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We have initiated two new investigations in the past year, both directed at increasing our understanding of the collision dynamics of electronically excited molecules that are of importance in the atmosphere.

A major focus has been on the dynamics of nitric oxide, NO, which occurs at virtually all levels of the atmosphere and is highly excited in the thermosphere.  In these experiments, we use two-color, double resonance excitation to prepare single ro-vibrational levels in the E Rydberg state.  We prepare a narrow distribution of population in rotational levels in the A state using laser light at approximately 226 nm, exciting the (0,0) A – X vibronic transition.  A second laser, operating at approximately 600 nm, then prepares a single rotational level in the E state using the (0,0) E – A transition.  We detect emission from the highly excited NO using a spectrograph/CCD combination. 

Our goal in this work is to explore the collision-induced electronic energy pathways that occur within the NO Rydberg states.  Previous work by Luque and Crosley (J. Phys. Chem. 104, 2567 (2004)) demonstrated that efficient electronic energy transfer occurs when NO in the D Rydberg state collides with diatomic nitrogen.  In our experiments, we have extended these investigations to the E Rydberg state using single collision conditions and with a higher degree of spectral resolution than utilized in the previous investigation.

Thus far, we have optimized the E – A – X double resonance excitation scheme and demonstrated that we prepare single rotational levels.  Preliminary experiments using added collision partners suggest that Ar is very inefficient at inducing electronic energy transfer.  We have also attempted experiments with diatomic nitrogen as a collision partner.  We find that in NO (E) + N₂ collisions, electronic quenching of the E state (to presumably form the X state or some other non-emitting state) competes efficiently with electronic energy transfer.  We do not observe any emission from the D, C or A electronic states.  This result is in contrast to the work of Luque and Crosley, in which emission from the C and A states is observed following excitation of the D state, with N₂ as a collision partner.  Our goal is to explore this unusual observation in greater detail and with additional collision partners.  We hope to understand the origin of the rather different dynamics exhibited by the D and E Rydberg states.

In a second set of experiments, we are investigating the collision-induced dynamics of SO₂, following excitation of the C(¹B₂) electronic state.  We prepare SO₂ in single vibronic levels of the C state using the well characterized C – X absorption system at approximately 230 nm.  Our goal is to document the degree and selectivity of collision-induced vibrational energy transfer within the C state, and to explore whether electronic energy transfer occurs to the lower lying A or B states.  These experiments are in a very preliminary stage; we have observed evidence for vibrational energy transfer within the C state, but have not yet made any conclusive assignments.

In the meantime, we have completed our work on the electronic energy transfer pathways that accompany Br₂ + Xe,  and Br₂ + CF₄ collisions when the bromine is excited to the E ion-pair state.  This work is now being prepared for publication.  A particular focus of this work has been the comparison with similar experiments on I₂ in the E state.  To facilitate this comparison, we have re-analyzed some unpublished data from our lab on I₂ + Xe collisions to extract both electronic energy transfer branching ratios and overall cross sections.  The accumulation of this data has allowed us to propose propensity rules for electronic energy transfer.  First, there is a significant mass effect in the cross sections for electronic energy transfer, with Xe collisions being more efficient than Ar collisions which are more efficient than He collisions.  This trend is independent of the halogen collision partner and the specific identity of the initial and final electronic states, as long as they are not in precise resonance.  Second, the trends in the distribution of vibrational energy in the final electronic state are in accord with the vibrational sudden approximation.  Lighter collision partners with higher velocity (He) produce vibrational distributions that roughly follow the vibrational overlaps between the initial and final electronic states, while heavier collision partners with smaller collision velocities (Xe) produce distributions that favor pathways with small vibronic energy gaps.  Third, there is significant enhancement of the electronic energy transfer cross sections when a collision partner (CF₄) with internal degrees of freedom is used.  Clearly, electronic energy transfer is enhanced when the collision partner is capable of absorbing excess vibronic energy into rotational and/or vibrational degrees of freedom.  This effect outweighs collision partner mass as a determining factor in the efficiency of collision-induced electronic energy transfer.