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Physical Review A

Pressure-dependent quenching of Rb 5Pstates in liquid helium

Toshiya Kinoshita, K. Fukuda, T. Matsuura, T. Yabuzaki

Published Jun 1, 1996• 34 citations
Publisher

Abstract

The decay times of ${\mathit{D}}_{1}$ emission lines of Rb and Cs atoms in superfluid helium were measured at 1.6 K as functions of helium pressure in a range from the saturated vapor pressure to about 25 atm. In the case of Cs, the measured decay time is almost independent of the helium pressure and is given by the lifetime of the 6${\mathit{P}}_{1/2}$ state of the free atom. However, we have found that the decay time of the Rb ${\mathit{D}}_{1}$ line decreases with increasing pressure. This suggests the coexistence of two processes leading from the excited P state to the ground state, one an ordinary ${\mathit{D}}_{1}$ emission transition and the other a process decaying via a lower energy state before reaching the ground state. The pressure-dependent quenching observed only for the Rb atom can be explained by the competition of the above two processes. A possible mechanism for quenching of the Rb 5P states is discussed in terms of the interaction potentials of the alkali-metal-atom--${\mathrm{He}}_{\mathit{N}}$ system, where two or six helium atoms are localized near the alkali core in a particular anisotropic configuration. The dynamics of processes leading from the excited P states of Na and Cs atoms are also discussed. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.4054

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The decay times of D1{\mathit{D}}_{1}D1​ emission lines of Rb and Cs atoms in superfluid helium were measured at 1.6 K as functions of helium pressure in a range from the saturated vapor pressure to about 25 atm. In the case of Cs, the measured decay time is almost independent of the helium pressure and is given by the lifetime of the 6P1/2{\mathit{P}}_{1/2}P1/2​ state of the free atom. However, we have found that the decay time of the Rb D1{\mathit{D}}_{1}D1​ line decreases with increasing pressure. This suggests the coexistence of two processes leading from the excited P state to the ground state, one an ordinary D1{\mathit{D}}_{1}D1​ emission transition and the other a process decaying via a lower energy state before reaching the ground state. The pressure-dependent quenching observed only for the Rb atom can be explained by the competition of the above two processes. A possible mechanism for quenching of the Rb 5P states is discussed in terms of the interaction potentials of the alkali-metal-atom--HeN{\mathrm{He}}_{\mathit{N}}HeN​ system, where two or six helium atoms are localized near the alkali core in a particular anisotropic configuration. The dynamics of processes leading from the excited P states of Na and Cs atoms are also discussed. \textcopyright{} 1996 The American Physical Society.
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.
The excitation, emission spectra, and decay curves of the emission intensity of thulium atoms implanted in liquid and solid helium were observed in the presence and absence of an external magnetic field. The observed narrow line (width $\ensuremath{\sim}0.1$ nm) of the excitation spectrum at 590.60 nm is assigned as a zero-phonon transition from the electronic ground state 4f13(2F7/2o)6s2{4f}^{13}{(}^{2}{F}_{7/2}^{o}{)6s}^{2}4f13(2F7/2o​)6s2, 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 1010{}^{10}10--1011{}^{11}11 atoms/cm3{}^{3}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 4f12(3H6)5d5/26s2{4f}^{12}{(}^{3}{H}_{6}{)5d}_{5/2}{6s}^{2}4f12(3H6​)5d5/2​6s2 (6,5/2)7/2{(6,5/2)}_{7/2}(6,5/2)7/2​ state of free thulium atoms. The excited state is expected to be a mixed state of the 4f125d6s2{4f}^{12}{5d6s}^{2}4f125d6s2 and 4f136s6p{4f}^{13}6s6p4f136s6p configurations. The metastable state 4f13(2F5/2o)6s2{4f}^{13}{(}^{2}{F}_{5/2}^{o}{)6s}^{2}4f13(2F5/2o​)6s2 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.
We have measured the pressure broadening and shift of the excitation and emission lines of the 6s2{\mathit{s}}^{2}s2 1^{1}1S0{\mathit{S}}_{0}S0​\ensuremath{\rightarrow}6s6p 1^{1}1P1{\mathit{P}}_{1}P1​ 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.
Measurements have been made of the lifetime for the escape of negative ions trapped in rotating helium II as a function of temperature and pressure. These measurements concern lifetimes in the range from 10 to 1000 sec, and have been made between the temperatures of 1.09 and 1.68\ifmmode^\circ\else\textdegree\fi{}K from the vaporization pressure to the solidification pressure. At constant pressure the lifetime decreases rapidly with increasing temperature. The curves of constant lifetime in the pressure-versus-temperature plane have negative slopes dPdT\frac{\mathrm{dP}}{\mathrm{dT}}dTdP​ and positive second derivatives d2PdT2\frac{{d}^{2}P}{d{T}^{2}}dT2d2P​. The results have been interpreted in terms of the bubble model of the negative ion and the vortex-line model of the rotating superfluid. The interpretation suggests that the ion radius equals approximately 19.5 A at saturated vapor pressure and decreases to a value between 12 and 14 \AA{} at the solidification pressure. However, a significant discrepancy between theory and experiment casts some doubt on the radius values obtained.
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.