Wavelength-modulated atomic absorption spectroscopy has been experimentally studied for real-time flux monitoring in molecular beam epitaxy. Gallium was chosen for the purpose of demonstration. Extension of this technique to other group III element (Al and In) flux measurements 14 should be straightforward, as well as. to any other element for which an adequate tunable laser source is available. From the measured absorbance spectra, the scanning parameters (central wavelength and scanning width) were calculated and experimentally optimized. The photon flux dependency of the signal saturates at large flux, which is explained by stimulated transitions within a simple three-level scheme. The variation of the flux-monitoring signal was also studied as a function of the detected even harmonics (from 2f to 6f). For Ga, measurements at 4f or 6f show the best characteristics with respect to signal-to-noise ratio. (c) 2007 American Vacuum Society.