Person:

Doyle, John

Cold and dense samples of naphthalene ((C_{10}H_8)) are produced using buffer gas cooling in combination with rapid, high flow molecule injection. The observed naphthalene density is (n \approx 10^{11} cm_{−3}) over a volume of a few (cm^3) at a temperature of 6 K. We observe naphthalene–naphthalene collisions through two-body loss of naphthalene with a loss cross section of (\sigma_{\Lambda-\Lambda} = 1.4 × 10^{-14} cm^2). Analysis is presented that indicates that this combination of techniques will be applicable to many comparably sized molecules. This technique can also be combined with cryogenic beam methods to produce cold, high flux, continuous molecular beams.

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.

Spin relaxation due to atom–atom collisions is measured for magnetically trapped erbium and thulium atoms at a temperature near 500 mK. The rate constants for Er–Er and Tm–Tm collisions are 3.0×10(^{-10}) and 1.1×10(^{-10}) cm(^3) s(^{-1}), respectively, 2–3 orders of magnitude larger than those observed for highly magnetic S-state atoms. This is strong evidence for an additional, dominant, spin relaxation mechanism, electronic interaction anisotropy, in collisions between these “submerged-shell,” L≠0 atoms. These large spin relaxation rates imply that evaporative cooling of these atoms in a magnetic trap will be highly inefficient.

We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of ∼600  mK. We measure a small N + NH trap loss rate coefficient of (\kappa_{loss^{(N+NH)}}) = 9(5)(3)×10(^{-13})  cm(^3) s(^{-1}). Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N + NH elastic-to-inelastic collisions remains large (>100) into the mK regime.

We predict that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10(^{12})  cm(^{-3}). We explore the spin relaxation of these molecules in buffer-gas-loaded magnetic traps and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong (T^{-6}) thermal dependence of the spin-change rate of silver (Ag) trapped in dense (^{3})He is accounted for by the formation and spin change of Ag(^{3})He van der Waals molecules, thus providing indirect evidence for molecular formation in a buffer-gas trap.

The collisional magnetic reorientation rate constant (\vartheta_{\mathbb{R}}) is measured for magnetically trapped atomic dysprosium (Dy), an atom with large magnetic dipole moments. Using buffer gas cooling with cold helium, large numbers (>10(^{11})) of Dy are loaded into a magnetic trap and the buffer gas is subsequently removed. The decay of the trapped sample is governed by collisional reorientation of the atomic magnetic moments. We find (\vartheta_{\mathbb{R}} = 1.9 \pm 0.5 \times 10^{-11} , \text{cm}^{3} , \text{s}^{-1}) at 390 mK. We also measure the magnetic reorientation rate constant of holmium (Ho), another highly magnetic atom, and find (\vartheta_{\mathbb{R}} = 5 \pm 2 \times 10^{-12} , \text{cm}^3, \text{s}^{-1}) at 690 mK. The Zeeman relaxation rates of these atoms are greater than expected for the magnetic dipole-dipole interaction, suggesting that another mechanism, such as an anisotropic electrostatic interaction, is responsible. Comparison with estimated elastic collision rates suggests that Dy is a poor candidate for evaporative cooling in a magnetic trap.

Buffer gas cooling is used to trap NH molecules with 1/e lifetimes exceeding 20 s. Helium vapor generated by laser desorption of a helium film is employed to thermalize 10(^5) molecules at a temperature of 500 mK in a 3.9 T magnetic trap. Long molecule trapping times are attained through rapid pumpout of residual buffer gas. Molecules experience a helium background gas density below 1×10(^{12}) cm(^{−3}).

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between (OH) and (ND_3) at a mean collision energy of (3.6 cm^{−1} (5 K)). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (^{2}\Pi) radical and a closed-shell polyatomic molecule.

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.