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In our preliminary experiments with tetracene, we were able to obtain two kinds of different LIF excitation spectra in cold nitrogen gas: 1) a relatively sharp line centered at the gas phase position (ca. 446.5 nm) and 2) a very broad spectrum that deviated only slightly from the dye laser profile. Unfortunately, we were not able to find any systematic behavior in these experiments and therefore we did not proceed further. From the spectroscopy point of view, tetracene turned out to not be an ideal test candidate since it has short radiative lifetime (ca. 20 ns) that is close to the excitation laser pulse temporal profile (9 ns), and it is a heavy molecule with the rotational levels efficiently thermally populated (except at very low temperatures) resulting in broadening of the LIF excitation spectrum. Despite these drawbacks, these experiments were able to demonstrate that our laser desorption method works correctly but gives very little information on the efficiency of the process. We have therefore shifted our efforts to study glyoxal, which can be conveniently produced from glyoxal-water polymer by heating it to ca. 200 oC. Note that this glyoxal-water polymer can also be doped by highly absorbing metal nanoparticles or embedded in activated charcoal, which would maximize the surface contact area with the surrounding polymer and significantly enhance the desorption process. From the spectroscopic point of view, glyoxal exhibits strong fluorescence, has a relative long radiative lifetime (ca. 2 microsec.), and exhibits a structured fluorescence and LIF excitation spectra even above room temperatures. This allows for careful calibration measurements all the way from room temperature gas phase conditions to superfluid helium. Note that these preparatory measurements combining laser desorption and LIF are not straightforward and primitive versions of these types of experiment have only been reported recently. The previously published work has also examined this process in air and it has been demonstrated that the method works also in the presence of dense gas where collisions between the laser desorbed molecules and N₂/O₂ are frequent. This is an encouraging finding as our target system has relatively high vapor pressures (when carried out above superfluid helium level). A preliminary LIF excitation spectrum of glyoxal in the gas phase has now been recorded. Gas phase glyoxal was generated by IR laser desorption of glyoxal exposed activated charcoal sample. We have also looked at the singlet and triplet state emissions from this system as they occur in very different timescales. After we have optimized our measurement system using glyoxal, we will go back and apply this method to tetracene. We have recently used scanning electron microscopy to study the adsorption of glyoxal on graphite to optimize the sample preparation process. We have also set up a mass spectrometer to study the laser desorbed molecules.

On the other front, we have been working on stabilization of methyl radicals at low temperatures. This project started as a side project during the previously mentioned superfluid helium experiments. We have now published two papers in the Journal of Chemical Physics and the Journal of Physical Chemistry A on this topic, which acknowledge the support from ACS-PRF. Our experimental and theoretical work has concentrated on using methyl radicals as probes for the environment where they are trapped. The methyl radicals were observed as by products of decomposition of hydrocarbon compounds.