Vibrational and translational enhancement of endothermic reactions: K + HCl(υ = 0,1) and K + HF(υ = 0,1)

Abstract

The relative integral cross section for the two endothermic reactions K + HCl(υ = 0 and 1) → KCl + H and K + HF(υ = 0 and 1) → KF + H has been measured as a function of the collision energy E using the crossed molecular beam technique. The vibrationally excited state (υ = 1) has been populated thermally by heating the beam source to temperatures around 2000 K. The variation of the collision energy from thermal up to around 2.1 eV was achieved by seeding the K-beam with various carrier gases. The molecular reaction product was detected by surface ionization in connection with a time-of-flight method. The total energy threshold of the reactions has been found to be equal to or only slightly above the corresponding endothermicities. This suggests a vanishing or very low barrier crest on the potential energy hypersurfaces which is contradictory to recent theoretical results. The inclusion of tunneling in case of K + HF leads to a negligible rise of the barrier heights. The efficacy of translational and vibrational energy in promoting the reactive process has been directly compared over a wide range of collision energies. For K + HCl the vibrational enhancement of the reactivity descends with increasing E from approximately a factor of 10 at E = 0.08 eV to around unity for E ⩾ 0.5 eV. The good agreement of this experimental result with phase space calculations suggests that the marked enhancements are predominantly caused by the long-range attraction between reagents in connection with an “early” barrier on the potential energy surface. In case of K + HF vibrational energy is by a factor of up to 380 more favourable in promoting the reaction than the same amount of translational energy. Again, with rising collision energy its efficacy decreases but promotes the reaction still by a factor of 70 at E = 1.7 eV. Since phase space theory fails here the reaction is certainly non-statistical and we conclude that the observed large efficacy of vibrational energy is due to a “late” barrier. The proposed barrier positions for the two systems are in accordance with theoretical results.