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We use our adiabatic pseudospectral method (APS) to calculate energy levels of the H2O2 molecule up to 5000 cm-1. Reasonably high accuracy (a few wave numbers) is achieved for a fully six dimensional calculation in a few hours of CPU time on an IBM 580 workstation. This contrasts with previous calculations on the same system which required 50–100 times more computational effort for a similar level of accuracy. The method presented here is both general and robust, and will allow routine studies of six dimensional potential surfaces and the associated spectroscopy, while making calculations on still larger systems feasible. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose an efficient path integral scheme for calculating the quantum dynamics of an arbitrary one-dimensional system coupled nonlinearly to many anharmonic noninteracting ‘‘bath’’ degrees of freedom. The starting point is an improved discretization of the path integral in terms of numerically constructed propagators [Chem. Phys. Lett. 193, 435 (1992)]. The resulting influence functional is comprised of one-dimensional correlation functions with step-structured time-dependent potentials and therefore is similar in structure to that employed in the spin-boson calculations of Coalson [J. Chem. Phys. 86, 995 (1987)]. In the present case, though, the influence functional is nonlinear and is computed using numerical iterative wave function propagation methods. Numerical tests on a system coupled to ten anharmonic oscillators demonstrate the efficiency of the proposed scheme, which requires numerical effort that scales only linearly with the number of anharmonic bath degrees of freedom. [ABSTRACT FROM AUTHOR]

Weakly bound complexes of CF3Br molecules or of CF3Br and CH3OH were prepared in supersonic jet expansions and studied by photoionization mass spectrometry. Argon was present in the jet expansion to promote cooling of the complexes. Measured ionization potentials are CF3Br, 11.404±0.014 eV; (CF3Br)2, 11.10±0.02 eV; (CF3Br·CH3OH), 10.76±0.05 eV. The appearance potential of CF+3 from CF3Br was found to be 11.56±0.02 eV, and evidence is presented that this value is adiabatic. From this appearance potential, ΔH0f0(CF+3)=86.6±1.1 kcal mol-1. The magnitude of the spin–orbit splitting in X 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X 2E CF3Br+, CF2Br+ from A 2A1 CF3Br+, Br+ from B 2A2 CF3Br+, and CF+2 from D 2E CF3Br+. (CF3·CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluster is ionized to A 2A1 CF3Br+. In contrast, (Br·CH3OH)+ does come from dimers as well as larger clusters. The cross sections for the production of (CF3Br)+2 and (CF3Br·CH3OH)+ from trimers or larger progenitors are enhanced when intracluster CF3Br is ionized to CF3Br+ in the B or higher electronic states. Possible reasons for these state-specific reactions are given. Finally, a new method is presented for determining neutral cluster distributions in a molecular beam, and the method is shown to be in quantitative agreement with the near-threshold technique previously published by Grover et al. [J. Phys. Chem. 95, 6473 (1991)]. [ABSTRACT FROM AUTHOR]

Three algorithms for global energy minimization based on the simulated annealing of the classical density distribution are presented. The first is based on annealing the classical density distribution directly in temperature and is the classical analog of imaginary time quantum dynamics. Another two algorithms are based on the approximate solution of the classical Liouville equation for the dynamics of a system coupled to a heat bath using a rigid temperature constraint and Fokker–Planck dynamics. These three methods are compared with standard simulated annealing based on molecular dynamics. The results for a series of Lennard-Jones clusters demonstrate that by annealing the continuous density distribution (representing a volume of phase points) the likelihood of finding the global minimum is dramatically enhanced. [ABSTRACT FROM AUTHOR]

We present a new method for solving the Redfield equation, which describes the evolution of the reduced density matrix of a multilevel quantum-mechanical system interacting with a thermal bath. The method is based on a new decomposition of the Redfield relaxation tensor that makes possible its direct application to the density matrix without explicit construction of the full tensor. In the resulting expressions, only ordinary matrices are involved and so any quantum system whose Hamiltonian can be diagonalized can be treated with the full Redfield theory. To efficiently solve the equation of motion for the density matrix, we introduce a generalization of the short-iterative-Lanczos propagator. Together, these contributions allow the complete Redfield theory to be applied to significantly larger systems than was previously possible. Several model calculations are presented to illustrate the methodology, including one example with 172 quantum states. [ABSTRACT FROM AUTHOR]

A three dimensional model Hamiltonian is used to mimic and interpret the results of full molecular dynamics simulations of an ion-molecule activationless recombination process in a solvent of structureless atoms. By making an adiabatic separation of variables it is shown that the gas phase capture model, suitably modified to incorporate the dynamical role of the solvent motion, can be used also in solution. Specifically, a motion along one uncoupled coordinate describes the capture process. The angular momentum for this coordinate is constant during the approach motion and thereby it provides a suitable criterion for capture. The motion of the approaching reactants is shown to be in the strong coupling adiabatic limit. In this limit there is a combination of two effects: A weak ion-molecule attractive interaction at large separations and a substantial solvation of the ion by the liquid. Thus the solvent is able to follow the motion along the reaction coordinate and to take part in the crossing of the centrifugal barrier. A second implication of the model is the efficient deactivation of the ion pair as a result of nonadiabatic V-T transitions. These transitions are confined to the ion-pair polarization well region, i.e., to the left of the adiabatic region of the centrifugal barrier. If a ‘‘solvent-separated’’ ion pair is formed the recombination process is delayed and the reorganization of the solvent is required to facilitate a successful capture. To model this effect a nonlinear, space dependent, coupling term is used in the model Hamiltonian. Comparison is made throughout between the results of full molecular dynamics simulations, computational results for the model Hamiltonian, and the predications of the adiabatic separation. The role of strong solvation in activationless recombination reactions is discussed in terms of the adiabatic separation and its breakdown. The conclusions are compared, and contrasted, with the case of... [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed for the dissociation and association (D/A) reactions N2O4[larger_closed_square]2 NO2 in the gas phase and in liquid N2O4. The trajectory was initialized from an equilibrium distribution of all variables in liquid N2O4, except the reactive mode, the NN distance of a reactant NO2 pair, was excited above the dissociation limit of the Morse-like potential between NO2 fragments, and the dynamics were calculated for 500 fs both forward and backward in time. Characteristics of the translational and vibrational energy relaxations of the reactant were studied in detail. Energy ERT, which is defined to be the sum of the potential and kinetic energies of interfragment motion, is found to play a key role in the D/A dynamics; a reactant pair is associated when ERT<0 and the pair is dissociated when ERT>=0. The transition state to the D/A reactions is hence defined by the last associated phase curve ERT=0 in the phase space. Energy transfer between intrafragment vibrational modes and the interfragment translational mode, which occurs at the inner turning point of the interfragment potential, is found to be the dominant prompter of the D/A reactions. The vibration–translation (V–T) energy transfer is found to excite the relative translational motion between fragments or gives rise to dissociation, and T–V energy transfer often causes deactivation of the relative translational motion or association in both the gas and liquid phases. In minor cases, the D/A reaction is found to occur by an energy transfer between reactant relative translational mode and solvent modes. The reaction rates are determined essentially by the rates of energy transfers among relative translational mode, intrafragment vibrational modes, and solvent modes. [ABSTRACT FROM AUTHOR]

An undercooled liquid exhibits crystalline fluctuations, some of which grow into crystal of macroscopic dimension, while smaller fluctuations disappear. We present a model which allows for exact analytic characterization of the inhomogeneous critical nucleus, the smallest fluctuation which will give rise to crystal growth, in terms of a single spatially varying order parameter for the degree of crystallinity. The model is built around the square-gradient approximation for the free energy with a simple double-parabolic form for the homogeneous component. We study the radius, free energy of formation, and profile of the critical nucleus as functions of the liquid undercooling and compare these with results from an earlier nonclassical theory and from the classical capillarity approximation. The time evolution of the order parameter is described by a phase-field equation which is easily solved numerically for growth dynamics of initially supercritical fluctuations or for the regression of subcritical profiles. [ABSTRACT FROM AUTHOR]

This is Part II of a series of papers in which we address the role of microscopic solvation in the association reaction between a free iodine atom and an iodine doped van der Waals cluster: I+I(Ar)n→I2+nAr. The influence of microscopic solvation on the I+I to I2 reactivity, reaction mechanism, energetics, and product energy partitioning is the major focus of our study. The overall reaction for I+I(Ar)12→I2+12Ar can be characterized by three fundamental processes: (1) capture of the incident iodine atom by the I(Ar)12 cluster; (2) diffusive migration of the captured I atom on the surface or in the interior of the cluster, leading ultimately to an encounter with the other I atom to form a highly excited I*2 molecule; (3) vibrational relaxation of the nascent I*2 product, leading to evaporative cooling and decomposition of the cluster. Part I [J. Chem. Phys. 98, 8551 (1993)] dealt with the capture process. This article focuses on the chemical dynamics of the subsequent processes of diffusion, vibrational energy transfer, and evaporative cooling. The stabilization of the chemically activated I*2 molecule through evaporative cooling eliminate the need of a third body collision as required in isolation gas phase recombination. The overall distribution of final energies is nonstatistical for the chemically activated I*2Arn. The final vibrational energy of I2 exhibits a nonthermal structure even after all the argon atoms are evaporated. In addition to monoatomic sequential evaporation, a ‘‘fissioning’’ mechanism, leading to the formation of at least one multiatom fragment, is observed. The relationship between structure and dynamics is explored. The dynamics of vibrational relaxation, diffusion of the captured iodine, evaporation, and fragmentation pattern, final I2 energy partitioning are found to be strongly dependent upon structure and temperature of the doped cluster. A spectroscopic experimental verification of the above... [ABSTRACT FROM AUTHOR]

The effects of global coupling through the gas phase in oscillatory surface chemical reactions are investigated using a model which represents the complex Ginzburg–Landau equation with an additional integral term. Depending on the parameters of the model, global coupling is found to have either a synchronizing or desynchronizing effect. Respectively, the breakdown of global coupling requires the presence of strong supercritical inhomogeneities or spontaneously occurs in a uniform system. [ABSTRACT FROM AUTHOR]

We present a general, fully quantum mechanical theory for molecule surface scattering at finite temperature within the time dependent Hartree (TDH) factorization. We show the formal manipulations which reduce the total molecule–surface–bath Schrödinger equation into a form which is computationally convenient to use. Under the TDH factorization, the molecular portion of the wavefunction evolves according to a mean-field Hamiltonian which is dependent upon both time and temperature. The temporal and thermal dependence is due to stochastic and dissipative terms that appear in the Heisenberg equations of motion for the phonon operators upon averaging over the bath states. The resulting equations of motion are solved in one dimension self consistently using quantum wavepackets and the discrete variable representation. We compute energy transfer to the phonons as a function of surface temperature and initial energy and compare our results to results obtained using other mean-field models, namely an averaged mean-field model and a fully quantum model based upon a dissipative form of the quantum Liouville equation. It appears that the model presented here provides a better estimation of energy transfer between the molecule and the surface. [ABSTRACT FROM AUTHOR]

One- and two-color time-domain probing of the resonant dichroic response of iodine in n-hexane following femtosecond B-X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third-order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B-X absorption and stimulated emission are positive but absorption from the B-state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B-X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive signal for blue probe wavelengths arises from a repulsive state reached by solvent-induced predissociation of the B state. Dephasing of B state vibrational coherence results from this solvent-induced predissociation of iodine. We discuss the evolving reaction in terms of possible dissociative potential energy curves a1g(3Π) and a’0g+(3Σ-). The time evolution of the bluest probe dichroism signals is representative of continuing atom separation; the experiments have not yet probed large enough internuclear separations to evidence a buildup of dissociated product or momentum reversal, i.e., caging. [ABSTRACT FROM AUTHOR]

The rotational reorientation times (τr) of three kinds of dyes—cresyl violet (a monocation), nile red (neutral but polar), and resorufin (a monoanion) were measured in a series of binary mixtures of hexamethylphosphoramide (HMPA)–water at 298 K using the picosecond fluorescence depolarization spectroscopy. A hook-type profile of τr vs η was observed for all three dyes. The contribution due to the dielectric friction is minimal and is irrelevant in explaining the rotational dynamics of nile red. The dielectric friction correction is modest for cresyl violet and resorufin. The solvation effects in the binary mixture are dominant for understanding the rotational dynamics of cresyl violet. The microscopic structure features of the binary mixture are brought out by the study of resorufin. [ABSTRACT FROM AUTHOR]

Classical trajectory calculations are performed to investigate how microscopic solvation influences the H+CH3→CH4 reaction mechanism, rate constant, energetics, product energy, and angular momentum partitioning; and how these solvation effects depend on the solute–solvent interaction strength. Without solvation, the final energy and rotational angular momentum of CH4 strongly depend on the H+CH3 relative translational energy. However, for HAr2+CH3 with a normal H–Ar Lennard-Jones interaction strength εHAr0, a spectator-stripping mechanism dominates the reactive collisions so that both the final CH4 energy and rotational angular momentum do not significantly depend on the relative translational energy. The association cross section to form CH4 is slightly larger for HAr2+CH3 than for H+CH3. When the H–Ar interaction strength for HAr2 is increased from 1 to 100εHAr0, it is found that (1) the association cross section to form CH4 is insensitive to the H–Ar interaction strength, suggesting a long-range transition state; (2) the reaction mechanism changes from a spectator-stripping model to a complex one, which alters the character of the CH4+Ar2 product energy and angular momentum partitioning; and (3) the formation of the Ar2–CH4 complex leads to stabilized CH4 product, with substantial energy transfer from CH4 for the strongest H–Ar interaction strength of 100εHAr0. [ABSTRACT FROM AUTHOR]

A treatment of the effect of vibrational predissociation of van der Waals complexes on vibrational relaxation rates [Chem. Phys. 29, 253 (1978)] has been extended by including V–V energy transfer between complexed and uncomplexed species and the possibility of different V–T/R relaxation rates for complexed and uncomplexed species. These modifications allow us to reproduce experimentally observed relaxation data for HCl(v=1) over two orders of magnitude in xenon density. The dependence of the vibrational relaxation rate on density is discussed as are the implications of the observed behavior on isolated binary collision formulations of vibrational energy relaxation processes. [ABSTRACT FROM AUTHOR]

The fourth-rank tensorial nonlinear response function which controls the response of a chromophore in the condensed phase to three arbitrarily polarized external fields is calculated. Internal (vibrational and rotational) degrees of freedom are incorporated using wave packets in Liouville space (the doorway/window picture). The rotational contribution of the chromophore is expressed in terms of a conditional probability for the rotational diffusion model. We apply this formalism to transient dichroism and birefringence spectroscopies. [ABSTRACT FROM AUTHOR]

Theory and practice of reaction dynamics on two-dimensional potential energy surfaces is investigated. Nonequilibrium multidimensional barrier crossing, occurring when the initial density is located near the ridgeline separating reactants and products, is treated by solving the time-dependent Smoluchowski equation as a function of diffusion anisotropy. For a locally separable potential, and slow relaxation in the perpendicular mode, the problem reduces to a one-dimensional Smoluchowski equation with a sink term. It may be further approximated as a first-order reaction with a time-dependent rate coefficient. These approximations are compared with exact two-dimensional propagations on a potential surface representing CO binding to α heme. The intermediate-time power-law decay of the survival probability is analyzed with the aid of the above approximations. The power also shows some kind of critical behavior near the isotropic diffusion limit, where these approximations are no longer valid. For fast relaxation, a nonmonotonic survival probability is observed. The long time decay of the survival probability is governed by the equilibrium rate coefficient. We calculate its anisotropy dependence and compare it with two asymptotic expansions for the lowest eigenvalue of the Smoluchowski operator-for the one-dimensional sink-Smoluchowski operator and the fully two-dimensional operator. The difference between the fast relaxation limit of these one- and two-dimensional equations may provide a quantitative explanation for previous problems in extrapolating to high temperatures using the Agmon–Hopfield model. The implications of these results to heme protein dynamics are discussed and new experiments are proposed. [ABSTRACT FROM AUTHOR]

I2Arn clusters yield visible and near-UV emissions when excited to the states which correlate with the first ion-pair manifold of I2. These states may be accessed by 193 nm excitation of ground-state clusters, or near-UV excitation of electronically metastable I2(A’ or A)Arm. Comparisons of the cluster and I2/Ar matrix spectra suggest that such excitations result in fragmentation and ‘‘melting’’ of the clusters prior to emission. 532 nm photodissociation of I2 within the clusters is followed by geminate recombination which populates the A’, A, and X states. The probability for ejection of the recombined I2 from the clusters during the relaxation process appears to be size dependent. These results represent tentative steps towards the study of I(2P3/2)+I(2P3/2) recombination in Ar clusters. They also indicate that dissociation, recombination, and fragmentation processes may be used as a general method for generating metastable species in supersonic expansions. [ABSTRACT FROM AUTHOR]

Femtosecond infrared coherent transients have been measured for the stretch vibration of CO on Cu(111). The free induction decay exhibits a dephasing time (T2) of 2±0.3 ps (and 2±0.1 ps assuming a single exponential decay between 2 and 3 ps). The decay was best fit by exponential relaxation, thereby suggesting that the CO vibrational band is almost entirely homogeneously broadened. The surface sum frequency spectrum was also measured at two coverages (0.10 and 0.45 L) using spectrally narrowed pulses. Interferences were observed leading to a determination of the relative phase and amplitude of the resonant and nonresonant second-order susceptibility in this system. The magnitude of the nonresonant susceptibility was only weakly dependent on coverage, suggesting that the nonresonant polarizability originates in the bulk Cu. Time and frequency domain results were in good agreement. [ABSTRACT FROM AUTHOR]

Fluorescence line narrowing spectroscopy is extended to the study of liquid state dynamics. Measurements are made on solutions of dimethyl-s-tetrazine in glycerol from room temperature to below the glass transition. The structural broadening identified in the preceding paper is related to a relaxation function by the use of a simple configuration coordinate model. This relaxation function is ∼150 times faster than the structural relaxation function measured by other experiments, showing that electronic state solvation is sensitive to a unique aspect of the structural dynamics of the liquid. [ABSTRACT FROM AUTHOR]

For the Cl-+CH3Cl SN2 nucleophilic substitution reaction transition-state theory predicts that crossing the central barrier region of the potential-energy surface is the rate-controlling step. In this work classical trajectories are initialized at the central barrier. Four different models are considered for the potential-energy surface. A significant amount of central barrier recrossing is observed in the trajectories, which suggests that transition-state theory is an incomplete model for calculating the Cl-+CH3Cl SN2 rate constant. Two types of recrossings are observed in the trajectories: intermediate recrossings in which trajectories linger near the central barrier and complex recrossings in which trajectories trapped in the Cl-···CH3Cl complex return to the central barrier region. Intermediate recrossings are important if, in the trajectory initial conditions, zero-point energy is added to the vibrational modes orthogonal to the reaction coordinate. Rice–Ramsperger–Kassel–Marcus (RRKM) theory predicts extensive dissociation of the Cl-···CH3Cl complex to Cl-+CH3Cl and negligible complex recrossings in the trajectory calculations. In contrast to this prediction, negligible Cl-+CH3Cl formation is observed and continual complex recrossings occur, on a time scale longer than the complex’s RRKM lifetime. These results indicate the ergodic assumption is invalid for the Cl-···CH3Cl complex. Phase-space bottlenecks which give rise to the intermediate and complex recrossings are considered. [ABSTRACT FROM AUTHOR]

This paper reports on a dynamical study of the reaction of an effusive F atom beam with iodine molecules condensed on a temperature-controlled LiF(001) surface. The observation of IF ground state products by laser-induced fluorescence and of IF(A,B) excited species by visible chemiluminescence can be explained by means of a reaction scheme consisting of two channels. In the first channel IF (X 1Σ+) ground state molecules are formed on the surface and desorb into the gas phase above a threshold temperature of about 200 K, where they are probed by laser-induced fluorescence. The vibrational population ratio [N’(v=0)/N’(v=1)] shows equilibrium of the desorbing molecules with the surface. The population distribution of the IF(v=0) rotational manifold can be well-characterized by a rotational temperature which follows the surface temperature. Surface equilibration of products implies that, unlike in the gas phase reaction F+I2, the products have no memory of their exoergic formation, indicating a Langmuir–Hinshelwood mechanism. In a second reaction channel I2F, as the intermediate in the reaction of fluorine atoms with adsorbed iodine molecules, stabilizes at the surface and desorbs into the gas phase above a surface temperature of 300 K. There it reacts with F atoms according to the chemiluminescence process F+I2F→IF*+IF. This gas phase reaction is identified by the exoergicity limit of the IF (B 3Π) state vibrational population distribution, obtained from analysis of the B→X chemiluminescence. In comparison with Kahler and Lee’s molecular beam synthesis of I2F from I2 and F2 with subsequent secondary encounter with the product F atom, the above reaction scheme represents an alternative induction of the F+I2F reaction. It can explain visible IF chemiluminescence from fluorine–iodine systems under experimental conditions in which F atoms are present and I2 wall adsorption is possible. In this work, evidence... [ABSTRACT FROM AUTHOR]

The dynamics of carbon monoxide oxidation on Pd, Rh, and Pt foils were probed under nearly collision-free conditions using high resolution infrared chemiluminescence. Auger electron spectroscopy was used to verify the absence of impurities on the surfaces. The reactants were supplied to the surface through a free jet nozzle source, while a Fourier transform infrared spectrometer operating at 0.012 cm-1 resolution was used to fully resolve the rotational structure of several vibrational transitions in the product CO2 (22 in the case of Pd). In all cases, the product CO2 is vibrationally excited and the apparent vibrational temperatures are in the same order as the peak reaction rates, i.e., Pd>Pt>Rh. The surface coverage of oxygen on Pd was varied by changing the CO[ATOTHER]@B:[/ATOTHER]O2 ratio and the surface temperature and, in both cases, increasing oxygen coverage causes an increase in vibrational excitation of product CO2. On Pt and Rh, the apparent temperatures of different vibrational modes are similar, while on Pd, those levels involved in Fermi resonances that are traditionally called the symmetric stretch levels are selectively populated. From these results, evidence that the activated complex is bent on Pd relative to Pt and Rh and, in all cases, is aligned more or less along the surface normal is presented. [ABSTRACT FROM AUTHOR]

Molecular dynamics of SF6 in small cylindrical pores in amorphous silica is studied by computer simulations. The solid is represented by an atomic model that takes into account microscopic structure of the surface. Adsorbent is modeled as an assembly of six Lennard-Jones potentials. Rotational and translational correlation functions are calculated and discussed in terms of temperature, density, pore diameter, and surface potential. Changes in the motion along the pore axis and pore radius are analyzed. It is shown that surface interactions determine translational and rotational diffusion of molecules in the monolayer, but at distances greater than two molecular diameters, the dynamics can be compared to that in the bulk liquid. [ABSTRACT FROM AUTHOR]

Rotational spectra of van der Waals dimers between an argon atom and CH2CHF, CH2CF2, and CHFCF2 have been obtained by pulsed-supersonic nozzle Fourier transform microwave spectroscopy. Analysis of the derived spectroscopic constants shows that the dimers have structures such that for CH2CHF, CH2CF2, and CHFCF2 the Ar atom is positioned over the FCCH, FCF, and FCCF atomic chains with Ar-molecular center-of-mass distances of 3.62 Å, 3.51 Å, and 3.56 Å, and angles between the Ar–cm axis and molecular planes of 48.2°, 72.9°, and 60.5°, respectively. Structures for the three dimers are also predicted with a simple multisite model which describes the anisotropy of the dispersive interaction; both the Ar acceptor site and the atom–atom distances are satisfactorily reproduced. [ABSTRACT FROM AUTHOR]

A formulation of a trajectory surface-hopping method is presented which starts with some of the basic ideas of the standard method, extends these to include more than two states, and enlists the classical-path equations not only to propagate through the nonadiabatic region but also to effect the transitions, or surface hops, themselves. The latter is accomplished by letting the Hamiltonian matrix elements in the time-dependent Schrödinger equation become complex, allowing manipulation of the fluctuation in the various adiabatic state populations. The procedure automatically conserves total energy and angular momentum as well as probability, and ensures that energetically inaccessible states are not significantly populated. In regions of extended degeneracy, the method resembles the standard classical-path approach with no surface hopping. In fact, the evolution of the wave function can be controlled to behave either as in the surface-hopping extreme or in the pure semiclassical extreme, allowing the method to be tailored to suit individual systems. The procedure is illustrated by application to a well-studied collision induced predissociation, Ne+He+2→Ne++He+He, where vibration in the entrance channel, Ne+He+2, leads to the strong nonadiabatic behavior responsible for the large observed cross sections. A few preliminary calculations for singly charged argon trimer ions produced in the ionization process Arn→Ar+n→Ar+2+(n-2)Ar, demonstrate that the formulation can be easily extended to include many degrees of freedom. [ABSTRACT FROM AUTHOR]

Mixed van der Waals clusters containing Cr(CO)6 and methanol are generated in the free-jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time-of-flight mass spectrometry. We find that the multiphoton dissociation and ionization dynamics of solvated Cr(CO)6 are strikingly different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula SnCr(CO)+x (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula SnCr(CO)+x (x=5,6), first appears in the mass spectrum at n+x=7.We discuss two possible dynamical schemes for MPI of Cr(CO)6 /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)6 -containing clusters gives rise to solvated Cr(CO)+6 primary photoions, which subsequently photodissociate to yield the observed coordinatively unsaturated daughter fragments; and an alternative scheme, in which initial photodissociation in the neutral manifold gives rise to one or more coordinatively unsaturated primary photoproducts, each of which subsequently undergoes photoionization. We consider, in a qualitative fashion, the mass spectral fragmentation patterns predicted by these two alternative schemes under conditions of extremely high laser fluence (where essentially all one-photon processes are saturated), and compare these predictions with the observed MPI mass spectra following high-fluence irradiation at both 248 and 350 nm. Assuming that any secondary photodissociation in the ionic manifold occurs statistically, our high-fluence results are inconsistent with a dynamical scheme in which MPI precedes photodissociation.We suggest that the dynamics are correctly described by initial photodissociation in the neutral manifold, followed by MPI of the... [ABSTRACT FROM AUTHOR]

Investigates state-resolved detection of bromine oxide radicals generated in reactive collision processes. Use of two-photon excitation technique with subsequent monitoring of the vacuum-ultraviolet fluorescence; Determination of the radical product excitation spectra; Existence of high-lying radiative C state.

We present and test an approximate method for the semiclassical calculation of vibrational spectra. The approach is based on the mixed time-averaging semiclassical initial value representation method, which is simplified to a form that contains a filter to remove contributions from approximately harmonic environmental degrees of freedom. This filter comes at no additional numerical cost, and it has no negative effect on the accuracy of peaks from the anharmonic system of interest. The method is successfully tested for a model Hamiltonian and then applied to the study of the frequency shift of iodine in a krypton matrix. Using a hierarchic model with up to 108 normal modes included in the calculation, we show how the dynamical interaction between iodine and krypton yields results for the lowest excited iodine peaks that reproduce experimental findings to a high degree of accuracy. [ABSTRACT FROM AUTHOR]

We demonstrate that surface plasmon oscillations excited at an adsorbate covered metal–vacuum interface can effectively couple to the electronic system of the adsorbed molecule. Using p-polarized light (hν=3.5 eV) incident at the surface plasmon resonant angle in Kretschmann’s attenuated-total-reflection (ATR) configuration, we observe a strong enhancement of the photodissociation rate of Mo(CO)6 from a 180 Å Al film, evaporated on a quartz prism in UHV. [ABSTRACT FROM AUTHOR]

The quantum-classical Liouville equation offers a rigorous approach to nonadiabatic quantum dynamics based on surface hopping type trajectories. However, in practice the applicability of this approach has been limited to short times owing to unfavorable numerical scaling. In this paper we show that this problem can be alleviated by combining it with a formally exact generalized quantum master equation treatment. This allows dramatic improvements in the efficiency of the approach in nonadiabatic regimes, making it computationally tractable to treat the quantum dynamics of complex systems for long times. We demonstrate our approach by applying it to a model of condensed phase charge transfer where our method is shown to be numerically exact in regimes where fewest-switches surface hopping and mean field approaches fail to obtain either the correct rates or long-time populations. [ABSTRACT FROM AUTHOR]

A simple surface hopping method for nonadiabatic molecular dynamics is developed. The method derives from a stochastic modeling of the time-dependent Schrödinger and master equations for open systems and accounts simultaneously for quantum mechanical branching in the otherwise classical (nuclear) degrees of freedom and loss of coherence within the quantum (electronic) subsystem due to coupling to nuclei. Electronic dynamics in the Hilbert space takes the form of a unitary evolution, intermittent with stochastic decoherence events that are manifested as a localization toward (adiabatic) basis states. Classical particles evolve along a single potential energy surface and can switch surfaces only at the decoherence events. Thus, decoherence provides physical justification of surface hopping, obviating the need for ad hoc surface hopping rules. The method is tested with model problems, showing good agreement with the exact quantum mechanical results and providing an improvement over the most popular surface hopping technique. The method is implemented within real-time time-dependent density functional theory formulated in the Kohn-Sham representation and is applied to carbon nanotubes and graphene nanoribbons. The calculated time scales of non-radiative quenching of luminescence in these systems agree with the experimental data and earlier calculations. [ABSTRACT FROM AUTHOR]

Given the limited intermolecular spaces available in dense liquids, the large amplitudes of highly excited, low frequency vibrational modes pose an interesting dilemma for large molecules in solution. We carry out molecular dynamics calculations of the lowest frequency ('warping') mode of perylene dissolved in liquid argon, and demonstrate that vibrational excitation of this mode should cause identifiable changes in local solvation shell structure. But while the same kinds of solvent structural rearrangements can cause the non-equilibrium relaxation dynamics of highly excited diatomic rotors in liquids to differ substantially from equilibrium dynamics, our simulations also indicate that the non-equilibrium vibrational energy relaxation of large-amplitude vibrational overtones in liquids should show no such deviations from linear response. This observation seems to be a generic feature of large-moment-arm vibrational degrees of freedom and is therefore probably not specific to our choice of model system: The lowest frequency (largest amplitude) cases probably dissipate energy too quickly and the higher frequency (more slowly relaxing) cases most likely have solvent displacements too small to generate significant nonlinearities in simple nonpolar solvents. Vibrational kinetic energy relaxation, in particular, seems to be especially and surprisingly linear. [ABSTRACT FROM AUTHOR]

We present an approach for calculating nonlinear spectroscopic observables, which overcomes the approximations inherent to current phenomenological models without requiring the computational cost of performing molecular dynamics simulations. The trajectory mapping method uses the semi-classical approximation to linear and nonlinear response functions, and calculates spectra from trajectories of the system's transition frequencies and transition dipole moments. It rests on identifying dynamical variables important to the problem, treating the dynamics of these variables stochastically, and then generating correlated trajectories of spectroscopic quantities by mapping from the dynamical variables. This approach allows one to describe non-Gaussian dynamics, correlated dynamics between variables of the system, and nonlinear relationships between spectroscopic variables of the system and the bath such as non-Condon effects. We illustrate the approach by applying it to three examples that are often not adequately treated by existing analytical models - the non-Condon effect in the nonlinear infrared spectra of water, non-Gaussian dynamics inherent to strongly hydrogen bonded systems, and chemical exchange processes in barrier crossing reactions. The methods described are generally applicable to nonlinear spectroscopy throughout the optical, infrared and terahertz regions. [ABSTRACT FROM AUTHOR]

We report on the ion-pair state emission spectra of Iodine isolated in Neon, Argon and Krypton matrices in the spectral range from 300 nm to 700 nm, and present VUV-absorption spectra in the 50 000–60 000 cm-1 region. Spectroscopic analysis of the UV and visible emission bands of the lowest-lying ion-pair state D[sup ′](2[sub g]) in Ne and Ar provides unambiguous evidence for both vertical (energy) and horizontal shifts (equilibrium distance) due to solvation in a rare gas matrix, and suggests that the ion-pair states maintain their distinct character. In Krypton the previously reported broad emission band at 420 nm is shown to consist of two narrow bands, which have counterparts in the visible. They are assigned to emission from D[sup ′](2[sub g]) and a second ion-pair states of I[sub 2], most probably the δ(2[sub u]) state. Partial charge delocalization to the matrix can only play a minor role in Kr since very similar emission is also found for secondary sites in Ar. These findings should be highly relevant for a description of ion-pair states in nonpolar media. They are also useful for the interpretation of time-resolved experiments that use the ion-pair state emission as probe signal. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The phase separation dynamics of a binary fluid containing randomly distributed fixed impurities is studied in two dimensions (d=2). The impurities act as osmotic force centers and favor one component of the fluid. We found, as expected, that hydrodynamic flow promotes the coalescence of the domains in the early stage of phase separation; at later stages for sufficiently high particle density and strong preferential interaction strength, the domain growth slows down and finally is pinned at a finite domain size independent of the hydrodynamics. The density of impurities in the unfavorable phase is shown to satisfy a scaling form involving the total impurity density n[sub 0] and the ratio R/R[sub 0] with R the domain size and R[sub 0]=n[sub 0][sup -1/d] the average distance between the impurities. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Adiabatic rigid-body diffusion Monte Carlo is used to study the structure and spectroscopy of complexes of OH(A˜ [sup 2]Σ[sup +]) with several neon atoms. Although the potential energy surfaces for these systems have many low-lying minima, the ground state wave functions are localized in the global minimum. This trend is found to persist in the first few vibrationally excited states of Ne[sub 2]OH/D. Low-lying vibrational states that are localized in the potential minimum that corresponds to the linear Ne-OH/D-Ne configuration of the complex are also found. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The relationship between the bulk density and pressure of configurations corresponding to local minima on the potential energy surface of molecular models of ethane, n-pentane, and cyclopentane (the equation of state of their energy landscape) has been explored. Like simpler, atomic fluids, these systems exhibit a limiting bulk density below which minimum energy configurations are no longer spatially homogeneous, but consist instead of a locally dense fraction and large, system-spanning voids. In the case of n-pentane, the sampling of the minima on the energy landscape was found to depend strongly on temperature, due to changing Boltzmann factors associated with the different conformers in the liquid. The pressures of the minimum energy configurations, in contrast, were found to be essentially independent of the liquid temperature in all cases. The highest amount of isotropic tension (negative pressure) that minimum energy configurations can sustain is reached at the limiting densities, and is of similar magnitude (approximately 250 MPa) for all three model substances. Crystalline configurations of ethane and n-pentane, in contrast, were found to exhibit higher isotropic tensile strength than their amorphous counterparts. A pronounced segregation of end groups on the boundary of large voids was observed in the minimum energy configurations of low bulk density pentane. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B([sup 3]Π[sub 0u]) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X([sup 1]Σ[sub g]) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm-1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman packet. This momentum filter, which should be ubiquitous in condensed media, leads to a signal with deeper modulation than in the gas phase. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study the characteristics of vibronic eigenfunctions of a multidimensional nonadiabatic system and their consequences in the quantum spectra. As an illustrative example, we investigate the properties of highly excited eigenfunctions of Heller's multimode nonadiabatic system. The system consists of two diabatic states and two-dimensional (two-mode) harmonic potentials that are nonadiabatically coupled with the Condon approximation and with an appropriate magnitude of the Duschinsky angle. "Quantum chaos" thus produced has no classical counterpart. In addition to rather characterless chaotic eigenfunctions that are uniformly widespread in configuration space, we have found highly excited localized eigenfunctions of two extreme types which favor either the diabatic picture or adiabatic picture. As a result, the features of the associated quantum spectra are strongly affected by the initial preparation of a wave packet. This finding suggests that one can control the rate of nonadiabatic transitions such as that for electron transfer by using laser techniques or by choosing appropriate solvents. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A wave packet prepared on the 1 [sup 1]B[sub 1] potential-energy surface of cis-1,3,5-hexatriene (CHT) is characterized by a very short lifetime of approx. 20 fs in this state. We present here model calculations of the excited-state dynamics of CHT that are consistent with the experimentally determined population decay time scale and yield an accurate description of the absorption, preresonance and resonance Raman (RR) spectroscopy of the 1 [sup 1]B[sub 1] state. The greater diffuseness and complexity of the free jet 1 [sup 1]B[sub 1] absorption band of CHT as compared to the 1 [sup 1]A[sub g]→1 [sup 1]B[sub u] transition of trans-1,3,5-hexatriene can be explained by a faster optical dephasing rate and more densely spaced vibronic level structure in the S[sub 2] state of the cis isomer primarily due to the presence of two very active low-frequency S[sub 1]-S[sub 2] coupling modes, ν[sub 30] and ν[sub 31]. The first measurement of the one-photon 1 [sup 1]A[sub 1]→2 [sup 1]A[sub 1] transition of CHT has been reported only ten years ago and the S[sub 1] state has since been thoroughly studied by different techniques. The simulations of the excitation and RR emission profiles of the 2 [sup 1]A[sub 1] state performed for this work are shown to be in quantitative agreement with the observed spectra. One of the most important and controversial questions arising from the spectroscopic information about the 2 [sup 1]A[sub 1] state concerns the nature of the intensity carrier for the one-photon S[sub 0]→S[sub 1] excitation process. It can be shown that the oscillator strength for one-photon transitions into the 2 [sup 1]A[sub 1] vibronic manifold is exclusively borrowed from the electronic 1 [sup 1]B[sub 1] configuration. One model Hamiltonian is defined for the representation of wave packet motion in the 1 [sup 1]A[sub 1], 2 [sup 1]A[sub 1], and 1 [sup 1]B[sub 1] states and the nuclear coordinate space comprises eight dimensions. The relevant n... [ABSTRACT FROM AUTHOR]

High-resolution helium nanodroplet isolation spectra of the first overtone (2ν[sub 1]) of the acetylenic stretch of several substituted acetylenes (RC≡C-H) at T=0.38 K, have been observed for the first time. A tunable 1.5 μm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide (HCN), monodeuteroacetylene (DCCH), cyanoacetylene (NCCCH), propyne (CH[sub 3]CCH), trifluoropropyne (CF[sub 3]CCH), 3,3-dimethylbutyne ((CH[sub 3])[sub 3]CCCH), and trimethylsilylacetylene ((CH[sub 3])[sub 3]SiCCH) have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (ν[sub 1]) transitions observed in a helium droplet by Nauta et al. [Faraday Discuss. Chem. Soc. 113, 261 (1999) and references therein]. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A hydrogen-bonded complex of OH with CO is identified along the reaction coordinate for the OH+CO↔HOCO→H+CO[sub 2] reaction. The existence of this linear OH-CO complex is established by infrared action spectroscopy, which accesses vibrational stretching and bending modes of the complex. Complementary electronic structure calculations characterize the OH-CO and OH-OC complexes, the transition state for HOCO formation, and the reaction pathways that connect these complexes directly to the HOCO intermediate. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper presents a new approach to the study of quantum dynamics in condensed phases. The methodology is comprised of two main components. First, a formally exact method is described which allows the description of the liquid as a collection of coupled (through kinetic and potential coupling) harmonic modes. The modes are related to the Fourier modes of the component particle densities. Once the modes have been defined, a canonical transformation from the standard classical interparticle Hamilton function describes a new Hamilton function, which is exactly equivalent and defined on these harmonic coordinates. The final step in this section is the transformation of this Hamilton function into a quantum Hamiltonian operator. The second step in the process is the derivation of a new quantum mechanical evolution operator which is exact and allows the correction from a reference evolution operator, which is formed by adiabatic evolution on an approximate potential. A particular approximate potential which we suggest will be useful, is the collection of harmonic modes given in the Zwanzig Hamiltonian, weighted by the spectral density. Application of the reference interaction propagator methodology can then correct the approximate adiabatic evolution operator based on the approximate potential to the exact Hamiltonian of Fourier modes described above. A test problem of a double well nonlinearly coupled to a harmonic oscillator shows that the methodology obtains rapid numerical convergence. The paper closes with a description of how the methodology would be applied to a many-dimensional (hundreds of degrees of freedom) picture of reaction in a condensed phase. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper describes a simple model for state resolved associative desorption, and by assuming detailed balance, of activated dissociative chemisorption. The equations that result from this model are identical to those traditionally used empirically in fits to experimental data. This allows better understanding of parameters used to fit experiments and how to compare these with theory. The role of model parameters in describing activated dissociation when the barrier is almost exclusively along a vibrational coordinate is also examined. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The centroid molecular dynamics technique is applied to the case of chloride-water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J. R. Reimers, R. O. Watts, and M. L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride-water interaction potential of Dorsett, Watts and Xantheas [J. Phys. Chem. A 103, 3351 (1999)]. We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the red shift associated with hydrogen bonding. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Transport of molecules through microporous crystals can be influenced by transport limitations at the mouths of pores. In the limit of high temperatures, the kinetics of adsorption into micropores is dominated by steric effects. To assess the role of molecular shape and pore size on pore mouth adsorption kinetics, discontinuous molecular dynamics has been used to simulate a variety of hard-sphere molecules adsorbing into slit, circular, and elliptical pores. Particular attention has been given to pores with length scales similar to zeolites. By simulating a range of straight chain, branched, and cyclic molecules, the selectivities that can arise due to differences in adsorption kinetics as functions of molecular size and shape have been probed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The A˜ continuum photoexcitation of ICN in an Ar matrix is studied using an implementation of the molecular dynamics with quantum transitions method of Tully. Five excited electronic potential energy surfaces of the ICN molecule, [sup 3]Π[sub 0+], [sup 1]Π[sub 1](A[sup ′],A[sup ″]), [sup 3]Π[sub 1](A[sup ′],A[sup ″]), as well as its ground state, are included in these calculations. The couplings between electronic states at large I-CN internuclear distances are modeled using a diatomic in molecules treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. The electronic motion, as well as the I-CN distance and the corresponding bending angle, are treated quantum mechanically using wave-packet techniques. The rotation and translation of the ICN molecule in the Ar cage are treated classically, as well as the motion of the Ar atoms. In contrast with previous calculations, in which all nuclear degrees of freedom were treated classically, we found a 2% of CN cage exit during the first 0.5 ps of the dynamics. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]