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Volume 104, Issue 1, 01 January 1996

Low‐frequency Raman scattering of aqueous solutions of L‐xyloascorbic acid and D‐araboascorbic acid
View Description Hide DescriptionDepolarized low‐frequency Raman spectra of aqueous solutions of L‐xyloascorbic acid and its epimer D‐araboascorbic acid have been investigated as a function of concentration at 30 °C. The influence of fluorescence in the low‐frequency Raman spectral intensity of D‐araboascorbic acid aqueous solution was removed by the background correction. The reduced χ″(ν̄) spectrum, which corresponds to the imaginary part of the dynamical susceptibility due to the dynamical structure of water in aqueous solutions, was analyzed with the superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The effect of L‐xyloascorbic acid on the dynamical structure of water is less than that of D‐araboascorbic acid.

Lattice dynamics and hyperfine interactions of C_{60}Fe(CO)_{4}
View Description Hide DescriptionDetailed ^{57}Fe Mossbauer experiments over the range 85<T<210 K have been carried out on the tetra‐carbonyl iron derivative of C_{60}(I) as well as on several related organometallic solids. The observed isomer shift and quadrupole splitting parameters are consistent with a bonding interaction between the metal center and the ligand which involves a single carbon–carbon double bond of the buckminsterfullerene. The temperature dependence of the recoil‐free fraction, compared to analogous data for maleic anhydride Fe(CO)_{4}(II), suggests that the two previously observed low frequency lattice modes in C_{60} persist in I. Contrary to expectations, there is no significant temperature‐dependent vibrational anisotropy (Gol’danskii–Karyagin effect) evident in the above temperature range.

Pressure‐induced amorphization of covellite, CuS
View Description Hide DescriptionCuS, or covellite (hexagonal symmetry), was compressed in a diamond anvil cell at room temperature up to a pressure of 45 GPa, and studied using x rays from both a Mo K_{α} source and a synchrotron. The x‐ray diffraction spectrum of CuS disappears by about 18 GPa. The presence of Cufluorescence lines in all spectra and the reappearance of diffraction lines upon decompression confirm that CuS undergoes reversible pressure‐induced amorphization at this pressure. A third‐order Birch–Murnaghan equation of state fit to the diffraction data below 11 GPa yields a bulk modulus of 89±10 GPa with a pressure derivative of −2±2 for covellite. Further compression up to 45 GPa shows three to four diffraction lines of very low intensity, implying some high pressure ‘‘ordering’’ of the amorphous phase. The Raman spectra obtained indicate that the changes in structure are probably due to the twisting or the distortion of covalently bonded CuS_{4}–CuS_{4} units in different directions.

Infrared spectroscopy of small size‐selected water clusters
View Description Hide DescriptionInfrared molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small water clusters [(H_{2}O)_{ n }, n=2,3,4,5]. The spectral region between 3300 and 3800 cm^{−1} was covered with an injection‐seeded optical parametric oscillator. Size‐specific information has been obtained by dispersing the clusterbeam with a secondary helium beam and measuring the depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H_{2}O)_{2}, with the bonded OH stretch being localized at 3601 cm^{−1}. For each of the larger water clusters (n=3,4,5), which have cyclic structures, two absorption bands could be identified, one belonging to the free OH stretch and the other being due to the excitation of the OH ring vibration. The measurements on free water clusters were complemented by studies on small water complexes formed on large argon clusters. The positions of the absorption bands observed in these spectra are close to those found for (H_{2}O)_{ n } in argon matrices.

Periodic orbit analysis of molecular vibrational spectra: Spectral patterns and dynamical bifurcations in Fermi resonant systems
View Description Hide DescriptionSemiclassical periodic orbittheory is used to analyze the quantum density of states for three model molecular vibrational Hamiltonians describing stretch/bend modes with and without 2:1 (Fermi) resonant coupling. Periods of classical periodic orbits as a function of energy are extracted directly from the quantum spectrum using a Gaussian windowed (Gabor) Fourier transform. The quantum (E,τ) plots so obtained provide an informative representation of the level structure. Qualitative similarities and differences between spectra (i.e., resonant vs nonresonant) are immediately apparent; in this sense, the quantum (E,τ) plot is an efficient device for analysis of spectral patterns. At a more detailed level of analysis, we show that, for sufficiently small effective values of ℏ, the quantum (E,τ) plots reflect in full detail the intricate periodic orbitbifurcation structure for Fermi resonant Hamiltonians previously described by Li, Xiao, and Kellman [J. Chem. Phys. 92, 2251 (1990)].

Photoelectron spectroscopy of manganese–sulfur cluster anions
View Description Hide DescriptionManganese–sulfur cluster anions (Mn_{ n }S^{−} _{ m }, n=1–10, m=1–10) have been studied using a magnetic‐bottle type photoelectron spectroscopy (PES). The Mn_{ n }S^{−} _{ m } cluster anions were formed in a laser vaporization cluster source. The most stable cluster anions were found to have the compositions of n=m and n=m±1. The electron affinities of neutral manganese sulfide clusters were measured from the onsets of the PES spectra. A new electronically excited state at 0.75 eV above the ground state was found for MnS. From the size dependence of the PES spectra, it was found that Mn_{2}S^{−} _{2}, Mn_{3}S^{−} _{3}, and Mn_{4}S^{−} _{4} are structural frameworks in Mn_{ n }S^{−} _{2}, Mn_{ n }S^{−} _{3}, and Mn_{ n }S^{−} _{4} series, respectively. The electronic properties and geometricalstructures of the clusters are discussed.

Fermi resonance and mode specificity in the vibrational autoionization of NO_{2}
View Description Hide DescriptionIonization‐detected absorption spectra of autoionizing Rydberg series converging to the (100), (010), (02^{0}0), and (02^{2}0) vibrational states of NO^{+} _{2} have been recorded using three‐color triple resonant excitation. Resonances produced by relaxation of the core, excited in one quantum of the bending vibration, (010), are found to be much sharper than those associated with decay by relaxation in symmetric stretch, (100), consistent with earlier findings on mode specificity of vibrational autoionization in this system. In contrast, series excited in the symmetric overtone of the bend, (02^{0}0), are found to exhibit dynamics that resemble much more those of series built on one quantum of symmetric stretch, indicating that the bending specificity that characterizes (010) is lost in the overtone. This loss of mode specificity with increasing vibrational energy is ascribed to moderate (100)–(02^{0}0) Fermi resonance in the NO^{+} _{2} core, exemplifying how even small vibrational coupling can affect mode‐specific pathways in polyatomic molecules.

Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free‐jet expansion: Determination of the structure and electronic ground state of nickel dichloride
View Description Hide DescriptionBy use of a free‐jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl_{2} at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, ^{58}Ni^{35}Cl_{2}, ^{60}Ni^{35}Cl_{2}, and ^{58}Ni^{35}Cl^{37}Cl in natural abundance. Spin components with Ω values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ν_{1}, ν_{2}, and ν_{3} have been determined for nickel dichloride in the upper state and for the bending wave number ν_{2} in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is ^{3}Σ^{−} _{ g } in character. Evidence is presented from the nickelisotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically ^{3}Π_{ u } in character; it probably derives from an electronic ^{3}Δ_{ g } state. The zero‐point averaged bond lengths are determined for both states as r _{0} ^{′}=0.209 435(13) nm and r _{0} ^{″}=0.205 317(14) nm. The fine structure parameters for the X̃ ^{3}Σ^{−} _{ g } state are interpreted in terms of low lying ^{1}Σ^{+} _{ g } and ^{3}Π_{ g } states, which are shown to lie a few thousand reciprocal centimeters above the ground state.

Direct vibrational energy transfer in zeolites
View Description Hide DescriptionWith two‐color picosecond infrared laserspectroscopy the dynamics of O–H and O–D stretch vibrations in zeolites are investigated. Zeolites appear to be good model systems to study transfer of vibrational energy in a solid. For the O–D vibrations, transient spectral holes are burnt in the inhomogeneously broadened absorption bands by saturating the absorption with a strong pump pulse. From the spectral hole widths the homogeneous absorption linewidths are obtained. The excited population lifetimes are determined using a time‐resolved pump–probe technique, and in combination with the homogeneous linewidth the pure dephasing time is revealed as well. For high concentrations of O–H oscillators the vibrational stretch excitations are found to diffuse spectrally through the inhomogeneous absorption band. This spectral diffusion process is explained by direct site‐to‐site transfer of the excitations due to dipole–dipole coupling (Förster transfer). The dependences of the transient spectral signals on oscillator concentration and the results of one‐color polarization resolved experiments confirm this explanation. The spectral transients are satisfactorily described by simulations in which the site‐to‐site transfer by dipole–dipole coupling is taken into account.

Diffusion‐limited geminate recombination of O+O_{2} in solid xenon
View Description Hide DescriptionThe thermally activated recombination reaction O+O_{2}→O_{3} is observed in solid xenon matrices and in free‐standing crystals of xenon at 14–25 K on the time scale 10^{2}–10^{5} s. The reactants are prepared as spatially separated O...O_{2} pairs immobilized in solid Xe at 10 K by 266 nm photodissociation of O_{3} precursor molecules. The temperature dependence of the ozone recovery rate yields an activation energy for diffusion of O atoms in solid xenon of 2.0±0.5 kJ/mol. This value also represents an upper limit to the potential energy barrier to the O+O_{2}recombination reaction itself. In dilute samples (mole fraction of ozone less than 2×10^{−4}) more than 90% of the initial O_{3} is recovered during the warming cycle. Only a small fraction of O atoms (<20%) escape geminate recombination with the partner oxygen molecule within the pair and react with other O_{2} molecules or O atoms. The experimental results are interpreted within the framework of a continuous diffusion model in which the initial spatial distribution of reactants is nonrandom.

Semiclassical calculation of cumulative reaction probabilities
View Description Hide DescriptionIt is shown how the rigorous quantum mechanical expression for the cumulative reaction probability (CRP) obtained by Seideman and Miller [J. Chem. Phys. 96, 4412; 97, 2499 (1992)], N(E)=4 tr[ε̂_{ r }⋅Ĝ*(E)⋅ε̂_{ p }⋅Ĝ(E)], which has been the basis for quantum calculations of the CRP for simple chemical reactions, can also be utilized with a semiclassical approximation for the Green’s function,Ĝ(E)≡(E+iε̂−Ĥ)^{−1}=(iℏ)^{−1}∫^{∞} _{0} exp(iEt/ℏ)exp(−i(Ĥ−iε̂)t/ℏ). Specifically, a modified Filinov transformation of an initial value representation of the semiclassical propagator has been used to approximate the Green’s function. Numerical application of this trajectory‐based semiclassical approximation to a simple one‐dimensional (barrier transmission) test problem shows the approach to be an accurate description of the reaction probability, even some ways into the tunneling regime.

Simulation of chemical reaction initiation through high velocity collisions of NO clusters with a surface
View Description Hide DescriptionSome computational results have been obtained for a system of diatomic molecules clustered together and driven to impact on a surface at sufficient energy to induce an observable quantity of chemical reactions. The diatomic molecules were modeled to be energetically similar to nitric oxide, NO, which is a detonable material when in the condensed phase. The system was intended to simulate an experiment devised to examine the initiation phase of a detonation of liquid NO stimulated by impact with a high‐speed flyer plate. Classical trajectories were computed for six different cluster sizes, from 4 molecules to 50, and the clusters were directed into a wall at five different impact speeds ranging from 3.0 to 11.8 km s^{−1}. The interatomic forces used for the computations were based on a modification of an empirical potential suggested by Tersoff. The characteristics of the products (O_{2}, N_{2}, NO, and N and O atoms) are examined, as well as the dynamic features of the collisions of the clusters with the wall. The conditions of the cluster impacts produced atom densities that were nearly triple the initial density of the clusters. The reactions in the n=50 cluster are complete in less than 300 fs. These conditions are unusual for studies of chemical reactions so that the many‐body effects are expected to be important. They are conditions experienced in the initiation of explosive detonations.

Dissociation pathways and binding energies of (LiH)_{ n }Li^{+} and (LiH)_{ n }Li^{+} _{3} clusters
View Description Hide DescriptionThe metastable decomposition of hydrogenated lithium cluster ions (LiH)_{ n }Li^{+} _{ m }(m=0, 1 and 3; n≤15) is studied by using a reflectron mass spectrometer. These clusters are found to decompose by evaporation of a LiH or a Li_{2}H_{2} molecule. The binding energy of these clusters are determined, using a statistical model which has been adapted to mixed clusters. Comparison with other mixed clusters suggests that (LiH)_{ n }Li^{+} clusters form compact cubic structure similar to pieces of a crystal lattice. For (LiH)_{ n }Li^{+} _{3} clusters, the dissociation channels are more surprising, and the localization of the two excess electrons is discussed, as well as the possible existence of an energy barrier for the dissociation.

Effective potential methods in variational treatments of electron‐molecule collisions. I. Theoretical formulation
View Description Hide DescriptionWe investigate the use of current effective core potentials to simplify variational treatments of electron scattering by target molecules containing one or more heavy atoms. The nonlocal character of these potentials poses severe computational problems for general algebraic variational methods that do not rely on specific analytic schemes for computing matrix elements. We show that standard l‐dependent pseudopotentials can be represented in a way that facilitates the numerical evaluation of the required collision integrals.

Effective potential methods in variational treatments of electron‐molecule collisions. II. Application to HBr
View Description Hide DescriptionWe report the results of variational calculations on low energy e ^{−}+HBr collisions using the complex Kohn method. We compare the results of all‐electron numerical calculations with those in which effective core potentials are used. We present total, differential, and momentum transfer cross sections for electronically elasticscattering, as well as dissociative excitation cross sections for the low‐lying electronic states that dissociate to ground‐state neutral atoms. We find excellent agreement between the all‐electron and core‐potential results for all processes considered.

193 nm laser photofragmentation time‐of‐flight mass spectrometric study of HSCH_{2}CH_{2}SH
View Description Hide DescriptionThe kinetic energy release spectra for SH resulting from the 193 nm laser photofragmentation of HSCH_{2}CH_{2}SH have been measured. On the basis of the observed maximum kinetic energy for the formation of HS+CH_{2}CH_{2}SH, a value of 74±2 kcal/mol is derived for the bonddissociation energy of HS–CH_{2}CH_{2}SH at 0 K [D_{0}(HS–CH_{2}CH_{2}SH)]. Angular distributionmeasurements for SH yield an anisotropic parameter β=−0.4±0.1 for the HS+CH_{2}CH_{2}SH channel, indicating that the C–S bond fission is fast with respect to molecular rotation. The energetics for the formation of HS+CH_{2}CH_{2}SH from HSCH_{2}CH_{2}SH have been investigated using the Gaussian‐2 (G2) and G2(MP3) ab initio quantum chemical procedures. The G2/G2(MP3) calculations give a prediction of 72.5 kcal/mol for D _{0}(HS–CH_{2}CH_{2}SH), in excellent agreement with the experimental value. Ab initio first‐order configuration interaction calculations have also been made to examine the possible excited state of HSCH_{2}CH_{2}SH involved in the photodissociation process and to rationalize the observed angular distribution for the HS+CH_{2}CH_{2}SH channel.

Vibrational frequency shift of H_{2} in rare gas clusters and solutions: Comparison of semi‐classical theory and experiment
View Description Hide DescriptionA recently developed semi‐classical statistical mechanical formulation [de Souza et al., J. Chem. Phys. 99, 9954 (1993)] is combined with accurate H_{2}‐rare gas potentials [Le Roy and Hutson, J. Chem. Phys. 86, 837 (1987)] to predict H_{2} vibrational frequency shifts in rare gas clusters and low density solutions. The results are compared with available experimental measurements as well as with predictions derived assuming a Lennard‐Jones (LJ) atom–atom potential. The Le Roy–Hutson potential has a minimum cluster energy and maximum H_{2}bond softening in the linear atom–diatom geometry, in contrast to the T geometry predicted using the LJ potential. The Le Roy–Hutson potential also yields better agreement with experimental temperature and density dependent H_{2} frequency shifts. A classical approximation to the ground state frequency of H_{2}‐rare gas clusters is suggested which relates the probability density of the cluster configuration to the classical Boltzmann distribution at a temperature equal to the cluster zero point energy.

Investigation of the reliability of density functional methods: Reaction and activation energies for Si–Si bond cleavage and H_{2} elimination from silanes
View Description Hide DescriptionIn order to test the reliability of plane‐wave and Gaussian‐orbital based DFT methods for calculating reactionenergies and activation barriers, detailed calculations are performed for several reactions involving gas phase silanes and a simple model of H_{2}desorption from the Si(100)2×1 surface. This study is motivated in particular by apparent discrepancies between the results of cluster‐model and slab‐model calculations of the activation energy for H_{2}desorption from the Si(100)2×1 surface. The DFT results obtained with several different exchange‐correlation functionals are compared with the results of calculations with the generally reliable QCISD(T) method and, where possible, with experiment. It is found that the functionals usually employed in plane‐wave DFT calculations significantly underestimate the activation energies. The Becke3LYP functional, on the other hand, is found to give reaction and activation energies close to experiment and to those from QCISD(T) calculations.

A chemical potential equalization method for molecular simulations
View Description Hide DescriptionA formulation of the chemical potential (electronegativity) equalization principle is presented from the perspective of density‐functional theory. The resulting equations provide a linear‐response framework for describing the redistribution of electrons upon perturbation by an applied field. The method has two main advantages over existing electronegativity equalization and charge equilibration methods that allow extension to accurate molecular dynamics simulations. Firstly, the expansion of the energy is taken about the molecularground state instead of the neutral atom ground states; hence, in the absence of an external field, the molecular charge distribution can be represented by static point charges and dipoles obtained from fitting to high‐level ab initio calculations without modification. Secondly, in the presence of applied fields or interactions with other molecules, the density response can be modeled accurately using basis functions. Inclusion of basis functions with dipolar or higher order multipolar character allows molecules or chemical groups to have correct local anisotropicpolarizabilities. A modified semiempirical form of the hardness matrix has been introduced that can be evaluated efficiently using Gaussians, and requires only one parameter per basis function. Applications at two basis‐set levels demonstrate the method can accurately reproduce induced dipole moments and estimated chemical potentials obtained from density‐functional calculations for a variety of molecules. Inclusion of basis functions beyond the conventional spherical‐atom type is essential in some instances. The present formulation provides the foundation for a promising semi‐empirical model for polarization and charge transfer in molecular simulations.

Quantum control of multidimensional systems: Implementation within the time‐dependent Hartree approximation
View Description Hide DescriptionThe exact formulation of quantum control is now well known and sufficiently general to describe multidimensional quantum systems. The implementation of this formalism relies on the solution of the time‐dependent Schrödinger equation (TDSE) of the system under study, and thus far has been limited for computational reasons to simple quantum systems of very small dimensionality. To study quantum control in larger systems, such as polyatomic molecules and condensed phases, we explore an implementation of the control formalism in which the TDSE is solved approximately using the time‐dependent Hartree (TDH) approximation. We demonstrate formally that the TDH approximation greatly simplifies the implementation of control in the weak response regime for multidimensional systems. We also present numerical examples to show that the TDH approximation for the weak response case is sufficiently accurate to predict the laser fields that best drive a quantum system to a desired goal at a desired time, in systems containing more than one degree of freedom, by considering a two‐dimensional quantum system and comparing the optimal fields obtained by solving the TDSE exactly to those obtained using the TDH approximation.