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AIP conference proceedings

Laser spectroscopy in superfluid helium

T. Yabuzaki, Akihiro Fujisaki, K. Sano, Toshiya Kinoshita, Yoshiro Takahashi

Published Jan 1, 1993• 0 citations
Publisher

Abstract

We have developed the laser sputtering method which can produce various kinds of neutral atoms and molecules at high density in the superfluid helium. We report here on the laser spectroscopy of alkaline‐earth atoms and alkali‐dimers, chosen as samples in the present demonstrating experiment. The line shapes, shifts and broadening of atomic spectra are found to be well explained by a simplified spherical bubble model.

DOI
10.1063/1.43783

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AIP conference proceedings•Jan 1, 1995•1 citation
We have developed the laser sputtering method which can produce various kinds of neutral atoms and molecules at high density in the superfluid helium. We report here on the laser spectroscopy of alkaline‐earth atoms and alkali‐dimers, chosen as samples in the present demonstrating experiment. The line shapes, shifts and broadening of atomic spectra are found to be well explained by a simplified spherical bubble model.
We have developed a new method for producing a high density of neutral atoms in liquid helium; laser sputtering within liquid helium. When we irradiate samples immersed in liquid helium with a pulsed laser, a large number of clusters are introduced into the liquid. When we further irradiate these clusters with an additional pulse laser, the clusters are sputtered and decomposed, which results in the creation of a high density of isolated atoms in the liquid. For alkaline earth atoms this method is demonstrated, and the results of laser spectroscopy are given.
The excitation and emission spectra of alkali-metal atoms (Cs and Rb) in liquid helium are studied. The helium pressure is changed from the saturated vapor pressure to about 25 atm at 1.6 K, under which conditions the liquid helium is in a superfluid state. The pressure shift and broadening of the observed spectra corresponding to the D lines are explained qualitatively by a theoretical calculation based on the spherical atomic bubble model. The Cs D2{\mathit{D}}_{2}D2​ excitation spectra are found to have double peaks, indicating the existence of anisotropic oscillations of the bubble surface. The disappearance of the D2{\mathit{D}}_{2}D2​ emission lines and the pressure-dependent quenching of the Rb D1{\mathit{D}}_{1}D1​ emission lines are also discussed.
Lineshapes associated with the absorption and emission spectra of neutral atoms in liquid helium and cold helium gas are discussed. A neutral atom in liquid helium forms a microcavity due to a strong short-range repulsive force between the helium and impurity atoms, producing very broad and largely blueshifted absorption bands and relatively narrow emission lines with much smaller shifts. A broad and weak emission band observed by exciting the D2 line of Ag atoms in liquid helium has confirmed the hypothesis that quenching of D2 emission lines for heavy group 1a and 1b atoms are due to the formation of exciplex. In cold helium gas, sudden decrease of the emission lifetimes below 5.2 K by the excitation of the D2 line of Ag atoms is attributed to the condensation of He atoms on the metastable 2D5/2 state. Very narrow absorption lines of Eu innershell electron transitions have revealed that superfluid helium behaves as a static and highly homogeneous low temperature matrix.
By D1{\mathit{D}}_{1}D1​-line optical pumping of alkali-metal atoms (Cs and Rb) in superfluid helium (He II), we could achieve large spin polarization of the atoms in the ground states, more than 50% for both electron and nuclear spins, especially for the Cs atoms. The alkali-metal atoms were implanted into He II by laser sputtering. We detected optically the magnetic resonances between the ground-state Zeeman sublevels, and found that the g values and hyperfine constants in He II were the same as in vacuum within the experimental error. We discuss possible applications of these spin-polarized atoms in He II.
Advances in laser cooling of neutral atoms have made possible a new form of high-resolution laser spectroscopy: photoassociation of ultracold atoms. Colliding neutral atoms, confined in a laser trap, are photoassociated to bound excited states of the dimer molecule by absorbing a photon from a tunable laser. The technique can probe long range and “purely long range” molecular states that are difficult or impossible to detect by traditional means and, because of the extremely low energy of the colliding atoms (<1 mK), is capable of high resolution (<0.001 cm-1). The spectra are useful for atomic lifetime measurements, determination of atomic ground-state scattering information, and measurement of curve-crossing probabilities. Theoretical and experimental work in the field, including multiple resonance techniques and photo association line shapes, are reviewed.
Recent advances in laser cooling techniques have made possible a new type of spectroscopy. In the first experiments of this kind alkali atoms are laser cooled to temperatures around 1 mK and trapped, using laser fields, at densities around 1011 cm−3. Atoms colliding under such ultracold conditions can be resonantly photoassociated to bound states of the dimer molecules. These molecules can in turn be photoionized, providing a sensitive detection of the photoassociation spectrum and revealing in extremely high resolution the structure of the dimer molecules. The particular advantages of this technique include high resolution and signal‐to‐noise and the ability to reach extremely long range states of the molecule, accessible only through such slow collisions.