Characterisation of desorbing molecules by REMPI-TOF spectroscopy: the system H 2 (D 2)−V(1 0 0)+O

Abstract

The internal state distribution (rotation, vibration) and the translational energy distribution of state selected molecules can be measured with the help of a REMPI (resonance enhanced multi-photon ionisation)-TOF (time-of-flight) spectrometer. We have performed measurements of this type on hydrogen and deuterium molecules desorbing from an oxygen covered V(1 0 0) surface. The rotational state population of the hydrogen and deuterium molecules in the vibrational ground state exhibits an overpopulation for J<3 and an underpopulation for 3< J<8. In addition we observe a slight overpopulation for J>9 for desorbing deuterium. The calculation of the mean rotational energy E rot leads in all cases to values significantly below the respective thermal equilibrium values (rotational cooling). For the vibrational state distribution we observe in all cases an overpopulation for molecules in the first excited vibrational state (vibrational heating). The translational energy distribution of desorbing deuterium strongly depends on the surface quality. On the flat, well annealed oxygen covered V(1 0 0) surface the mean translational energy of desorbing deuterium is hyperthermal, whereas the translational energy of molecules desorbing from a rough surface is closer to thermal.