Arsenic incorporation during GaAs/GaAs(100) molecular beam epitaxy is studied in situ with laser single-photon ionization time-of-flight mass spectrometry and reflection high-energy electron diffraction (RHEED). Incident and scattered fluxes of Ga and As, species in front of the growing GaAs wafer are ionized repetitively by a pulsed laser beam of 118 nm (10.5 eV) photons. The methods to obtain and interpret time-of-flight mass spectra and the simultaneous RHEED measurements are described. The real time behaviors of incident Ga and desorbing As-2 and As-4, obtained without mass spectral cracking, are studied during growth of GaAs layers with AS(4) and when growth is arrested as a function of substrate temperature and Ga/As-4 flux ratio. During growth only with AS(4), both As-2 and As-4 are desorbed or scattered in varying amounts depending on flux and substrate temperature conditions. Without an incident gallium flux, desorbing AS(4) decreases while desorbing AS(2) increases with increasing surface temperature. During gallium deposition and GaAs growth, the amounts of desorbing arsenic fluxes decrease linearly with increasing Ga/As-4 flux ratio, but the arsenic incorporation rate saturates at a Ga/As-4 flux ratio greater than or equal to 2, i.e. (Ga/As greater than or equal to 1/2). The total integrated incorporation of arsenic increases linearly with increasing Ga/AS(4) flux ratio when the surface is allowed to recover with an incident arsenic flux after the gallium flux is terminated. In the range of substrate temperatures optimum for layer-by-layer GaAs growth with As-4, As-4 incorporation dominates at low temperatures, while As-4 and As-2 incorporations contribute equally at high temperatures. Surface reaction sequences and mechanisms of arsenic incorporation are discussed and compared with measured RHEED results and previous experimental and theoretical results.