Volume 105, Issue 2, 08 July 1996
Index of content:

Many‐body effects in weakly bound anion and neutral clusters: Zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of Ar_{ n }Br^{−} (n=2–9) and Ar_{ n }I^{−} (n=2–19)
View Description Hide DescriptionThe anion zero electron kinetic energy (ZEKE) spectra of the van der Waals clusters Ar_{2‐3}Br^{−} and Ar_{2‐3}I^{−} have been measured, and partially discriminated threshold photodetachment (PDTP) experiments have been performed on Ar_{4‐9}Br^{−} and Ar_{8‐19}I^{−}. The experiments yield size‐dependent adiabatic electron affinities (EAs) and electronic state splittings of the halogen atom in the neutral clusters formed by photodetachment. These results are compared with simulated annealing calculations using model potentials for the anion and neutral clusters, making use of the neutral and anion pair potentials determined from previous work on the diatomic rare gas–halide atom complexes [Y. Zhao, I. Yourshaw, G. Reiser, C. C. Arnold, and D. M. Neumark, J. Chem. Phys. 101, 6538 (1994)]. A simple first‐order degenerate perturbation theory model [W. G. Lawrence and V. A. Apkarian, J. Chem. Phys. 101, 1820 (1994)] of the neutral cluster potentials was found to agree well with the size‐dependent splitting of the halogen ^{2} P _{3/2} state observed in the ZEKE spectra. However, the binding energies calculated from the pair potentials alone were found to be inconsistent with the experimental electron affinities, and it was necessary to include various nonadditive terms in the simulated annealing calculations to obtain reasonable agreement with experiment. Many‐body induction in the anion clusters was found to be the dominant nonadditive effect. The exchange quadrupole effect—i.e., the interaction of the exchange induced electron charge distribution distortion among argon atoms with the halide charge—was also found to be important. This comparison between experiment and theory provides a sensitive probe of the importance of nonadditive effects in weakly bound clusters.

High‐pressure Brillouin scattering study of dense argon and nitrogen
View Description Hide DescriptionWe have measured the Brillouin shift as a function of hydrostaticpressure in dense argon and nitrogen at different constant temperatures. We show that for argon there is a linear relation between the index of refraction and the reduced density. A comparison of the sound velocities determined ultrasonically and from our Brillouin data shows a negative velocity dispersion. This is in agreement with theoretical predictions and measurements of the temperature dependence of the sound velocity along the vapor‐pressure equilibrium curve using Brillouin scattering. The fact that the negative velocity dispersion increase with pressure at constant temperature is smaller at lower temperatures is, however, puzzling and cannot be explained within the framework of the above‐mentioned theoretical model.

Proton tunneling assisted by the intermolecular vibration excitation. Temperature dependence of the proton spin‐lattice relaxation time in benzoic acid powder
View Description Hide DescriptionTemperature dependence of the proton spin‐lattice relaxation time (T _{1}) in powdered benzoic acid dimer and in its deuterated analog is calculated. The model assumes that two protons (deuterons) synchronously move in the double‐minimum potential of the dimer. The two‐dimensional potential energy surface was constructed previously, which adequately describes the static properties of the hydrogen‐bonded complex. The important characteristics of this potential are a very strong mode coupling and a very high proton potential barrier (≳25 kcal/mol), whereas the experimental activation energy for the proton transfer is known to be on the order of 1 kcal/mol only. This apparent discrepancy is removed by our suggestion that the proton transfer is driven by the transitions between OHO fragment vibrational levels under the action of random forces of the surrounding. The excitation of the low‐frequency intermolecular vibrations assists such transfer mechanism strongly. Using four fitting parameters to allow for the medium repolarization, the calculated T _{1} temperature dependence is found to be in good agreement with the experiments in the natural and deuterated benzoic acid dimer. The agreement is best at high temperature where the apparent activation energy for proton transfer was found to be 2.3 kcal/mol.

Spectroscopy of CrF: Rotational analysis of the B ^{6}Π–X ^{6}Σ^{+} band system in the 1.2 μm region
View Description Hide DescriptionThe B ^{6}Π–X ^{6}Σ^{+} band system of the CrF radical has been recorded in emission with FTS techniques in the region between 6965 and 9240 cm^{−1}, using a resolution of 0.025 cm^{−1}. A rotational analysis including the (0,0), (1,1), (1,0), (0,1) and (1,2) bands of this system has been carried out, and a set of molecular parameters has been derived for the B ^{6}Π state. The present analysis has also resulted in substantially better determined values for the λ and γ parameters of the X ^{6}Σ^{+}ground state. An improved set of molecular parameters has been derived for the previously analyzed, heavily perturbed A ^{6}Σ^{+} state. The present analysis gives strong support for the interpretation that the local perturbations in the v=0–3 levels of A ^{6}Σ^{+} are due to interactions with the vibrational levels v=3–6 of the B ^{6}Π state. Earlier suggestions involving a low‐lying ^{4}Π state as the dominating perturber of the A ^{6}Σ^{+} state must now be considered as erroneous. A previously suggested interpretation of the 7400 cm^{−1} band as being possibly due to a quartet transition has also been shown to be in error. The derived first order spin‐orbit parameter value of B ^{6}Π [A=47.0382(18) cm^{−1}] and the equilibrium bond distances of X ^{6}Σ^{+} [1.7839 Å], A ^{6}Σ^{+} [1.8919 Å], and B ^{6}Π [1.8277 Å] have been discussed, and it has been proposed that the 9σ (Cr_{4s–4p }) molecular orbital of CrF is slightly bonding, while 4π (Cr_{3dπ}) is nonbonding and 10σ (Cr_{3dσ}) antibonding.

TTF–TCNE a charge transfer π–molecular crystal with partial ionic ground state: Optical properties and electron‐molecular vibrations interaction
View Description Hide DescriptionThe electronic ground statestructure of the charge transfer molecular complex TTF–TCNE is investigated on the basis of its electronic and vibrational spectra. Highly oriented polycrystalline films and the spectra of the fully deuterated complex, TTF‐d _{4}–TCNE, allow one to obtain a full exploitation of the spectra. Using the vibrational frequencies as local probes of the electronic structure one finds a value of 0.5±0.1 for the degree of charge transfer of this molecular solid. This partial degree of charge transfer and the alternation of self‐dimers of TTF and TCNE along the one‐dimensional electronic π‐structure reveal themselves in the vibrational spectra and particularly in the charge transfer vibronic resonances present in the infrared and Raman spectra. These resonances and the electronic spectrum related to the charge transfer excitations are understood on the basis of a Holstein–Hubbard model which allows the determination of the electron‐intramolecular vibration coupling constants of TTF and TCNE.

Size‐selected vibrational spectra of phenol‐(H_{2}O)_{ n } (n=1–4) clusters observed by IR–UV double resonance and stimulated Raman‐UV double resonance spectroscopies
View Description Hide DescriptionOH and CH stretching vibrations of bare phenol, phenol‐(H_{2}O)_{ n } clusters (n=1–4), and partially deuterated clusters in the S _{0} state were observed by using IR–UV double resonance and stimulated Raman‐UV double resonancespectroscopies.Characteristic spectral features of the OH stretching vibrations of the phenol as well as of the H_{2}O sites were observed, which are directly related to their structures. The cluster structures were investigated by comparing the observed spectra with the calculated ones obtained by the ab initio molecular orbital calculation with (self‐consistent field) SCF 6‐31G and SCF 6‐31G* basis sets given by Watanabe and Iwata. It was found that for the clusters with n≥2, the isomer of ring form hydrogen‐bonded structure is most stable and the simulated IR spectra based on the calculated structure showed good agreements with the observed ones. For a particular cluster, which was assigned as an isomer of the n=4 cluster, an anomalous IR spectrum was observed. Two forms of the isomer are proposed with respect to the structure of water moiety: (1) an ‘‘ice’’ structure and (2) an ‘‘ion‐pair’’ structure. The relative IR absorption cross sections of each bands were also investigated for the clusters with n=1 to 4. It was found that the IR absorption cross section of the phenolic OH stretching vibration of the n=1 cluster increases by a factor of 6 compared to that of bare phenol and it further increases with the cluster size.

Theoretical studies of geometric structures of phenol‐water clusters and their infrared absorption spectra in the O–H stretching region
View Description Hide DescriptionThe structures of the phenol‐(H_{2}O)_{ n } clusters (n≤4) are determined with ab initio molecular orbital methods, and their infrared spectra are calculated to analyze the experimental spectra reported in the preceding paper. The experimental infrared spectra of phenol‐(H_{2}O)_{ n } clusters for n≤4 have ‘‘window regions,’’ which are intervals of two types of OH stretching modes of the water molecules. The calculated IR spectra of isomers with a ringstructure will reproduce these window regions. The ring is formed by a network of the hydration bonds of the −OH group of the phenol and water molecules. For n=4, two kinds of spectra are reported in the experiments. One spectrum has a window region similar to that of n≤3. It is, therefore, identified to the isomers of a ringstructure. The other one has several bands in the window region. The calculations for several isomers and large clusters suggest that this spectra might be attributed either (i) to the mixture of several branched isomers, (ii) to the products of evaporation of large clusters, or (iii) to the product of the proton transfer reaction in phenol‐(H_{2}O)_{4} cluster.

The coalescence range of the α and β processes in the glass‐forming liquid bis‐phenol‐C‐dimethylether (BCDE)
View Description Hide DescriptionIn this paper, we introduce a method of analysis of the dielectric relaxation data, which results to be very suitable for studying the coalescence of the α and β processes in the time domain. We show that, at least in the case of the glass‐forming liquid BCDE (bis‐phenol‐C‐dimethylether), the coalescence process can be well described by assuming that both, the α and β processes, behave as statistically independent processes. This simple assumption allows to describe the coalescence range of the two processes by extrapolating the low temperature behavior (where both processes are well separated), contrary to more classical approaches in which one needs to introduce a temperature at which the mechanism of the β process changes.

The potential energy surface of He–HCN determined by fitting to high‐resolution spectroscopic data
View Description Hide DescriptionTwo potential energy surfaces for He–HCN are determined by least‐squares fitting of parameterised functional forms to data from high‐resolution microwave and millimeter‐wave spectroscopy [Drucker et al., J. Phys. Chem. 99, 2646 (1995)]. The two potentials both have significantly deeper wells than suggested by the ab initiosupermolecule calculations of Drucker et al. Both potentials have linear or near‐linear equilibrium geometries, He–H–C–N, but the shapes of the well depth functions away from the linear geometry are significantly different. The existing experimental data are thus not sufficient to probe this potential feature in detail. Predictions of spectroscopicproperties that would allow the new potentials to be tested and refined are given.

Cage exit probability versus excess energy in the photodissociation of matrix‐isolated HCl
View Description Hide DescriptionDissociation efficiencies for excitation of the repulsive A ^{1}Π state of HCl were recorded in Xe, Kr, and Ar matrices for photonenergies between 5 and 10 eV from the content of dissociation products and quantum efficiencies were derived with the absorption spectra. Influence of temperature and preparation conditions was investigated. The quantum efficiency rises monotonically in Xe from an excess energy of 1.4 eV above the gas phase dissociation energy on, saturates around 2.4 eV and remains then essentially constant up to 4 eV. In Ar and Kr, it saturates around 2 eV and in Ar an absolute efficiency of about 0.18 is determined at 3.7 eV. Results of molecular dynamics calculations and a statistical model agree qualitatively but the observed saturation at low excess energies is not well described and the absence of a temperature effect in Ar needs further consideration.

The bending dynamics of acetylene
View Description Hide DescriptionThe dynamics and spectroscopy of (J=0) acetylene bending degrees of freedom are investigated using a reduced dimensional Hamiltonian. This Hamiltonian is obtained by applying an adiabatic approximation to average the vibrational Hamiltonian over the ground state in the three stretch coordinates. Within this approximation, an effective bend force field is obtained by adjusting force constants in the adiabatic potential to improve agreement between experimental and theoreticaleigenvalues. With minor modification, a global bend force field is determined that qualitatively describes the vinylidene vibrations and quantitatively describes the acetylene vibrations. This surface is compared to the results of a recent ab initio calculation. A dispersed fluorescencespectrum out of the excited Ã state, calculated from this model, is found to agree well with results of a recent experimental study.

Quantum, semiclassical and classical dynamics of the bending modes of acetylene
View Description Hide DescriptionThe dynamics and spectroscopy of an adiabatic Hamiltonian, derived by McCoy and Sibert [J. Chem. Phys. 105, 459 (1996)], describing the bending motions of acetylene are presented and discussed. The resulting eigenfunctions of this model are interpreted using classical, semiclassical, and quantum mechanical descriptions of the vibrations. Using perturbation theory, the Hamiltonian describing the bends is reduced to two coupled hindered rotor Hamiltonians. This simple Hamiltonian predicts that local mode dynamics of the bending motion first occurs at about 6000 cm^{−1} of excitation. This prediction is confirmed by the quantum mechanical studies. The hindered rotor Hamiltonian also predicts that l‐type doubling leads classically to stable periodic orbits corresponding to planar motion. The extent of planar type motion is quantified using both semiclassical and quantum mechanical models.

Fluctuations near limit cycles in chemical reaction systems
View Description Hide DescriptionWe investigate fluctuational properties near a limit cycle for a homogeneous chemical reaction system using a master equation approach. Our method of solution is based on the WKB expansion of the probability density in the inverse of the system size. The first two terms of this series give the leading asymptotic behavior. The eikonal equation for the leading order term has the structure of a Hamilton–Jacobi equation. Its solutions are determined by the associated characteristic equations, which also give fluctuational trajectories. In the vicinity of the limit cycle, the characteristic equations are the variational equations for the associated Hamiltonian system, and its solutions may be expressed as linear combinations of Floquet eigenfunctions. These eigenfunctions fall into three sets according to whether the real part of the characteristic exponent is less than, equal to, or greater than zero. Eigenfunctions corresponding to characteristic exponents with the real part less than zero span the stable subspace; they describe exponentially fast relaxation to the limit cycle in the deterministic system. Eigenfunctions corresponding to characteristic exponents with the real part greater than zero span the unstable subspace; they describe most probable fluctuational trajectories away from the limit cycle. The remaining two eigenfunctions are associated with a double zero characteristic exponent and span the center subspace. One eigenfunction is due to the translational invariance of the periodic orbit and the other (generalized eigenfunction) to the one‐parameter family of periodic orbits in Hamiltonian systems. The generalized eigenfunction describes diffusion along the limit cycle of a probability distribution front for which the gradient is perpendicular to the isochrons of the limit cycle. We develop an explicit formula for the time evolution of an initially localized density based on all these eigenfunctions. We show that relaxation of the density is exponentially fast in directions transverse to the limit cycle and slow (linear in time) along the limit cycle. In addition, we give a simple formula for the probability diffusion coefficient that characterizes dephasing along the orbit. A formula for the stationary distribution is ob‐ tained from the nonstationary density by removing the center and stable subspace. For this dens‐ ity, we give a new derivation of an identity: The marginal probability density along the limit cycle equals a constant times the inverse of the speed on the cycle, which is the invariant density along the limit cycle of the deterministic system.

Reactions of O(^{3} P) with alkynes: The CO and H atom channels
View Description Hide DescriptionThis is the second in a series of papers on the reaction of O(^{3} P) with alkynes in which the internal state distribution of some products are studied. The first paper dealt with acetylene whose two product channels are CO+CH_{2} and H+HCCO. The present paper deals with the reactions of a series of higher alkynes; however, just the CO release and the H atom release channels were studied. The CO product was rotationally and vibrationally cold in every case. We therefore infer that, except possibly for acetylene, the initial ketocarbene undergoes intersystem crossing to a singlet state and isomerizes to a substituted ketene which then dissociates through a linear C–C–O transition state. The absence of CO vibration energy implies that the energy taken from the initially formed C–O bond to facilitate a 1,2 migration is not returned. The large H atom translational energy implies that the H atom is released simultaneously with the formation of a radical of high resonance energy. Finally, the CO and H atom yields decrease in the longer alkynes, presumably because the dominant reaction channel becomes C–C bond breaking leading to radical pair formation.

Collisional recombination reaction H+O_{2}+M→HO_{2}+M: Quantum mechanical study using filter diagonalization
View Description Hide DescriptionWe report the results of calculations for the collisional recombination rate of the reaction H+O_{2}+M→HO_{2}+M. This study uses the recently developed quantum mechanical theory of recombination (within the strong collision approximation) based on the flux–flux correlation function analysis [J. Phys. Chem. 99, 12387 (1995)]. The quantum dynamics calculations are based on the new very efficient computational procedure of filter diagonalization [J. Chem. Phys. 103, 10074 (1995)]. This procedure allows one to obtain individual complex eigenenergies and eigenfunctions of a large non‐Hermitian matrix representation of the Hamiltonian with an absorbing potential. The computed rates are compared to the Lindemann steady‐state approximation result. The latter leads to an overestimation of the rate since it does not take into account the resonance interference effects. This becomes more critical at higher temperatures where many broad high energy overlapping resonances contribute to the rate.

Photofragmentation of mass‐selected ICl^{−}(CO_{2})_{ n } cluster ions: Solvation effects on the structure and dynamics of the ionic chromophore
View Description Hide DescriptionPhotofragmentation studies at 644 nm and 740 nm of ICl^{−}(CO_{2})_{ n } cluster ions (n=0–8) have been carried out in a tandem time‐of‐flight mass spectrometer.Photodissociation of these cluster ions at a wavelength at which bare ICl^{−} produces only I^{−} results in the formation of three classes of fragment ions: I^{−}, Cl^{−}, and ICl^{−} based clusters. The I^{−} based clusters correspond to the direct photoproduct in which a Cl atom has escaped the cluster ion. The ICl^{−} and Cl^{−} based clusters are a result of a nonadiabatic electronic transition to the ground state mediated by the solvent. The relative importance of these photofragment channels strongly depends on the cluster ion size. An ICl^{−} caged product is first observed for ICl^{−}(CO_{2})_{2}, increasing rapidly to a maximum at n≊6 and then decreasing. This caging efficiency is dramatically different from the I^{−} _{2}(CO_{2})_{ n } cluster ions where complete caging was observed for 16 solvent molecules. The Cl^{−} photofragment channel increases smoothly for the cluster size range studied and becomes the dominant channel for n=8. The relative yields of the ICl^{−} and Cl^{−} based products reflect the extent to which solvation influences the photodissociation pathways of ICl^{−}.

Accurate calculation of quantum and diffusion propagators in arbitrary dimensions
View Description Hide DescriptionA new approach to calculating the dynamics and equilibrium thermodynamics of an arbitrary (quantum or stochastic) system is presented. Its key points are representing the full propagator as a product of the harmonic‐oscillator propagator with the configuration function, and expanding the configuration function (its exponent) in a power series in a given function of t. Recursion relations are obtained for the expansion coefficients which can be analytically evaluated for any number of degrees of freedom. This representation is particularly attractive for two reasons. Being structurally similar to the standard Taylorlike expansions for the propagator already known in the literature, it nevertheless shows a dramatic improvement over the latter in that it converges significantly better over a much broader range of t. Another attractive feature of the present expansion is that it is amenable to subsequent approximations. With this technique a minimal computational effort is required for constructing an improved global approximation for the propagator which is exact not only if t goes to zero, but also in the limit t→∞. Numerical applications to the coordinate space density matrix, quantum‐mechanical time correlation function, and Fokker–Planck conditional probability show an accurate description of dynamical (statistical) properties to be already achieved for arbitrarily large times (small temperatures) with just the first term of the present expansion taken into account. Its use in a path integral means that a dramatic reduction of the number of integration variables which is required for convergence will be achieved even though simulations over very long times are desirable.

The reflection of predissociation dynamics in pump/probe photoelectron distributions
View Description Hide DescriptionWe present simulations of pump/probe photoionization experiments on a diatomic molecule with two nonadiabatically coupled electronic states. The NaI molecule is used as an example. The nuclear wave packet dynamics in two coupled electronic states is mapped into the kinetic energy distribution of photoelectrons which are recorded at different delay times between the pump pulse, preparing the initial wave packet and the ionizing probe pulse. In this way details of the indirect fragmentation process can be observed in real time.

Bond selective infrared multiphoton excitation and dissociation of linear monodeuterated acetylene
View Description Hide DescriptionQuantum mechanical simulations of vibrational excitation of monodeuterated linear acetylene (HCCD) with linearly polarized, frequency‐swept, intense but nonionizing infrared laser pulses are performed. The aim is selective dissociation of either H or D atoms by optimal shaping of the laser pulses. We use a discrete variable representation and a compact (<400 states) bright‐state expansion to represent the wave function during and after the pulse. Wave packet propagations in the bright‐state expansion are at least an order of magnitude faster than discrete variable representation wave packet propagations. This enables optimal‐control calculations to find the best parameters for the laser pulses. The dynamics of CH‐bond breaking with infrared pulses are very different from the dynamics of CD‐bond breaking. This is a direct consequence of CH being the highest‐frequency mode in the molecule. Selective CH‐bond breaking is possible with two synchronized pulses, the first being quasi‐resonant with the Δv=1 transitions in the CH stretch between v=0 and v=8, and the second being quasiresonant with Δv=2 transitions at higher v. H‐atom yields as high as 7.7%, with H to D yield ratio as high as 2.1, are demonstrated. Selective CD‐bond breaking is possible using a single, subpicosecond, frequency‐swept pulse. D‐atom yields as high as 3%, or D to H atom yield ratios as high as 3.9, are calculated.

State‐selective photofragment imaging of iodine atoms via photodissociation of CF_{3}I at 277 nm
View Description Hide DescriptionThe photodissociation of CF_{3}I cooled in a supersonic molecular beam has been investigated at 277 nm by state‐selective photofragment imaging. Fragmented iodine atoms of two spin–orbit states are state‐selectively ionized and projected onto a two‐dimensional position‐sensitive detector, to obtain their speed and angular distribution. The anisotropy parameter for an excited iodine atom I^{*}(^{2} P _{1/2}), β(I^{*}), is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150–350 fs from rotational correlation function. Contrary to earlier reports, the parallel‐like distribution for the ground state iodine atom I(^{2} P _{3/2}) at this wavelength, shows a more favorable curve‐crossing dissociation path (68%) from ^{3} Q _{0} to ^{1} Q _{1} and a less favorable direct dissociation path (32%) from ^{3} Q _{1}. The recoil energy distribution of I is found to be broader than that of I^{*} and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previous works, and the strong photon energy dependence of the energy partitioning in CF_{3}+I* channel and curve crossing are interpreted in terms of the final state interaction and curve crossing probability, respectively.