The mechanisms responsible for Ar+-laser-induced growth enhancement of GaAs by chemical beam epitaxy with triethylgallium and tris(dimethylamino) arsenic as precursors are investigated here using numerical models developed for simulating the epitaxial growth and Ar+-laser-induced temperature rise. Numerical simulations taking the laser pyrolytic effect into account alone cannot reproduce observed growth-rate enhancements. Procedures are established for examining catalytic and photolytic effects of laser irradiation on epitaxial growth. Calculation of photoinduced surface excess carrier concentrations corresponding to experimental conditions reported in the literature reveals that the catalytic effect should be negligible. Amount of precursors in the first and second adsorption layers is looked into quantitatively. It is concluded that photodecomposition of chemisorbed TEGa decomposition products, along with laser-induced temperature rise, is responsible for observed Ar+-laser-induced GaAs growth enhancements. Likely photodecomposition reactions are postulated. The triethylgallium absorption cross section required to reproduce observed growth enhancement is estimated. Group-V species are found to control the growth-enhancement temperature window because of their site-blocking effects.