Buffer gas

Abstract: In a number of recent studies, information about the structure of large polyatomic ions has been deduced from gas phase ion mobility measurements by comparing mobilities measured in helium to those estimated for assumed geometries using a hard sphere projection approximation. To examine the validity of this approach, we have compared mobilities calculated using the hard sphere projection approximation for a range of fullerenes (C20−C240) to those determined from trajectory calculations with a more realistic He−fullerene potential. The He−fullerene potential we have employed, a sum of two-body 6-12 interactions plus a sum of ion-induced dipole interactions, was calibrated using the measured mobility of C60+ in helium over an 80−380 K temperature range. For the systems studied, the long-range interactions between the ion and buffer gas have a small, less than 10%, effect on the calculated mobility at room temperature. However, the effects are not insignificant, and in many cases it will be necessary to cons...

Abstract: Recent advances1,2,3,4,5 in the magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have opened important areas of research, such as Bose–Einstein condensation and ultracold atomic collisions. Similarly, the ability to trap and cool molecules should facilitate the study of ultracold molecular physics and collisions6; improvements in molecular spectroscopy could be anticipated. Also, ultracold molecules could aid the search for electric dipole moments of elementary particles7. But although laser cooling (in the case of alkali metals1,8,9) and cryogenic surface thermalization (in the case of hydrogen10,11) are currently used to cool some atoms sufficiently to permit their loading into magnetic traps, such techniques are not applicable to molecules, because of the latter's complex internal energy-level structure. (Indeed, most atoms have resisted trapping by these techniques.) We have reported a more general loading technique12 based on elastic collisions with a cold buffer gas, and have used it to trap atomic chromium and europium13,14. Here we apply this technique to magnetically trap a molecular species—calcium monohydride (CaH). We use Zeeman spectroscopy to determine the number of trapped molecules and their temperature, and set upper bounds on the cross-sectional areas of collisional relaxation processes. The technique should be applicable to many paramagnetic molecules and atoms.

Abstract: A new cooling technique for heavy ions stored in a Penning trap has been developed. The axial and cyclotron motions are cooled by buffer gas collisions. The outward radial diffusion caused by the buffer gas is counteracted by an azimuthal quadrupole rf field at the sum frequency of the magnetron and cyclotron motions. A mass selectivity of 500 in the cooling is achieved while the axial energy distribution is observed to be in equilibrium with the buffer gas temperature (T = 300 K).

Abstract: The ability to cool and slow atoms with light for subsequent trapping allows investigations of the properties and interactions of the trapped atoms in unprecedented detail. By contrast, the complex structure of molecules prohibits this type of manipulation, but magnetic trapping of calcium hydride molecules thermalized in ultra-cold buffer gas and optical trapping of caesium dimers generated from ultra-cold caesium atoms have been reported. However, these methods depend on the target molecules being paramagnetic or able to form through the association of atoms amenable to laser cooling, respectively, thus restricting the range of species that can be studied. Here we describe the slowing of an adiabatically cooled beam of deuterated ammonia molecules by time-varying inhomogeneous electric fields and subsequent loading into an electrostatic trap. We are able to trap state-selected ammonia molecules with a density of 10(6) cm(-3) in a volume of 0.25 cm3 at temperatures below 0.35 K. We observe pronounced density oscillations caused by the rapid switching of the electric fields during loading of the trap. Our findings illustrate that polar molecules can be efficiently cooled and trapped, thus providing an opportunity to study collisions and collective quantum effects in a wide range of ultra-cold molecular systems.

Abstract: The Overhauser nuclear polarization effect involving dipolar interactions between an optically polarized atom and the nucleus of a suitable buffer gas was observed in He/sup 3/ gas used as the buffer for the optical pumping of rubidium vapor. The rubidium was polarized and the degree of nuclear polarization of He/sup 3/ was determined. The polarization was reduced by relaxation processes. The relaxation time was proportional to the density and doubled in going from 300 to 77 deg K. (M.C.G.)