The influence of potential energy surface topologies on the dissociation of H2

Abstract

In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state-to-state cross sections in both the dissociated and the scattered flux. The potential energy surface is two-dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule.

Dissociation of H2 on Cu(100): Dynamics on a new two-dimensional potential energy surface

“…Work done on the H 2 ϩCu system and aimed at calculating the ''threshold'' or ''barrier'' to the reaction includes experiments, 1-11 electronic structure calculations, [12][13][14][15][16][17][18][19] and dynamical studies. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] The experimentally obtained reaction probabilities or sticking coefficients can be interpreted in terms of reaction thresholds, which are related to the barrier height obtained from electronic structure studies in a nontrivial way. 33,36 Tested against experiment, dynamics studies are the ultimate test of the accuracy of proposed potential energy surfaces ͑PES's͒, in addition to being helpful in interpreting the experimental findings.…”

Reactive and diffractive scattering of H2 from Pt(111) studied using a six-dimensional wave packet method

“…This was unexpected because vibrational enhancement is usually associated with late barriers. 6,7 However, our analysis showed that in the entrance channel the force constant associated with the vibration decreased as the molecule approached the barrier. The vibrational energy that is released in this process can flow into translation along Z, thereby enhancing reaction.…”

“…2͒ calculations. It is not due to a reduced mass effect 7 because the barriers where reaction will predominantly occur ͑barriers for the parallel orientation͒ are all located in the entrance channel. Instead vibrational enhancement is due to a decrease of the force constant associated with the molecular vibration, as the molecule approaches the barrier.…”

Ab Initio Molecular Dynamics Calculations versus Quantum-State-Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas–Surface Reaction

“…Note that the theory underestimates the experimental vibrational efficacy because the energy shift between the laser-off and the ν 1 -excited reaction probability curves is lower for the AIMD results than that for the experiments. One reason for this 50 could be that the barrier obtained with the PBE functional occurs at a too small value of the length of the breaking bond, as also found for H 2 + Cu(111). 41 The ratio between CH and CD bond cleavage is, within the error bars of the calculations, equal to the statistical value of 1:3 for laser-off beams, but only CH cleavage is observed when the CH stretch is pre-excited (Table 2).…”