Cold collisions of polyatomic molecular radicals with S-state atoms in a magnetic field: An ab initio study of He + ${\rm CH}_2(\tilde{X})$ CH 2(X̃) 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.

Proposal for Laser Cooling of Complex Polyatomic Molecules

“…Later Stuhl et al [21] pointedo ut that to reduce the number of required laser frequencies J 0 ¼ J 00 À 1t ype transitions could be used to eliminate rotational branching. Following these initial proposals, laser cooling and slowing has been demonstrated on the diatomic radicals SrF, [22,23] YO, [50,117] and CaF, [24,25] and led to the creationo f am agneto-optical trap (MOT) for SrF. [29,31] Polyatomic molecules possess extra atoms compared to diatomics,l eadingt oa dditional degrees of freedom and complexity.…”

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…180 In this case the dissociation occurs through coupling between Zeeman levels of the bound and unbound channels and the magnitude of the coupling is varied by changing the external magnetic field. The possibility of sympathetic cooling of magnetically trapped polyatomic molecules by 1 S 0 -state atoms, including alkaline earth atoms, was explored by Tscherbul et al 181,182 Coupled-channel quantum calculations of He + CH 2 indicate that the collision induced spin-relaxation rate is too slow to cause trap loss. 181 Spin relaxation also occurs slowly in collisions of spinpolarized CaH with Li atoms in a magnetic field despite the significant anisotropy of the Li-CaH interaction, 183 making Li atoms a promising cooling medium for CaH molecules.…”

Cold collisions of heavy Σ2 molecules with alkali-metal atoms in a magnetic field: Ab initio analysis and prospects for sympathetic cooling of SrOH(Morita1, Kłos2, Buchachenko3 et al. 2017Phys. Rev. ASelf Cite252441View full textAdd to dashboardCite

“…We showed that despite the strong angular anisotropy of the interactions between 2 Σ molecular radicals and alkali-metal atoms, the inelastic cross sections for interspecies collisions are strongly suppressed due to the weakness of the spin-rotation interaction in 2 Σ molecules [29]. Small polyatomic molecular radicals such as methylene (CH 2 ), methyl (CH 3 ), and amidogen (NH 2 ) were found to have small spin relaxation cross sections with S-state atoms, and hence suggested as promising candidates for sympathetic cooling experiments in a magnetic trap [30,31]. Magnetic trapping of CH 3 radicals has been accomplished in a recent experiment [32].…”

“…Cooling and trapping polyatomic molecular radicals is expected to provide new insights into many-mode vibrational dynamics, photochemistry, and chemical reactivity at ultralow temperatures [20,[30][31][32][35][36][37] [20,36].…”