Semiclassical study of the collision of a highly excited Rydberg atom with the molecules HF and HCl

The semiclassical impact-parameter method is applied to the processes of state changing and energy transfer in the collision of a highly excited Rydberg atom ({ital n}{ge}20) with the polar molecules HF and HCl. The relative motion of the molecule and atomic nucleus is taken to be rectilinear; the electron-molecule and ion core-molecule interactions are represented by cutoff dipole forms. Cross sections for transitions involving quantum numbers {ital n} and {ital l} of the atom and rotational quantum number {ital j} of the molecule are obtained for a range of collision energies and initial atomic and molecular states. Comparisons are made with the results of earlier classical studies and with the quantum-mechanical impulse approximation. Collision rates are calculated and compared with experimental values for {ital l} mixing and {ital n} and {ital j} changing. The agreement between experiment and theory is shown to be satisfactory, within the uncertainties of both the measurements and the theory. Cases of agreement and disagreement between various theories are examined. One finding of the present work is that the quantum-mechanical impulse approximation appears to significantly overestimate the values of various state-changing cross sections when the internal energy defect is small. The validity of the impulse approximation for collisions of Rydberg atoms with polar molecules is discussed.