H + emission by MeV-ion impact: Charge state dependence of energy and angular distributions

Abstract

A linear time-of-flight mass spectrometer equipped with two acceleration grids and a position sensitive stop detector was used to measure the initial energies and emission angles of H + ions as a function of the charge state of the incident heavy ion beam. For this purpose, three different targets – a bare gold foil, a bare carbon foil and a 1500 A ̊ thick glycine film deposited on a gold foil – were irradiated by a 127 MeV I q+ beam ( q=17–32) under an angle of incidence of 55° against the surface normal. The measurements were performed at a pressure of about 10 −7 mbar. The results are axial, radial and total energy distributions and corresponding mean energies and emission angles. As in previous experiments, the axial energy distribution of H + measured for the glycine sample is shifted to shorter flight times, i.e. larger energies compared to the distributions obtained with the bare foils. This shift is not charge state dependent. The mean total energy increases with the charge state as 0.14, 0.21 and 0.25 eV per charge unit, respectively, for the gold, carbon and glycine targets. The angular distributions are generally asymmetric; we observed that the H + ions are ejected preferentially backwards along the line-of-incidence of the primary ions. This effect is much more expressed for the glycine film than for the bare foils. The results are discussed in the frame work of available models of H + production and desorption.