State-resolved rotationally inelastic cross sections of CaCl (X 2Σ+) with polar molecule targets

State-to-state rotationally inelastic integral cross sections for the scattering of Ca35Cl(X 2Σ+) by the polar molecules HCl, HCN, and SO2 have been determined. CaCl in the incident N=2 e level is prepared by electric quadrupole state selection of a supersonic beam, and final states are detected by single-mode cw dye laser fluorescence excitation in the B 2Σ+–X 2Σ+ band system. The cross sections clearly display a propensity for conservation of fine-structure level, i.e., e → e, in collisional rotational transitions, predicted by Alexander [J. Chem. Phys. 76, 3637 (1982)] from examination of the fully quantum description of the dynamics. The magnitudes of the cross sections are also found to be large, as expected from the long range of the electrostatic dipole–dipole interaction. However, in spite of the relatively high collision energy, the sudden approximation considerably overestimates the cross sections and does not explain their nonmonotonic ordering with target dipole moment. The adiabatically corrected sudden (ACS) theory, on the other hand, satisfactorily explains the absolute magnitudes of the cross sections and, more importantly, provides an explanation for the dependence on target molecule properties. The peculiar dependence of the cross sections on target molecule dipole moment was found to be a result of differences in the adiabatic damping of the sudden cross sections due to the energy level spacings, and hence rotational constants, in the target molecule.