Rotational Excitations in CO-CO Collisions at Low Temperature: Time-Independent and Multiconfigurational...

The rotational excitation in collisions between two carbon monoxide molecules was studied while combining the use of both time-independent and time-dependent formalisms. All of the calculations made use of a recently published four dimensional PES for CO dimer. Time-independent scattering calculations were performed in the lower part of the collision energy range, thus limiting the number of open channels and computational cost. The PES features a low-energy path for geared motion that appears to affect the excitation propensities in low-energy collisions. For reactants colliding without initial rotational excitation, symmetric excitations (both monomers excited equally) are strongly favored. This behavior deviates significantly from an exponential gap model based on endo- or exoergicity. Comparable time-dependent calculations were performed in an extended energy range made feasible by the lower cost of those calculations. The wave packet propagation in the time-dependent approach was performed with the multiconfiguration time-dependent hartree (MCTDH) method and analyses via the flux method, and the Tannor and Weeks approach was used to calculate the transition probabilities in the energy range up to 1000 cm(-1). We deduce from the cross sections the corresponding reaction rates for temperatures between 10 and 250 K. MCTDH was found to yield well-converged results, where the methods overlap, validating the use of MCTDH as an efficient tool to study collision processes.