ИСТИНА

Degeneracy of magnetic sublevels in isotropic space makes the experimental study of the orientation and alignment effects in atomic collisions difficult. External magnetic field allows one to discriminate magnetic sublevels in energy, but Zeeman splitting is normally small comparing to thermal collision energy or its spread. However, at low temperatures the situation reverses. Cold collisions in magnetic field can be therefore used to probe the cross sections of the transitions inelastic in the magnetic quantum number and, in turn, underlying effects of interatomic interaction anisotropy, the same as stipulate orientation and alignment in free space. In particular, collision-induced Zeeman relaxation of the low-field seeking magnetic states is the dominant source of losses in the buffer gas loading into the magnetic trap. Measurements of the relaxation rates supplemented by thorough theoretical analysis greatly enhance our understanding of the nature of interaction anisotropy for different open-shell atoms. In the non-relativistic picture, the only source of interaction anisotropy is the orbital electronic angular momentum, which introduces the dependence of interatomic interaction potential matrix elements on its projection onto the collision axis. The next important effect is represented by the vectorial spin-orbit (SO) coupling. The first-order (intra-multiplet) SO coupling rearranges the magnetic sublevels according to the total (orbital plus spin) electronic angular momentum J eventually suppressing the interaction anisotropy for some J levels. For instance, an atom in the 2P1/2 level interacts with the He atom isotropically, whereas anisotropic interaction takes place for the same atom in the 2P3/2 level. Previous and recent experiments with the Al, Ga and In atoms at the temperatures close to or below 1 K [1] provide clear indication of the suppressed Zeeman relaxation in the ground 2P1/2 level, in full accord with the theoretical predictions based on the rigorous quantum scattering calculations with accurate ab initio interaction potentials. By contrast, the second-order (inter-multiplet) SO coupling may induce the interaction anisotropy. This is the case for the atoms in S state with the spin S > 1/2 interacting with excited states of P symmetry. SO interaction effectively “transfers” excited-state anisotropy to the ground state. An example is the pnictogen atoms (N-Bi) in their 4So states. For heavy Sb-He and Bi-He collision complexes with very strong SO coupling, theoretical calculations predicted large interaction anisotropy – the splitting of the ground 4Σ¯ state into Ω = 1/2 and 3/2 components [2]. It largely enhances Zeeman relaxation making magnetic trapping hardly possible. The measurements fully confirmed this prediction. This work was supported by the Program of the Fundamental Research by Division of Chemistry and Material Sciences 01 of RAS. 1. Connolly, C. B., Au, Y. S., Chae, E., Tscherbul, T. V., Buchachenko, A. A., Lu, H.-I, Ketterle, W., and Doyle, J. M., "Spin-orbit suppression of cold inelastic collisions of aluminum and helium", Phys. Rev. Lett., 110(17), 173202-1-5 (2013). 2. Connolly, C. B., Au, Y. S., Chae, E., Tscherbul, T. V., Buchachenko, A. A., Ketterle, W., and Doyle, J. M., "Zeeman relaxation induced by spin-orbit coupling in cold antimony-helium collisions", Phys. Rev. A, 88(1), 012707-1-8 (2013).