CAPTURE OF LITHIUM BY HE-4 CLUSTERS - SURFACE-ADSORPTION, PENNING IONIZATION, AND FORMATION OF HELI+

We investigate the capture of lithium atoms by a beam of large cold he lium clusters and subsequent ion production by electron impact. Li ato ms are efficiently picked up by the He droplets, although with a cross section significantly lower than that for other atoms and molecules. The mass spectrum reveals the presence of Li atom and dimer ions, as w ell as the weakly bound complex HeLi+, confirming that capture by He-c luster beams can be used for efficient soft ionization of fragile spec ies. The electron-energy dependence of the Li+, Li-2(+), and HeLi+ yie ld shows that they are formed primarily by Penning ionization in a col lision with a metastable He atom in the droplet. This leads to the con clusion that lithium metal atoms are not submerged in the helium clust ers but locate on the surface, corroborating theoretical predictions f or bulk helium surfaces and spectroscopic measurements. (C) 1997 Ameri can Institute of Physics.