Photoionization studies of germanium and tin clusters in the energy region of 5.0-8.8 eV:: Ionization potentials for Gen (n=2-57) and Snn (n=2-41)

Photoionization thresholds for Ge-n (n less than or equal to 57) and Sn-n (n less than or equal to 41) are examined by laser photoionization with detection by reflectron time-of-flight mass spectrometry. Stimulated Raman anti-Stokes scattering of narrow bandwidth 193, 248, and 266 nm radiation is used to produce ionization light sources in the vacuum ultraviolet region (200-141 nm). A very similar size dependence of the ionization potentials (IPs) is found for germanium and tin clusters with fewer than 12 atoms, featuring a major maximum at n=10. The rather high IP of Ge-10 compared with its neighbors is consistent with the results of a photodissociation study of Ge-n(+). We also find a rapid decrease in the IPs for Ge-n between n=15 and 26, which is very similar to that for silicon clusters reported in our previous paper. On the other hand, the IPs of medium size Sn-n (n=15-41) clusters are found to decrease slowly without such a gap. The remarkable difference in the size dependence of the IPs for the Si-n, Ge-n, and Sn-n clusters is discussed in relation to the existence of a structural transition in the medium-size Si-n and Ge-n clusters. (C) 1999 American Institute of Physics. [S0021-9606(99)01133-2].