Dynamics of the SiF 4-sensitized decomposition of PH 3 by a pulsed CO 2 tea laser

Abstract

The dynamics of the SiF 4-sensitized decomposition of PH 3 in an IR laser photon field were examined within the framework of a model which takes into account three effects: energy absorption, energetics of the process and shock wave expansion. The absorption of energy causes heating of the gas mixture initially present in the absorption volume and subsequent unimolecular decomposition of PH 3 molecules. Since the decomposition process is endothermic it brings about a gradual decrease in the temperature and thus quenches the process. The primary absorption creates a shock wave causing expansion of hot molecules into the cool surrounding volume. The latter process brings about cooling of the hot gas and also quenches the decomposition process. Taking into account the above-mentioned effects, the temperature profiles ( T vs. time and geometry of the reaction zone) were evaluated on the basis of statistical thermodynamics, assuming that, at any stage, the system attains thermal equilibrium. Yields of decomposition were then derived using k uni values calculated by the Rice-Ramsperger-Kassel-Marcus method. This approach was used to examine the effects of irradiation time (number of pulses), fluence, pressure, initial composition and presence of foreign gases (helium, nitrogen) on the yield of PH 3 decomposition. The derived dependences were compared with the experimental decomposition yields reported previously.