Collisional depolarization in laser-induced fluorescence of OH

Laser-induced fluorescence in a flame can be polarized when excited by a polarized laser beam. This can produce apparently anomalous rotational line intensities resulting in errors in concentration and temperature measurements. Collisional depolarization does occur, but it must compete with vibrational and rotational energy transfer and quenching and is therefore not complete. We have measured collisional depolarization of. LIF in OH at room temperature, using a discharge flow cell. The cross sections for F-1(4) in the A state (v' = 0) are 3 Angstrom(2) for the He collider, 15 Angstrom(2) for nitrogen, 6 Angstrom(2) for oxygen, 9 Angstrom(2) for carbon dioxide, 20 Angstrom(2) for argon, and an upper limit of 30 Angstrom(2) for water. For He as a collider, the cross sections for F-1(2) and F-1(1) are 6 Angstrom(2) and 9 Angstrom(2), respectively.