Molecular beam measurements of inelastic cross sections for transitions between defined rotational states (j,m) of CsF in collisions with He, Ne, Ar, Kr, CH4, CF4, SF6, C2H6, N2, CO, CO2, N2O, CH3Cl, CH3Br, CF3H, and CF3Br

Abstract

Absolute inelastic cross sections for rotational quantum state to state transitions have been measured in small angle grazing collisions of CsF molecules in collisions with He, Ne, Ar, Kr, Xe, CH4, CF4, SF6, C2H6, N2, CO, CO2, N2O, CH3Cl, CH3Br, CF3H, CH3Cl, CF3Br. For the rare gas collision partners the following Δj=1, Δm=0 transitions were measured: (j,m) = (1,0) → (2,0), (2,0) → (1,0), (2,0) → (3,0), (3,0) → (2,0) as well as the Δj=2, Δm=0 transition (3,0) → (1,0). In addition the Δj=1, Δm=1 transitions (1,1) → (2,0) and (2,0) → (3,1) and the pure Δm=1 transitions (1,1) → (1,0) and (3,0) → (3,1) were measured. In each case the signs of m and Δm are undetermined. For the other scattering partners only the Δj=1, Δm=0 and Δj=2, Δm=0 transitions were measured. In the collision region the quantization axis of the CsF molecules is defined by a weak electric field (300 V/cm) directed perpendicular to the direction of the velocity selected CsF and the target nozzle beam and thus perpendicular to the relative velocity vector. The apparatus and the techniques used to determine the absolute cross sections are given and the formulas used to derive the various types of cross sections from the pressure dependences are derived. Because of the favorable geometry and nearly monoenergetic conditions it was possible to calculate by a Monte Carlo method a transmission function for each rare gas and transition studied. The tabulated transmission functions make it possible to compare theoretical predicted differential cross sections directly with the reported values. An approximate analytic expression is also given for the (2,0) → (3,0) transition which can be used for all scattering targets. This approximate transmission function is then used to obtain absolute cross sections in the center of mass system for all systems. These cross sections are correlated with the predictions of a Born-approximation theory for the various predominant potential terms. The correlation shows that for the first 8 partners the induced dipole–quadrupole induction term is predominant. For the molecules C2H6, N2, CO, CO2 and N2O an electrostatic dipole–quadrupole potential is dominant, while for the polar symmetric top molecules the dipole–dipole potential is producing the transitions. The results suggest that the method can be used to determine quadrupole moments.