The excimer laser induced photodecomposition of the arsine precursor in metal organic vapour phase epitaxy (MOVPE) was monitored in situ with temporal resolution by coherent anti-Stokes Raman scattering (CARS). Simultaneously, the local gas temperature was derived from the CARS spectra. The results show that the 193 nm ArF line leads to a nearly 100% decomposition efficiency at low total pressures but to a considerably reduced efficiency at higher total pressures. The temporal development of the efficiency occurs in two steps. From the CARS-derived local gas temperature it is concluded that the first step essentially results from photodissociation, while in the second one thermal decomposition is dominant.
