Crossed molecular beams kinetics: BaO recoil velocity spectra from spin-forbidden Ba+SO2

Laboratory (lab) angular and time-of-flight (TOF) recoil velocity spectra of BaO product produced by crossing a thermal Ba beam with an SO2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer–quadrupole mass filter detector. Product center-of-mass (c.m.) recoil angle and energy distributions have been fit to the data by convoluting the c.m.lab transformation over measured speed distributions of both beams and the bandpass of the TOF analyzer for four experiments which employed different sharp distributions in incident collision energy. By energy conservation, the BaO which is observed must be produced by the spin-forbidden Ba (1S)+SO2(1A1) →BaO(X 1Σ+)+SO(X 3Σ−) reaction, at least in the three lower collision energy experiments. Here, reaction proceeds via formation of a long-lived BaSO2 complex to give a symmetric product angular distribution which is very sharply peaked in the forward and backward directions. Results of this and of previous studies of this reaction would probably be consistent with a phase space theory analysis of the reaction energy partitioning because of the large uncertainty in the reaction exoergicity. In the highest collision energy experiment, BaO is scattered preferentially backward via a direct reaction mechanism.