Molecular beam surface ionization detection: II. Field reversal and surface ionization study of Na on Re with adsorbed oxygen

The desorption and surface ionization of Na on a polycrystalline Re surface with various amounts of adsorbed oxygen have been studied by field reversal, surface ionization and thermoelectronic emission methods. In this work the unique properties of the field reversal method are taken advantage of, i.e. that both neutral and ionic desorption rate constants can be determined simultaneously. Absolute ionization coefficients have been measured by field reversal and have been compared with values found by the “oxygen coverage” method and by static surface ionization. The application to beam flux density determinations is discussed. The simultaneous variation of the neutral and ionic desorption rate constants during oxygen adsorption and the temperature dependence of them have been studied. The Re surface in 2 × 10^?8 Torr of oxygen and at 1300–1500 K is shown to be very stable and to behave differently than in studies at higher temperatures. The very rapid change in both desorption rate constants at an effective work function Φ^e = 5.35 V is here correlated with the results of LEED experiments (Gorodetskii and Knysh) and is proposed to indicate a change from a stable Re oxide surface at low Φ^e (and oxygen coverage) to a different surface structure at higher Φ^e. Desorption energies have been determined at various values of Φ^e. The neutral desorption energy at low oxygen pressure is 2.70 ± 0.06 eV, which agress well with earlier, here corrected modulated beam results. The energy (Schottky) cycle for surface ionization is shown to be closed at low Φ^e, which has been difficult to show with other methods in any other case.