Semiclassical methods in molecular collisions

Abstract

Part 1 A classical path method using hyperbolic orbits and perturbation theory has been used to calculate rotational excitation cross sections for polar-ion-electron collisions. Good agreement with corresponding Coulomb-Born calculations is obtained close to threshold. The focussing effect of the Coulomb field is shown to be important for close collisions. Previous calculations including the dipole potential only are shown to underestimate substantially the ∆J = +1 rotational cross section particularly for weak dipoles. Calculations using the quadrupole interaction only are shown to be unreliable. Cross sections including an empirical estimate of short- range effects have been performed for HD+, CH+ and H30+ at electron energies up to a few electron volts. Part 2 The Strong Coupling Correspondence Principle (SCCP) method is applied to rotationally inelastic HF-HF and HC1-HC1 collisions. Transitions probabilities and cross sections have been calculated for different transitions and energies. Good agreement with corresponding quantum mechanical close coupling (CC) is found only for some transitions. Comparison with other theories suggests ti.at all theories are unreliable for adiabatic collisions. The first-order correspondence principle (FOCP) is consistently unreliable, overestimating the transition probability. The body-fixed correspondence principle (BFCP) approximation, the M-conserving correspondence principle (MCCP) and the decoupled-L-dominant correspondence principle (DLDCP) approximation are derived and applied to the molecule-molecule collision. Comparison with SCCP shows that MCCP is the better approximation. BFCP is good for short-range adiabatic collisions while DLDCP is good at large impact parameters only for some transitions.