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

A linear time-of-flight mass spectrometer equipped with two acceleration gr ids 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 stale o f the incident heavy ion beam. For this purpose, three different targets - a bare gold foil, a bare carbon foil and a 1500 Angstrom thick glycine film deposited on a gold foil - were irradiated by a 127 MeV Iq+ beam (q = 17-3 2) under an angle of incidence of 55 degrees against the surface normal. Th e measurements were performed at a pressure of about 10(-7) mbar. The resul ts 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 fl ight times, i.e. larger energies compared to the distributions obtained wit h 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 ej ected preferentially backwards along the line-of-incidence of the primary i ons. This effect is much more expressed for the glycine film than for the b are foils. The results are discussed in the frame work of available models of H+ production and desorption. (C) 2000 Elsevier Science B.V. All rights reserved.