validity of the limiting law according to which the fluid density in the capillary approaches its fugacity in vanishingly narrow pores is confirmed. The truncated graphical expansion around the exact narrow pore limit is found to represent a useful approximation for very narrow capillaries. In systems with the pore diameter exceeding a few diameters of the spheres and at the reduced bulk densities of the fluid ρbσ3≤0.6, the Percus–Yevick approximation provides a satisfactory description of the fluid distribution.

Notes: Excited He* (2 3S) atoms in normal liquid 4He(1 1S) environment exist inside "bubbles'' that have sizes and shapes characteristic of the electronic state of He* as well as the thermodynamic state of the liquid. The bubbles are stabilized by the repulsive interaction of the Rydberg-like excited electron with bath He atoms. We employ classical computer simulation methods to characterize these "bubble states'' in a high pressure (gigapascals) regime. We analyze for the presence of clusters (He@B|n) within the bubbles, and find the results sensitive to the electronic state involved as well as the pressure. The He*n along with the He atoms on the bubble's inner surface behave like a single-shell solventberg at lower pressures, whereas a two-shell structure emerges in the high pressure regime. The simulated bubble radius varies between 6–8 a0 in the pressure range 14–0.5 GPa.

Notes: A Gibbsian thermodynamic description of a simple coherent interface in a nonhydrostatically stressed, N component, crystalline solid is developed. Interfacial excess properties are defined in a manner consistent with the thermodynamics of nonhydrostatically stressed crystalline solids. This formalism is then used to describe the relationship between the independent thermodynamic variables and the interfacial energy. A result of the crystalline nature of the solid is that the interfacial energy and the interfacial stress are distinct quantities. This difference, along with the presence of a nonhydrostatic stress, results in fundamentally different forms for the standard interfacial thermodynamic relationships. In particular, we show that the Gibbs adsorption isotherm for a coherent spherical misfitting particle in a matrix contains terms which are related to both interfacial and bulk stresses and that these terms can be dominant in certain cases.

Notes: Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (〈first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.

Notes: All 13 elastic constants of a vapor grown, uncut, anthracene single crystal were determined from acoustic phonon velocities obtained by the Brillouin scattering method. The phonon velocities are plotted for three crystallographic planes containing the crystal axis. The relationships between phonon velocities and lattice dynamics are discussed. A minimizing procedure is introduced for converting phonon velocities of low symmetry systems into elasticity coefficients. This is shown to have several advantages over previous methods used. The results are compared with those obtained by previous studies of anthracene elastic constants.

Notes: The interacting ν1 and ν3 bands of SnH4 have been studied by infrared absorption and infrared-microwave double resonance spectroscopy. The infrared spectrum has been taken with Doppler-limited resolution using a tunable diode laser. Pure rotational transitions within the ν3 state and rovibrational transitions between the ν1 and ν3 state have been observed by infrared-microwave double resonance. The infrared and microwave data have been fitted simultaneously to the ν1 and ν3 Hamiltonian coupled by one vibration–rotation interaction term. Seventeen spectroscopic constants have been determined for each of the five most abundant isotopic species.

Notes: Single crystals of [(Cp*Ru)2(η6,η6-[22](1,4)cyclophane)2+][(TCNQ)2−4] (Cp*=η5-C5Me5, TCNQ=tetracyanoquinodimethane) have been examined by EPR spectroscopy at temperatures down to 4 K. The anisotropic spectrum measured in the 77–150 K range is characteristic of an excitonic triplet species (I). This behavior is attributed to interactions between electrons located in one of the TCNQ acceptor stacks present in the title compound. The direction of maximum electronic interaction coincides with the stacking axis of the crystals (the crystallographic a axis). Intensity measurements establish that I is a thermally excited triplet with a J=560±30 cm−1, exhibiting zero field splittings of ||D||=187 MHz (66G) and ||E||=32 MHz (10 G). A second although weaker feature in the spectrum is tentatively assigned to a second triplet (species II) having very much smaller values of ||D|| and J than those of I. Species II is attributed to an impurity present in very low concentrations.

Notes: We have determined quenching rate coefficients for N2(a 1Πg, v'=0) by N2, O2, H2, CO2, Ar, CH4, and CO using two-photon laser excitation. Quenching by N2 appears to proceed via collisional coupling to N2(a' 1Σ−u, v=0) with a rate coefficient of 2.2±0.2×10−11 cm3 molecule−1 s−1. Quenching by Ar is nearly as efficient. Gas-kinetic rate coefficients are obtained for quenching by CO, O2, H2, CO2, and CH4. Collisional energy transfer from N2(a,0) to CO(A) is observed in these experiments. However, quenching by O2, H2, CO2, and CH4 is thought to be reactive.

Notes: Vibrational relaxation rates for the v=2 level of the X 2Πi state of the OH radical have been measured in a low pressure flow system, using a novel two-laser pump-and-probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser-induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm3 s−1 units) for NH3: (1.20±0.15)×10−10; CH4: (2.3±0.2)×10−12; CO2: (6.7±1.1)×10−13; N2O: (4.6±0.6)×10−13; O2: (2.6±0.54)×10−13; N2 and H2: ≤10−14. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A 2Σ+ excited state of OH, where attractive forces appear to play a role.

Notes: Optical–optical double resonance spectroscopy is used to probe Rydberg series converging to the first ten rotational levels of NO+ X 1Σ+, v+=0. Above the lowest ionization threshold, rotational autoionization of Rydberg series converging to higher thresholds is observed. Predissociation of these Rydberg states is found to compete with rotational autoionization in much the same manner as predissociation competes with vibrational autoionization in the region of the first few vibrational limits of NO+. The presence of this competing decay process, which has a decay rate similar to that of rotational autoionization, permits the comparison of rotational autoionization rates for different changes in rotational quantum number (ΔN+). Rotational autoionization by ΔN+=2 is found to be faster than by ΔN+=1 or 3. This results from the requirement that ΔN+=even processes require interactions between levels that both have even or both have odd values of orbital angular momentum l, while ΔN+=odd processes require interactions between levels of which one has even l and the other has odd l. In NO, the latter interactions are known to be quite weak. The electric field dependence and pressure dependence of the ionization threshold are also discussed.

Notes: We report a study of the classical scattering of a point particle from three hard circular discs in a plane, which we propose as a model of an idealized unimolecular fragmentation. The system possesses a fractal and chaotic metastable classical state. On the basis of a coding of the system dynamics, we develop a method to construct the invariant probability measure and to calculate the particle escape rate, the Hausdorff dimension, the Kolmogorov–Sinai entropy per unit time and the mean largest Lyapunov exponent of the repellor. The relations between these characteristics of the system dynamics are discussed. In particular, we show that, in general, chaos inhibits escaping from the metastable state. The theory is compared with numerical simulations. We also introduce the classical tools necessary for the semiclassical quantization of the dynamics; the latter is discussed in the following paper.

Notes: We compare the sensitivities to initial conditions for both direct (regular) and long-lived (chaotic) trajectories in classical scattering calculations with the corresponding properties of trajectories of position and momentum expectation values for quantum wave packets. The collinear H+H2 reaction is used as an example. The results show that the high sensitivity seen in chaotic trajectories is not reflected in the quantum dynamics. We conclude that it is possible for a classical ensemble consisting of only regular trajectories to respond trajectory by trajectory to perturbations in much the same way as a quantum wave packet. (There will of course be cases that are exceptions to this rule.) The response of an ensemble consisting of chaotic trajectories may on the average be similar to that of a wave packet, but not at the level of individual trajectories. In addition, the sensitivities of these trajectories to variations in the potential are analyzed. We conclude that the large contributions to the sensitivities from particular long-lived trajectories must approximately cancel when an exact ensemble average is taken. An algorithm is presented to smoothly account for the contributions to the sensitivities from these trajectories.

Notes: Emission spectra of SiF3 radical were observed from photodissociative excitation of SiF4 at 99.1, 95.5, and 92.3 nm. The spectra show a broad visible band in the 350–800 nm region, a UV band in 290–340 nm, and a weak band in 240–280 nm. The visible band resembles the chemiluminescence spectra observed from etching of silicon by F or XeF2.

Notes: Model calculations are presented to interpret the large H–F and C–H stretching vibrational dependencies of the interconversion tunneling splittings and the corresponding infrared vibrational-tunneling state selection rules in (HF)2 and (HCCH)2. The model consists of two potential curves in the tunneling coordinate, coupled by an interaction term that allows the vibrational excitation to be exchanged between the two monomer units, permitting tunneling to occur. The interaction term is approximated by resonant infrared transition–dipole coupling. The magnitudes of the calculated vibrational dependencies, their isotopic shifts, and the predicted selection rules are in agreement with previous experimental observations.

Notes: Observation of charge transfer states in phenothiazine crystal which have lower energies than Frenkel excitons is reported. Two weak absorption peaks are observed at 15 K around 420 nm (2.9 eV) in the absorption edge, the absorption coefficient of which can be modulated up to a few percent by an external electric field. The modulation is so large, because the transitions are not overlapped with strong exciton absorption bands and susceptible to direct observation. The final states of the transitions have large dipole moments (∼50 D). The conductivity band gap is estimated to be 3.35 eV.

Notes: Measurements in the ν2 (umbrella) vibrational spectrum of CD3 are reported. The data include transitions up to and including the v2=4–3 bands and cover a wider range of rotational excitation than has been previously reported [Frye, Sears, and Leitner, J. Chem. Phys. 88, 5300 (1988)]. The spectroscopic data have been combined with all previously published data pertaining to the methyl radical and an estimate of the vibrational potential function for the species has been made. From this, we predict the positions of so far unobserved vibrational bands for all the various symmetric isotopic modifications and also extract an equilibrium structure. Preliminary results on the population distribution of v2 levels produced on the photofragmentation of acetone-d6 at 193 nm are reported and it is suggested that the methyl v3=1 and v3=0 levels may be produced in this reaction with a population inversion.