Effects of hydrogen adsorption on the performance of the W(001) spin-polarized low-energy electron diffraction electron polarimeter

Since hydrogen is the main residual gas in a metal ultrahigh vacuum chamber, the effect of hydrogen adsorption on the performance of an electron polarimeter based on spin-polarized low-energy electron diffraction (SPLEED) from the W(001) surface, has been investigated experimentally. The experiments involved both dosing the analyzer crystal to calibrated coverages of hydrogen, and allowing residual gas in the vacuum chamber to adsorb on the analyzer crystal surface. The sensitivity (Sherman function, S) and efficiency (figure of merit, F) were monitored using SPLEED rotation curves. The W(001) SPLEED polarimeter is found to be resiliant to hydrogen adsorption. The sensitivity and efficiency require only a small scaling, which is linear in hydrogen coverage up to about 0.5 of saturation coverage. In a residual hydrogen partial pressure of 1 × 10−10 Torr, the analyzer crystal need be cleaned only every 2 h. The polarimeter may also be operated with a saturation coverage of hydrogen on the analyzer crystal. Under these conditions, the sensitivity falls to S=−0.19±0.02 and the efficiency to F = 4 × 10−5.