Temperature Dependence of Desorption in HigMy Photo-Excited CdS

Dependence of laser-induced desorption on both substrate temperature and emission angle have been studied in CdS by using the pulse mass counting method. The effective translational temperature (Ttrans) of particles ejected toward the angle of 0° from the surface normal is much higher than that toward 45°. The Ttrans of cach particlc, S, S2 and Cd, is different among them. Ttrans does not exhibit the surface temperature even when the time-of-flight spectra can be fitted with a Maxwellian velocity distribution. Arrhenius plots of desorption yields for the substrate temperature before laser irradiation show rather complicated features. Estimation of the temperature rise at surface by calculating the thermal diffusion equation gives the linear relationship in the Arrhenius plot. The enthalpy for desorption is estimated to be 0.44µ0.1 eV. This value is considerably smaller than that (2.2 eV) obtained in the free vacuum vaporization (ref. 13), suggesting the contribution of the electronic excitation effect.