Our work to date on the grant project has focused on the modifications required to our existing apparatus, along with the development of software controls and calibration procedures.  We have now implemented our NO laser-induced fluorescence detection scheme and are detecting very cold (< 10 K rotational temperature) NO with signal-to-noise levels that exceed our expectations.  Preliminary experiments investigating the inelastic collision dynamics of NO with Cl atoms have not yet succeeded, perhaps due to inadequate power in the 355 nm laser used to photolyze Cl₂.  A 193 nm excimer laser, which will be used to produce O atoms (also for collision with NO) using SO₂ as a precursor is now on site and will be operational by the end of 2007.  Our work on O + NO collisions will begin in earnest at that time.

        In the meantime, we have taken the opportunity to pursue a related project that builds on expertise developed in our lab over the past several years.  The ion-pair states of the diatomic halogen molecules provide a nearly ideal environment for studying the details of collision-induced electronic energy transfer.  In a typical experiment, double resonance laser excitation is used to prepare single rotational levels in a single vibronic level in the E(0_g⁺) ion-pair state of I₂ or Br₂.  The E state is one of 6 closely spaced electronic states, with a difference in T_e values of only 1500 cm^-1.  Collisions of the ion-pair excited molecule with added collision partners results in population transfer to nearby ion-pair states, with electronic and vibrational populations that can deduced from wavelength resolved emission spectra.  In previous work, we have examined the electronic energy transfer pathways that accompany collisions between I₂(E) and He, Ar and CF₄.  We found that within a particular final electronic state, the vibrational distribution reflects a balance between a propensity to minimize the overall vibronic energy gap and a preference for final states with large Franck-Condon overlap with the initially prepared state.  The experiments involving CF₄ reveal an additional component in the inelastic collision dynamics: electronic energy transfer pathways that result in intermolecular energy transfer into the lowest frequency vibrational degree of freedom in the CF₄ molecule. 

        With this work in mind, we have extended our experiments to consider collisions of Br₂(E) with He, Ar and CF₄.  The results obtained when He and Ar are the collision partners are broadly in accord with those observed in I₂(E) collisions.  A new feature of this work is that our theoretical collaborator, Alexei Buchachenko (Moscow State University, Russia) has performed quantum scattering calculations on model potential energy surfaces for Br₂(E) + He and Br₂(E) + Ar.  Theory is in excellent agreement with experiment, which gives us confidence that we will reach our ultimate goal of developing a set of easily applied propensity rules for predicting the outcome of collisions in which electronic energy transfer is a major inelastic channel.  This work has been presented by faculty and students at poster sessions; a manuscript describing our experimental and theoretical collaboration is now in preparation.  Experiments investigating Br₂(E) + CF₄ collisions are now underway; our goal is to explore the importance of the intermolecular energy transfer channel that was previously identified in I₂(E) + CF₄ collisions.  Buchachenko plans parallel theoretical calculations on this system as well.