Electronic Quenching of OH(2Σ+) in Flames and Its Significance in the Interpretation of Rotational Relaxation

Abstract

The quenching of fluorescence of electronically excited OH(2Σ+) molecules in the burnt gases above a lean acetylene-oxygen flame burning at a pressure of 3.5 mm Hg has been measured for resolved rotational lines. From these data the lifetime in this flame of the 2Σ+ state with respect to electronic quenching collisions is found to be 0.45×10—7 sec, about one-tenth of the radiative lifetime of the free molecule. Quenching measurements as a function of pressure and composition give cross sections for quenching by O2, CO2, and H2O of 7, 16, and 35 A2. These values exceed by a factor of 10 or more the cross sections to be expected for simple exchange of rotational with translational energy. For argon the quenching cross section is less than 2 A2. The large cross sections for quenching by the flame gases, and the resultant short lifetime of electronically excited OH molecules in the flame mean that these molecules undergo a radiationless loss of electronic energy in collisions before they have time for either a radiative transition or a collision leading to simple rotational energy transfer. The rotational distribution in the electronic spectrum emitted by OH in flames is thus determined almost exclusively by collision processes involving transfer of electronic energy. Rotational temperatures derived from these distributions bear in general no direct relation to the translational or rotational temperatures which may be associated with the other species in the flame, and are determined principally by specific excitation processes.