Nonadiabatic dynamics of NO colliding with atoms and molecules.

The nitric oxide (NO) radical has been a paradigm molecule in cold and controlled collision studies for many years. Its ²Π electronic ground state is spatially degenerate. This results in a linear Stark effect which allows manipulation of a molecular beam of NO with electric fields. In a Stark-decelerator experiment in Nijmegen, integral and differential cross sections for collisions of NO with helium were observed at collision energies down to 0.2 cm^-1 with an energy resolution of 0.02 cm^-1 [1]. Because of the electronic (near) degeneracy, nonadiabatic couplings are strong and we use diabatic states in the quantum scattering calculations. This approach is well known, but because of the unprecedented experimental resolution and the low collision energies, we have re-visited several common approximations in our calculations. We computed potential energy surfaces at the CCSDT(Q) level and explored nonadiabatic effects beyond the usual two-state model. In particular, we developed a method to estimate the diagonal Born-Oppenheimer correction (DBOC) and determined its effect on cross sections. The DBOC from electronic structure codes (we used CFOUR) diverges for open-shell systems.

For open-shell molecule-molecule collisions there are several methods to construct diabatic electronic states which are required for quantum scattering calculations. I will show the accuracy of several methods by comparison to derivative couplings computed with high level ab inito methods [2] and demonstrate the power of the multiple-property based diabatization method that we had initially developed to study a completely different process: collision-induced absorption spectroscopy of oxygen molecules [3].

[1] T. de Jongh, M. Besemer, Q. Shuai, T. Karman, et al. Science 368, 626 (2020)
[2] T. Karman, M. Besemer, A. van der Avoird, and G.C. Groenenboom, J. Chem. Phys. 148, 094105 (2018)
[3] T. Karman, M. A. J. Koenis, A. Banerjee, D. H. Parker, et al. Nature Chem. 10, 549 (2018)