Another noteworthy point is that incident beams at a high nozzle temperature, having a higher fraction of vibrationally excited CO 2 , especially in the low-lying bending modes, do not show a measurable increase in S 0 for the same E i (red open circles, Figure , T n = 743 -763 K). The total bending mode population at T n = 750 K is estimated to be 34% (26.5% in (010) and 7.5% in (020) state, see SI-4). This estimate allows us to put an upper limit to the relative efficiency of vibrational and translational energy in overcoming the reaction barrier, often described in terms of vibrational efficacy. In particular, S 0 measurements at nearly the same incident energy but at different nozzle temperatures help us understand this aspect better. Consider the measurements at E i ∼ 0.6 eV with T n = 300 K and 750 K (Figure (a), blue triangle, point #2 and hollow red circle, point #1), and measurements at E i ∼ 1.4 eV at T n = 300 K and 745 K (Figure (a), blue triangle point #5 and hollow red circle point #2). At 750 K, the mean vibrational energy of the incident CO 2 beam is 0.08 eV (assuming no vibrational relaxation in the expansion process). For the same energy provided in the form of translational energy, S 0 increases approximately by a factor of two.