Person:

Lu, Hsin-I

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 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.

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}).

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 report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of (^{14})N and (^{15})N atoms with densities (5(\pm)2) × (10^{12}) (cm^{-3}) and measure their magnetic relaxation rates at milli-Kelvin temperatures. These measurements, together with rigorous quantum scattering calculations based on accurate (ab) (initio) interaction potentials for the (^{7}\Sigma^{+}{u}) electronic state of (N{2}) demonstrate that dipolar relaxation in N+N collisions occurs at a slow rate of ~(10^{-13}) (cm^{3})/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.

We present a quantitative study of suppression of cold inelastic collisions by the spin-orbit interaction. We prepare cold ensembles of (>10^{11} Al(^2P_{1/2})) atoms via cryogenic buffer-gas cooling and use a single-beam optical pumping method to measure their magnetic (mJ-changing) and fine-structure (J-changing) collisions with (^3He) atoms at millikelvin temperatures over a range of magnetic fields from 0.5 to 6 T. The experimentally determined rates are in good agreement with the functional form predicted by quantum scattering calculations using ab initio potentials. This comparison provides direct experimental evidence for a proposed model of suppressed inelasticity in collisions of atoms in (^2P_{1/2}) states [T. V. Tscherbul et al., Phys. Rev. A 80, 040701(R) (2009)], which may allow for sympathetic cooling of other (^2P_{1/2}) atoms (e.g., In, Tl and metastable halogens).

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}).

Employing a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a “slowing cell” placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of (v_f = 65 m s^{−1}), a narrow velocity spread, and a flux of 109 molecules per pulse. In the other regime, our slowest beam has a forward velocity of (v_f = 40 m s{−1}), a longitudinal temperature of (3.6 K), and a flux of (5 \times 10^8) molecules per pulse.

This thesis demonstrates a new cooling and trap loading technique for molecules, leading to trapping of calcium monofluoride (CaF).

Employing a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a ‘‘slowing cell’’ placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of vf=65ms-1, a narrow velocity spread, and a flux of 109 molecules per pulse. In the other regime, our slowest beam has a forward velocity of vf=40ms-1, a longitudinal temperature of 3.6 K, and a flux of 5 x 10 8 molecules per pulse.