Dynamical Characteristics of Laser Vaporization Process in Group IV Elements

Abstract

Emission characteristics of electrons and ions desorbed from the solid target of group IV elements have been investigated by nitrogen laser excitation in the intensity range from 5 to 200 MW/cm². The results on Si, for example, show the presence of a threshold in the laser intensity above which emission of both electron and positive ion increases sharply by more than three orders of magnitudes in an approximately equal rate, and then tend to saturate with further increase in the intensity. The observed features can not be described in terms of ordinary vaporization process in which each atom/ion is thermally ejected individually from isolated atomic sites on the surface. The presence of threshold and its characteristics indicate that optically excited atoms are desorbed not individually into vacuum but in various forms of large clusters as a result of electronic excitation above a critical density. The measured results by means of a 127° energy analyzer indicate that the subsequent fragmentation of clusters takes place in a time period between 1 to 10 µs. The major decomposition process toward the final few steps is represented by Si_k ^a+ → Si_k−1 ^(a−1)+ + Si⁺ + E_r, where the released kinetic energy E_r < 3 eV.