Volume 110, Issue 7, 15 February 1999
 COMMUNICATIONS


Concept of interbasin mixing and extension of the Lyapunov exponent in multiple potentialbasin dynamics as in structural isomerization of clusters
View Description Hide DescriptionFor Hamilton dynamics on a potential that has multiple local basins as in structural isomerizationreaction of clusters, a notion of interbasin mixing is introduced that is responsible for Markovtype stochastic appearance of molecular structures. An extension of the Lyapunov exponent to quantify the time scale to reach interbasin mixing is proposed. The present dynamics also serves as a prototype of multichannel chemical reactions.
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 THEORETICAL METHODS AND ALGORITHMS


Can Monte Carlo simulation describe dynamics? A test on LennardJones systems
View Description Hide DescriptionA new estimation of physical time in Monte Carlo simulations is derived from the requirement that the selfdiffusion coefficient measured by Monte Carlo and molecular dynamics simulations have the same value. The dynamics of the particles using both simulation methods are compared by measuringvelocity autocorrelation functions. Simulations of pure LennardJones liquids and a binary LennardJones solution show that at small time scales the particle dynamics are different, but at larger time scales they become similar. As a critical test crystal growth from the melt is simulated using the proposed time scale. Both for Monte Carlo and molecular dynamics a linear dependence of the growth rate on undercooling is found and the measured proportionality constant (the kinetic coefficient) is equal to within 6%, i.e., within the statistical error of both methods.

Molecular dynamics algorithms for path integrals at constant pressure
View Description Hide DescriptionExtended system path integral molecular dynamics algorithms have been developed that can generate efficiently averages in the quantum mechanical canonical ensemble [M. E. Tuckerman, B. J. Berne, G. J. Martyna, and M. L. Klein, J. Chem. Phys. 99, 2796 (1993)]. Here, the corresponding extended system path integral molecular dynamics algorithms appropriate to the quantum mechanical isothermal–isobaric ensembles with isotropiconly and full system cell fluctuations are presented. The former ensemble is employed to study fluid systems which do not support shear modes while the latter is employed to study solid systems. The algorithms are constructed by deriving appropriate dynamical equations of motions and developing reversible multiple time step algorithms to integrate the equations numerically. Effective parallelization schemes for distributed memory computers are presented. The new numerical methods are tested on model (a particle in a periodic potential) and realistic (liquid and solid parahydrogen and liquid butane) systems. In addition, the methodology is extended to treat the path integral centroid dynamics scheme, [J. Cao and G. A. Voth, J. Chem. Phys. 99, 10070 (1993)], a novel method which is capable of generating semiclassical approximations to quantum mechanical time correlation functions.

Reversible molecular dynamics for rigid bodies and hybrid Monte Carlo
View Description Hide DescriptionA timereversible molecular dynamics algorithm is presented for rigid bodies in the quarternion representation. The algorithm is developed on the basis of the Trotter factorization scheme, and its structure is similar to that of the velocity Verlet algorithm. When the rigid body is an asymmetric top, its computationally inconvenient Eulerian equation of motion is integrated by combining the computationally convenient solutions to the Eulerian equations of motion for two symmetric tops. It is shown that a larger time step is allowed in the timereversible algorithm than in the Gear predictor–corrector algorithm. The efficiency of the hybrid Monte Carlo method for a molecular system is also examined using the timereversible molecular dynamics algorithm in the quarternion representation.

Unambiguous formalism of molecular vibrations: Use of redundant coordinates and canonical matrices
View Description Hide DescriptionThis paper introduces a new approach to the problem of describing molecular vibrations in redundant valence coordinates. A consistent definition for canonical force field, which is different of that by Kuczera and Pupyshev et al., is outlined. This new definition is also generalized to other matrices like kineticenergy matrices and transformation matrices between different sets of vibrational coordinates. The importance of such canonical matrices follows from the fact that they are uniquely determined and allow us to consider the redundant coordinates as though they were independent.
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 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Photodissociation of OClO and Ar/OClO and clusters studied by the resonance enhanced multiphoton ionizationtime of flight method
View Description Hide DescriptionThe photodissociation of jetcooled OClO following excitation into the state at around 350 nm was investigated in homogeneous OClO and large heterogeneous Ar/OClO and clusters (estimated cluster size by probing the O and ClO photofragments using the resonance enhanced multiphoton ionizationtime of flight technique. Action spectra, photofragment excitation spectra and photofragment speed distributions were recorded and compared to those for monomerdissociation. OClO was found to occupy both surface and interior sites in the heterogeneous clusters with the percentage of surface and interior dissociation processes being for large cluster sizes. Both O and ClO photofragments generated in the cluster interior are translationally thermalized with K and the ClO fragments are strongly rotationally and vibrationally relaxed. This is most important for vibration as monomerdissociation yields ClO containing up to 8 vibrational quanta at this photolysis wavelength. Photodissociation on the cluster surface is found to proceed with little interaction with the cluster host. The distribution of counterfragment masses leads to a broadening of the speed distributions compared with monomerdissociation. In addition, cluster chemistry was found to occur in OClOrich heterogeneous clusters as manifested by detection of O photofragments with velocities exceeding the highest thermodynamically possible value. This result, consistent with that from homogeneous OClO cluster dissociation, indicates the presence of small OClO aggregates on the surface and within heterogeneous clusters. From a standpoint of atmospheric chemistry, clusters yield a substantial fraction of thermalized primary photofragments, in contrast to OClO monomerdissociation.

Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products
View Description Hide DescriptionVacuum ultraviolet (VUV) laser photoionization is combined with timeofflight(TOF)mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce and respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm^{2} and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.

Quantum dynamics of overtone relaxation in 30mode benzene: A timedependent local mode analysis for
View Description Hide DescriptionWe report the results of a detailed quantum dynamical investigation of energy flow in 30mode benzene. The overtone was examined by propagating an initial bright state in an active space of dimension for 9.6 ps. Two sets of initial conditions were investigated: one in which the overtone energy was distributed as in one of the symmetries appropriate for dipole excitation from the ground state and the other in which the overtone energy was localized in a single CH stretching motion. The rate and extent of energy flow out of the CH chromophore is dictated by the choice of initial conditions. Conclusions from the model where energy is localized in a single CH stretch indicate that the meta inplane CH wags and para outofplane CH bend are the major recipients of bursts of energy for the ipso CH stretch retains only 10% of the overtone energy by 9.6 ps. For the model, the initially excited CH stretches retain 30% of the overtone energy in long time; the ipso outofplane CH bend is activated before the ipso inplane CH wag and with less energy, mimicing the results of previous classical investigations. For both models examined, energy is equipartitioned within the CC stretches, inplane CH wags, and outofplane CH bends by 9.6 ps; the magnitude of energy taken up by the CC stretches and inplane CH wags is the same. CCC inplane bends and CCCC outofplane ring torsions are of relatively minor importance (on the time scale considered) in intramolecular vibrational energy redistribution in benzene.

Single and multiple collision effects observed in the femtosecond spectroscopy of rare gas collision complexes: a statistical description
View Description Hide DescriptionWe use a simple statistical model to simulate recent femtosecondexperiments on the caging dynamics of in high pressure rare gas environments [Wan et al., J. Chem. Phys. 106, 4353 (1997)]. The collisioninduced bound state motion in and its appearance in the pump/probe signal is perfectly reproduced in our calculation. In the case where He is used as the collision partner we find multiple collisions to be important. The numerical results can be explained analytically under simplified assumptions.

Photofragment angular momentum distributions in the molecular frame: Determination and interpretation
View Description Hide DescriptionPhotolysis of a molecule typically yields openshell photofragments having angular momenta. A procedure is described for the measurement of the photofragment angular momentum distribution in terms of polarization parameters which are expressed in the molecular frame and which may be related to the transition dipole matrix elements. The index (p) indicates either a parallel transition (∥), a perpendicular transition (⊥), or a mixed transition (∥,⊥) having both parallel and perpendicular character. This procedure has the advantage that it decouples the angular momentum distributions in the molecular frame from the photofragment angular distributions in the laboratory frame, which gives new insight into the photodissociationdynamics. For cases in which and with linearly polarized photolysis light, the photofragment angular momentum distribution arising from pure parallel transitions can be described with only one parameter, photofragment angular momentum distributions arising from pure perpendicular transitions require only two parameters, and photofragment angular momentum distributions arising from mixed transitions, having both parallel and perpendicular character, can be described with five parameters: the two (coherent) interference terms and in addition to the three incoherent terms mentioned above. We describe procedures for the measurement of the complete angular momentum distribution of stateselected photofragments using laser detection (such as REMPI) and some form of laboratory velocity selection (such as timeofflight mass spectrometry,Dopplerspectroscopy, or ion imaging). The laserdetection probability of a single photofragment is presented in the form where is the angle between the recoil direction and the photolysispolarization, Θ and Φ are the spherical polar angles describing the orientation of the probe polarization about the recoil direction, and β is the spatial anisotropy parameter. The physical significance of the is discussed; in particular, the and describe the photofragment mstate distribution along the recoil direction; the describe how broken cylindrical symmetry in the parent molecule is reflected in the photofragment angular momentum distribution in a plane perpendicular to the recoil direction; and the are related to the asymptotic phase difference associated with the interfering channels, and are thus sensitive to the shapes of the dissociative surfaces.

Measurements of Clatom photofragment angular momentum distributions in the photodissociation of and ICl
View Description Hide DescriptionWe have studied the complete Clatom molecularframe photofragment angular momentum distributions from the photodissociation of and ICl in the 320–560 nm region using timeofflightmass spectroscopy with laser detection. The experimental signals were analyzed using the polarizationparameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of through the state accessed via a parallel transition, yielding Clatom photofragments with polarizations described by the single parameter The second case is the 320 nm dissociation of through the state accessed via a perpendicular transition, yielding Clatom photofragments with polarizations described by the two parameters and The third case is the dissociation of ICl in the 490–560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength region, the polarizations of the resulting Clatom photofragments are completely described by the approximately constant incoherent parameters, and whereas the interference contributions to the polarization, the and oscillate sinusoidally with excitation wavelength in a fashion that is sensitive to the shapes of the dissociative surfaces.

Structural studies of alkali methylidyne radicals: High resolution spectroscopy of NaCH and KCH
View Description Hide DescriptionHigh resolution spectroscopic measurements have been carried out for alkali methylidyne radicals. The pure rotational spectra of NaCH and KCH, along with their deuterium isotopomers, have been recorded in the frequency range 328–529 GHz using millimeter/submillimeter direct absorption techniques. These molecules were created in a dc discharge by the reaction of metal vapor and or These data indicate that KCH and NaCH are linear molecules with ground electronic states arising from a configuration. Spectroscopic constants for KCH and NaCH have been determined from the data, including rotational, spin–spin, and spin–rotation parameters, as well as bond lengths. In comparison with other alkali and transition metalbearing molecules, these results suggest some degree of covalent bonding in the alkali methylidynes, with carbon atom undergoing sp hybridization.

Determination of a methane intermolecular potential model for use in molecular simulations from ab initio calculations
View Description Hide DescriptionThe possibility of obtaining an accurate sitesite potential model suitable for use in molecular dynamics (MD) simulations of methane from ab initio calculations has been explored. Counterpoisecorrected (CPC),supermolecule,ab initio energies at the level were computed for eleven relative orientations of two methane molecules as a function of CC separation distance. CC, CH, and HH interaction parameters in a pairwiseadditive, sitesite potential model for rigid methane molecules were regressed from the ab initio energies, and the resultant model accurately reproduced the ab initio energies. The model suggests that CH attractions are dominant in weakly binding the methane dimer. CPC energies for methane trimers, tetramers, and a pentamer were also calculated at the same level. The results indicate that the mer energy per pair of interactions monotonically converges with increasing but that the assumption of pairwise additivity commonly used in MD simulations is reasonably valid. A limited number of higherlevel calculations using and MP4/augccpVTZ were also performed to investigate the possibility of obtaining the intermolecular potentialmodel from higher accuracy calculations without a substantial increase in computer resources. Results suggest that a Nemethane probe method is not useful in this regard, but that limited, highlevel computations, coupled with more extensive lowerlevel values, may be used to improve the model at minimal cost.

Conformational energy and dynamics of 9ethylfluorene
View Description Hide DescriptionThe excited state and cation ground state of jet cooled 9ethylfluorene have been studied experimentally using resonant enhanced multiphoton ionization and zero electron kinetic energy (ZEKE) photoelectron spectroscopy. The spectroscopy has identified two conformations of the ethyl chain which are labeled symmetric and unsymmetric both of which exist in the supersonic expansion. Density functional quantum chemical calculations are used to calculate the ground state and cation energies of each conformer as well as the barrier to conformer interconversion via a bond rotation. Dynamics on the surface are measured using picosecond and nanosecond ZEKE photoelectron spectroscopy. Fast irreversible vibrational redistribution is measured at energies ⩾990 cm^{−1} and the ZEKE spectra are shown to have a unique signature for each of the two isomers. Picosecond and nanosecond ZEKE spectroscopy are used to search for conformer interconversion but even at the highest energy probed (2648 cm^{−1}) no evidence is seen for a dynamic barrier crossing. Statistical density of states calculations are used to predict the relative populations of each conformer expected as a function of excess energy as well as related Rice–Ramsperger–Kassel–Marcus calculations to predict the expected isomerization rates.

Structure and dynamics of 9ethylfluorene van der Waals complexes
View Description Hide DescriptionThe neutral excited state and the ion ground state of 9ethylfluorene van der Waals complexes have been studied for Resonance enhanced multiphoton ionizationspectroscopy of the state of the argon clusters reveals multiple isomeric structures for each of the cluster sizes studies coupled with the two monomerconformations. The cluster shows three isomers, one of the symmetric 9ethylfluorene and two of the unsymmetric. The clusters has four possible isomers all of which are assigned to a (11) conformation, although each represents a unique structure with different argon binding sites. The cluster collapses down to two dominate isomers, one for each conformation of the parent. Mass analyzed threshold ionization (MATI) spectroscopy was used to investigate the ion, as well as assisting in isomer assignment of the spectrum. IVR and dissociation of the argon complexes have also been studied with MATI spectroscopy.Ab initio calculations are used to determine the binding energy for all conformers and isomers of the complex. These values are in excellent agreement with the experimentally bracketed values, and prove useful in isomer assignments. Redistribution of the 208 cm^{−1} band of the symmetric conformation shows dynamics suggesting interconversion of all isomers to a new unassigned structure. Preferential dissociation of the argon located on the ethyl chain side of the sym9ethylfluorene complex is observed in both isomers.

Neon and argon matrix ESR and theoretical studies of the and radicals
View Description Hide DescriptionElectron spin resonance(ESR) studies are reported for the first time on the various isotopomers of the radical isolated in neon and argon matrices. The radicals were generated in neon matrices by the reaction of laserablated cadmium metal and various methyl precursors, and in argon matrices by xirradiation. The neon matrix values measured were and and estimates were derived for and The argon matrix values measured were and and an estimate was derived for The ESR experimental values for the neon and the argon matrices agree with the reported gasphase value [J. Chem. Phys. 101, 6396 (1994)]. The matrix ESR values show small shifts compared with the gasphase results (5% greater for the neon matrix and 12% greater for the argon matrix). At 4.3 K in the neon matrices, additional ESR lines assigned to tunneling phenomena were observed. The radical geometry obtained from ab initio calculations was consistent with that reported from the various experimental results. Multireference single and double excitation configuration interaction (MRSDCI) calculations of the hyperfine interactions gave values that were consistently below the experimental values of and for H, and MRSDCI calculations for the CdH radical showed an analogous trend.

Photodissociation of Jdependent anisotropy revealed in photofragment images
View Description Hide DescriptionPhotofragment imaging is used to measure the statespecific angular distributions of produced in the ultraviolet photodissociation of REMPI (resonance enhanced multiphoton ionization) through the state is used to ionize individual rotational states of between and produced when is dissociated by light of approximately 203 nm. The anisotropy parameter, β, used to characterize the angular distributions of the ion images, is greater than one for the lower rotational levels probed, and decreases to approximately zero for the highest rotational levels of The positive beta parameter indicates that the products arise primarily from a parallel transition to the state in the parent molecule, and the decrease in β with increasing J of is due to increased bending in the parent during dissociation. The ion images also allow us to assign transitions in a spectroscopically perturbed region of the photofragment REMPI spectrum and to reveal the production of products in the first vibrationally excited state.

The potential energy surface and neardissociation states of He
View Description Hide DescriptionThe potential energy surface for the ground state of He is calculated using ab initio QCISD(T) calculations and a correlationconsistent basis set. The geometries chosen include all combinations of 21 intermolecular distances three HH distances and seven Jacobi angles The final potential is fitted to a functional form that incorporates the correct longrange behavior. Closecoupling calculations of both lowlying and neardissociation vibrationrotation states are carried out. The results are expected to be of assistance in assigning the microwave spectra of He in neardissociation states [Carrington et al., Chem. Phys. Lett. 260, 395 (1996)].

reaction. IV. Rotationally resolved total cross sections
View Description Hide DescriptionThe photochemical reaction has been studied in a crossedbeam experiment with electronic excitation of Cs atoms and laserinduced fluorescence detection of CsH products. The reactive cross section is measured at a collision energy of 0.09 eV: The rotational distributions of CsH products measured on for and on for are close to statistical ones with 16% of products in A quasiclassical 3D trajectory calculation of this reaction yields both rotational and vibrational distributions of CsH products and their angular scattering probability. The reactive cross section measured with excitation is an order of magnitude smaller than the cross section which is reevaluated in this article.
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 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


A new interpretation of the OHstretch spectrum of ice
View Description Hide DescriptionA new interpretation is presented of the vibrational band shapes of the proton disordered ice Ih and Ic in the OH and OD stretch region. The vibrationally excited states are shown to be related to those of a periodic system, whose unit cell is composed of four oscillating dipoles in a tetrahedral arrangement around an Oatom. Analysis of the excitations in this periodic system and of the perturbation induced by disorder is used to provide a detailed assignment of the band shapes of the measured spectra.
