Kinetic-energy dependence of the secondary-electron yield for low-energyO(S5)metastables on a Cu-Be-O surface

The secondary-electron yield ${\gamma }_m$ for $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S\right)$ metastables on a Cu-Be-O multiplier surface was measured as a function of metastable kinetic energy from near thermal to several eV. This measurement, which seems to be the first of its kind, was made possible by the simultaneous detection of the metastables by secondary-electron emission from surfaces and in-flight radiative decay. Unlike the yield ${\gamma }_m$, the photon yield ${\gamma }_p$ for photons from radiative decay cannot depend on metastable velocity. The dependence of ${\gamma }_m$ on metastable kinetic energy was thus obtained by pulsing the metastable beam with a time-of-flight technique and comparing the time-of-flight distributions for direct particle detection and radiative decay. It was found that the yield ${\gamma }_m$ was constant to within 10% for metastable kinetic energies from about 0.05 to 3 eV. This range of available kinetic energies was determined by the process of dissociative excitation of $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S\right)$ metastables by electron impact on ${\mathrm{O}}_2$. According to theoretical arguments, constancy of the yield ${\gamma }_m$ may also apply to other metastable species and higher kinetic energies.