The lowest quartet-state of the ketenyl (HCCO) radical: Collision-induced intersystem crossing and the ν2 vibrational mode

Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer

“…Through decades of experimental and theoretical work, collisional relaxation of highly vibrationally excited molecules can now be well described, both in terms of the mechanisms and the magnitude of the energy transferred. − Beyond factors associated with the specific collider and the driving force for the energy transfer, a critical variable associated with the excited molecule that can significantly influence the energy transfer rate is the complexity of its potential energy surface (PES). In particular, the existence of low-lying excited electronic states or structural isomers on the PES has previously been shown to enhance the rate of energy transfer. ,,− …”

“…It has been demonstrated that, when a molecule has sufficient excitation to access low-lying excited electronic states, the mixing of electronic character into the vibrational levels enhances the transition dipole driving the intermolecular energy transfer process and dramatically enhances the energy transfer rate. ,,, On the other hand, the appearance of structural isomers at sufficiently high vibrational energy results in increases in vibrational level density and therefore a decrease in energy gap for energy transfer, causing a substantial increase in the rate of energy transfer. ,,, The magnitude of energy transferred as a function of internal energy in energy regions where there are no excited electronic states or isomers tends to increase linearly with the excitation energy. However, above the onset of an energetically accessible electronic state, the energy transfer plot exhibits a distinct elbow in which the magnitude of the energy transferred increases by an order of magnitude or more.…”