QUANTITATIVE-DETERMINATION OF DESORPTION TIME DELAYS FOR ION-BOMBARDED (100) ALKALI-HALIDE SINGLE-CRYSTALS

The time-of-flight (TOF) spectra of neutral alkali atoms which desorb from the {100} surface plane of RbI and KI single crystals have been o btained using multiphoton resonance ionization (MPRI) in tandem with o ur experimental method for detection of energy and angle resolved neut ral (EARN) atoms. The desorption events were initiated by a 400 ns pul se of 5 keV noble gas ions (He+ and Ar+) impinging along the [100] cry stallographic direction. The relative contributions of the collisional and electronic channels of desorption have been determined for variou s target temperatures with both incident projectiles. In all cases the obtained results are accurately described by the Maxwell-Boltzmann di stribution above 160 degrees C. However, at lower target temperatures desorption time delays become apparent and have been quantitatively de termined as a function of projectile mass and the target temperature. The delays increase with decreasing target temperature and appear not to be sensitive to the mass of the incident projectile. The obtained d elays are discussed with respect to a recently proposed model in which the emission of neutral alkali atoms results from the recombination o f a surface alkali ion with an adjacent excited F center. Furthermore, based on the observed experimental trends in the relative intensity o f the thermal and ballistic channels, electronic processes are suggest ed to be primarily responsible for the production of lattice defects w hile the eventual ballistic formation of F and H centers appears at be st a secondary process in the observed desorption trends.