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 (H₂, HD, CO, O₂, 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 β to the real component α of the scattering length generally increases with decreasing rotational constant for linear molecules. With the exception of CO, β becomes significantly smaller than α as the energy gap for rotational transitions increases. In all cases, β decreases with rotational energy gap for relatively large rotational excitation, allowing for convenient fits to an exponential energy gap formula. Excited rotational levels of H₂ and HD appear to be collisionally stable due to the very low values of β/α. Rotationally excited H₂O also appears to be a viable candidate for He buffer gas cooling due to relatively small values of β.