Capture of lithium by 4He clusters: Surface adsorption, Penning ionization, and formation of HeLi

We investigate the capture of lithium atoms by a beam of large cold helium clusters and subsequent ion production by electron impact. Li atoms 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 well as the weakly bound complex HeLi+, confirming that capture by He-cluster beams can be used for efficient soft ionization of fragile species. The electron-energy dependence of the Li+, Li2+, and HeLi+ yield shows that they are formed primarily by Penning ionization in a collision with a metastable He atom in the droplet. This leads to the conclusion that lithium metal atoms are not submerged in the helium clusters but locate on the surface, corroborating theoretical predictions for bulk helium surfaces and spectroscopic measurements.