Quantitative determination of desorption time delays for ion bombarded {1 0 0} 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 obtained using multiphoton resonance ionization (MPRI) in tandem with our experimental method for detection of energy and angle resolved neutral (EARN) atoms. The desorption events were initiated by a 400 ns pulse of 5 keV noble gas ions (He⁺ and Ar⁺) impinging along the 〈100〉 crystallographic direction. The relative contributions of the collisional and electronic channels of desorption have been determined for various target temperatures with both incident projectiles. In all cases the obtained results are accurately described by the Maxwell-Boltzmann distribution above 160°C. However, at lower target temperatures desorption time delays become apparent and have been quantitatively determined 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 delays are discussed with respect to a recently proposed model in which the emission of neutral alkali atoms results from the recombination of a surface alkali ion with an adjacent excited F center. Furthermore, based on the observed experimental trends in the relative intensity of the thermal and ballistic channels, electronic processes are suggested to be primarily responsible for the production of lattice defects while the eventual ballistic formation of F and H centers appears at best a secondary process in the observed desorption trends.