Investigation of small lithium and potassium clusters by Knudsen effusion mass spectrometry using broad band photoionization

A novel Knudsen cell-mass spectrometer system using broad-band photoionization has been developed and used for the study of the equilibrium vaporization of lithium and potassium in the temperature ranges of 540 to 830 and 350 to 460 K, respectively. The ions M+, M2- M3+ (M = Li, K), and Li4+ were detected and their intensities determined except that of Li4+. Partial pressures of M, M2, and M3 (M = Li, K) were tentatively evaluated from the ion intensities and enthalpies of dissociation of Li2 and K2 determined according to the second- and third-law methods as well as of Li3 and K3 according to the third-law method. The data obtained for the dimer K2 indicate an unusually high photoionization cross section for the ion K2+ relative to that of K+. This effect increases the K2+/K+ ion intensity ratio in alkali metal cluster beams, also generally analyzed using broad-band photo-ionization, by a factor of 11. Enthalpies of atomization for Li3, and K3, ΔH°0, were obtained at 152 ± 3 and 90 ± 2 kJ mol-1, respectively, neglecting possible errors of the molecular parameters of these species and assuming no unusual photoionization cross section of Li3+ and K3+ relative to that of Li+ or K+, respectively.