Classical dynamics of state-resolved hyperthermal O(3P) + H2O(1A1) collisions

Abstract

Classical dynamics calculations are performed for O((3)P) + H2O((1)A1) collisions from 2 to 10 km s(-1) (4.1-101.3 kcal mol(-1)), focusing on product internal energies. Several methods are used to produce ro-vibrationally state-resolved product cross sections and to enforce zero-point maintenance from analysis of the classical trajectories. Two potential energy surfaces are used: (1) a recently developed set of global reactive surfaces for the three lowest triplet states which model OH formation, H elimination to make H + OOH, O-atom exchange, and collisional excitation and (2) a non-reactive surface used in past classical and quantum collision studies. Comparisons to these previous studies suggest that for H2O vibrational excitation, classical dynamics which include gaussian binning procedures and/or selected zero-point maintenance algorithms can produce results which approximate quantum scattering cross sections fairly well. Without these procedures, the classical cross sections can be many orders of magnitude greater than the quantum cross sections for exciting the bending vibration of H2O, especially near threshold. The classical cross section over-estimate is due to energy borrowing from stretching modes which dip below zero-point values. For results on the reactive surfaces, the present calculations show that at higher velocities there is an unusually large amount of product internal excitation. For OOH, where 40% of available collision energy goes into internal motion, the excited product vibrational and rotational energy distributions are relatively flat and values of the OOH rotational angular momentum exceed J = 100. Other product channel distributions show an exponential fall-off with energy consistent with an energy gap law. The present detailed distributions and cross sections can serve as a guide for future hyperthermal measurements of this system.

Capturing quantum effects with quasi-classical trajectories in the D + H+3 → H2D+ + H reaction

“…Energies and classical vibrational actions are also computed for the product vibrational modes through an analysis of the product motion, following ref. 22. The method of separating rotational and vibrational motion is derived in ref.…”

“…(5) ''ZPE-HA'': energy binning, eqn (4), ( 19) and ( 20), using the harmonic approximation (HA) to compute the vibrational energy through the vibrational actions following ref. 22 and ignoring trajectories below the product ZPE.…”

“…We have found that the harmonic approximation for the vibrational action often undershoots the true action. 22 This results in more trajectories with energy below the ZPE, so that usually the weights ZPE-E are larger than ZPE-HA. We can make the further observation that in the energy regime here, the number of reactive trajectories is much smaller than the number of nonreactive trajectories.…”

“…The ZPE-E method is likely more accurate than ZPE-HA because the latter method performs a separation of the vibrational motion into modes and computes the action of each mode to get the vibrational energy. 22 The separation into modes can be problematic, especially for low-frequency modes or highly excited modes, and prone to numerical bias. The ZPE-E method computes the product vibrational energy directly using eqn ( 1)-( 14) of ref.…”

“…QCT with Gaussian binning has been remarkably successful in matching measurements and quantum scattering results for reaction rates and product state distributions including the reactions: O( 3 P) + HCl, OH + CO, and O( 3 P) + H 2 O. [20][21][22][23][24] ''QCT in a quantum spirit'' which can straightforwardly prepare reactant states and analyze product state distributions, if sufficiently accurate, would be highly desirable. Can a relatively inexpensive classical trajectory method, suitably weighted, work for D + H + 3 ?…”