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We have performed quantitative measurements of the variation of the on-resonance absorption coefficients ${\ensuremath{\kappa}}_{0}$ of the four hyperfine components of the Cs ${D}_{1}$ transition as a function of laser power $P$, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate ${\ensuremath{\gamma}}_{1}$) yield algebraic ${\ensuremath{\kappa}}_{0}(P)$ dependences that do not reproduce the experimental findings from Cs vapor in a paraffin-coated cell. However, numerical results that consider spin-exchange relaxation (at a rate ${\ensuremath{\gamma}}_{\mathrm{se}}$) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of ${\ensuremath{\kappa}}_{0}$, the optical pumping saturation power ${P}_{\mathrm{sat}}$, and the ratio ${\ensuremath{\gamma}}_{\mathrm{se}}/{\ensuremath{\gamma}}_{1}$, are well described by the experimental conditions and yield absolute values for ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{\mathrm{se}}$. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section.

We have studied both theoretically and experimentally the optical pumping of Cs atoms trapped in (body-centered-cubic and hexagonal-close-packed) crystalline ${}^{4}\mathrm{He}$ matrices. The theoretical approach is based on rate equations for which time-dependent and asymptotic solutions are obtained in the case of depopulation and repopulation pumping. Comparison with experiments show that repopulation pumping, i.e., a process in which spin polarization in the excited state is not destroyed, is the dominant pumping mechanism in both crystalline phases.

In a weak magnetic field coherent dark resonances in cesium vapor are split into up to 15 resolved components, depending on field direction and laser polarizations. We find that the selection rules are different for vapor cells with and without buffer gas due to a change in multipolarity of the two-photon coupling. At low laser intensities or sufficiently high buffer-gas pressure optical pumping between different dark resonances can be neglected so that a simple model allows one to calculate the relative line strengths, giving complete agreement with the experimental spectra. @S1050-2947~98!00907-X#

We report an experimental investigation of selective reflection spectra using a bichromatic field with a resonant \ensuremath{\Lambda}-type coupling to the ${\mathrm{D}}_{1}$ transition of Rb vapor atoms. The reflection spectra are found to have a more complex structure than equivalent spectra in transmission experiments. This structure is traced back to the effect of wall collisions on velocity selective optical pumping and saturation processes. The spectral line shapes are well described by a theoretical model developed in the text. In addition we report the observation of dark resonances in reflection spectroscopy.

We report the results of electron-spin-resonance studies of Cs atoms trapped in the body-centered-cubic phase of a solid {sup 4}He matrix. Free-induction-decay times longer than 100 ms have been measured in spin-transient experiments. In optical-rf double-resonance experiments we have obtained linewidths of less than 20 Hz (full width at half maximum). The magnetometric sensitivity of our sample is 26 nG/{radical}Hz. We have also demonstrated that magnetic resonance in such samples can be detected with high sensitivity using forward-scattered optical radiation. {copyright} {ital 1996 The American Physical Society.}

We have investigated the phase shift caused by a static electric field on a beam of Rb atoms carrying alignment (but no orientation) using an all optical technique with spatially separated pump and probe regions. Quantitative measurements of the phase shift agree with theoretical predictions to within 1.4%. An experimental demonstration of the velocity independence of the effect based on time-resolved measurements is given.

We present an experimental study of the laser-induced fluorescence spectra of Cu atoms in bulk liquid and solid helium matrices as well as in the dense plasma created by the laser ablation of copper in liquid helium. We observe transitions of the valence electron and of inner-shell electrons. The former produce structureless line shapes, a large broadening, and a blueshift. The latter practically are not shifted with respect to the free atom and possess a substructure consisting of a zero-phonon line and phonon wings. We suggest a qualitative interpretation of the observed spectra based on the atomic bubble model that takes the lifting of the $|{M}_{J}|$ degeneracy due to the bubble deformation and matrix-phonon interactions into account.

We present an exhaustive overview of optical absorption and laser-induced fluorescence lines of Ba atoms in liquid and solid helium matrices in visible and near-infrared spectral ranges. Due to the increased density of isolated atoms, we have found a large number of spectral lines that were not observed in condensed helium matrices before. We have also measured the lifetimes of metastable states. The lowest ${}^{3}{D}_{1}$ 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 extend our previous discussion of the resonant nonlinear Faraday effect in terms of the precession of an induced quadrupole moment to the realistic experimental situations where depopulation, repopulation, and hyperfine optical pumping contribute. We derive, to lowest order in the light intensity, an analytical expression for the magnetorotation angle near an electric-dipole resonance in an atomic medium of arbitrary spins. We find excellent quantitative agreement with absolute measurements performed on the Cs ${\mathit{D}}_{2}$ line.

We present time-resolved measurements of the laser-induced fluorescence from Cs atoms and ${\mathrm{Cs}}^{*}{\mathrm{He}}_{N}$ exciplexes in a solid $^{4}\mathrm{He}$ matrix. The linear ${\mathrm{Cs}}^{*}{\mathrm{He}}_{N=2}$ exciplex is strongly quenched by a radiationless transformation into a ring-shaped ${\mathrm{Cs}}^{*}{\mathrm{He}}_{N=6,7}$ exciplex, which decays radiatively with a lifetime of $88\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. The analysis of this lifetime provides information on the formation rate of ${\mathrm{Cs}}^{*}{\mathrm{He}}_{N=6,7}$ and on perturbations of the electronic states of the Cs atom by the bound helium atoms.

We give a detailed account on our semiempirical calculations of the forbidden electric tensor polarizability ${\ensuremath{\alpha}}_{2}^{(3)}$ of the ground state of free Cs atoms and of Cs atoms implanted in a solid $^{4}\mathrm{He}$ matrix. The results are compared with measurements of ${\ensuremath{\alpha}}_{2}^{(3)}$ in the free atoms [Europhys. Lett. 76, 1074 (2006)] and in He-trapped atoms [Phys. Rev. A 75, 042505 (2007)]. Features with respect to calculations by other authors are the inclusion of off-diagonal hyperfine interactions and an analysis of contributions from continuum states, which turn out to be negligible. For both samples the results of the calculations are in good agreement with the experimental values, thereby settling a long-standing discrepancy.

We present a systematic experimental study of the absorption and fluorescence spectra of the 6S{sub 1/}2-6P{sub 1/2} transition in Cs atoms isolated in solid {sup 4}He matrices. The bubble model developed earlier for alkali-metal atoms in liquid He is revised and applied to the present system. The analysis of the dependencies of absorption and fluorescence wavelengths on He pressure in liquid and solid He (cubic and hexagonal) environments leads us to modify the bubble model by taking the elastic deformation of solid He by the atomic bubble into account.

We have measured the electric tensor polarizabilities of the hyperfine levels of Cs atoms embedded in a body-centered cubic $^{4}\mathrm{He}$ crystal. The polarizabilities are inferred from the shift of optically detected magnetic resonance lines in each hyperfine level induced by static electric fields up to $50\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$. We recorded the magnetic resonances both by scanning the rf frequency and in a configuration in which the system is operated as a phase-stabilized magnetometer. The results from both measurements agree well with model calculations taking the effect of the solid helium matrix and our recent extension of the theory of forbidden tensor polarizabilities into account. We have also performed the first measurement of the differential tensor Stark splittings of the $F=3$ and $F=4$ hyperfine levels of the ground state, thus confirming the recently revised sign of this shift which affects the blackbody correction of primary frequency standards.

We report the observation of emission spectra from ${\mathrm{Rb}}^{*}{\mathrm{He}}_{n}$ exciplexes in solid $^{4}\mathrm{He}$. Two different excitation channels were experimentally identified, viz., exciplex formation via laser excitation to the atomic $5{P}_{3∕2}$ and to the $5{P}_{1∕2}$ levels. While the former channel was observed before in liquid helium, on helium nanodroplets, and in helium gas by different groups, the latter creation mechanism occurs only in solid helium or in gaseous helium above $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The experimental results are compared to theoretical predictions based on the extension of a model, used earlier by us for the description of ${\mathrm{Cs}}^{*}{\mathrm{He}}_{n}$ exciplexes. We also report the observation of fluorescence from atomic rubidium in solid helium, and discuss striking differences between the spectroscopic features of $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{He}$ and $\mathrm{Cs}\text{\ensuremath{-}}\mathrm{He}$ systems.

We present experimental and theoretical studies of the absorption, emission, and photodissociation spectra of Rb{sub 2} molecules in solid helium. We have identified 11 absorption bands of Rb{sub 2}. All laser-excited molecular states are quenched by the interaction with the He matrix. The quenching results in efficient population of a metastable (1) {sup 3}{pi}{sub u} state, which emits fluorescence at 1042 nm. In order to explain the fluorescence at the forbidden transition and its time dependence we propose a new molecular exciplex Rb{sub 2}({sup 3}{pi}{sub u})He{sub 2}. We have also found evidence for the formation of diatomic bubble states following photodissociation of Rb{sub 2}.

We present a theoretical study of the spectra produced by optical–radio-frequency double resonance devices, in which resonant linearly polarized light is used in the optical pumping and detection processes. We extend previous work by presenting algebraic results which are valid for atomic states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary geometries. The only restriction made is the assumption of low light intensity. The results are discussed in view of their use in optical magnetometers.

The theory of selective reflection from the interface of a dielectric medium and an atomic vapor is reconsidered. Both the local-field correction and the nonexponential decay of the electromagnetic field inside the vapor are accounted for in a purely analytical form. It is shown that the large influence of both effects on the shift of the spectral line is due to an interference between contributions from atoms moving towards the reflecting surface and from those moving away. We show that the local-field correction reduces the blueshift due to the field decay by one-third of its value. A simple formula relating the shift of the maximum of the reflection spectrum to the transition dipole moment and the number density of the atoms is given.