| Laser Induced Fluorescence (LIF) is one of the chief methods for detecting ambient hydroxyl radicals. To measure the absolute concentration of this important atmospheric chemical species, accurately known quenching rate constants due to the dominant gases in the air are of paramount importance. Unfortunately, these rate constants have only been measured under conditions remote from those of ambient air. This dissertation reports the measurement of the rate constants of water, argon, nitrogen and oxygen under ambient conditions.;As the LIF is carried out at atmospheric pressure, the OH fluorescence life-time becomes extremely short (about 1 ns). Time Correlated Photon Counting was used to study this short fluorescence phenomenon. The unique feature this study was that the deactivation processes of the excited OH could be described completely by a kinetic model. Detailed theoretical treatments of the LIF processes are presented. The data obtained are consistent with the accepted model, thereby demonstrating the usefulness and validity of the experimental approach.;The rate constants (cc molecule('-1) sec('-1)) for quenching by water and argon were accurately measured. The vibrational relaxation rate constant by water (3.4 (+OR-) 0.6 x 10('-10)) is the first value ever reported, while the electronic quenching rate constants for water (k(,Q0) = 7.9 (+OR-) 0.47 x 10('-10), k(,Q1) = 1.48 (+OR-) 0.74 x 10('-10)) are the first measured at atmospheric pressure. The electronic quenching rate constants of the ('2)(SIGMA)(v' = 1) state (k(,Q1)) and the ('2)(SIGMA)(v' = 0) state (k(,Q0)) by argon are k(,Q0) = 0.48 (+OR-) 0.14 x 10('-12) and k(,Q1) = 0.24 (+OR-) 0.06 x 10('-11). The vibrational relaxation rate constant by argon is k(,10) = 0.34 (+OR-) 0.06 x 10('-11). The rate constants for oxygen (k(,Q1) (+OR-) k(,10) = 1.83 (+OR-) 0.43 x 10('-9) and k(,Q0) = 2.6 (+OR-) 0.6 x 10('-10)) are significantly higher than the limited measurements available at low pressure. The rate constants for nitrogen are k(,Q0) = 0.71 (+OR-) 0.08 x 10('-11) and k(,10) = 8.07 (+OR-) 0.65 x 10('-11). All these values are the first obtained at atmospheric pressure and although shown to be consistent with the widely scattered low pressure values, they allow more reliable analysis of ambient OH measurements, and result in a 31.2% increase in the published values. |
