Mark Thachuk

A semiclassical approach to intense-field above-threshold dissociation in the long wavelength limit. II. Conservation principles and coherence in surface hopping

This paper is a companion to our recently published semiclassical formalism for treating time-dependent Hamiltonians [J. Chem. Phys. 105, 4094 (1996)], which was applied to study the dissociation of diatomic ions in intense laser fields. Here two fundamental issues concerning this formalism are discussed in depth: conservation principles and coherence. For time-dependent Hamiltonians, the conservation principle to apply during a trajectory hop depends upon the physical origin of the electronic transition, with total energy conservation and nuclear momentum conservation representing the two limiting cases. It is shown that applying an inappropriate scheme leads to unphysical features in the kinetic energy of the dissociation products. A method is introduced that smoothly bridges the two limiting cases and applies the physically justified conservation scheme at all times. It is also shown that the semiclassical formalism can predict erroneous results if the electronic amplitudes for well-separated hops are ...

Linewidths and shifts of very low temperature CO in He: A challenge for theory or experiment?

The pressure broadening and shifting coefficients for pure rotational transitions of CO in a He bath gas at very low temperatures are calculated from the best available potential energy surfaces, and compared with very recent measurements by M. M. Beaky, T. M. Goyette, and F. C. De Lucia [J. Chem. Phys. 105, 3994 (1996)]. The results obtained for two recent empirical potentials determined from fits to Van der Waals spectra, and for a recent high quality purely ab initio surface, are consistent with one another. The best of the spectroscopic potentials also yields good agreement with high temperature virial coefficients and transport properties. Predictions from all three of these potentials agree with linebroadening and shifting measurements at temperatures above ∼20 K, but are in substantial disagreement with the measurements at temperatures below 4 K. At present, the source of this discrepancy is not known.

A SEMICLASSICAL APPROACH TO INTENSE-FIELD ABOVE-THRESHOLD DISSOCIATION IN THE LONG WAVELENGTH LIMIT

A new semiclassical formalism has been developed to treat Hamiltonians having explicit time dependence, with particular application to the dissociation of diatomic ions in intense laser fields. Based on this formalism, a hopping algorithm is presented which specifies how classical trajectories should be moved between coupled electronic surfaces. The theory is laid out in a rigorous, general form and an analysis is also presented for the case where only two electronic surfaces are strongly coupled. In addition, valuable physical insight into the hopping process is obtained by considering the theory in a number of physically relevant limiting cases. From this insight a number of guidelines are proposed which detail the manner in which trajectory hopping should be implemented when time‐dependent potential energy surfaces are present, including the effects of phase coherence and conservation principles.

Tracer diffusion in hard sphere fluids from molecular to hydrodynamic regimes

Molecular dynamics is employed to investigate tracer diffusion in hard sphere fluids. Reduced densities (rho*=rhosigma(3), sigma is the diameter of bath fluid particles) ranging from 0.02 to 0.52 and tracers ranging in diameter from 0.125sigma to 16sigma are considered. Finite-size effects are found to pose a significant problem and can lead to seriously underestimated tracer diffusion constants even in systems that are very large by simulation standards. It is shown that this can be overcome by applying a simple extrapolation formula that is linear in the reciprocal cell length L(-1), allowing us to obtain infinite-volume estimates of the diffusion constant for all tracer sizes. For higher densities, the range of tracer diameters considered spans diffusion behavior from molecular to hydrodynamic regimes. In the hydrodynamic limit our extrapolated results are clearly consistent with the theoretically expected slip boundary conditions, whereas the underestimated values obtained without extrapolation could easily be interpreted as approaching the stick limit. It is shown that simply adding the Enskog and hydrodynamic contributions gives a reasonable qualitative description of the diffusion behavior but tends to overestimate the diffusion constant. We propose another expression that fits the simulation results for all densities and tracer diameters.

A Charge Moving Algorithm for Molecular Dynamics Simulations of Gas-Phase Proteins

A method for moving charges in a coarse-grained simulation of gas-phase proteins is presented which uses a Monte Carlo approach to move charges between charge sites. The method is used to study the role of charge movement in the dissociation mechanism of protein complexes in order to better understand experimentally observed mass spectra from CID studies. The charge hopping process is analyzed using energy distributions and a pair correlation plot. Hopping rates, charge distributions, and structural parameters (radius of gyration and RMSD) are also calculated. The importance of charge movement for the unfolding of protein complexes is demonstrated. The algorithm is implemented in the GROMACS molecular dynamics software package. In this study, transthyretin (TTR) tetramer is used with the MARTINI force field as a model system, and comparisons to experiments are made. The hopping and unfolding are found to be controlled by the Coulomb repulsion among the charges in the complex.

Classical analysis of diatomic dissociation dynamics in intense laser fields

The dissociation of a diatomic ion in an intense laser field is studied using a one‐dimensional model with a Morse function representing the nuclear interaction potential, and coupling to a linear dipole moment representing the interaction with the laser field. A perturbative treatment is generally not possible because the field strengths employed are large enough to significantly distort the potential surface. Instead, classical trajectories are used to investigate some qualitative features of the dissociation process, with the goal of introducing some simple models to explain these features. A modified barrier suppression model is proposed which predicts the field strength at which trajectories first start to dissociate, and a ‘‘wagging tail’’ model is proposed which predicts the maximum kinetic energy of the dissociation products. Both these models provide physical insight into the dissociation process, and can be used to qualitatively understand experimental results.

Transport and relaxation properties of isotopomeric hydrogen–helium binary mixtures. II. HD, D2, T2–He mixtures

The comprehensive comparison between calculated bulk non-equilibrium properties of hydrogen–helium isotopomeric mixtures and experiment that has previously been carried out for H2–helium mixtures [2004, Molec. Phys., submitted] has been extended to mixtures of HD, D2 and T2 with 3He and 4He. For HD–4He mixtures, comparison is also made, where possible, with previous calculations of Köhler and Schaefer [1983, Physica A, 120, 185]. The phenomena examined herein include low temperature interaction second virial coefficients, binary diffusion and thermal conductivity coefficients, rotational relaxation, transport property field effects and flow birefringence. Scattering calculations have been carried out for the HD–He PES of Schaefer and Köhler [1985, Physica A, 129, 469], and for both the Köhler–Schaefer and Tao [1994, J. chem. Phys., 100, 4947] potential surfaces for the D2–He and T2–He interactions. Comparisons between calculated and experimental results for HD, D2, T2–He mixtures confirm the conclusion, reached earlier from the H2–He comparisons, that these potential surfaces are very close to the correct one for the hydrogen–helium interaction, and that the small differences between them cannot be distinguished readily by measurements of bulk gas phenomena unless the attendant experimental uncertainties are better than ±0.3%.

Molecular dynamics study of the collision-induced rotational alignment of N2+ drifting in helium

The full velocity-angular momentum distribution function for gas-phase N2+ drifting in helium is calculated using a molecular dynamics method, and utilized to examine collision-induced rotational alignment in detail. These results are also compared with experimental measurements, most especially those of Anthony et al. [J. Chem. Phys. 112, 10269 (2000)] and those appearing in the preceding article [Anthony et al., J. Chem. Phys. 114, 6654 (2001)]. Both the calculations and experiments show a number of interesting features including, drift velocities which depend upon rotational state, and quadrupolar alignment parameters which change from negative at high velocities to positive at low velocities.

Molecular-dynamics study of rotational alignment of NO+ drifting in helium—velocity and angular momentum distribution functions

Collision-induced rotational alignment of NO+ ions drifting in a helium buffer gas is studied with molecular dynamics using the ab initio potential surface of S. K. Pogrebnya et al. [Int. J. Mass Spectrom. Ion Proc. 149/150, 207 (1995)], obtained via a coupled-cluster singles–doubles approximation. We examine average translational and rotational temperatures, velocity and angular momentum distributions, and the dependence of these quantities on the applied electric field. The distributions show that angular momentum is preferentially aligned perpendicular to the electric field vector. We investigate the mechanism of this alignment through a multipolar moment expansion, and propose and demonstrate the accuracy of a bi-Maxwellian analytic form for describing the angular momentum distribution.

MOBILITIES OF NO+ DRIFTING IN HELIUM : A MOLECULAR DYNAMICS STUDY

A new molecular dynamics (MD) method is introduced, and used to study NO+ ions drifting in helium under the influence of a uniform electric field. Mobilities, average values of squared velocities, and self-diffusion coefficients parallel and perpendicular to the electric field are reported for two recent ab initio potential surfaces: a coupled cluster singles–doubles with perturbative treatment of triple excitations [CCSD(T)] surface [S. K. Pogrebnya et al., Int. J. Mass Spectrom. Ion Processes 149/150, 207 (1995)] and a MP4SDTQ/6-311+G(2df,p) surface [L. A. Viehland et al., Chem. Phys. 211, 1 (1996)]. Average values of angular momentum and alignment parameters are also reported and compared. In all cases, no significant differences were found in the calculated values for the two different potential surfaces. Finally, mobility values are compared with experimental measurements [J. A. de Gouw et al., J. Chem. Phys. 105, 10398 (1996)] and good agreement is obtained for both potential surfaces.