H-2(+), H-3(+) and H- emission by MeV ion impact: charge state dependence of energy and angular distributions

A 1 MeV/n I-127(q+) beam of the Tandem accelerator at Orsay, France, has be en used to study the dependence of hydrogen ion emission on the charge stat e of the incident primary ions. The samples investigated were bare gold and carbon foils and a 150 nm glycine film vacuum-evaporated on an Au foil. Th e measurements were performed at a vacuum pressure of about 10(-7) mbar: th e sample surfaces were covered by a contamination layer. The linear time-of -flight instrument used was equipped with two acceleration grids and a posi tion sensitive stop detector, in order to measure the initial velocity comp onents and the angle of emission for each detected ion. The results were ax ial, radial and total energy distributions and angular distributions for th e hydrogen ions H+, H-2(+), H-3(+) and axial energy distributions for H-, a ll measured at constant beam energy for the charge states 17+, 20+, 26+, 30 + or 32+ under an angle of incidence of 55 degrees against the surface norm al. The angular distributions of all ions were asymmetric relative to the surfa ce normal, for the glycine target more than for the bare metals. The mean e nergies increase slightly with the charge state, the increase was in the or der of 0.5-2% per charge state. The change of the mean emission angle had n egative as well as positive values in the order of 0-0.2 degrees per charge state. Comparing glycine and metal targets, the angular distributions obta ined with the metals seemed to be wider around the main direction of emissi on. A few suggestions about possible desorption/ionization processes are gi ven. (C) 2000 Elsevier Science B.V. All rights reserved.