Cold collisions of ground state oxygen molecules with Helium have been investigated in a wide range of cold collision energies (from 1 µK up to 10 K) treating the oxygen molecule first as a rigid rotor and then introducing the vibrational degree of freedom. The comparison between the two models shows that at low energies the rigid rotor approximation is very accurate and able to describe all the dynamical features of the system. The comparison between the two models has also been extended to cases where the interaction potential He -O 2 is made artificially stronger. In this case vibration can perturb rate constants, but fine-tuning the rigid rotor potential can alleviate the discrepancies between the two models.Typeset using REVT E X 1

The intermediate and long-range behavior of the three lowest doublet potential energy surfaces for the F(2P j )-H2 and Cl(2P j )-H2 systems has been studied, using a harmonic expansion of the potential, where the dependence on the relative orientation of the half-filled orbital of the open-shell atom and the molecular axis has been given in terms of bipolar spherical harmonics, whereas the coefficients modulate the effect of the variation of the intermolecular distance. The contribution of van der Waals, electrostatic, and charge-transfer interactions to the strength and the intermolecular distance dependence of each radial term are derived from previous molecular beam scattering experiments and from correlation formulas. The latter provide the link of these quantities to basic properties of the interacting partners. Besides describing elastic and inelastic channels, these surfaces also provide accurate information on the entrance channel for reactions.

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We investigate the collisional stability of magnetically trapped ultracold molecules, taking into account the influence of magnetic fields. We compute elastic and spin-state-changing inelastic rate constants for collisions of the prototype molecule 17 O 2 with a 3 He buffer gas as a function of the magnetic field and the translational collision energy. We find that spin-state-changing collisions are suppressed by Wigner's threshold laws as long as the asymptotic Zeeman splitting between incident and final states does not exceed the height of the centrifugal barrier in the exit channel. In addition, we propose a useful one-parameter fitting formula that describes the threshold behavior of the inelastic rates as a function of the field and collision energy. Results show a semi-quantitative agreement of this formula with the full quantum calculations, and suggest useful applications also to different systems. As an example, we predict the low-energy rate constants relevant to evaporative cooling of molecular oxygen.