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

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We present global potential energy surfaces for the three lowest triplet states in O3P H2OX1A1 collisions and present results of classical dynamics calculations on the O3P H2OX1A1 yields OHX2Pi OHX2Pi reaction using these surfaces. The surfaces are spline-based fits of approx. 20,000 fixed geometry ab-initio calculations at the CASSCFMP2 level with a O4s3p2d1fH3s2p 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 kms 16.9 kcalmol collision energy to 11 kms 122.5 kcalmol 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 kcalmol collision energy ro-vibrationally excited OHX2Pi products are a significant and perhaps dominant contributor to the observed 1-5 micron spectral emission from O3P H2OX1A1 collisions. Another important result is that OHX2Pi 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 kcalmol collision energy, vibration dominates the OH internal energy, and above approx. 50 kcalmol 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 H2OX1A1 by O3P and also OHX2Pi OHX2Pi collisions.

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  • Physical Chemistry

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