Simultaneous laser-induced fluorescence and quadrupole-mass-spectroscopy studies of electron-stimulated desorption of ground-state lithium atoms from lithium fluoride crystals.

Abstract

Under certain experimental circumstances, a number of authors have observed an unexpected increase in the yield of desorbed lithium atoms (a ``delayed maximum'') following the cessation of electron bombardment. The origin of this delayed maximum remains uncertain and continues to be a subject of active study. In all previous experiments that monitored delayed emission, a quadrupole mass spectrometer (QMS), which is relatively insensitive to the kinetic energy of the desorbing particles, was used for the detection of the desorbed lithium atoms. An important question is whether the delayed maximum arises from lithium atoms rather than dimers (which would appear as atoms following the ionization state of the QMS), and whether their velocity distribution is thermal as is the case for alkali-metal atoms desorbed during electron bombardment. To address these issues, we performed simultaneous laser-induced fluorescence (which is sensitive to the velocity of the desorbed atoms) and quadrupole-mass-spectrometry experiments, under experimental conditions where a delayed maximum could be observed. Our results prove that the occurrence of the delayed maximum is caused by the desorption of lithium monomers, not dimers, and indicate that the velocity distribution of the lithium atoms contributing to the delayed maximum does not appear to correspond to a Maxwell-Boltzmann velocity distribution. In addition, we observed an unexpected shift of the velocity distribution of the emitted atoms during bombardment as measured by laser-induced fluorescence that cannot be explained by any reasonable change in the surface temperature of the crystal during electron impact. Further, our data show that the substrate temperature dependence of the lithium desorption yield is the same for both experimental measurement techniques.