M.W. Reynolds

Adiabatically changing the phase-space density of a trapped Bose gas

We show that the degeneracy parameter of a trapped Bose gas can be changed adiabatically in a reversible way, both in the Boltzmann regime and in the degenerate Bose regime. We have performed measurements on spin-polarized atomic hydrogen in the Boltzmann regime, demonstrating reversible changes of the degeneracy parameter (phase-space density) by more than a factor of 2. This result is in good agreement with theory. By extending our theoretical analysis to the quantum degenerate regime we predict that, starting close enough to the Bose-Einstein phase transition, one can cross the transition by an adiabatic change of the trap shape. [S0031-9007(97)02357-0] PACS numbers: 03.75.Fi, 67.65.+z, 32.80.Pj

Lyman‐α spectroscopy of magnetically trapped atomic hydrogen

Recently it became possible to study magnetically trapped atomic hydrogen gas by Lyman‐α spectroscopy. We discuss the typical conditions at which the gas is trapped and how this reflects itself in the observed spectra. We given a description of the experimental apparatus. The use of the system is illustrated by a spectroscopic study of evaporative cooling.

VUV spectroscopy of magnetically trapped atomic hydrogen

We discuss the experimental and theoretical aspects of absorption spectroscopy of cold atomic hydrogen gas in a magnetostatic trap using a pulsed narrow-band source (bandwidth ≈ 100 MHz) at the Lyman-α wavelength (121.6 nm). A careful analysis of the measured absorption spectra enables us to determine non-destructively the temperature and the density of the trapped gas. The development of this diagnostic technique is important for future attempts to reach Bose-Einstein condensation in trapped atomic hydrogen.

Atomic deuterium adsorbed on the surface of liquid helium

We investigate deuterium atoms adsorbed on the surface of liquid helium in equilibrium with a vapor of atoms of the same species. These atoms are studied by a sensitive optical method based on spectroscopy at a wavelength of 122 nm, exciting the 1S-2P transition. We present a direct measurement of the adsorption energy of deuterium atoms on helium and show evidence for the existence of resonantly enhanced recombination of atoms residing on the surface to molecules.

Optical Observation of Atomic Hydrogen on the Surface of Liquid Helium

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Base degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The optical spectrum of the adsorbed hydrogen atoms was not known a priori. It shows a resonance that is much broader than that of a hydrogen atom in vacuo, and it is shifted to lower frequencies. From the fluorescence intensity we determine that we have reached a surface density corresponding to one atom per square De Broglie wavelength. This means that our experiments take place at the edge of quantum degeneracy. In the regime where the adsorption isotherm is known we can use the measured hydrogen densities to infer the temperature of the helium surface. We use this information to determine the thermal conductance between the surface and the bulk of liquid helium. We find quantitative agreement between the measured temperature drops and the prediction of ripplon-phonon coupling theory.

Evaporative cooling of magnetically trapped atomic hydrogen

Abstract We present a new model describing the dynamics of the evaporative cooling of a sample of trapped particles. We compare the results of the model with recent optical measurements of the evolution of the density and the temperature of magnetically trapped atomic hydrogen during evaporative cooling.

Optical excitation of atomic hydrogen adsorbed on liquid helium

Abstract We calculate the interaction between an optically excited hydrogen atom H * ( n=2 ) and the free surface of liquid helium ( l -He). The l -He is modelled by a solid with a hexagonal surface lattice and the potential energy of the H * is derived from known HeH * pair potentials. Delocalization of the H * parallel to the surface averages the interaction to give potential curves ( A 2 ∑ + , B 2 Π , and C 2 ∑ + in spectroscopic notation) for motion perpendicular to the surface. In contrast to the excimer (HeH * ) case, all of the potential curves possess barriers which prevent the H * from approaching the l -He too closely. Like ground-state H, H * is only weakly physisorbed. We obtain the optical lineshape for Lyman-α excitation of H physisorbed on the surface of l -He. The quenching of H(2 S ) by l -He is also discussed.

Apparatus for optical study of atomic hydrogen on the surface of liquid helium

Spin-polarized atomic hydrogen adsorbed on the surface of liquid helium is the most promising candidate for the observation of quantum degeneracy in a two-dimensional Base gas. In this article we describe our experimental apparatus which is being used to realize this goal. The apparatus employs a system of superconducting and iron magnets to supply electron and proton spin-polarized hydrogen to a cold cell (T ≍ 0.1K) at sufficient flux to compensate recombination losses and attain the regime of two-dimensional quantum degeneracy. The gas in the cell is probed using light resonant with the Lyman-α atomic transition.

Resonance-enhanced two-photon spectroscopy of magnetically trapped atomic hydrogen

We show that resonant enhanced two photon spectroscopy is a very powerful and promising diagnostic method for trapped atomic hydrogen and antihydrogen. This method, in which photons of different wavelengths are used to excite H atoms from the 1S to the 3D or 3S level, uses the resonant enhancement of the intermediate 2P level to obtain high sensitivity while at the same time offering potentially much higher spectral resolution than single photon (Lyman α) excitation.

Nonlinear hydrodynamics and ripplon interactions on superfluid helium

We discuss the elementary processes which determine the lifetime of ripplons at low temperatures, considering interaction of ripplons with ripplons and phonons. The interaction Hamiltonian is constructed from the nonlinear hydrodynamic equations of motion using the approach of Khalatnikov and Pokrovsky [Physica A166, 288 (1990)]. The rate of spontaneous ripplon decay is calculated. Interaction of ripplons with thermally excited ripplons and phonons is considered. In the limit of low temperature the ripplon subsystem decouples from the phonon subsystem, which is promising for the observation of surface second sound.