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Vibrational cooling of large molecules in supersonic expansions: The case of C60 and pyrene Bradley M. Gibson and Jacob T. Stewart, Department of Chemistry, University of Illinois at Urbana-Champaign Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign Why study C60? Symmetry Figure from: Astrochemical Relevance J. Cami, J. Bernard-Salas, E. Peeters, and S.E. Malek. Science 329, 1180 (Sep. 2010) {2} How do we look for C60? β’ High- temperatureC(~850 K) Pressure oven Vapor 60 β’ Cooling via continuous supersonic expansion β’ CW-CRDS detection, ~1185 cm-1 Figures from: B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of gas phase C60. UIUC, 2011. {3} What do we expect to see? β’ Branch heads possible S/N= ππ§ ππ β² πΏπππ‘ βπ£(πππΈπ΄ ) βB β’ Pinhole nozzle: ~2 β’ Slit nozzle: ~40 Figure from: J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation. {4} What did we actually see? Estimated: Observed: (NEA ~0.6 ppm) Figures from: J. T. Stewart, B. Brumfield, B. M. Gibson, B. J. McCall. In preparation. {5} Why didnβt we see anything? High-Temperature Ground State Oven Population Alignment 1.0 Fraction in Ground State 0.8 0.6 0.4 0.2 0.0 0 Figure from: 50 100 200 150 Vibrational Temperature (K) 250 300 B. Brumfield. Development of a quantum cascade laser based spectrometer for high-resolution spectroscopy of gas phase C60. UIUC, 2011. {6} Do other molecules cool efficiently? Pyrene β’ Oven temp ~430 K β’ Estimated from absorption depth β’ Tvib 60-90 K D2O β’ Oven temp ~800 K β’ Estimated from hot band β’ Tvib > 1000 K {7} How does vibrational cooling work? V-T Transfer v=2 v=1 kBT kBT v=0 Translational Energy: β Vibrational Energy: β See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {8} How does vibrational cooling work? V-T Transfer v=2 kBT v=1 kBT v=0 Translational Energy: β Vibrational Energy: β See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {9} How does vibrational cooling work? V-T Transfer -4 10 -6 Fraction in Ground State (After Cooling) 10 -8 10 -10 10 -12 10 -14 10 -16 10 -18 10 -20 10 300 400 500 600 Initial Temperature (K) 700 800 {10} How does vibrational cooling work? Intramode Relaxation v=2 v=1 v=0 See also: M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {11} How does vibrational cooling work? Intermode Relaxation Bend See also: Stretch M. E. Sanz, M. C. McCarthy and P. Thaddeus. J. Chem. Phys. 122, 194319 (2005) {12} How does vibrational cooling work? Cluster Predissociation Bend See also: Intermolecular Stretch G. Ewing. Chem. Phys. 29, 253 (Apr. 1978) {13} How can we produce colder vapor? Laser Ablation β’ Initial temperature >1900K Laser Desorption β’ Initial temperature uncertain β’ Ground state detectable by R2PI See also: E. E. B. Campbell, I. V. Hertel, Ch. Kusch, R. Mitzner, and B. Winter. Synth. Mat. 77, 173 (1996) R. E. Haufler, L-S. Wang, L. P. F. Chibante, C. Jin, J. Conceicao, Y. Chai, and R. E. Smalley. Chem. Phys. Lett. 179, 449 (1991) {14} How can we produce colder vapor? Supercritical Fluid Expansion β’ Initial temperature solvent dependent (~450 K) β’ CO2 w/ toluene co-solvent β aids cooling β’ Low vapor flux (~1013 molecules s-1) See also: C. H. Sin, M. R. Linford, and S. R. Goates. Anal. Chem. 64, 233 (1992) {15} Conclusions β’ β’ β’ β’ C60 signal not yet observed Lack of signal likely due to poor vibrational cooling Efficient cooling highly dependent upon initial temperature New vaporization technique required {16} Acknowledgements β’ McCall Group β’ Brian Brumfield β’ Claire Gmachl {17}