STAY TIME MEASUREMENTS FOR XE, KR, AND CO2 PHYSIOSORBING ON NI AND CU SURFACES

A pulsed molecular beam apparatus is used to measure mean stay times for gases physisorbing on cooled surfaces. Most of the data are for Xe on nickel surfaces. Data are also presented for Kr and CO2 on nickel, Xe on copper, and Xe on ion-sputter-cleaned nickel. All targets are polycrystalline. Surface temperatures range from 92 to 125 K and measured stay times range from 10-5 to 10-3 s. Heats of adsorption and pre-exponential factors deduced from the data indicate that the adsorption is localized (immobile) and suggest that the sputter-cleaned targets may be approximately clean. A model relating the shape of the detector signal to the mean stay time is presented and its validity is assessed. Measured speed distributions for the desorbing molecules exhibit an excess of slow molecules compared to that expected for simple effusion. At lower surface temperatures where longer stay times are observed, a peculiar detector signal dip is observed which appears to indicate that the adsorbing beam pulses temporarily reduce the steady state desorption rate of background atoms. © 1978.