Gunnar Nyman

Efficient microcanonical sampling for a preselected total angular momentum

Expressions for the molecular momentum density of states as a function of spatial configuration in an angular momentum resolved microcanonical ensemble are derived. These expressions are then used to formulate an efficient sampling scheme for the generation of spatial configurations or full phase space vectors in an ensemble where both energy and angular momentum are predetermined. Applications to simple diatomic (OH) and triatomic (H2O) molecular models are presented.

A low‐energy quasiclassical trajectory study of O(3P)+OH(2Π)→O2(3Σ−g)+H(2S). II. Rate constants and recrossing, zero‐point energy effects

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent double many body expansion III potential‐energy surface by Varandas et al. (1988). Recrossing factors are calculated and found to depend strongly on how the vibrational zero‐point energy is treated. Detailed and thermal rate constants for the temperature range 20–500 K are presented. Comparisons with experiments, trajectory calculations by Miller (1986), and by Quintales et al. (1988), and a previously proposed extended Langevin model are made. It is noted that the coupling of the electronic and nuclear motion in OH may have a large effect on the thermal rate constant below 200 K. Close complex formation rate constants are calculated and found to agree well with the experimental rate constants.

A low‐energy quasiclassical trajectory study of O(3P)+OH(2Π) →O2(3Σ−g)+H(2S). I. Cross sections and reaction dynamics

Quasiclassical trajectory calculations for the title reaction have been carried out using the recent DMBE III (double many body expansion) potential‐energy surface by Varandas et al. (1988). The dynamics of complex formation were studied in detail and two different types of complexes, referred to as close and distant complexes, were observed. Corresponding cross sections and reaction cross sections are reported as a function of diatom rotational quantum number and atom–diatom relative translational energy in the range 0.25–2.0 kcal/mol. The cross sections decrease with increasing translational energy. The rotational‐state dependence of the reaction cross section is found to be complicated. One interesting observation is that the cross section for the rotational ground state is much smaller than for other rotational states. The features in the cross sections are related to the shape of the potential‐energy surface and the reaction dynamics.

Complex formation in O+OH collisions: a two-step mechanism

Abstract A thorough theoretical analysis of O( 3 P) + OH( 2 Π) collisions by quasiclassical trajectory calculations and statistical rate coefficient estimation indicates that complex formation is of a two-step character. An oxygen atom approaching the diatomic molecule from the asymptotic uncoupled region first encounters a centrifugal barrier. Inside this barrier is a region of intermediate coupling strength, separated from an inner strong-coupling region by another potential barrier. The cross sections for complex formation and reaction to form O 2 + H show a strong dependence on the rotational quantum number. Despite these peculiarities the variational transition state theory and the canonical effective potential theory give a reasonable account of the rate coefficients.

Efficient microcanonical sampling for triatomic molecular systems: Exact distributions verified

Statistical distributions of several quantities, such as linear and angular momenta, for a triatomic molecular system similar to an excited H2O molecule were obtained with an efficient microcanonical sampling method, previously described by Severin et al. The distributions were recorded as a function of the total angular momentum. Using this sampling procedure to obtain initial values for classical trajectory calculations and comparing with the trajectory quantities after variable times in the range 0.7 fs to 0.2 ps, it was verified that the sampling method gives exact distributions. This may open a way of reducing computer time in future trajectory calculations of unimolecular reactions, particularly important at low total energies. We also illustrate the breakdown of equipartition of energy between the various degrees of freedom in highly excited complexes. In some cases equipartition is not even valid for potential energy surfaces built solely from harmonic terms.

Surface scattering of NO from graphite: A statistical description of energy distributions

In the present theoretical study, inelastic scattering of NO from graphite surfaces is analyzed with a statistical model. The results are in good agreement with previous classical trajectory calculations by Pettersson et al. (1988). Angular distributions and the ‘‘rotational cooling’’ effect found in experiments published by Frenckel et al. (1982), Segner et al. (1983), and Hager and Walther (1984) are successfully reproduced. The model describes a small part of the graphite surface together with a scattering diatom as a collision complex, which decomposes in a unimolecular fashion. The surface is assumed to be flat, whereby the diatom angular momentum component along the surface normal and the linear momentum parallel to the surface are conserved. Otherwise the diatom translation and rotation are allowed to exchange energy with the surface, which is characterized by a set of harmonic oscillators. The experimentally observed ‘‘rotational cooling’’ effect is clearly demonstrated to be due to the conservati...

A low‐energy quasiclassical trajectory study of N++H2 and N++D2. Dynamics, cross sections, and rate constants

A quasiclassical trajectory study investigating the N++H2(D2)→NH+(ND+)+H(D) reactions has been performed. Two potential‐energy surfaces differing only in their thermicities were employed. Both surfaces are based on the recent ab initio calculations for the N+(3P)+H2(1Σ+g)→NH+ (2Π)+H(2S) reaction by Wilhelmsson et al. Dynamics, forward–backward scattering, product energies, and trajectory lifetimes are discussed. Reaction cross sections and thermal rate constants are presented and compared with experimental results. In general, the agreement with experimental data is good. The thermicity of the reaction can, however, not be established by the trajectory calculations.

A classical trajectory study of inelastic scattering of NO from graphite surfaces: Rotational energy distributions

The inelastic scattering of NO molecules from graphite surfaces is studied by classical trajectory methods. The experimental results from Frenkel et al. (1982), Segner et al. (1983), and Hager and Walther (1984) are analyzed. A model using a small isolated part of the graphite surface in interaction with the NO molecule gives results in good agreement with experiment. The parameter values in the model are fixed at the values previously found to reproduce the angular distributions well [Nyman and Pettersson (1987)]. For this system, the experimental results give a ‘‘rotational cooling’’ such that the rotational temperature of the inelastically scattered molecules becomes smaller than the surface temperature. This effect is reproduced accurately by the calculations, giving a rotational temperature of 250 K, independent of the surface temperature above 300 K. The main factor controlling this inelastic rotational cooling is the low initial value of the normal component of the total angular momentum. A ‘‘rotat...

A low energy quasiclassical trajectory study of N++H2. Potential energy surface effects

A low energy quasiclassical trajectory study has been performed on three potential energy surfaces, obtained by modifying the recent ground state ab initio potential energy hypersurface by Wilhelmsson et al. for the N+(3P)+H2(1Σ+g)→NH+ (2Π)+H(2S) reaction. The importance of explicit consideration of the asymptotic ion–induced dipole and ion–quadrupole interactions in the entrance channel is demonstrated. Reaction cross sections as a function of collision energy (approximately in the range 0.06–0.4 eV in c.m.) are presented.

Complex formation in X+ + H2 collisions. Statistical estimation of ion-quadrupole capture rate constants

Abstract The capture cross section and rate constant of ion-molecule collisions have been studied in the case when the molecule is diatomic and has a vanishing dipole moment but a nonvanishing quadrupole moment obtained from quantum chemistry as a function of bond length. In addition the ion-induced dipole interaction is accounted for. Calculations have been carried out at three levels of theory: variational transition state theory, effective potential theory and adiabatic capture theory. In the latter the channels are defined by the vib-rot states of the diatomic. Application has been made to Ar + or N + + H 2 collisions. The results show that in this case the ion-induced dipole interaction is far more significant than the ion-quadrupole interaction. The roles of vibrational-rotational coupling and orbital angular momentum are considered by comparison of weak ( L orb conserving) and strong coupling theories. The range of validity of the perturbation treatment of the coupling is analyzed. Special attention is directed to the magnitude of the quantum effects introduced through the channel eigenvalues of the adiabatic capture theories.