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IQSE AMO QO Seminar Series

"Is one-dimensional dynamics useful for understanding molecular vibrational energy transfer?"

Brian Stewart
Wesleyan University in Connecticut


One of the time-honored models of molecular vibrational energy transfer involves the collinear collision of an atom A with a harmonically-bound molecule BC. Work from the 1930s by Landau and Teller and from the 1960s by workers such as Rapp, Mahan and Secrets employs one-dimensional dynamics to model the essential features of vibrational energy transfer. Even three dimensional "breathing sphere" models, such as the oft-cited SSH model, ultimately rely upon one-dimensional dynamics that decouple molecular vibration from rotation.

There is ample reason to expect such a decoupling: the dramatically disparate timescales that generally characterize these degrees of freedom ensure that typical molecular collisions are rotationally sudden but vibrationally nearly adiabatic at ordinary temperatures. This implies that vibration and rotation may be treated separately; calculating the efficiency of vibrational energy transfer then simply involves consideration of the component of the collisional impulse along the internuclear axis.

It is therefore surprising that recent investigations imply that vibrational energy transfer may never be dominated by collinear dynamics. Experimental and computational work in our research group shows that the vibrational energy transfer dynamics in nonreactive collisions of lithium dimer with rare gases is dominated by T-shaped impacts; work by the Zare group shows that hydrogen atom collisions with deuterium dimer proceed through a "frustrated reaction" involving a relative velocity perpendicular to the molecular axis.

In the nonreactive case, molecular rotation plays a surprisingly important role even in collisions of initially unexcited molecules. I will discuss these findings and their likely importance in all atom-diatom collisions. The results help refine the understanding of molecular collision dynamics and also of the collisional three-body problem in general.

Tuesday, October 31, 2017
IQSE Seminar Room, 12:00 Noon
(578 MPHY)

Institute for Quantum Science and Engineering
Texas A&M University

(Pizza and salad to be served at 11:30 am)

Host: George Welch