. A comparison with S 0 values at similar E i , but at lower T n (obtained using a more dilute mixture of CO 2 in H 2 ), a slight decrease was observed. Qualitatively speaking, at E i larger than 2 eV, this decrease is consistent with that expected from broadened E i distribution of the incident beam at higher nozzle temperatures, especially when combined with the fact that the S 0 only increases very slowly at energies larger than 2 eV. However, at lower E i a corresponding increase is not observed, indicating that the spread in E i is not the sole contributing factor here (SI-6). Given the limitations of the present set of experiments where the E i has been estimated indirectly, the spread in E i , rotational and vibrational state distribution can not be controlled independently, and that we have only a few such measurements, we are unable to conclusively understand this trend. A direct E i measurement of CO 2 especially over the entire temporal incidence beam profile can possibly help in understanding this issue better. Nonetheless, this observation does rule out any significant enhancement in S 0 caused by the vibrationally excited incident CO 2 molecules under our experimental conditions. This is important as the low-lying bending mode is expected to be populated significantly at higher T n .