A New Take On Vibrational Excitation

A previously unknown mechanism describing vibrational excitation of molecules undergoing collisions may play a role in all collisions between pairs of neutral species that have the potential to form chemical bonds with one another, according to a new study (Nature 2008, 454, 88). In collisions between a hydrogen atom and a hydrogen molecule, for example, some of the kinetic energy associated with the atom's and the molecule's translational energy can be converted to vibrational and rotational energy in the molecule. Conventional wisdom holds that the energy-transfer process in this classic textbook reaction proceeds by way of a head-on collision between the atom and molecule (shown at top). The event sends the colliding partners rebounding in opposite directions, with the atom being backscattered along the path of its initial approach and away from the now-excited molecule. According to the new mechanism (bottom), which was proposed by Stuart J. Greaves of the University of Bristol, in England; Richard N. Zare of Stanford University; and coworkers, glancing collisions can lead to formation of a transitory bond between the atom and molecule. That process stretches the molecule and renders it vibrationally excited. When the momentary bond breaks, the atom continues along its initial path.