Vibrational energy transfer is a fundamental process in molecules which is closely related to chemical reactivity. Supersonic jet expansions have been an important tool in spectroscopy and chemical physics. These expansions are used to produce cold molecules under collision-free conditions. Among the various degrees of freedom that are collisionally relaxed, our focus is on vibrationally inelastic collisions between the analyte molecule and the carrier gas. A chirpedpulse Fourier-transform millimeter wave spectrometer (CP-FTmmW) is employed to observe vibrational relaxation (VR) of low-frequency vibrational modes in small molecules SO 2 , CHF 3 , CH 3 CN and a medium sized molecule CH 2 CHCN. Systematic study of several supersonic expansion parameters extracts empirical relationships between VR and collision conditions. This includes a study of VR in molecules seeded in helium considering different valve types (Even-Lavie valve vs. General Valve), instrumental parameters (nozzle temperature, stagnation pressure, orifice dimensions), and variation of the seeded molecule concentration. The identity of the collision partner is explored using several carrier gases (neon, argon, nitrogen, and hydrogen) and comparing the observed VR with that of helium. A universal inverse-linear relationship between the extent of VR and the frequency of the vibrational mode has been revealed by the experiments using helium. This was strikingly different from what was observed for other choices of carrier gases, where mode-specific VR was observed. For CH 3 CN (which has a degenerate bending mode, 2v 8 0,2 ), efficient l-relaxation was observed. Separate use of two complementary laser-based techniques, laser induced fluorescence and millimeter wave optical double resonance, led to characterization of the velocity slip effect, the onset of clustering, and effects of Van der Waals bonding, studied as analyte concentrations were increased. Apart from demonstrating the power of a multiplexed form of rotationally resolved spectroscopy (CP-FTmmW), a 'roadmap' is generated to aid the design of future experiments by tailoring the choices of supersonic conditions. Empirical and intuitive approximate models are assembled that will aid in understanding vibrationally inelastic scattering and VR across a wide range of expansion parameters.

Photoelectric charging experiments monitor the uptake of pyrene onto NaCl aerosol particles coated with either oleic acid or myristic acid. In both cases, thin coatings produce a small net decrease in pyrene uptake. In the larger coverage limit, the uptake of the myristic acid coated particles remains nearly constant whereas the oleic acid coated particles exhibit greater uptake rates than the bare NaCl particles. Fitting the results with a multilayer kinetic model yields uptake rate coefficients as well as parameters that describe the distribution of organic molecules on the aerosol particle surface. The model accounts for the decrease in uptake associated with thin coatings of oleic acid through a concomitant reduction in surface area. The adsorption rate constants for the myristic and oleic acid coated surfaces are 50 and 80 times faster, respectively, than for NaCl. The desorption rates for pyrene on the fatty acid surfaces are faster, as well. For myristic acid coatings, the fast desorption (over 400 times the rate of desorption from NaCl) results in slower net adsorption, whereas for oleic acid (approximately 12 times the desorption rate from NaCl), the net uptake rate increases with coverage. The results also suggest that both myristic acid and oleic acid spread incompletely on the aerosol surfaces under the conditions of these experiments. In the optimized kinetic model, the fatty acids cover approximately 50% of the surface when the nominal coating thickness is approximately 6 nm. The surface is over 90% covered with a nominal coating thickness of 20 nm, which is approximately 10% of particle diameter in these experiments. Very thin oleic acid coatings reduce the surface area of particles consistent with the preferential coverage of highly corrugated or porous regions.

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The Great Oxygenation Event (GOE), the introduction of O 2 into the Earth's atmosphere approximately 2.4 billion years ago, is a critical signpost in the development of life on Earth. The vanishing of sulfur isotope anomalies, called Sulfur Mass-Independent Fractionation (S-MIF), in the rock record is thought to be correlated with oxygenation of the early atmosphere. However, the mechanism for the generation of S-MIF in an anoxic atmosphere is unknown. Here, I propose a mechanism that involves spectroscopic perturbations in the B-X UV band system of S 2 . This proposal is based on a global deperturbation analysis done by Green and Western a b and work that I presented previously at this conference in 2015 (MG12) and 2016 (MG08). Specifically, perturbations of the "bright" B state by a "dark" B" state cause some isotopologues to have longer average excited state lifetimes than others. I demonstrate a difference between the shorter-lifetime symmetric (e.g. 32 S-32 S) isotopologues of S2, for which nuclear permutation symmetry causes half of the rotational lines to be missing, and the longer-lifetime asymmetric isotopologues (e.g. 33 S-32 S). I also comment on general features of the B/B" system of S 2 that make it uniquely well-suited to generate a large MIF isotope effect. a M.E. Green, C.M. Western, A deperturbation analysis of the B 3 Σu − (v' = 0-6) and the B" 3 Πu (v' = 2-12) states of S 2 , J. Chem. Phys. 104 (3) (1996) 848-864. b M.E. Green, C.M. Western, Upper vibrational states of the B" 3 Πu state of 32 S 2 , J. Chem. Soc., Faraday Trans. 93 (3) (1997) 365-372.