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 w ith alkali and alkaline earth atoms is detected by Langmuir-Taylor surface ionization. We measured detection efficiencies and time constants as a func tion of the temperature of a hot rhenium surface. Compared to results on fr ee K, Na, and Ba atoms we find practically no differences in the detection properties; apparently the helium host environment does not significantly i nfluence the mechanism of surface ionization. In addition, data for Li, Ca, and Sr on detection probabilities and residence times of the ions on the h ot 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].