Contact-potential measurements of the adsorption of Cs, O 2 and H 2 on (110) Ta

Abstract

The electron-beam retarding potential method has been used to measure the work function changes, Δφ, resulting from the adsorption of Cs, O 2, and H 2 on a tantalum ribbon that was a single crystal with (110) orientation. The thermionic work function of the bare (110) Ta specimen was found to be 4.73 eV. For Cs, the maximum decrease in work function was Δφ = − 3.18eV, and at high coverage Δφ tended toward a limiting value of − 2.94 eV. For O 2, the work function decreased in the low coverage range, the maximum decrease being Δφ ≅ − 0.45eV; at higher coverages (e.g., exposures greater than 2.7 × 10 −8 Torr-minute), the work function increased above the bare surface value, the limiting value being Δφ ≅ + 0.8eV. Although H 2 adsorption led to non-parallel I–V curves that made data interpretation difficult, the apparent decrease in work function was less than 0.1 eV in magnitude, even for H 2 pressures as high as 10 −6 Torr and exposures of 10 −5 Torr-min. Cs adsorption on oxygenated (110) Ta consistently produced a maximum decrease in work function that exceeded the value observed for Cs adsorption upon unoxygenated (110) Ta. If the thermionic work function of 4.73 eV is used for the bare surface, the minimum work function achieved in these CsOTa runs was ∼ 1.3 eV.