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Physical Review Letters

Optical Excitation of Atomic Hydrogen Bound to the Surface of Liquid Helium

Allard P. Mosk, M. W. Reynolds, Tom W. Hijmans, Jook T. M. Walraven

Published Nov 16, 1998• 22 citations
Publisher

Abstract

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Bose degeneracy in two dimensions. The atoms are excited by $121.6$ nm light and detected both in fluorescence and in absorption. The excitation spectrum shows a resonance that is much broader than that of a free atom, and shifted to lower frequencies. From the absorption signal we determine that we have reached a surface density corresponding to one atom per square de Broglie wavelength. We also use the hydrogen fluorescence to probe the thermal resistance between the free surface and the bulk of the liquid helium.

DOI
10.1103/physrevlett.81.4440

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We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Bose degeneracy in two dimensions. The atoms are excited by 121.6121.6121.6 nm light and detected both in fluorescence and in absorption. The excitation spectrum shows a resonance that is much broader than that of a free atom, and shifted to lower frequencies. From the absorption signal we determine that we have reached a surface density corresponding to one atom per square de Broglie wavelength. We also use the hydrogen fluorescence to probe the thermal resistance between the free surface and the bulk of the liquid helium.
We propose the explanation of a small yet finite frequency shift of a hyperfine transition in two-dimensional atomic hydrogen bound to the surface of superfluid helium at T < 0.1 K. The nonzero shift is caused, first, by the interaction between adsorbed atoms, which effectively reduces their binding energy to the helium surface by a value proportional to their density and, consequently, reduces the deviation of the hyperfine constant from the free-atom value. Second, the transition between the hyperfine states b and c is shifted owing to a difference in the interaction of the b and c atoms with the residual atoms in the state a that appear due to one and two-body nuclear relaxation. This shift is also linear in the density of the two-dimensional gas. The net effect of the two contributions qualitatively agrees with the experimentally observed value of the shift.
We describe experiments where two-dimensional (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 $5\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\ensuremath{-}2}$ at temperatures $\ensuremath{\approx}100\mathrm{mK}$ corresponding to the maximum 2D phase-space density $\ensuremath{\approx}1.5.$ By independent measurements of the surface density and its decay rate we make a direct determination of the three-body recombination rate constant and get the upper bound ${L}_{3b}\ensuremath{\lesssim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}{\mathrm{cm}}^{4}/\mathrm{s}$ which is an order of magnitude smaller than previously reported experimental results.
Possible acoustic modes in a superfluid 2D gas of hydrogen atoms adsorbed on the surface of liquid helium at T ≲ 0.1 K are considered depending on the oscillation frequency and times of energy and momentum transfers both between 2D subsystems of hydrogen atoms and ripplons and into the bulk liquid or substrate. Analogues of the usual and second sounds are realized in 2D hydrogen at high frequencies. In the case of weak coupling with the bulk liquid and substrate, ripplons provide an addition to the normal hydrogen component, which leads to a change in the speed of the second sound. In the most interesting range of low frequencies, an analogue of the fourth sound is realized, when ripplons and the normal hydrogen component are immobile and only the superfluid hydrogen component moves. In this case, when the rate of heat transfer into the bulk liquid is much lower than the sound frequency, oscillations of the temperature of hydrogen can be observed in phase with density oscillations. Methods for exciting acoustic modes are discussed.
We have observed the 1S—3S and 1S—3D Doppler-free two-photon transitions in atomic hydrogen using a powerful narrow-band tunable light source at 205 nm. The excitation is monitored by looking at the fluorescent light emitted on the Balmer α transition. By this method we have been able to detect atomic hydrogen and atomic deuterium at low density (around 107-108 at/cm3). One of the possible applications of this method is the search of superheavy hydrogen atoms. These atoms composed of a heavy proton and an electron are predicted by some supersymmetry theories.1 Because of their important mass shift, they should be easily distinguishable from hydrogen, deuterium, or tritium. Using a preliminary centrifugation, we believe that we can improve the present limit (~10−15) on the abundance of these superheavy atoms.