Incident translation and rotation energies and their distributions were determined using spectroscopic methods in combination with our newly developed ion imaging setup. Due to the lack of availability of efficient resonance-enhanced multiphoton excitation (REMPI) schemes for CO 2 , we chose to use CO as a proxy instead. These measurements were carried out under identical experimental conditions as used for CO 2 . It should be noted that systematic differences in the rotational cooling of the incident beam can arise due to the larger rotation constant of CO (compared to CO 2 ). Hence, our inferences are valid under the assumption that small changes in the rotation state distributions will not affect the S 0 values by much.