Study of atom-wall collisions by optical pumping

Abstract

Precise hyperfine frequency measurements have been performed on Rb 87 atoms contained in hydrocarbon wall coated cells. Frequency detection is achieved optically by hyperfine filtering and transmission monitoring. Atom-wall collisions which perturb the F=2 and F= 1 hyperfine levels unequally result in a transition frequency shifted from the free-atom value by about -85 cps. This shift compares favorably with atomicbeam hyperfine measurements of Goldenberg, Kleppner, and Ramsey for Cs atoms contained in hydrocarbon storage boxes. The large Rb87 shifts observed by Alley using a coherent pulse technique are not compatible with these results. Temperature-dependent studies in the region 0° to 72°C reveal that the hyperfine frequency increases approaching the free atom value as the temperature is raised, a decrease in the Rb residence time on the surface being indicated for an increase in temperature. From this information an adsorption energy of 0.1 eV and an average adsorption time of 5×10-11 sec are derived, the latter corresponding to 50 Rb oscillations in the potential well of the hydrocarbon surface. At 72°C the transition frequency is a maximum, and a further increase in temperature up to 105°C produces a frequency decrease, a behavior which correlates with the melting range of the polydisperse hydrocarbon used. This result is interpreted to mean that the adsorption energy and hence the dwell time increase as the polymer becomes more amorphous and is consistent with the idea that the amorphous state contains voids which provide a larger area of contact for a Rb atom than does the crystalline state.