Velocity dependent vibrational and rotational energy distributions of sputtered sulfur molecules

Abstract

S 2 molecules are sputtered by keV ion bombardment from sulfur. With a laser induced fluorescence technique the vibrational and rotational energy distributions are measured. With an additional time-of-flight method a velocity selection is attained. This enables us to determine the internal energy of the S 2 as a function of the molecular velocity. The shape of the population distribution is found to resemble Boltzmann behaviour, yielding effective “temperatures” for vibration and rotation. The vibrational energy is almost independent of the velocity, T vip ∼− 1500 K. However, the average rotational excitation energy increases with both molecular velocity and vibrational quantum number. Approximated by Boltzmann behaviour, the “temperature” T rot varies from 300 to 1600 K. Sputtering with He + instead of Ar + ions gives identical results except for a higher ground vibrational level population. The results are compared with a double and single collision model of molecule sputtering. It is concluded that the single collision mechanism cannot explain the results. The following qualitative features of the energy partition are correctly predicted with the double collision model: the vibrational population for high molecular kinetic energies, the rotational populations of the ground vibrational state, increasing rotational excitation with vibrational quantum number and increasing rotational excitation with molecular velocity.