Molecular beam electric resonance spectra of reaction products: Vibrational energy of LiF from Li + SF6

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Abstract
Reactively scattered LiF from crossed molecular beams of Li and SF6 is analyzed by electric resonance spectroscopy. Stark transitions between the J = 1, MJ = 0 and J = 1, MJ = 1 rotational states are measured for the v = 0–3 vibrational levels. Intensity ratios for these levels correspond to a Boltzmann distribution. The vibrational temperature is Tv (LiF)=920±30°K with the parent beams at T(Li)∼ 900°K and T(SF6)=400°K; it increases appreciably with increase in T(Li) over a range of ∼ 150°K but is independent of T(SF6) over a range of 450°K. The magnitude of Tv (LiF) is only about half that corresponding to equipartition of energy in an LiSF6 complex. The observed refocusing properties of the LiF indicate its translational and rotational energy are also relatively low. Most of the approximately 60 kcal/mole reaction exoergicity thus goes into internal excitation of the SF5 radical (enough to allow dissociation to SF4+F). These results, together with previous data on the LiF cross section and angular distribution, indicate that the reaction does not proceed via a long-lived LiSF6 complex, in contrast to the analogous Cs+SF6 reaction. Vibrational relaxation was observable only for SF6 fluxes considerably higher than those used in the measurements. In another experiment, however, vibrational relaxation was measured for a thermal LiF beam (at 1100°K) scattered at an angle of 20° by the crossed SF6 beam (at ∼ 300°K). The vibrational temperature of the scattered LiF is markedly lowered (to ∼ 750°K). The dipole moment of 7Li19F is also determined for the v = 0–5 vibrational states.