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Julia Piskorski

David Patterson


Schematic of experimental setup.

Cryogenic helium buffer gases have cooled a diversity of molecules to temperatures ~1 K. Driven by collisional physics studies, as well as mixture and chiral analysis, we are now cooling larger (often biological) molecules. A detailed understanding of helium-molecule sticking is currently unknown, as is any (related) possible molecular size limit of buffer gas cooling. We explore He-molecule collisions at energies well below their binding energy, where the pair would form a dimer (or a larger cluster) under equilibrium conditions. Molecules from an oven source (300-500 K) flow into a cold (5 K) cell, where they undergo a few hundred collisions with He that is at a density of ~10^14 cm^-3. Using laser fluorescence, we observe translational and internal cooling of benzonitrile, benzonitrile dimers, trans-stilbene and the significantly larger molecule Nile Red. Narrow, unperturbed spectral lines in trans-stilbene indicate the gas-phase monomer. Our spectra of Nile Red -- the first ever taken in the gas-phase -- also indicates cold monomer. In Nile Red, we also see broad spectral features that are possibly consistent with the creation of a molecule-molecule dimer population. We see no evidence of high He-molecule dimer formation rates (in either species) and conclude that for these molecules (as well as smaller molecules cooled in earlier work [1]) the lifetime of possible helium-molecule dimers is below ~1 microsecond.

  • [1] D. Patterson and J. Doyle, Cooling molecules in a cell for FTMW spectroscopy," Molecular Physics, vol. 110, pp. 1757{1766, 2012.