Collisional reorientation of symmetric-top molecules in stark fields

• Published in: International journal of quantum chemistry Vol. 71; no. 1; pp. 75 - 82
• Main Authors: Hu, Wan-Ping, Harris, Sean A., Harland, Peter W., Phillips, Leon F.
• Format: Journal Article
• Language: English
• Published: New York John Wiley & Sons, Inc 1999
• Subjects: beam-defocusing collisions; collisional reorientation; long-range collisions; selection rules for collisions; Stark states
• ISSN: 0020-7608; 1097-461X
• DOI: 10.1002/(SICI)1097-461X(1999)71:1<75::AID-QUA8>3.0.CO;2-G

Calculated cross sections for collisional defocusing of a beam of symmetric‐top molecules in a hexapole field are compared with new and recent experimental values. The calculations show that collision processes involving changes in K have much smaller cross sections than those for 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 consequence of ΔM transitions induced by a time‐varying field which comprises the dipole‐induced dipole potential and the anisotropic part of the London potential between the symmetric top and its collision partner. Because the dipole–dipole potential depends on 1/r3, its time variation is much slower, and so it is much less effective than the potentials which vary as 1/r6 at inducing transitions by this mechanism during long‐range (∼1 nm) collisions at moderate relative velocity (∼500 m/s). ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 75–82, 1999