Ab initio study of Tm-He interactions and dynamics in a magnetic trap

The dynamics of magnetic relaxation of Tm atoms in cold helium gas is investigated using ab initio interaction potentials between Tm(2F) and He atoms. The interaction of Tm(2F) with He gives rise to four adiabatic potentials of Σ+, Π, Δ, and Φ symmetries, which are found to be degenerate to within 0.1 cm-1 at all interatomic distances larger than R=5.4 Angst . The small splitting between the interaction potentials leads to suppression of Zeeman transitions in Tm-He collisions. Our quantum scattering calculations yield a small rate constant of 1.5x10-15 cm3 s-1 for Zeeman relaxation of the Tm(2F7/2) atoms in the maximally stretched M=7/2 sublevel at the temperature 0.8 K, in reasonable agreement with the measured value (5.0±2.1)x10-15 cm3 s-1. The sensitivity of Zeeman-relaxation cross sections to details of the interaction potentials is examined. To our knowledge, this is one of only two comparisons of ab initio quantum scattering calculations with experimental measurements of inelastic collisions at subkelvin temperatures near one Kelvin. Our work thus demonstrates the feasibility of describing cold collisions from first principles