Langmuir-Taylor surface ionization of alkali (Li, Na, K) and alkaline earth (Ca, Sr, Ba) atoms attached to helium droplets

A beam of superfluid helium droplets (He-N, N approximate to 10(4)) doped with alkali and alkaline earth atoms is detected by Langmuir-Taylor surface ionization. We measured detection efficiencies and time constants as a function of the temperature of a hot rhenium surface. Compared to results on free K, Na, and Ba atoms we find practically no differences in the detection properties; apparently the helium host environment does not significantly influence the mechanism of surface ionization. In addition, data for Li, Ca, and Sr on detection probabilities and residence times of the ions on the hot rhenium surface will be presented for the first time. Time constants and desorption energies are extracted; for alkaline earth atoms the latter are found to be significantly higher compared to the desorption of the alkalis. (C) 2000 American Institute of Physics. [S0034- 6748(00)03809-0].