Mutual vibrational quenching in CO + H2 collisions

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Vibrational quenching of CO and H2 is studied quantum mechanically for collisions where both molecules are vibrationally excited. A five-dimensional (5D) coupled states (CS) approximation is used to formulate the dynamics. The approximation is tested against six-dimensional (6D) results for CO + H2 with single vibrational excitation using both the CS approximation and the full close-coupling (CC) method. The 5D approximation is shown to provide a practical and reliable numerical approach for obtaining state-to-state cross sections in the computationally challenging case of mutual rovibrational de-excitation. State-resolved and partially-summed cross sections are presented for this astrophysically important collision system over a wide range of energies, and prospects for developing a database of rovibrational quenching rate coefficients are discussed.