Depolarisation of the spatial alignment of the rotational angular momentum vector by hyperfine interaction

Abstract

The investigation of stereodynamic processes in gas phase and gas-surface reactions is increasingly performed by optical methods. With these techniques the orientation and the quadrupole alignment of the molecular rotational angular momentum vector can be prepared or assessed. The knowledge about these alignment parameters yields insight into azimuthal and polar constraints of the interaction potential of the process investigated. The spatial alignment of the angular momentum vector is, however, depolarised by its interaction with the nuclear spin of atoms within the molecule. Starting from well-developed theory this influence is explicitly calculated for a number of systems – hydrogen, deuterium, acetylene and nitric oxide – which in recent years found wide-spread use in investigations of gas-surface reactions and in gas phase studies. It is shown that in all cases studied at low values of the rotational angular momentum its spatial direction is strongly influenced by the nuclear spin of the molecules over the time scale of a typical experiment. But even at larger values of the angular momentum a significant average depolarisation occurs.