Research remains divided between three areas: construction of a pulsed-jet infrared spectrometer, ab initio investigations of weakly bound complexes of atmospheric interest, and microwave spectroscopic investigations of these complexes.  Two undergraduate students have been involved in these investigations this year.

            The first student continued her ab initio investigations of hydrochlorofluorocarbon (HCFC) complexes by extending her investigations to complexes containing radicals.  Although these calculations are not quite complete, our results indicate that the structures of these complexes will be guided by CH…O interactions, much like the results obtained previously for closed-shell species.  The dimers investigated include CHF₂–CO₂ and CHF₂–OCS.  The second student's goal has been to perform ab initio optimizations of complexes of SO₂ and O₃ with small linear molecules.  This project returns to the initial proposal topic of studying complexes containing SO₂, NO_x and CO₂.  Results have been obtained for complexes of O₃ with CO₂, N₂O, OCS and CS₂ and SO₂ with CO₂ and CS₂.  None of the ozone complexes have been studied experimentally, but experimental structures do exist for the SO₂ complexes.  Our structure for SO₂-CS₂ is in agreement with the experimental results, but SO₂-CO₂ is observed to have C_2v symmetry, while the ab initio data indicate C_s symmetry with a low barrier at the C_2v configuration.  One possible explanation is that there is a very low barrier inversion motion in the complex, leading to an average structure that is quite different from the equilibrium structure.  Further work will involve completion of the series of ab initio calculations and hopefully revisiting some of the SO₂ complexes experimentally and investigating the possibility of producing O₃ complexes in our pulsed discharge nozzle (see below).

            The experimental part of the research has been a microwave spectroscopic investigation of the CHClF₂–OCS complex.  Although the spectrum of CHCl₂F–OCS was partially assigned last year, the hyperfine structure from the quadrupolar chlorine nuclei has proven too complicated, so far, to obtain a full assignment.  To simplify the problem, we undertook an investigation of the CHClF₂–OCS complex, in which the hyperfine structure promises to be less complex.  Ab initio results indicate that a CH…O interaction dominates the structure, with two possible isomers – one in which both fluorine atoms are roughly parallel to the OCS, and one in which the chlorine atom is parallel to the OCS.  A search over several hundred megahertz led to identification of over 100 transitions.  Testing of all these transitions with only CHClF₂ present led to the elimination of about 2/3 of the lines.  Within the remaining transitions, several clusters have spacings very similar to predictions based on the ab initio calculations; however, there are more groups of transitions with these distinctive spacings than would be expected, and so far efforts to predict and identify new transitions based on these tentative assignments have been unsuccessful.  Interestingly, a reevaluation of the theoretical data indicates the energy difference between the two possible CHClF₂–OCS isomers is so small that both could be present in the supersonic expansion.  This is one possible explanation of the larger than expected number of observed transitions.  Work to assign this spectrum continues.

            The instrumentation aspect of the research has recently made several advances.  Most of the early part of 2007 was spent in obtaining a new plasma cartridge for the nitrogen laser.  Once this was installed, significant progress was made on the laser alignment, but the ringdown mirrors have not yet been installed because of our decision first to pursue a project geared towards finding a simple system with which to test the pulsed discharge nozzle which has also been constructed over the past year.  The machining work for this project was completed at the University of Illinois early in the year.  (Construction was funded by a separate grant.)  A high voltage power supply (funded by this grant) has recently been tested and is ready to connect to the nozzle.  A new window in the vacuum chamber will allow us to visually check for emission from the discharge plasma.  We will then investigate several species that will emit in the visible range before attempting to produce species such as ozone and HCFC radicals that are more relevant to this research project.  Our ultimate goal is to construct a pulsed-jet infrared spectrometer.  We are currently weighing the chances of success with a cavity ringdown spectrometer (as initially planned) or a rapid scan IR spectrometer (less sensitive to alignment issues).  We plan to make a final decision as to which direction to take the infrared spectrometer after a visit in November 2007 to a colleague who's group has both types of system in operation.