Potential Surfaces and Dynamics of the O(3P) + H2O(X1A1) yields OH(X2Pi) + OH(X2Pi) Reaction

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Abstract : We present global potential energy surfaces for the three lowest triplet states in O(3P) + H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P) + H2O(X1A1) yields OH(X2Pi) + OH(X2Pi) reaction using these surfaces. The surfaces are spline-based fits of approx. 20,000 fixed geometry ab-initio calculations at the CASSCF+MP2 level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1,000-2,500 K range using these surfaces. We also compute the total, ro-vibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approx. 4 km/s (16.9 kcal/mol collision energy) to 11 km/s (122.5 kcal/mol collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approx. 40 kcal/mol collision energy ro-vibrationally excited OH(X2Pi) products are a significant and perhaps dominant contributor to the observed 1-5 micron spectral emission from O(3P) + H2O(X1A1) collisions. Another important result is that OH(X2Pi) products are formed in two distinct ro-vibrational distributions. The active' OH products are formed with the reagent O-atom, and their ro-vibrational distributions are extremely hot. The remaining spectator' OH is relatively ro-vibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approx. 50 kcal/mol collision energy, vibration dominates the OH internal energy, and above approx. 50 kcal/mol rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2Pi) + OH(X2Pi) collisions.