Direct detection of a metal desorption channel: electron-stimulated desorption of aluminum from methanol-dosed Al(111)

Abstract Multiphoton laser ionization mass spectrometry has been used to investigate neutrals produced in 1 keV electron-stimulated desorption (ESD) from Al(111) which was methanol-dosed to produce the monolayer-coverage CH 3 O/Al(111) system. The major neutral desorption channel consists of CH 3 O, the methoxy species, detected primarily as the C + , CH + , and CO + photofragments. The novel observation of the desorption of metal substrate was facilitated by the overlap of 193 nm laser radiation with a strong Al autoionization transition. Differences between the laser power dependence of ESD-produced Al + signal and that from sputtered aluminum atoms, photoionized in a reference experiment, suggest that ESD releases an aluminum-containing molecule which is efficiently photofragmented to Al atoms; no molecular precursor was detectable directly. Velocity distributions of the methoxy photofragments and the Al + photoion have been measured. The former have distributions which peak at ∼ 1100 m/s, corresponding to a kinetic energy of 0.19 eV for the parent methoxy desorbate. The aluminum distribution peaks at ∼ 800 m/s and exhibits less population in the high-velocity region. These differences indicate that the detected aluminum is independent of the major desorption channel: neutral methoxy. An absolute yield measurement for aluminum of 1.7 × 10 −6 atoms per incident 1 keV electron was substantially lower than that for the methoxy channel (yield ≈ 10 −3 ). In addition, it is shown that increasing the temperature of the surface to ∼ 600 K, where methoxy is known to be decomposed on Al(111), leads to a drastic decrease in the desorbed aluminum signal. The findings are discussed in the context of current desorption theories.