Hsin-I Lu

The buffer gas beam: an intense, cold, and slow source for atoms and molecules.

Beams of atoms and molecules are stalwart tools for spectroscopy and studies of collisional processes. The supersonic expansion technique can create cold beams of many species of atoms and molecules. However, the resulting beam is typically moving at a speed of 300−600 m s^(−1) in the laboratory frame and, for a large class of species, has insufficient flux (i.e., brightness) for important applications. In contrast, buffer gas beams can be a superior method in many cases, producing cold and relatively slow atoms and molecules (see Figure 1) in the laboratory frame with high brightness and great versatility. There are basic differences between supersonic and buffer gas cooled beams regarding particular technological advantages and constraints. At present, it is clear that not all of the possible variations on the buffer gas method have been studied. In this review, we will present a survey of the current state of the art in buffer gas beams, and explore some of the possible future directions that these new methods might take.

Magnetic Trapping and Zeeman Relaxation of NH (X 3 )

Imidogen (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 imidogen in collisions with He-3 is measured to be 3.8 +/- 1.1 E-19 cm^2 at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of gamma = 7 E4 in agreement with the recent theory of Krems et al. (PRA 68 051401(R) (2003))

Mechanism of Collisional Spin Relaxation in \(^3\)Σ Molecules

We measure and theoretically determine the effect of molecular rotational splitting on Zeeman relaxation rates in collisions of cold 3Sigma 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/B_{e};{2} dependence of the collision-induced Zeeman relaxation rate coefficient on the molecular rotational constant B_{e}. The measured 3He rate coefficients are much larger than the 4He 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.

Magnetic Trapping of Atomic Nitrogen (\(^{14}\)N) and Cotrapping of NH (\(X\)\(^{3}\)\(\Sigma\) -)

Author(s): Hummon, MT; Campbell, WC; Lu, HI; Tsikata, E; Wang, Y; Doyle, JM | Abstract: We observe magnetic trapping of atomic nitrogen (N14) and cotrapping of ground-state imidogen (N14 H, X Σ-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× 1011 cm-3 at 550 mK. The 12±4 s 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 N14 H is accomplished, with 108 NH trapped molecules at a peak density of 108 cm-3.

Magnetic trapping of NH molecules with 20 s lifetimes

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 . Contents

Cold N+NH collisions in a magnetic trap

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 k(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.

Collisional properties of cold spin-polarized nitrogen gas: Theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

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.

ColdN+NHCollisions in a Magnetic Trap

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 k(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.