The processes involved in the sensitized fluorescence excitation of the OH molecules have been investigated by photometric measurements of the intensities of lines of the (0,0), (1,0) and (1,1) OH bands. Collisions between metastable ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ mercury atoms and water molecules produce unexcited OH molecules which are then excited by further collisions with ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ mercury atoms to levels whose energies are equal to or less than the energy of the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ mercury atoms. Abnormal rotational energy is produced in the excitation of the OH molecules. Collisions between nitrogen and OH molecules transfer vibrational energy of the OH molecules to rotational energy and also reduce the rotational energy of the OH molecules somewhat toward thermal values. Collisions between helium and OH molecules are very much less effective in transferring energy than nitrogen collisions.

• Received 10 July 1939

DOI:https://doi.org/10.1103/PhysRev.56.466