X-ray photoemission spectroscopy of alkali-metal antimonide photocathodes: Valence-band spectra

${\mathrm{Na}}_2$KSb, ${\mathrm{Na}}_3$Sb(Cs), ${\mathrm{Na}}_2$KSb(Cs), and ${\mathrm{Cs}}_3$Sb photocathodes were prepared in ultrahigh vacuum and studied by x-ray photoemission spectroscopy. These measurements for the first time give sufficient experimental data on the entire valence band of these compounds for comparison with the spectra of related compounds. The spectra obtained show that alkali-metal antimonides are of the essentially-ionic-bond type, in agreement with chemical-shift measurements. The structure of the valence band evolves systematically with increasing ionicity and anion-anion distance. We perform a tight-binding calculation and give an interpretation of the experimental valence-band spectra. Previously published calculations are not in agreement with experimental spectra.