The Kinetics and Dynamics of AN Atmospheric Reaction System Using Chemiluminescence, Laser-Induced Fluorescence and Stimulated Raman Excitation: Hydrogen Sulfide + Nitrous Oxide

Acid rain and the greenhouse effect are two of the most important environmental issues of this century. Oxidation of reduced sulfur compounds is responsible for acid rain, and the increase in concentration and subsequent reactivity of vibrationally excited greenhouse gases are important to understand global warming. The following reaction system addresses these two issues and forms the basis for the current research. rm HS + N_2 O(nu_1, nu_2,nu_3)to HSO + N_2 (1). rm HSO + N_2Oto SO _2 + N_2 + H (2). Chemiluminescence (CL) and laser-induced fluorescence (LIF) were used to detect products in real time. Stimulated Raman excitation (SRE) and coherent anti-Stokes Raman scattering(CARS) were used to prepare, monitor and quantify the amount of vibrational excitation achieved in the N_2 O reactant. The measured reaction rate coefficient for reaction 1 with N_2O(0,0,0) and the vibrational state distribution of the HSO product indicates the dominant pathway at room temperature is probably direct abstraction. LIF experiments demonstrate SO_2 to be a direct product in reaction 2. Our rate coefficient results suggest that N_2O could be a dominant tropospheric oxidant for reduced sulfur compounds. One quantum of excitation in the linear stretch in N_2O(100) promotes the rate of reaction 1 by a factor of 4.2 over unexcited N _2O, while excitation of the isoenergetic first overtone of the bending motion in N_2 O(020), does not enhance the reaction rate. Results are also presented on SRE of radicals. The results provide a clear demonstration of mode-specific chemistry in this reaction system. Our results also suggest a possible way to use lasers to influence reaction product branching ratios and possibly alter reaction mechanisms.