Relaxation and recombination in spin-polarized atomic hydrogen

Bell, D. A.; Hess, Harald F.; Kochanski, Greg; Buchman, Saps; Pollack, Lois; Xiao, Yong; Kleppner, Daniel; Greytak, Thomas J.

doi:10.1103/physrevb.34.7670

Cited by 57 publications

(15 citation statements)

References 46 publications

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“…This is the first directly measured value based on independent determinations of the recombination rate and surface density. It agrees well with the calculations of de Goey et al [15], but is an order of magnitude lower than the L 3b values reported in several previous works [9,10,11]. In all previous measurements of surface recombination σ was inferred into the rate equations through the adsorption isotherm.…”

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confidence: 91%

Thermal compression of two-dimensional atomic hydrogen gas

et al. 2004

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We describe experiments where 2D atomic hydrogen gas is compressed thermally at a small "cold spot" on the surface of superfluid helium and detected directly with electron-spin resonance. We reach surface densities up to 5×10 12 cm −2 at temperatures ≈ 100 mK corresponding to the maximum 2D phase-space density ≈ 1.5. By independent measurements of the surface density and its decay rate we make the first direct determination of the three-body recombination rate constant and get the upper bound L 3b 2 × 10 −25 cm 4 /s which is an order of magnitude smaller than previously reported experimental results.PACS numbers: 05.30.Jp, 67.65.+z, 82.20.Pm When adsorbed on the surface of superfluid helium spin-polarized atomic hydrogen (H↓) is an ideal realization of a two-dimensional (2D) boson gas [1]. Helium provides a translationally invariant substrate and its surfacenormal potential supports only one bound state for hydrogen with the binding energy E a =1.14(1) K [2]. Even for such a weak interaction, lowering the surface temperature T s well below 1 K leads to a large adsorbate density σ. At high H↓ coverages three-body recombination is expected to be the dominant density decay mechanism setting the main obstacle to the achievement of the quantum degeneracy regime, where the thermal de Broglie wavelength Λ is larger than the average interatomic spacing. Degenerate 2D H↓ is expected to exhibit collective phenomena such as the Kosterlitz-Thouless superfluidity transition and the formation of a quasicondensate.Two methods of local compression of adsorbed H↓ have been employed to overcome limitations caused by recombination and its heat. Magnetic compression has been successfully used to achieve quantum degeneracy [3,4]. In this method the recombination heat is removed from the compressed H↓ by ripplons of the helium surface and the cooling efficiency depends on the length of the heat transfer path. By decreasing the size of the compressed region to 20 µm we were able to achieve σΛ 2 ≈ 9 [3]. However, the small size of the sample together with large magnetic field gradients did not allow to implement direct diagnostics of adsorbed H↓. In the thermal compression method [5,6] cooling a small part of the sample cell wall well below the temperature of the rest of the wall leads to an exponential increase of σ on such a "cold spot". In this method the recombination heat is transferred from the ripplons to the phonons of helium [7] and then to the substrate beneath the spot. Therefore a larger spot is preferable as long as the total recombination rate on the spot becomes a limitation. The larger sample size and the homogeneity of the magnetic field make thermal compression better suited for direct studies of adsorbed H↓.In the present work we use sensitive electron-spin resonance [8] to diagnose 2D H↓ gas thermally compressed to σΛ 2 ≈ 1.5 and discuss the limitations and possible improvements of the "cold spot" method to reach and detect the Kosterlitz-Thouless transition. By independent measurements of the recombination rate a...

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confidence: 91%

Thermal compression of two-dimensional atomic hydrogen gas

et al. 2004

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“…3 obtained here at T 2 202͑2͒ mK and s b 5.9͑4͒ 3 10 13 cm 22 . At low densities, K bbb remains constant, K bbb 8.4͑3.5͒ 3 10 225 cm 4 s 21 , which agrees with the above fitting result and data given elsewhere [16]. However, starting at s b l 2 2 ഠ 3, K bbb decreases steadily and at the highest degeneracy it is suppressed by a factor of 11 (2).…”

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Observation of Quasicondensate in Two-Dimensional Atomic Hydrogen

et al. 1998

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We have performed magnetic compression experiments with the two-dimensional gas of hydrogen atoms on liquid 4 He at T 120 200 mK. For 2D phase-space densities higher than 3, the probability of three-body dipole recombination is observed to decrease gradually by an order of magnitude in comparison to its low-density value K bbb 8.4͑3.5͒ 3 10 225 cm 4 s 21 . This is attributed to local coherence developing in a 2D quasicondensate. We have also determined the value U͞k 5.1͑5͒ 3 10 215 K cm 2 for the mean-field energy of the gas. [S0031-9007(98)07787-4] PACS numbers: 03.75.Fi, 05.30.Jp, 67.65. + z, 68.35.Rh In addition to the occupation of a single quantum state, a Bose-Einstein condensate (BEC) is characterized by long-range order or, equivalently, global coherence. The concept of quasicondensate (QC) has been introduced [1] for the lesser known situation where a boson system is coherent only locally. Having similar particle interactions, BEC and QC would be hardly distinguishable by virtue of properties related to inelastic [2,3] or elastic [4] collisions. However, the difference is transparent in spatially uniform two-dimensional (2D) system where a BEC cannot exist at finite temperature T but a QC can.In fact, a QC may appear due to interactions which suppress density fluctuations and therefore allow the 2D system to be described by a single wave function. Although the phase of the function fluctuates spatially and thus thwarts a long-range order [1], the phase coherence persists within a finite length L~exp͑sl 2 ͒ [2]. Here, l p 2ph 2 ͞mkT is the de Broglie wavelength, s is the 2D gas density, and m is the particle mass. For T ! 0 the "local BEC" regions of size L tend to extend throughout the sample and the QC turns into a "true" condensate.The appearance of a QC is to be expected when the interaction energy becomes comparable with the kinetic energy, sU ϳ kT . Here, U 4ph 2 j͞m is the mean-field parameter for elastic interactions and j is a dimensionless interaction strength. It is known that at T 0 the ratio of densities of above-condensate and condensate particles is s 0 ͞s 0 ഠ j [5]. Therefore a QC may manifest itself preferably in weakly interacting (low-j) systems.This Letter reports on the first experimental evidence for a quasicondensate in 2D atomic hydrogen gas, where we have observed a significant reduction in three-body recombination probability [2,6]. Hydrogen atoms on liquid 4 He surface occupy a single bound state with low adsorption energy, E a ͞k Ӎ 1.0 K [7]. The H adatoms are localized quite far, 8 Å, from the liquid and their out-of-plane delocalization length, l h͞ p 2E a m Ӎ 5 Å, is large compared with, e.g., the 3D scattering length a 0 0.72 Å. Thus adsorbed hydrogen may be also regarded as quasi-2D in the sense that it obeys 2D statistics while particle interactions are three dimensional [2]. It may be shown [2] that j ഠ a 0 ͞l and consequently a QC in hydrogen on 4 He may be observed at sl 2 * l͞2a 0 Ӎ 3.5.It is not easy to achieve a high degree of quantum degeneracy, i.e., high s at ...

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“…In fact, some experimental studies of TBA do exist, but they have been carried out under conditions not relevant to primordial chemistry. Measurements have been carried out for spin-polarized atomic hydrogen, but at temperatures of 130 − 600 mK and magnetic fields of 3 − 9 T [13]. TBA measurements have also been performed with ultracold atoms at ∼ 10 − 500 nK, though not yet with atomic H [14].…”

Section: Three Body Associationmentioning

confidence: 99%

Laboratory measurements of primordial chemistry

Savin¹

2012

AIP Conference Proceedings

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Molecular hydrogen plays a key role in structure formation during the early Universe, sometimes cooling the gas and at other times heating it. We discuss three of the key chemical reactions controling the formation of H 2 during the epoch of primordial galaxy and first star formation. These are the associative detachment (AD) reaction H − + H → H 2 + e − , the mutual neutral (MN) reaction H − + H + → H + H, and the three body association (TBA) process H + H + H → H 2 + H. For each reaction we discuss the cosmological implications due to uncertainties in the relevant rate coefficients. In the case of AD we also present our recent experimental work to remove these uncertainties. For MN we discuss our ongoing experimental work. Lastly, for TBA we discuss the theoretical and experimental challenges facing attempts to improve the required reaction data.

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Relaxation and recombination in spin-polarized atomic hydrogen

Cited by 57 publications

References 46 publications

Thermal compression of two-dimensional atomic hydrogen gas

Thermal compression of two-dimensional atomic hydrogen gas

Observation of Quasicondensate in Two-Dimensional Atomic Hydrogen

Laboratory measurements of primordial chemistry

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