Person:

Doyle, John

We measure and theoretically determine the effect of molecular rotational splitting on Zeeman relaxation rates in collisions of cold (^3)Σ molecules with helium atoms in a magnetic field. All four stable isotopomers of the imidogen (NH) molecule are magnetically trapped and studied in collisions with (^3)He and (^4)He. The (^4)He data support the predicted 1/B(^{2}_{e}) dependence of the collision-induced Zeeman relaxation rate coefficient on the molecular rotational constant B(_e). The measured (^3)He rate coefficients are much larger than the (^4)He coefficients, depend less strongly on B(_e), and theoretical analysis indicates they are strongly affected by a shape resonance. The results demonstrate the influence of molecular structure on collisional energy transfer at low temperatures.

We 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.

We demonstrate that buffer-gas cooling combined with laser ablation can be used to create coherent optical media with high optical depth and low Doppler broadening that offers metastable states with low collisional and motional decoherence. Demonstration of this generic technique opens pathways to coherent optics with a large variety of atoms and molecules. We use helium buffer gas to cool (^{87}Rb) atoms to below (7 K) and slow atom diffusion to the walls. Electromagnetically induced transparency in this medium allows for (50%) transmission in a medium with initial optical depth (D>70) and for slow pulse propagation with large delay-bandwidth products. In the high-(D) regime, we observe high-contrast spectrum oscillations due to efficient four-wave mixing.

We report the creation of a Bose-Einstein condensate using buffer-gas cooling, the first realization of Bose-Einstein condensation using a broadly general method which relies neither on laser cooling nor unique atom-surface properties. Metastable helium ((^4)He*) is buffer-gas cooled, magnetically trapped, and evaporatively cooled to quantum degeneracy. 10(^{11}) atoms are initially trapped, leading to Bose-Einstein condensation at a critical temperature of (5 \mu K) and threshold atom number of 1.1×10(^6). This method is applicable to a wide array of paramagnetic atoms and molecules, many of which are impractical to laser cool and impossible to surface cool.

The (v = 1 \to 0) radiative lifetime of (NH (X^3 \Sigma^-, v=1, N=0)) is determined to be (\tau_{rad,\text{exp.}} = 37.0 \pm 0.5_{\text{stat}}{}^{+2.0} {-0.8\text{syst}}) miliseconds, corresponding to a transition dipole moment of (|\mu{10}| = 0.0540_{{-0.0018}^{+0.0009}}) D. To achieve the long observation times necessary for direct time-domain measurement, vibrationally excited (NH (X^3 \Sigma^-, v=1)) radicals are magnetically trapped using helium buffer-gas loading. The rate constant for background helium-induced collisional quenching was determined to be (k_{v=1}<3.9 \times 10^{-15}cm^3s^{-1}), which yields the quoted systematic uncertainty on τrad,exp.. With a new ab initio dipole moment function and a Rydberg-Klein-Rees potential, we calculate a lifetime of 36.99 ms, in agreement with our experimental value.

We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with (^3)He in a magnetic field. A rigorous quantum theory for spin-changing transitions is developed and applied to calculate the spin exchange and spin relaxation rates of Li and K atoms in cryogenic (^3)He gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for Li-(^3)He and K-(^3)He, which are in agreement with the present theory. Our calculations demonstrate that the alkali-metal atoms have extremely slow spin depolarization rates, suggesting a number of potential applications in precision spectroscopy and quantum optics.

We observe magnetic trapping of atomic nitrogen ((^{14})N) and cotrapping of ground state imidogen ((^{14})NH, (X)(^{3})(\Sigma) -). Both are loaded directly from a room temperature beam via buffer gas cooling. We trap approximately 1 x 10(^{11}) (^{14})N atoms at a peak density of 5 x 10(^{11}) cm(^{-3}) at 550 mK. The (12\pm{4} s 1/e) lifetime of atomic nitrogen in the trap is limited by elastic collisions with the helium buffer gas. Cotrapping of (^{14})N and (^{14})NH is accomplished, with 10(^{8}) NH trapped molecules at a peak density of 10(^{8}) cm(^{-3}).

We present a combined experimental and theoretical study of cold collisions between bismuth and helium atoms in strong magnetic fields and demonstrate that the spin-orbit interaction coupling between different nonrelativistic states of Bi leads to rapid Zeeman relaxation. The Zeeman relaxation of Bi in the ground electronic state is found to be very efficient due to the admixture of electronic excited states which show an interaction anisotropy due to their nonzero electronic orbital angular momentum. Our results indicate that dense ensembles of heavy relativistic atoms will generally be unstable in magnetic traps due to significant spin relaxation induced by spin-orbit interactions.

NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with He3 is measured to be 3.8±1.1×10−19  cm2 at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of γ=7×104, in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. Kłos, and G. Chałasiński, Phys. Rev. A 68, 051401 (2003)].

A quantitative, analytic model of evaporative cooling covering both the small- (<4) and large- (>4) η regimes is presented. h is the dimensionless parameter defined as the trap depth divided by the temperature of the trapped sample. Although some of the same general properties present at large η are also present at small η, there are significant quantitative differences. These differences must be taken into account in order to accurately extract from the trapping data quantitative measurements of, for example, collisional atomic cross sections.