Complex scattering lengths for ultracold He collisions with rotationally excited linear and nonlinear molecules

The translational and internal level cooling of atoms and molecules in ultracold gases results from a combination of elastic and inelastic collisional processes. While elastic collisions lead to rapid thermalization, exoergic inelastic collisions may lead to heating and trap loss. To date, most collisional studies have targeted low-lying levels of diatomic molecules. Here we investigate inelastic quenching and elastic scattering of rotationally excited linear (H-2, HD, CO, O-2, and CO2) and nonlinear (H2O and NH3) molecules in ultracold collisions with He and report the corresponding complex scattering lengths. It has been found that the ratio of the imaginary component beta to the real component alpha of the scattering length generally increases with decreasing rotational constant for linear molecules. With the exception of CO, beta becomes significantly smaller than alpha as the energy gap for rotational transitions increases. In all cases, beta decreases with rotational energy gap for relatively large rotational excitation, allowing for convenient fits to an exponential energy gap formula. Excited rotational levels of H-2 and HD appear to be collisionally stable due to the very low values of beta/alpha. Rotationally excited H2O also appears to be a viable candidate for He buffer gas cooling due to relatively small values of beta.