Probing the transition state in dissociative adsorption

Abstract

We have performed calculations for H₂ molecules being scattered from a metal surface. The emphasis of the work is aimed at understanding the role of rotational energy on the dissociative adsorption reaction. The time-dependent wavepacket formalism has been employed on a vibrationally adiabatic PES which explicitly contains two rotational degrees of freedom and one translational. The PES is activated (ca. 0.25–1.5 eV) and the value of the barrier depends strongly on the angle that the molecular bond axis makes with the surface. This PES enables a scattering event to be modelled which corresponds to a normally incident beam of hydrogen in the ground vibrational state and the J, m_J rotational state. We have found that the translational energy dependence of the dissociation probability of para- and normal hydrogen is approximately the same. This reflects the fact that the quantum of rotational energy is rather small in comparison to the activation barrier height. More interestingly it has been found that the form chosen for the PES gives rise to effective angular discrimination whereby molecules having only particular orientations will dissociate. This, in turn, gives rise to a characteristic rotational distribution which may be used to explore the symmetry of the dissociative transition state.