Photoionization and photoelectron spectroscopy of alkali halide monomers, dimers, and trimers

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, Li/sub 2/X/sub 2/, are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, X..cap alpha.. and ionic model calculations on this point are presented. Finally, the very weak stability of M/sub 2/X/sub 2/+ and M/sub 3/X/sub 3/+ and the high stability of M/sub 2/X+ and M/sub 3/X/sub 2/+ are discussed, and conjectures are made regarding the geometric structure of these entities. 13 figures, 5 tables.