| 概 要 |
In contrast to atoms, laser cooling of molecules is rather challenging dueto the complex internal structure of molecules. The difficulty lies in finding sufficiently closed cycling transitions [1]. To circumvent this limitation, alternative schemes are considered. Methods that use ultracold atoms to generate ultracold molecules are referred to as indirect methods [2][3].
On the other hand, methods that provide cold samples of naturally abundant molecules are classified as direct methods. Here, typically cold molecules are extracted from thermal reservoirs or produced by supersonic beam sources.
For applications where only the temperature of the external degrees of freedom is of importance electric velocity filtering can be used to obtain large samples of slow polar molecules [4][5]. Here, an electric guide is used to extract slow polar molecules from a thermal reservoir.
This approch is combined with the technique of buffer-gas cooling [6]. Here, an inert buffer gas enclosed in a container (buffer-gas cell) which is cooled by a refrigerator (helium refrigerator, pulse tube cooler etc.) is used to cool a gas of molecules [7]. The many collisions between the molecules and the cold buffer gas atoms dissipate the kinetic and internal energy of the molecules. Thereby a high-density guided beam of slow and internally cold polar molecules in the few Kelvin regime is produced.
To bridge the gap between Molecules in the few Kelvbin to the sub-Kelvin and sub-mK regime an efficient cooling scheme has to be used. In this talk, experimental realization of opto-electrical cooling [7], a general Sisyphus-type cooling scheme for polar molecules will be presented. As a first result, electrically trapped methyl-fluoride molecules have been cooled by a factor 4.6 to 77mK, resulting in an increase in phase-space density by a factor 7. The electric trap which holds the molecules is a key element of this approach [8]. Improvements in our trap design will allow cooling to sub-mK temperatures and beyond, opening a viable direct route towards a molecular BEC starting from a warm ensemble for a broad range of molecular species.
[1] H. J. Metcalf and P. van der Straten. Laser Cooling and Trapping. Springer, Berlin (1999)
[2] O. Dulieu, M. Raoult, and E. Tiemann. Cold molecules: a chemistry kitchen for physicists? J. Phys. B 39 (2006).
[3] L. D. Carr, D. DeMille, R. V. Krems, and J. Ye. Cold and ultracold molecules: science, technology and applications. New J. Phys. 11, 055049 (2009).
[4] S. A. Rangwala, T. Junglen, T. Rieger, P. W. H. Pinkse, and G. Rempe. Continuous source of translationally cold dipolar molecules. Phys. Rev. A 67, 043406 (2003).
[5] T. Junglen, T. Rieger, S. A. Rangwala, P. W. H. Pinkse, and G. Rempe. Slow ammonia molecules in an electrostatic quadrupole guide. Eur. Phys.
J. D 31, 365 (2004).
[6] J. D. Weinstein, R. deCarvalho, T. Guillet, B. Friedrich, and J. M. Doyle. Magnetic trapping of calcium monohydride molecules at millikelvin temperatures. Nature 395, 148 (1998).
[7] L. D. van Buuren, C. Sommer, M. Motsch, S. Pohle, M. Schenk, J. Bayerl, P. W. H. Pinkse, and G. Rempe. Electrostatic Extraction of Cold Molecules from a Cryogenic Reservoir. Phys. Rev. Lett. 102, 033001 (2009).
[8] M. Zeppenfeld, M. Motsch, P. W. H. Pinkse and G. Rempe. Optoelectrical cooling of polar molecules. Phys. Rev. A 80, 041401(R) (2009).
[9] B. G. U. Englert, M. Mielenz, C. Sommer, J. Bayerl, M. Motsch, P. W. H. Pinkse・ G. Rempe, and M. Zeppenfeld. Storage and Adiabatic Cooling of Polar Molecules in a Microstructured Trap. Phys. Rev. Lett. 107, 263003 (2011)
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