International Conference on Spectral Line Shapes (ICSLS)

Optical spectra of atoms in liquid helium and cold helium gas

Michio Takami

Published Jan 1, 1999• 0 citations

Abstract

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.

DOI: 10.1063/1.58324

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International Conference on Spectral Line Shapes (ICSLS)

Optical spectra of atoms in liquid helium and cold helium gas

Michio Takami

Published Jan 1, 1999• 0 citations

Publisher

Abstract

DOI: 10.1063/1.58324

Related papers

Optical properties of alkali-metal atoms in pressurized liquid helium

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Condensation of He Gas on Optically Excited Ag Atoms Below the Critical Temperature ( T<5.2K )

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S. I. Kanorsky, Markus Arndt, R. Dziewior +2 more

We have measured the pressure broadening and shift of the excitation and emission lines of the 6 ${\mathit{s}}^{2}$ $^{1}$ ${\mathit{S}}_{0}$ \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.We have measured the pressure broadening and shift of the excitation and emission lines of the 6${\mathit{s}}^{2}$ $^{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.

Physical review. B, Condensed matterSep 1, 199474 citations

{\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.

D2

line consist of three narrow lines and three broadbands, which are assigned to transitions of Ag atoms and AgHe and

{\mathrm{AgHe}}_{2}

exciplexes. Surprisingly long ( $\ensuremath{\gtrsim}300\mathrm{ns}$) effective emission lifetimes are observed at

T>{T}_{c}

(5.2 K). The long lifetimes are the result of population trapping in the

{}^{2}{D}_{5/2}

metastable state. At $T\ensuremath{\lesssim}{T}_{c}$, the effective lifetimes decrease to $\ensuremath{\lesssim}20\mathrm{ns}$ due to the condensation of He gas on the Ag atom in its

{}^{2}{D}_{5/2}

state.

{4f}^{13}{(}^{2}{F}_{7/2}^{o}{)6s}^{2}

, which indicates that the transition between the inner shells is weakly perturbed by surrounding helium atoms. The pressure dependence of the emission wavelength suggests that the symmetry of helium atoms distributed around a thulium atom in the solid phase is similar to that in the liquid phase. The emission intensity was stable and large in the solid phase since thulium atoms were trapped at a density of 10

{}^{10}

--10

{}^{11}

atoms/cm

{}^{3}

. The lifetime of the excited state was measured to be $7.09\ifmmode\pm\else\textpm\fi{}0.04$ $\ensuremath{\mu}$s, which was longer than that of the

{4f}^{12}{(}^{3}{H}_{6}{)5d}_{5/2}{6s}^{2}

{(6,5/2)}_{7/2}

state of free thulium atoms. The excited state is expected to be a mixed state of the

{4f}^{12}{5d6s}^{2}

and

{4f}^{13}6s6p

configurations. The metastable state

{4f}^{13}{(}^{2}{F}_{5/2}^{o}{)6s}^{2}

is populated by a radiative transition from this excited state and relaxes to the ground state through a magnetic dipole transition. The lifetime of the metastable state of a neutral thulium atom was measured to be $75\ifmmode\pm\else\textpm\fi{}3$ ms.

{}^{3}{D}_{1}

metastable state has lifetime of 2.6 s and can be used as an intermediate state in two-step excitations of high-lying states. Various matrix-induced radiationless population transfer channels have been identified.

We report on the systematic observation of emission spectra of

{\mathrm{Rb}}^{*}{\mathrm{He}}_{n}

exciplexes

(n=1,2,\dots{},6),

realized by exciting Rb atoms to the

{5}^{2}P

states

({\mathrm{Rb}}^{*})

in a cold

{}^{4}\mathrm{He}

gas. The observed broad spectral components are assigned to

{\mathrm{Rb}}^{*}{\mathrm{He}}_{n}

(n=1--6)

using theoretical spectra obtained from ab initio potential curves. The dynamics of the exciplex formation is discussed, based on the observed temperature dependence of the spectra. The He gas density dependence of the spectra of

{\mathrm{Rb}}^{*}\mathrm{He}

is understood as a change in the population distribution over the vibrational levels. The present results are compared with our previous work with Cs [K. Enomoto et al., Phys. Rev. A 66, 042505 (2002)], and differences are explained in terms of the difference in the fine-structure splitting. Furthermore, we show the emission spectrum observed after the excitation of Rb in liquid He and conclude that it is the fluorescence from the exciplex

{\mathrm{Rb}}^{*}{\mathrm{He}}_{6}.

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

{\mathit{s}}^{2}

^{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.