Person: Rasmussen, Julia Hege
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Rasmussen
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Julia Hege
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Rasmussen, Julia Hege
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Publication Cooling, Collisions and non-Sticking of Polyatomic Molecules in a Cryogenic Buffer Gas Cell(2014-10-21) Rasmussen, Julia Hege; Doyle, John M.; Greiner, Markus; Schwartz, MatthewWe cool and study trans-Stilbene, Nile Red and Benzonitrile in a cryogenic (7K) cell filled with low density helium buffer gas. No molecule-helium cluster formation is observed, indicating limited atom-molecule sticking in this system. We place an upper limit of 5% on the population of clustered He-trans-Stilbene, consistent with a measured He-molecule collisional residence time of less than \(1 \mu s\). With several low energy torsional modes, trans-Stilbene is less rigid than any molecule previously buffer gas cooled into the Kelvin regime. We report cooling and gas phase visible spectroscopy of Nile Red, a much larger molecule. Our data suggest that buffer gas cooling will be feasible for a variety of small biological molecules. The same cell is also ideal for studying collisional relaxation cross sections. Measurements of Benzonitrile vibrational state decay results in determination of the vibrational relaxation cross sections of \(\sigma_{22} = 8x10^{-15} cm^2\) and \(\sigma_{21} = 6x10^{-15} cm^2\) for the 22 (v=1) and 21 (v=1) states. For the first time, we directly observe formation of cold molecular dimers in a cryogenic buffer gas cell and determine the dimer formation cross section to be \(\sim10^{-13} cm^2\).Publication A cold and slow molecular beam(Royal Society of Chemistry (RSC), 2011) Lu, Hsin-I; Rasmussen, Julia Hege; Wright, Matthew; Patterson, Dave; Doyle, JohnEmploying a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a ‘‘slowing cell’’ placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of vf=65ms-1, a narrow velocity spread, and a flux of 109 molecules per pulse. In the other regime, our slowest beam has a forward velocity of vf=40ms-1, a longitudinal temperature of 3.6 K, and a flux of 5 x 10 8 molecules per pulse.Publication Intense Atomic and Molecular Beams Via Neon Buffer-gas Cooling(Institute of Physics, 2009) Patterson, David; Rasmussen, Julia Hege; Doyle, JohnWe realize a continuous, intense, cold molecular and atomic beam source based on buffer-gas cooling. Hot vapor (up to 600 K) from an oven is mixed with cold (15 K) neon buffer gas, and then emitted into a high-flux beam. The novel use of cold neon as a buffer gas produces a forward velocity distribution and low-energy tail that is comparable to much colder helium-based sources. We expect this source to be trivially generalizable to a very wide range of atomic and molecular species with significant vapor pressure below 1000 K. The source has properties that make it a good starting point for laser cooling of molecules or atoms, cold collision studies, trapping, or nonlinear optics in buffer-gas-cooled atomic or molecular gases. A continuous guided beam of cold deuterated ammonia with a flux of 3×10\(^{11}\) ND\(_{3}\) molecules s\(^{−1}\) and a continuous free-space beam of cold potassium with a flux of 1×10\(^{16}\) K atoms s\(^{−1}\) are realized.