Angular and energy effects in sputtering of hexadecanethiol monolayers self-assembled on Ag(111)

Sputtering of monolayers of hexadecanethiol (HDT) molecules CH3(CH2)(15)SH self-assembled on Ag(111) was studied with 1150 eV He+ bombardment under di fferent angles of incidence ranging from 27 degrees to 71 degrees and with different He+ ion energies (800-1900 eV for 50 degrees). Sputtering kinetic s in the dose range from 1 x 10(14) to 1 x 10(16) cm(-2) was monitored with ion scattering spectroscopy (ISS), X-ray and ultraviolet photoelectron spe ctroscopies (XPS, UPS) and metastable induced electron spectroscopy (MIES). A two-stage process in degradation of the layer under ion irradiation incl uding high-rate fragmentation of standing hydrocarbon chains with formation of C = C double bonds and ion desorption of the sulfur sublayer could be d istinguished. The initial sputtering yield of fragments increases with inci dent angle as Y-0 = Y-0(0 degrees)/cos beta. It lies in the range of 0.7-1. 9 molecules/ion (11-30 CH2-groups/ion). On the basis of a model of sputteri ng of the HDT film, a cross-section of chain fragmentation was evaluated to be about 1.52 +/- 0.1 Angstrom(2). This cross-section, as well as Y-0, dec reases with increasing ion energy, exhibiting a linear correlation with the calculated ion energy loss in the HDT monolayer. The angular and energy de pendencies of the sulfur desorption cross-section are compared with calcula tions based on Winters-Sigmund theory. (C) 1999 Elsevier Science B.V. All r ights reserved.