FIM studies of clean and graphitized rhodium using lithium and oxygen as imaging species

Selected areas of a clean or Li-doped Rh field emitter tip were studied by low-temperature O-2(+)-FIM and Li+-FDM in order to reveal the promoting inf luence of Li on the early stages of the oxidation process. The experiments were conducted by graphitizing a Rh tip specimen in a first step and reacti ng off subsequently the surface carbon to create two single "windows" with a clean Rh surface in the region of (1 0 0) and (1 2 3). Both windows were imaged by Li+-FDM. At high held strengths Li+ formation was restricted to t he border between the clean surface areas of the windows and the graphite-l ike overlayer, i.e. ring-like ion images appeared on an otherwise dark back ground. Since the Li source was originally present on the shank of the Rh t ip it must be concluded that Li atoms penetrate the graphite-like layer in order to form an intercalation structure ensuring high mobility to sustain steady supply for image formation in Li+-FDM. Field strengths and temperatu res could also be adjusted so that the initially clean Rh windows were prec overed by Li without desorption occurring. This enabled us to study the inf luence of Li on the rate of surface oxidation while imaging in O-2(+)-FIM a t 78 K. We found an enhancement of this rate (as compared to the clean surf ace) and explain this observation by an increase of the sticking probabilit y of O-2 on the Li-covered surface. In another experiment, we found that O- 2(+)-FIM of Rh surfaces covered by COad yield higher brightness than those covered by O-ad. (C) 1999 Elsevier Science B.V. All rights reserved.