Absorption Lineshape and Laser-Induced Fluorescence Measurements of Hydroxyl.

We performed absorption and fluorescence measurements of hydroxyl (OH) radical. In absorption measurements, the frequency-doubled output of a single-frequency CW-dye laser was tuned across the rotational-electronic lines. The absorption profile and its first derivative were respectively detected using the difference and the frequency modulation schemes. The lineshapes thus generated were analyzed using a Voigt profile by both the method of non-linear least squares fitting and the method utilizing the distance between the inflection points deduced from the first derivative measurements. We determined shift and broadening of OH transitions induced by collisions with molecular oxygen and nitrogen, as well as those with inert gases He, Ne, Ar, and Kr. Both shift and broadening induced by these collisions showed linear dependence on the density of buffer gas. The homogeneous linewidth for the P_1 (2) transition of OH in ambient air was determined. Results with Ar and Kr showed contributions not accounted for by the polarizability of the colliding species. We also studied the absorption lineshape of the OH transitions in the presence of high concentrations of water molecules. Our studies indicate that in such environments, the OH lineshape deviates from a Voigt profile because of collision-induced narrowing of the inhomogeneous part of the profile. This deviation from a Voigt profile is understood in terms of velocity -changing collisions due to near-resonance interaction with water molecules. It was found that the frequency of velocity -changing collisions depends linearly on the density of water molecules residing in particular rotational levels. The linear dependence observed between this collision-frequency and the resonance broadened linewidth of the OH transitions is discussed. We studied the pressure dependence of OH fluorescence signal and ozone interference. Measurements of pressure dependence of ozone interference indicate a good agreement between the experiments and the relevant theoretical considerations. Finally, the method of laser-induced fluorescence at low pressures was applied to measure ambient OH concentrations. The peak OH concentration was found to be about 3 times 10^6 molecules/cm ^3; the day time OH concentration was found to be in the range of middle 10^5 molecules/cm^3, and the nighttime average below the detection limit of 0.9 times 10^5 molecules/cm ^3. (Abstract shortened with permission of author.).