Volume 109, Issue 12, 22 September 1998
 COMMUNICATIONS


Velocity map imaging studies of the Lyman α photodissociation mechanism for H atom production from hydrocarbons
View Description Hide DescriptionH atoms produced in Lyman α photolysis of ethane, propane, and ethylene have been studied using velocity map imaging techniques. Two types of H atoms are identified, one formed along with an alkyl radical in the Rydberg state and the other by the subsequent decomposition of this Rydberg radical.
 Top

 THEORETICAL METHODS AND ALGORITHMS


Do bond functions help for the calculation of accurate bond energies?
View Description Hide DescriptionThe bond energies of eight chemically bound diatomics are computed using several basis sets with and without bond functions (BF). The bond energies obtained using the basis sets (with a correction for basis set superposition error, BSSE) tend to be slightly smaller that the results obtained using the basis sets, but slightly larger than the BSSE corrected results. The and basis sets yield reasonable estimates of bond energies, but, in most cases, these results cannot be considered highly accurate. Extrapolation of the results obtained with basis sets including bond functions appears to be inferior to the results obtained by extrapolation using atomcentered basis sets. Therefore bond functions do not appear to offer a path for obtaining highly accurate results for chemically bound systems at a lower computational cost than atom centered basis sets.

Model studies of nonadiabatic dynamics
View Description Hide DescriptionMixed quantumclassical methods are applied to an increasingly challenging series of model problems, and their accuracy is examined. The models involve one light and one heavy degree of freedom, and exhibit substantial nonadiabatic behavior. In all of the models the coupling between the light and heavy particles is linear (harmonic). In addition, different external potentials are applied to the heavy particle only. The energies of the light particle quantum states, as a function of the position of the heavy particle, define a sequence of ground and excited Born–Oppenheimer potential energy curves. Because the light particle experiences a purely harmonic potential, the potential energy curves are parallel and equally spaced for all of the models. In addition, the nonadiabatic couplings among potential energy curves persist for all times due to the nonvanishing linear coupling between light and heavy particles. The model problems were used to test two strategies for carrying out mixed quantumclassical dynamics in systems involving nonadiabatic transitions: mean field and surface hopping. The model calculations reported here suggest that, in cases where linear couplings dominate, the mean field mixed quantumclassical method displays useful accuracy and is robust to the introduction of anharmonic heavyparticle interactions. The model calculations also reveal special situations in which the surface hopping approximation is inappropriate.

Model calculations of resonant vibration to vibration transition probabilities in clusters
View Description Hide DescriptionResults are presented for the calculated probability for resonant transfer of vibrational excitation energy in clusters of identical molecules. The calculations are performed for twodimensional clusters in order to allow for calculations on larger clusters.Clusters of 5, 10, and 20 molecules are considered. The probability of resonant transfer is calculated by quantum mechanically propagating the wave function for the vibrational degrees of freedom of the molecules in the cluster, while the rotational and translational degrees of freedom evolve along classical trajectories. The transition probabilities are averaged over a canonical distribution of initial phase space points for the trajectories. If the probability for the transition of a vibrational quantum of energy from one molecule to some other molecule in an N molecule cluster is evaluated as independent two molecule vibrational problems, the results are found to be in excellent agreement with those obtained from the propagation of the full N molecule vibrational problem. The results also show that quantum coherenceeffects result in an early time nonlinear behavior in the transition probabilities that persists for several picoseconds in these systems.

A nonDyson thirdorder approximation scheme for the electron propagator
View Description Hide DescriptionAn efficient thirdorder propagator method to compute ionization potentials and electron affinities of atoms and molecules is presented. The development is based on the algebraic diagrammatic construction (ADC) representing a specific reformulation of the diagrammatic perturbation series of the electron propagator In contrast with previous approximation schemes, relying on the Dyson equation and approximations for the selfenergy part, the ADC procedure here is applied directly to the electron parts and respectively, of the electron propagator. This leads to decoupled secular equations for the ionization energies electron part) and electron affinitieselectron part), respectively. In comparison with the Dysontype approach, there is a substantial reduction of the secular matrix dimension opposed by a small additional expense in computing some second and thirdorder contributions to the secular matrix elements. The relationship of the nonDyson ADC(3) method to coupled cluster methods is outlined.

Polarizabilities of CO, HF, Ne, BH, and from ab initio calculations: Systematic studies of electron correlation, basis set errors, and vibrational contributions
View Description Hide DescriptionThe dipolepolarizabilities of CO, HF, Ne, BH, and have been investigated using coupledcluster methods. An extensive basis set study has been carried out and electron correlation effects have been studied employing a hierarchy of coupledcluster models. Vibrationalaveraged electronic polarizabilities as well as the pure vibrational polarizabilities have been calculated based on ab initio energy and property curves. The frequencydependency of the vibrationallyaveraged electronic polarizabilities and vibrational polarizabilities are discussed. The final results are compared with available experimental data.
 Top

 GAS PHASE DYNAMICS AND STRUCTURE: SPECTROSCOPY, MOLECULAR INTERACTIONS, SCATTERING, AND PHOTOCHEMISTRY


Methyl iodide band decomposition study by photofragment velocity imaging
View Description Hide DescriptionThe methyl iodide Aband photodissociation process has been studied in a cold molecular beam. Full threedimensional statespecific speed and angular distributions of the nascent fragments were recorded using (2+1) resonanceenhanced multiphoton ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the quantum yield and anisotropy parameters for both I and channels, the relative absorption strength to the contributing electronic states and as well as the probability for curve crossing are determined for excitation wavelengths across the full A band (240–334 nm). Parallel excitation to the state turns out to dominate the A band even more than previously thought.

Origin of the complex dynamics in structural isomerization of small clusters: The effects of potential topography
View Description Hide DescriptionThe dependence of lifetime distribution in isomerizationdynamics of like clusters on the potential topography is reported. Using the scaled Morse potential where is an internuclear distance, and the equilibrium distance of the isolated diatomic molecule, is the only independent parameter that uniquely specifies the system Hamiltonian, we have examined the dynamics for two typical values of In the high enough energy region, which is called the liquidlike phase, the cluster of a small (called a compact cluster) has an exponential form for the lifetime distribution. In contrast, dynamics on the potential with a large (a loose cluster) exhibits a large deviation from the exponential form, its mixing dynamics notwithstanding; it displays a large hole in the short lifetime region and a steep clifflike structure in the even shorter lifetime region embedded in the hole. Since the deviation is so clear, the present system offers an excellent example with which to investigate the dependence of phase space structure on the potential topography. We have identified the mechanisms of formation both of the hole and cliff. The hole is particularly important to understand how mixing dynamics can deviate from the statistically simple exponential distribution. By comparing a variety of quantities that characterize the system dynamics, we propose a conceptual picture to understand the basic difference between the dynamics on compact and loose potentials, in which the complexity of the dynamics is ascribed to the bifurcation of the reaction tubes and to the turningpoint distributions clinging to the branching places of such bifurcated tubes.

Degenerate four wave mixing of pyridazine from a slit nozzle
View Description Hide DescriptionUsing a pulsed supersonic slit nozzle, the nonfluorescing transition of pyridazine was investigated. The degenerate four wave mixing(DFWM) spectra showed numerous vibrational bands over a 1200 cm^{−1} region. Most of these bands were parallel transitions with a strong Q branch and weaker but observable P and R branches. Based on our previous model [H. Li and W. Kong, J. Chem. Phys. 107, 3774 (1997)], these transitions were simulated with success. The polarization dependence of the rotational branching ratios suggested that primary contributions to the DFWM signal were from large spaced gratings formed by ground state molecules. The lack of contributions from excited stategratings and small spaced gratings was attributed to the fast internal conversion process on the surface of pyridazine (0.3–3 ns), the washout time due to movements of the sample in a molecular beam, and the duration time of the excitation laser (7 ns). Two vibrational bands showed unexpected enhancement in the P or R branch, but for each band, one adjustment factor was sufficient to reproduce the spectra recorded under all different polarization combinations. Perturbations were observable from the rotationally resolved spectra, however in most cases, rotational progressions did not seem to be affected by the perturbation in terms of both line positions and intensities. A more detailed analysis of the supersonically cooled spectra, together with data from a room temperature gas cell and ab initio calculations, will be necessary to completely interpret the spectroscopy of pyridazine. This paper demonstrates that with the increased sensitivity achievable through a slit nozzle, DFWM is an effective technique for detailed spectroscopic studies, particularly for nonfluorescing species.

Femtosecond dissociation dynamics of methyl iodide clusters
View Description Hide DescriptionThe photodissociationdynamics of methyl iodide clusters using nm as pump and nm as probe are studied using a femtosecond two color pump–probe laser arrangement combined with a reflectron timeofflight (RTOF) mass spectrometer. This enables the state and Rydberg state of methyl iodide to be accessed with the pump beam. Of particular interest is a comparison of the femtosecond dynamics of the methyl iodide monomer with the clustered species. Clocking of the monomerdissociation shows a transient which is indicative of a fast C–I bond breakage as is to be expected upon excitation of methyl iodide into the fast dissociating state, or into the predissociative Rydberg state. Clusters, however, show a very different pump–probe transient composed of a fast decay and a subsequent dip in ion signal followed by a rise for pump–probe delay times greater than 2 ps. The cluster ion signal shows an enhancement for pump probe delay times up to 70 ps. The results are interpreted in terms of the electronic state diagram of the methyl iodide monomer and effects resulting from clustering of these species, shifts of electronic energy levels and caging of excited species in the cluster.

Ultrahighresolution spectroscopy of the transition of transglyoxal
View Description Hide DescriptionDopplerfree twophoton absorptionspectra of the transition of glyoxal and the effects of magnetic fields up to 14 kG are measured. The absolute energies of transition lines are measured with accuracy better than , and is determined to be . From the analysis of the perturbed and perturbing levels, the hyperfine constant of the deperturbed level is determined to be 88 MHz. The magnetic moment of the level is determined to be 0.86 from the magnitude of Zeeman splitting of a hyperfine component. From an analysis of the Zeeman splittings of unperturbed levels, which are transitions to the state and where no appreciable energy shifts are observed, the state is shown to be slightly mixed into the state: . The perturbation between the and levels is shown to occur through the vibronic interaction between the mixed and the states. The perturbation is appreciable if levels of the and states are accidentally close in energy. The energy spacing between levels of the and states changes with the magnetic field, and the resulting changes of the perturbation are observed. The mechanism of the intersystem crossing of a chosen single level is clarified in this study.

Reactive and nonreactive scattering of and from and
View Description Hide DescriptionDouble differential cross sections of sodium in both the ground state and the electronically excited state scattered from and have been measured at center of mass collision energies of 0.6 and 2.0 eV for and 1.0 eV for . For the experiments with a two frequency excitation scheme is applied, by which an excitation efficiency (determined by scattering experiments) of 31% is obtained. In the experiments, one finds a substantial transfer of the collision energy into the vibrational and rotational degrees of freedom of . A transfer of electronic energy of into is not observed in nonreactive collisions. For the NaF formed in the reaction, an angular distribution originating from a longlived complex, superimposed by a forward peak caused by a direct reaction mechanism is found. Especially the ground statemeasurements show an angulardependent velocity distribution of the product. Compared to forward and backward scattering in sideways collisions, a lower velocity is found. From to the general characteristic does not change. The gain of reactivity with the excitation is decreasing with the collision energy. For the experiments with and a quenching channel is found. The energy transferred to is in good agreement with the prior distribution, calculated in a simplified RRHO model. The statistical behavior and the shape of the angular distribution of the intensity suggest the interpretation by the formation of a longlived collisional complex.

Theoretical study of the reactions of and using the trajectory surface hopping method
View Description Hide DescriptionTrajectory surface hopping calculations have been carried out for collisions of and on three lowlying potential energy surfaces projected from the original six in the Kuntz and Roach diatomics in molecules surface for this system. The location and probability of hops between surfaces were determined using the new algorithm developed by Parlant and Gislason. In addition to the reactive channel and total charge transfer to and dissociative channels to, for example, and have been studied. Particular attention was paid to the dissociativecharge transfer isotope effect for the processes or near threshold the product is favored over which we attribute to preferential dissociation of excited products. This is the first theoretical study of these dissociation processes.

Internal rotation and Stark effect in
View Description Hide DescriptionThe avoidedcrossing molecularbeam method has been applied to in the ground torsional state Three “rotational” anticrossings have been measured corresponding to normally forbidden transitions in which both the rotational and leading torsional energy terms change. Each torsional sublevel with and given torsion–rotation symmetry Γ undergoes an avoided crossing with its counterpart with and the same Γ. Four “barrier” anticrossings have been measured corresponding again to normally forbidden transitions, but in which only the torsional energy changes. These transitions are and for and 2. From these seven zerofield splittings and nine existing branch microwave frequencies for nine torsion–rotation parameters have been determined including the effective rotational constant and the effective height of the barrier to internal rotation For each anticrossing studied, an estimate has been made of the contribution to the zerofield splitting from the nuclear hyperfine interactions. For and barrier anticrossings have been previously investigated. For each of these anticrossings, estimates of are made here as well. For all cases studied (including those for it is found that For by using conventional electricresonance molecularbeam methods, the electric dipole moment has been determined to an accuracy of for each of the rotational states and

Quantum mechanical simulations of inelastic scattering in collisions of large clusters:
View Description Hide DescriptionAn approach based on the TimeDependent SelfConsistent Field (TDSCF) is used to carry out quantum calculations of inelastic atom scattering from large, highly anharmonic clusters. The computation is carried out for lowenergy collisions of Ar with and all the vibrational modes of the cluster are included. The method treats the collider atom classically, but the dynamics of the interacting anharmonic modes of is handled quantum mechanically. The results provide insight into the collision physics of large systems having soft anharmonic modes, and into the role of quantum effects in such cases. The main findings are the following: (a) Large differences are found between quantum and classical results with regard to energy transfer into specific cluster modes. (b) Classical calculations wrongly predict efficient excitation of many stiff modes, including processes that are quantummechanically forbidden. (c) Single quantum excitations are the most important transitions at the collision energy used. (d) Atom–atom pair distribution functions of after the collision show insignificant differences from the corresponding precollision distribution functions. The results show that quantum calculations of collision dynamics of lowtemperature anharmonic clusters are feasible, and also necessary in view of the prediction of significant quantum effects.
 Top

 CONDENSED PHASE DYNAMICS, STRUCTURE, AND THERMODYNAMICS: SPECTROSCOPY, REACTIONS, AND RELAXATION


External field control of condensed phase reactions
View Description Hide DescriptionControl of tunneling in a symmetric double well coupled to a bath via an external field is demonstrated. Optimal control theory is employed to design a laser field which couples to the reaction coordinate and drives a localized wave packet on the reactant side to the product side at a specified target time τ. Both a very quantumlike system (high barrier) and a low barrier double well are examined; excellent results are obtained for a range of reduced bath viscosities.Analysis of the control fields and corresponding localization dynamics shows that the frequency components of the control field are more or less in resonance with eigenstates of the double well and that the laser field enhances the natural dynamics of the individual wells. Future extension to more complicated models where the field couples to the bath is discussed.

Calculation of the aqueous solvation free energy of the proton
View Description Hide DescriptionThe value of the proton hydration free energy, has been quoted in the literature to be from −252.6 to −262.5 kcal/mol. In this article, we present a theoretical model for calculating the hydration free energy of ions in aqueous solvent and use this model to calculate the proton hydration free energy, in an effort to resolve the uncertainty concerning its exact value. In the model we define as the free energy change associated with the following process: where the solvent is represented by a neutral nwater cluster embedded in a dielectric continuum and the solvated proton is represented by a protonated nwater cluster also in the continuum. All solvated species are treated as quantum mechanical solutes coupled to a dielectric continuum using a self consistent reaction field cycle. We investigated the behavior of as the number of explicit waters of hydration is increased from to As n increases from 1 to 3, the hydration free energy decreases dramatically. However, for the hydration free energy maintains a relatively constant value of −262.23 kcal/mol. These results indicate that the first hydration shell of the proton is composed of at least four water molecules. The constant value of the hydration free energy for strongly suggests that the proton hydration free energy is at the far lower end of the range of values that have been proposed in the literature.

On the local and nonlocal components of solvation thermodynamics and their relation to solvation shell models
View Description Hide DescriptionA procedure for identifying the local and nonlocal components of excess partial molar quantities is presented. This procedure is based on an analysis of the excess thermodynamic quantities expressed as integrals of local estimators over the system volume. The local components can be described in terms of the solvation shell model and are independent of the condition of solute insertion. The nonlocal components are related to the asymptotic behavior of distribution functions at large distances from the solute and they give rise to the ensemble dependence of the thermodynamic quantities of solvation. It is shown that in the constant pressure process, by using natural choices for the local estimators, the nonlocal components of the excess partial molar energy, enthalpy,entropy, and volume are absent. The relationship between our analysis and a phenomenological model which expresses excess thermodynamic quantities in terms of the solute volume, surface area, and diameter is also considered and the validity of the local interpretation is discussed.

A molecular dynamics study of sub and supercritical water using a polarizable potential model
View Description Hide DescriptionA series of molecular dynamics calculations for water has been carried out along an isochore at and an isotherm at 600 K in order to examine microscopic properties of water in the sub and supercritical states. A polarizable potential model proposed by Dang (RPOL model) was employed to take into account the state dependence of intermolecular interaction. Along the isochore, fluid structure changes from tetrahedral icelike structure at room temperature to simpleliquidlike one at high temperatures. Orientational correlation between a tagged molecule and its neighbors is reduced substantially with increasing temperature, though hydrogen bonds between two molecules persist even at 600 K. As temperature increases, the number of the hydrogen bonds per molecule decreases monotonically from 3.2 at 280 K to 1.9 at 600 K. The activation barrier for diffusion at 600 K is about half as large as that at room temperature. A collective polarization relaxation loses collective character above the temperature where the structural change occurs. Along the isotherm, on the other hand, the longranged tail of radial distribution functions was observed near the critical density OrnsteinZernike behavior, however, was not found owing to the present small system. The number of hydrogen bonds decreases almost linearly as a function of the density from 1.9 at to 0 in the gas limit. However, the hydrogen bonds were still found near the critical density. At densities below density dependence of the diffusion coefficients are qualitatively described by the simple kinetic theory for gases. At higher densities, the diffusion coefficients deviate from the prediction by the kinetic theory. Rotational correlation function at low density has the form similar to free rotors, while at high densities, the rotational relaxation may be described by rotational diffusion. It indicates that the rotational dynamics changes continuously around the critical density from a gaslike one to a liquidlike one.

Criticalpoint of the LennardJones fluid: A finitesize scaling study
View Description Hide DescriptionMonte Carlo simulations of the full, i.e. nontruncated, LennardJones fluid in the critical region are reported. The simulations are performed within the grand canonical ensemble in conjunction with hyperspherical boundary conditions in order to take account of the algebraic attractive part of the pair potential. Using mixedfield finite size scaling analysis and with the assumption of Ising criticality the critical temperature is estimated to be and the critical density Precised estimates of the critical energy per unit volume, pressure, chemical potential, and mixedfield parameters are also reported. The values obtained for the renormalization exponents and are compatible with the Ising values within the error bars.
