The scattering property of tertiarybutylarsine (TBAs) on a GaAs(001)-(4×6) surface at room temperature was studied by supersonic-molecular-beam scattering. Polar angle measurements show that the scattering signal is due to thermal desorption of trapped molecules combined with a direct-inelastic scattering. The measurement of the sticking coefficient shows a precursor-mediated adsorption behavior. The time decay curve can be divided into two components with activation energies of 6.9 and 8.3 kcal/mol, revealing that TBAs molecules can physisorb into two different potential wells on the GaAs (001)-(4×6) surface. ©1997 American Institute of Physics.