Photon and electron stimulated surface dynamics of single molecules. Final report on D.O.E. No. DE-FG0295ER14563

Abstract

The initial goal of this work was to build up an entirely new low-temperature scanning tunneling microscopy (STM) and ultrahigh vacuum system to examine the electron- and photon-induced chemistry of single molecules at low surface temperatures where thermal diffusion would be quenched. The photochemistry of methyl bromide on Pt(111) was first examined at 90 K using liquid nitrogen cooling. Br atoms were quite mobile even at 90 K, and were only visible by STM when coalesced along Pt step edges or in Br islands structures. The 193 nm photofragmentation of methyl bromide efficiently created monovacancies in the substrate at 90 K. It was found that at elevated temperatures there is considerable restructuring and reactive attack of the Pt surface by halogens, but for traditional, lower temperature studies of alkyl radicals prepared by thermal dissociative adsorption of alkyl iodides there is probably no problem with adsorbing I generating monovacancies on the surface. The dynamics of the ho t Br atoms formed by dissociative adsorption of Br{sub 2} was also examined. It was discovered that hot Br atoms from Br{sub 2} dissociative adsorption travel farther than hot O atoms from O{sub 2} dissociative adsorption; hot atom motion from different dissociative adsorption systems had not previously been compared for the same metal substrate. The experimental results strengthened the theoretical case that corrugation of the adsorbate/substrate potential is the key issue in determining hot atom travel. In addition, the data provided strong evidence for the transient existence of a weakly adsorbed and mobile Br{sub 2} precursor to dissociative adsorption. Some experiments imaging individual molecules at 15 K were also conducted.