Niels E. Henriksen

Dissociative chemisorption of N2 on rhenium: Dynamics at high impact energies

The dissociative chemisorption of N 2 on the (0001) rhenium crystal surface is studied theoretically at high impact energies. The dynamics of the molecule is accordingly treated classically excluding tunneling processes. This study extends previous low energy studies in three important ways: (1) all six degrees of freedom of the N 2 molecule are considered; (2) lateral variations (corrugation) are included in the molecule-crystal interaction potential; (3) energy exchange between the molecule and the surface is allowed for by treating the dynamics of the crystal atoms within a linear phonon forcing model. It is found that the energy transfer from the molecule to the phonons of the crystal is very significant. The smaller than unity dissociative sticking probability found experimentally even at the highest impact energies well above the barrier energy can be accounted for by the Landau-Zener probability of a transition from the dissociative potential energy surface (PES) to a non-dissociative PES.

Dissociative chemisorption of N2 on rhenium: Dynamics at low impact energies

Abstract The dissociative chemisorption of nitrogen on the (0001) rhenium surface is studied at low impact energies, where tunnelling processes are important. A quantum-classical model is used in which two coordinates, the distance from the surface and the vibrational coordinate, are treated quantum mechanically using the FFT (fast Fourier transform) technique. Also normal modes of the solid are quantized using a quantum boson approach and the remaining degrees of freedom are treated classically. Full corrugation of the surface and phonon coupling to infinite order as well as rotational motion of the diatom are included in the model.

The time propagation of the stationary states of a Morse oscillator by the Gaussian wave packet method

The Gaussian wave packet method has been developed for the simulation of processes like molecular collisions, photodissociation of molecules, and laser excitations of molecules. So far a necessary condition for an accurate result is that the fragment states are propagated accurately. We have considered one‐dimensional bound states described by a Morse potential, and carried out a systematic study of the ability of the Gaussian wave packet method to propagate the stationary states. It is found that the complete set of equations of motion as derived by the minimum error method (MEM) cannot be used in practical calculations because of numerical problems. These are eliminated by the introduction of simplifications such as the independent Gaussian approximation (IGA), where each wave packet is propagated independently. The conditions for an accurate propagation within that assumption are developed, and a simple method is devised to identify those states, which are propagated accurately. This procedure may be u...

Phase space representation of quantum mechanics: approximate dynamics of the morse oscillator

Abstract The accuracy of the Wigner propagation method is studied for stationary as well as non-stationary states of Morse oscillators. We investigate the possibility of improving the approach by introducing an effective potential. We find that the Wigner propagation method is accurate only for the ground state.

Molecular photodissociation dynamics: The time-dependent formulation

SummaryThe time-dependent formulation for nuclear dynamics in molecules induced by electronic excitation in a radiation field is reviewed. The present discussion is especially aiming at extracting physical observables for photodissociation and highlighting the connection to the nuclear dynamics of the process. The total dissociation probability, the probability associated with the formation of a given chemical product, and the probability that this product shows up in a specified quantum state is considered. The results are given as a function of the form of the light pulse, and special attention is given to situations where the duration of the light pulse is very short or very long.

Catalytic synthesis of ammonia using vibrationally excited nitrogen molecules : theoretical calculation of equilibrium and rate constants

Abstract The dissociation of nitrogen is the rate-limiting step in the catalytic synthesis of ammonia. Theoretical calculations have shown that the dissociative sticking probability of molecular nitrogen on catalytic active metal surfaces is enhanced by orders of magnitude when the molecules are vibrationally excited to states with quantum numbers 3–10. The rate and equilibrium constants for the process using vibrationally excited nitrogen molecules are calculated and expressions for the reaction rates are derived. A comparison with the ordinary process, where the nitrogen molecules are in the vibrational ground state, shows a great potential for a significant improvement of the yield in the process.

On the evaluation of branching ratios in molecular photofragmentation

Abstract An equation for the branching ratio between chemically distinct products is derived for a direct photofragmentation process. Via the Wigner phase-space representation of quantum mechanics, contact is made with descriptions in terms of classical mechanics.

Semiclassical model for laser fragmentation of polyatomic molecules

Abstract We present a semiclassical model for laser fragmentation of polyatomic molecules in an intense laser field. The model assumes a few active bonds which may break due to the interaction with the field and a quantum heat bath. The quantum heat bath consists of a set of harmonic oscillators (normal modes) coupled together through the Coriolis interaction. The model may also be used to study the dynamics of unimolecular decomposition.