Rotational Excitations in CO–CO Collisions at Low Temperature: Time-Independent and Multiconfigurational Time-Dependent Hartree Calculations

Abstract

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.

Three sources of errors in the Ehrenfest treatment of inelastic scattering and possible ways of resolving them

“…Such a case of strong coupling can be found in CO + CO. 43 Preliminary calculations for this system (unpublished) revealed poor agreement between MQCT and the full quantum results, even if the average velocity of Billing is used. This system is considered next.…”

“…The potential energy surface for CO + CO interaction is that by Dawes and co-workers. 42,43 In the first example, the excited state is 11(00) at energy E 2 = 7.69 cm 1 which corresponds to simultaneous excitations of both CO molecules into the first rotationally excited state. So, ∆E = 7.69 cm 1 (relative to the ground state energy E 1 = 0).…”

“…We discuss the limits of applicability of this method and test it by applying it to one real system, namely, to the inelastic scattering of CO + CO, using the potential energy surface from the recent literature. 42,43 Advantages and disadvantages of this method are outlined, and possible avenues for its development and applications are discussed.…”

Computational study of the rovibrational spectra of CO2–C2H2 and CO2–C2D2

“…13,43 These are experimentally determined ground state values 43 and are very close to a slightly more recent value. 44 For the stretch coordinate we use a potential optimised discrete variable representation with 20 functions defined in the range [4,20] bohr and computed in a sine basis. 38,45,46 To test convergence levels we also computed with 120 sine discrete variable representation functions.…”

“…Many dimers with identical linear monomers have slipped-parallel equilibrium configurations and their dynamics are dominated by a disrotatory path between different minima. [9][10][11][12]20 The CO 2 dimer is a good example. Motion along a low-energy disrotatory path connects equivalent slippedparallel minima and motion along a conrotatory coordinate, which also connects equivalent minima, is impeded by a large barrier (a PES plot is shown in Figure 7 of a recent fitting methods review).…”