Dynamics at the Gas Liquid Interface: Quantum State Resolved Studies of DCl, CO and OCS


The gas-liquid interface is explored via quantum state resolve direct absorption molecular
beam scattering. Both rovibrational populations and Doppler profiles are measured in order to
elucidate the dynamics at the gas-liquid interface. This work explores the collisional dynamics
between several different molecules (DCl, CO and OCS) and the liquid surface of the three
prototypical liquid scattering surfaces (PFPE, glycerol and squalane) as function of both the
incident collisional energy and the temperature of the surface. Each of these molecules has had an
interesting story to tell.

The scattering of deuterium chloride (DCl) was shown to follow the known empirical
scattering pathways. These can be split up into two distinct paradigms, thermal desorption (TD)
where molecules collide with the surface, trap in the surfaces bound well, thermalize with the
surface and then desorb at the temperature of the surface, and impulsive scattering (IS) where
molecules hits the surface hard, undergoes a few collisions and then scatters from the surface. In
this IS pathway, empirically it has been seen that the molecules scatter with a rotational distribution
that is temperature-like but hotter than the surface. It was shown that a simple impulsive scattering
model of DCl, when convolved with gaussian distributed surface roughness, can predict a
temperature like rotational distribution. Carbon monoxide scattering from these prototypical
liquids showed behavior that did not follow the two temperature model. At low collision energies,
a sub thermal scattering pathway was discovered. By comparing molecular dynamics between
molecules that undergo TD dynamics and CO, it was proposed that due to the shallow attractive
interaction CO undergoes with the surface is not deep enough for CO to promote the TD pathway
and thus a low energy IS pathway is probably seen. Due to the low energy vibrations of OCS,
populated vibrational states can be observed in both the incident and the outgoing molecules. Low 
energy collisional studies showed that vibrational energy transfer does not occur for TD
trajectories. At high collisional energies, the same behavior was seen for the IS pathway.
Remarkably, at these high energies, it was shown the vibrational energy transfer occurs along the
TD pathway.

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Department of Physics
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University of Colorado Boulder
JILA PI Advisors
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