Mass spectrometric observations of enhanced rates of association reactions:: nLiF=LinFn (n = 2, 3) at LiF single crystal surfaces

A mass spectrometric method was used to study the kinetics of lithium fluoride single-crystal sublimation. In electron impact ionization mass spectra, Li+, LiF+, Li2F+, and Li3F2+ ions originating from monomer (LiF), dimer (Li2F2), and trimer (Li3F3) molecular precursors were detected in the temperature range 970-1070 K. The dimer-to-monomer and trimer-to-monomer flux ratios were found to increase progressively with increasing temperature and also in comparison with those measured under equilibrium of crystalline LiF with its saturated vapor. The temperature dependence of the ion current ratio I(Li2F+)/I(Li+) measured over the interval 916-1087 K was shown to pass reproducibly through a minimum at about 975 K. The enhancement of the rate of association reactions at LiF crystal surfaces is discussed in light of the terrace-ledge-kink model of vaporization acid surface charge concept.