COLLISIONAL REORIENTATION OF SYMMETRICAL-TOP MOLECULES IN STARK FIELDS

Calculated cross sections for collisional defocusing of a beam of symm etric-top molecules in a hexapole field are compared with new and rece nt experimental values. The calculations show that collision processes involving changes in K have much smaller cross sections than those fo r changing M, and cross sections for changing J are smaller still. The beam defocusing produced by collisions with a background gas that is not the same species as the molecules in the beam occurs mainly as a c onsequence of Delta M transitions induced by a time-varying field whic h comprises the dipole-induced dipole potential and the anisotropic pa rt of the London potential between the symmetric top and its collision partner. Because the dipole-dipole potential depends on 1/r(3), its t ime variation is much slower, and so it is much less effective than th e potentials which vary as 1/r(6) at inducing transitions by this mech anism during long-range (similar to 1 Mn) collisions at moderate relat ive velocity (similar to 500 m/s). (C) 1999 John Wiley & Sons, Inc.