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.

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.

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 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}).

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)].

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 þ NH trap loss rate coefficient of kðNþNHÞ loss ¼ 9ð5Þð3Þ 1013 cm3 s1. 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 þ NH elasticto-inelastic collisions remains large (> 100) into the mK regime.

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 3He and 4He. The 4He data support the predicted 1=B2 e dependence of the collision-induced Zeeman relaxation rate coefficient on the molecular rotational constant Be. The measured 3He rate coefficients are much larger than the 4He coefficients, depend less strongly on Be, 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 observe magnetic trapping of atomic nitrogen (N14) and cotrapping of ground-state imidogen (N14H, XΣ−3). Both are loaded directly from a room-temperature beam via buffer gas cooling. We trap approximately 1×1011 N14 atoms at a peak density of 5×1011cm−3 at 550mK. The 12±4s 1∕e lifetime of atomic nitrogen in the trap is consistent with a model for loss of atoms over the edge of the trap in the presence of helium buffer gas. Cotrapping of N14 and N14H is accomplished, with 108 NH trapped molecules at a peak density of 108cm−3.