Rotational energy effect upon the branching fraction for reactive decay of the CsF+K collision complex

Abstract

The influence of the rotational energy of the CsF molecule upon its reactivity with K has been measured by the crossed molecular beam technique over a range of relative translational energy Ētr from 3–6 kcal mol−1. The experiments involve the endoergic (1.8 kcal mol−1) reaction CsF+K⇄[CsFK]→Cs+KF, for which the reactive branching fraction FR had been previously measured in this laboratory using velocity-selected but otherwise ’’thermal’’ (Boltzmann) beams. In the present investigation, beams of low rotational states of CsF have been prepared by means of a quadrupole state selector focusing field placed beyond the usual slotted-disk velocity selector. This arrangement provides so-called ’’low-J’’ beams of CsF, with known speed distributions, which are then crossed with a well-characterized K beam. The measurements consist of angular distributions of reactive (i.e., Cs) and nonreactive (i.e., CsF) scattering for the ’’low-J’’ beam compared to the ’’thermal’’ reactant beam at essentially the same Ētr. The reactive yield and thus the reactive branching fraction FR is found to be significantly smaller for the ’’low-J’’ beam relative to the ’’thermal’’ beam of CsF. An analysis of the state selection and focusing properties of the quadrupole lens sytem has been carried out which accounts satisfactorily for the observed velocity dependence of the CsF focused beam intensity. This analysis yields a calculated rotational state distribution P (J), from which the average rotational energy Ērot of the reactant CsF molecules is obtained at each of the Ētr values studied. Ērot ranges from 0.13 to 0.04 kcal mol−1 for Ētr in the range 3–6 kcal mol−1, whereas Ērot for the ’’thermal’’ CsF beam is 2.52 kcal mol−1. (Thus the effect of applying the rod voltage to the quadrupoles is to ’’switch off’’ some 2.4 kcal mol−1 of CsF rotational energy.) This analysis also predicts the observed enhancement of the total scattered signal due to the focusing, which implies that the cross section for complex formation is the same (within ≈±10%) for the low-J and the thermal reagent molecules. By combining the data on the Ētr-dependence of FR for the rotationally cold CsF beams with the previous results for thermal beams it is possible to compare the relative importance of rotational versus translational energy at a given total energy. It is found that rotational energy of the CsF is significantly less effective than relative translational energy in promoting reactive decay of the complex. The theoretical implications of this result are discussed.