We measure the ratio $gamma$ of the momentum-transfer to the vibrational quenching cross section for the X ($^1Sigma^+$), $ u=1$, $mathrm{J=0}$ state of molecular thorium monoxide (ThO) in collisions with atomic $^3$He between 800 mK and 2.4 K. We ob
serve indirect evidence for ThO--He van der Waals complex formation, which has been predicted by theory. We determine the 3-body recombination rate constant $Gamma_3$ at 2.4 K, and establish that the binding energy E$_b >$ 4 K.
We measure inelastic collisional cross sections for the ground $^3$F$_2$ state and the excited $^3$P$_0$ state of atomic thorium in cold collisions with $^3$He. We determine for Th ($^3$F$_2$) at 800 mK the ratio $gamma approx 500$ of the momentum-tr
ansfer to Zeeman relaxation cross sections for collisions with $^3$He. For Th ($^3$P$_0$), we study electronic inelastic processes and find no quenching even after $10^6$ collisions. We also determine the radiative lifetime of Th ($^3$P$_0$) to be $tau > 130$ ms. This great stability of the metastable state opens up the possibility for further study, including trapping.
Spin relaxation due to atom-atom collisions is measured for magnetically trapped erbium and thulium atoms at a temperature near 500 mK. The rate constants for Er-Er and Tm-Tm collisions are 3.0 times 10^-10 cm^3 s^-1 and 1.1 times 10^-10 cm^3 s^-1, r
espectively, 2-3 orders of magnitude larger than those observed for highly magnetic S-state atoms. This is strong evidence for an additional, dominant, spin relaxation mechanism, electrostatic anisotropy, in collisions between these submerged-shell L > 0 atoms. These large spin relaxation rates imply that evaporative cooling of these atoms in a magnetic trap will be highly inefficient.
