Velocity Distribution of Laser Photoionized Neutrals Ejected from Methanol-Dosed Aluminum(111) by Electron-Stimulated Desorption

Abstract

Nonresonant multiphoton ionization at 193 nm wavelength was employed for efficient detection of electron-stimulated neutral desorption from Al(111) dosed with MeOH to produce monolayer MeO coverage. Velocity spectra were measured by the flight time from the crystal surface to the focal region of the laser beam with a pulsed primary electron beam of 3 keV and the sample at 300 K. Either the C+ or HCO+ photofragment indicated the same non-Boltzmann velocity spectrum for the neutral parent precursor with a peak kinetic energy of 0.1 eV. Identical distributions were obtained when the cleaned crystal was preoxidized with O2 prior to MeOH dosing. As the crystal temp. was raised, HCO+ photoion signal declined steadily, while C+ increased until .apprx.550 K. The total cross section for loss of parent signal with dose of 3 keV electrons was measured to be (2 +- 1) * 10-17 cm2.