title: Cold collisions of polyatomic molecular radicals with S-state atoms in a magnetic field: an ab initio study of He + CH2(X) collisions.
abstract: We develop a rigorous quantum mechanical theory for collisions of polyatomic molecular radicals with S-state atoms in the presence of an external magnetic field. The theory is based on a fully uncoupled space-fixed basis set representation of the multichannel scattering wave function. Explicit expressions are presented for the matrix elements of the scattering Hamiltonian for spin-1/2 and spin-1 polyatomic molecular radicals interacting with structureless targets. The theory is applied to calculate the cross sections and thermal rate constants for spin relaxation in low-temperature collisions of the prototypical organic molecule methylene [CH(2)(X(3)B(1))] with He atoms. To this end, two accurate three-dimensional potential energy surfaces (PESs) of the He-CH(2)(X(3)B(1)) complex are developed using the state-of-the-art coupled-cluster method including single and double excitations along with a perturbative correction for triple excitations and large basis sets. Both PESs exhibit shallow minima and are weakly anisotropic. Our calculations show that spin relaxation in collisions of CH(2), CHD, and CD(2) molecules with He atoms occurs at a much slower rate than elastic scattering over a large range of temperatures (1 μK-1 K) and magnetic fields (0.01-1 T), suggesting excellent prospects for cryogenic helium buffer-gas cooling of ground-state ortho-CH(2)(X(3)B(1)) molecules in a magnetic trap. Furthermore, we find that ortho-CH(2) undergoes collision-induced spin relaxation much more slowly than para-CH(2), which indicates that magnetic trapping can be used to separate nuclear spin isomers of open-shell polyatomic molecules.
authors.affiliation: Chemical Physics Theory Group, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada. ttscherb@chem.utoronto.ca
authors.forename: T V
authors.initials: TV
authors.lastname: Tscherbul
authors.forename: T A
authors.initials: TA
authors.lastname: Grinev
authors.forename: H-G
authors.initials: HG
authors.lastname: Yu
authors.forename: A
authors.initials: A
authors.lastname: Dalgarno
authors.forename: Jacek
authors.initials: J
authors.lastname: Kłos
authors.forename: Lifang
authors.initials: L
authors.lastname: Ma
authors.forename: Millard H
authors.initials: MH
authors.lastname: Alexander