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Excited atoms in cavities of liquid He I: long-range inter-atomic repulsion and broadening of atomic lines.

Vadim V. Atrazhev, Jussi Eloranta, Nelly Bonifaci, Nguyen, Thi Hai Van, Frédéric Aitken, Van-Haeften, K., Gérard Vermeulen

Published Sep 9, 2012• 0 citations

Abstract

\n\t\t\t\tA theoretical analysis of the line broadening of localized atomic transitions in liquid helium is presented. It is shown that accurate information can be derived on the long-range part of the He*-He interaction as well as on the local structure near the He* emitters. The analysis confirms that in corona discharges in liquid helium the emitting He* atoms reside in cavities and that for known He*-He interaction the size of the cavities can be deduced from the line profile. The He*-He interaction was calculated using the full configuration interaction (CI) method as implemented in the Molpro package. The widths of atomic lines due to fluorescent transitions between different excited states of helium atoms were calculated as a function of external pressure in the range from 0.1 and 3.5 MPa using the static approximation method, and the input from the results of the CI calculation and cavity diameters calculated using the bubble model. The calculated widths showed excellent agreement with experimental data of liquid helium excited by corona discharges. A second, analytical analysis using a power function to represent the He*-He interaction showed qualitative agreement with the experimental data.\n\t\t\t

DOI: 10.1051/epjap/2012120418/pdf

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\n\t\t\t\tA theoretical analysis of the line broadening of localized atomic transitions in liquid helium is presented. It is shown that accurate information can be derived on the long-range part of the He*-He interaction as well as on the local structure near the He* emitters. The analysis confirms that in corona discharges in liquid helium the emitting He* atoms reside in cavities and that for known He*-He interaction the size of the cavities can be deduced from the line profile. The He*-He interaction was calculated using the full configuration interaction (CI) method as implemented in the Molpro package. The widths of atomic lines due to fluorescent transitions between different excited states of helium atoms were calculated as a function of external pressure in the range from 0.1 and 3.5 MPa using the static approximation method, and the input from the results of the CI calculation and cavity diameters calculated using the bubble model. The calculated widths showed excellent agreement with experimental data of liquid helium excited by corona discharges. A second, analytical analysis using a power function to represent the He*-He interaction showed qualitative agreement with the experimental data.\n\t\t\t

Abstract By means of a direct recording Fabry-Perot interferometer, the contours of several singlet lines of helium were studied over a wide range of pressure and temperature. The source was a microwave discharge cooled with liquid nitrogen or liquid helium. Analysis of the tracings allowed the Doppler and Lorentz components to be separated quantitatively. A detailed study was carried out for the line λ 7281 Å (21P - 31S). By extrapolation of the Lorentz width to zero pressure the radiation width of the level 2 was determined and led to an ƒ-value 0·377 + 0·035 of the resonance line λ 584 Å (11S— 21P) The resonance broadening is found to be independent of the speed of the atoms, over a range of a factor five in the speed, in agreement with theories of optical collision by Weisskopf and others. The absolute value of the resonance broadening constant is compared with various theories. The strong line λ 6678 Å (21P -31D) showed self-absorption due to imprisonment of resonance radiation.

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

{\mathit{D}}_{2}

excitation spectra are found to have double peaks, indicating the existence of anisotropic oscillations of the bubble surface. The disappearance of the

{\mathit{D}}_{2}

emission lines and the pressure-dependent quenching of the Rb

{\mathit{D}}_{1}

emission lines are also discussed.

We have measured the pressure broadening and shift of the excitation and emission lines of the 6

{\mathit{s}}^{2}

{\mathit{D}}_{1}

{\mathit{S}}_{0}

\ensuremath{\rightarrow}6s6p

^{1}

{\mathit{P}}_{1}

transition of Ba atoms implanted in liquid and solid helium at 1.5 K. The observed spectra are quantitatively explained within the adiabatic line-broadening theory by assuming a bubble structure of the trapping sites for the Ba atoms. The evolution of the bubble shape and size in the optical excitation-deexcitation cycle is modeled theoretically.

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.

The long-range interaction between an antiprotonic helium atom

\overline{p}{\mathrm{He}}^{+}

and helium atom in its ground state is studied. We calculate the dispersion coefficients

{C}_{6}

using the complex coordinate rotation (CCR) formalism in order to comply with the resonant nature of metastable states of the antiprotonic helium. We present as well numerical data on static dipole polarizabilities of antiprotonic helium states. The obtained coefficients

{C}_{6}

may be used to estimate the collisional shift and broadening of transition lines in a low-density precision spectroscopy of the antiprotonic helium.

We show that for atoms trapped in a liquid He matrix the excitation and emission spectra can be quantit- atively described by the bubble model which treats the matrix as an incompressible continuous medium. Com- parison with experimental results shows that the model also holds for a solid matrix. The model is only valid in the case where the attractive He-He interaction dominates the attractive part of the foreign atom-helium interaction. Its failure in the case of implanted Na atoms is discussed.