Buffer gas cooling

Translational energy thermalization of molecules in buffer-gas-cooling experiments is mediated by elastic collisions with helium atoms. In order to estimate the efficiency of buffer-gas-cooling experiments, it is necessary to understand the mechanism of Zeeman relaxation and evaluate the cross-sections for elastic scattering and... 

Under the conditions of T 1000 K and M/m 50, of order 100 collisions are required for the atoms or molecules to fall to within 30% of the He buffer-gas temperature T = 0.25 K. This 100 collisions typically corresponds to a time of order 0.1-10 msec, depending on the buffer-gas density this is consistent with our observations of buffer-gas cooling. Figure 13.2 shows the thermalization of laser-ablated VO in cold helium buffer gas for different delays after the ablation pulse. The spectra show complete translational thermalization in less than 10 msec, in accordance with the above simple model. 

Numerous species have been laser-ablated and buffer-gas cooled. Laser ablation of solid materials is well established as an important tool in many scientific and technological endeavors, including surface processing, surgery, mass spectrometry and... 

FIGURE 13.3 The saturated vapor density of helium at buffer-gas cooling temperatures. The He curve is extrapolated from ITS-90 [8] and the He curve is from Ref. [7] with the old TTS-90 curve (thin line) shown for reference. The shaded region represents the range of saturated vapor densities used for buffer-gas loading, which sets a minimum temperature of 180 and 500 mK for He and He buffer gas, respectively. 

A List of Molecules That have been Helium Buffer-Gas Cooled to Less than 10 K... 

Atoms That have been Helium Buffer-Gas Cooled... 

Typical quenching cross-sections by cold ( 1 K) helium yield a rotational quench on the order of every 10 to 100 elastic collisions whereas it can take more than 10 elastic collisions before a vibrational quench [12]. DeLucia and coworkers measured rotationally inelastic cross-sections with He of H2S, NO, and H2CO. Typical values were on the order of 1 to 10 x 10 cm at 1K [13,15,56], which is about 10 to 100 times smaller than a typical diffusive cross-section at 1 K. We therefore expect buffer-gas cooling to effectively thermalize the rotational temperature of the target molecules while leaving the vibrational temperature out of thermal equilibrium. Because the rotational and translational energy transfer cross-sections are similar, thermalization of both happens rapidly and in tandem in butfer-gas cooling. We do not detect nonequilibrium rotational populations. 

Harris, J.G.E., Michniak, R.A., Nguyen, S.V., Brahms, N., Ketterle, W, and Doyle, J.M., Buffer gas cooling and trapping of atoms with small effective magnetic moments, Europhys. Lett., 67(2), 198-204, 2004. 

Egorov, D.M., Buffer-gas cooling of diatomic molecules, Ph.D. thesis. Harvard University, 2004. 

Weinstein, J.D., deCarvalho, R., Amar, K., Boca, A., Odom, B.C., Friedrich, B., and Doyle, J.M., Spectroscopy of buffer-gas cooled vanadium monoxide in a magnetic trapping field, J. Chem. Phys., 109(7), 2656-2661, 1998. 

Newman, B., Brahms, N., Johnson, C., Kleppner, D., Greytak, T., and Doyle, J., Buffer-gas cooled cerium, 2008. Unpublished. 

Bakker, J.M., Stoll, M., Weise, D.R., Vogelsang, O., Meijer, G., and Peters, A., Magnetic trapping of buffer-gas-cooled chromium atoms and prospects for the extension to paramagnetic molecules, J. Phys. B, 39, S1111, 2006. 

In traps using buffer-gas cooling, photo fragmentation is an established method [15], where it is applied to polyatomic ions [15,16,85,86], In connection with sympathetic cooling of MgH+ molecular ions, two-photon dissociation was demonstrated and the branching ratio of the two possible dissociation channels. Mg - - H+ and Mg" " + H, was investigated [76]. In all of these experiments, pulsed lasers were employed. Photodissociation of HD+ is described in the following section. 

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