A laser-detonation source was used to produce hyperthermal beams containing argon (Ar beam) or a mixture of atomic and molecular oxygen (O/O2 beam). For both hyperthermal beams, a pulse of precursor gas (Ar or O2) was produced by a custom piezoelectric pulsed valve, and this pulse entered a conical nozzle through a 1 mm orifice. The hyperthermal gas pulse was generated by the shock wave produced when a 7 J pulse -1 CO2 TEA laser was focused into the throat of the nozzle and a high-temperature plasma was created. This pulsed hyperthermal source has been described previously and was used with a repetition rate of 2 Hz. The O/O2 beam was used for oxidation studies, and the Ar beam was used where non-reactive incident particles were desired to study only the effects of sample morphology on the scattering dynamics. The hyperthermal beams passed through a skimmer, which had a diameter of 2 mm, and were further collimated by 3 mm diameter aperture that was 3.5 cm from the skimmer. The total distance between the nozzle orifice and the sample surface (the center of rotation of the mass spectrometer detector) was 99.2 cm, and the distance from the surface to the electron-impact ionizer of the detector was 34.4 cm. A synchronized chopper wheel, with three centrally-symmetric 1.5 mm wide slots rotating at 300 Hz, was used to select a narrow velocity distribution from the overall beam pulse (before the pulse reached the surface). The O/O2 beam had an average velocity of 7918 m s -1 and a velocity width (full width at half maximum, fwhm) of 492 m s -1 . The beam was composed of O( 3 P) ,