Molecular Beam Electric Resonance Spectra of Reaction Products: Vibrational Energy Distribution in CsF from Cs+SF6

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Abstract
Reactively scattered CsF generated at the intersection of crossed molecular beams of Cs and SF6 is analyzed by electric resonance spectroscopy. The observed Stark spectrum arises from transitions between space‐quantized components (MJ → MJ′) of particular rotation‐vibration (J, υ) states of CsF. The transitions are well resolved for several rotational states (J = 1–4) . For each of these states, lines arising from the lowest four or five vibrational levels (υ = 0–4) are identified by comparison with the spectrum of a thermal CsF beam. Intensity ratios for these levels indicate that the reactively scattered CsF has an approximate Boltzmann distribution of vibrational energy. The vibrational temperature is Tυ(CsF) = 1120 ± 90°K with the parent beams at T(Cs) = 580°K and T(SF6) = 230°K ; it increases to Tυ(CsF) = 1270 ± 140°K for T(SF6) = 600°K . Concurrent angular distribution, product velocity analysis, and electric‐deflection studies show that the reactively scattered CsF is peaked symmetrically forward and backward with respect to the initial relative velocity vector and that its translational and rotational energy is small compared to the reaction exothermicity. The symmetry of the angular distribution indicates that a collision complex is formed which lives for several rotational and many vibrational periods before the products CsF and SF5 emerge. Since the present experiment shows that less than 6% of the approximately 40 kcal/mole exothermicity appears in the mean vibrational energy of the newly formed CsF bond, most of the exothermicity must go into excitation of the SF5 radical. The vibrational temperature of the CsF agrees well with that calculated assuming equipartition of the total available energy into all possible modes of motion of the CsSF6 complex.