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Journal of Physics B Atomic Molecular and Optical Physics

Window resonances below the strontium 6s threshold

M Aymar, J. M. Lecomte, C. C. Chu, Hok‐Sum Fung, Haibin Wu, T. S. Yih

Published Dec 14, 1998• 7 citations
Publisher

Abstract

Absolute photoabsorption cross sections of strontium have been measured in the energy range extending from the threshold up to above the 6s threshold. The synchrotron radiation source of the Synchrotron Radiation Research Center at Hsin-Chu was used as the background continuum. Strontium vapour was confined in a heatpipe oven. The absolute column density was determined by simultaneously measuring the temperature distribution profiles and the total pressure in the heatpipe. Absolute cross sections were determined using the Beer-Lambert law. The observed spectrum was analysed using a combination of a recent extension of the jj -coupled eigenchannel R -matrix adapted to handle highly correlated levels and the phase-shifted multichannel quantum defect theory. The main features of the observed spectrum, dominated by the presence of many window-type resonances, are well reproduced and the structures are identified. The observed absorption windows result from the fact that only continua are directly excited from the ground state.

DOI
10.1088/0953-4075/31/23/011

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Absolute photoabsorption cross sections of strontium have been measured in the energy range extending from the threshold up to above the 6s threshold. The synchrotron radiation source of the Synchrotron Radiation Research Center at Hsin-Chu was used as the background continuum. Strontium vapour was confined in a heatpipe oven. The absolute column density was determined by simultaneously measuring the temperature distribution profiles and the total pressure in the heatpipe. Absolute cross sections were determined using the Beer-Lambert law. The observed spectrum was analysed using a combination of a recent extension of the jj -coupled eigenchannel R -matrix adapted to handle highly correlated levels and the phase-shifted multichannel quantum defect theory. The main features of the observed spectrum, dominated by the presence of many window-type resonances, are well reproduced and the structures are identified. The observed absorption windows result from the fact that only continua are directly excited from the ground state.
Absolute total cross sections for electron scattering from Ar and Xe at electron energies ranging from 7 meV to 20 eV were obtained with the experimental technique employing the threshold-photoelectron source. The measured total cross sections are in good agreement with those obtained by other groups down to 100 meV, above which several experimental works have been reported. Scattering lengths for electron scattering from Ar, Kr, and Xe were determined from the present total cross sections and our recent results for Kr using the modified effective range theory. The values of the scattering length obtained in the present analysis differ from the values determined from the previous swarm experiments and beam experiments. The resonant structures in the total cross sections due to Feshbach resonances of Ar, Kr, and Xe with an improved energy resolution were also measured. Analyses of the resonant structure were carried out based on the spin-dependent resonant scattering theory in order to determine the values of the natural width of Feshbach resonances of Ar, Kr, and Xe precisely.
Abstract Using the reactance matrix approach, we systematically develop new multichannel quantum defect theory (MQDT) models for the singlet and triplet S, P, D and F states of strontium below the first ionization limit, based on improved energy level measurements. The new models reveal additional insights into the character of doubly excited perturber states, and the improved energy level measurements for certain series allow fine structure to be resolved for those series’ perturbers. Comparison between the predictions of the new models and those of previous empirical and ab initio studies reveals good agreement with most series; however, some discrepancies are highlighted. Using the MQDT wave functions derived from our models we calculate other observables such as Landé -factors and radiative lifetimes. The analysis reveals the impact of perturbers on the Rydberg state properties of divalent atoms, highlighting the importance of including two-electron effects in the calculations of these properties. The work enables future investigations of properties such as Stark maps and long-range interactions of Rydberg states of strontium.
We present measurements of photodetachment cross sections of negative gold ions stored in a RF trap, in the energy range from about to above threshold. A number of different laser devices have been used. The results complement those described in a previous paper. The photodetachment spectrum exhibits a strong resonance, located at above ground level and a much fainter one at . These resonances can be represented as Fano profiles and reveal the presence of autodetaching excited levels of (most probably and ). Furthermore, the opening of a new photodetachment channel near 360 nm (corresponding to the first excited level of neutral Au) is accompanied by an increase of the photodetachment yield.
We present a joint experimental and theoretical investigation to resolve the discrepancy in the ratio of relative peak cross sections for narrow atomic resonances among various experimental spectra and also between theory and experiment. Our study includes an effort to measure both the absorption and ionization spectra in a single experimental setup. We also present a careful analysis of the effect on the resonance structure due to the Doppler broadening at finite temperature when the Doppler width is greater than the natural linewidth of the resonance. In addition, we demonstrate that the column density strongly affects not only the absorption structure profile of a narrow atomic resonance but also the ionization spectra measured in an ionization chamber. From the good agreement reported in this article between the observed and the theoretically simulated spectra for the pressure-dependent peak cross sections and the effective asymmetry parameter for the lowest resonance of the He $(1,0){}_{2}^{\ensuremath{-}}$ series, we are able to characterize the monochromator (i.e., slit) function of a given light source, including its estimated energy resolution.
This paper reports on our studies of near-resonant scattering of laser light in a collisional environment. A pulsed dye laser with a peak power of 55 MW/cm2{\mathrm{cm}}^{2}cm2 was tuned near the 460.73-nm resonance line of strontium and the side emission was observed from an oven containing both strontium vapor and argon buffer gas. The emission was composed of three spectral components: Rayleigh scattering at the frequency of the laser ${\ensuremath{\omega}}_{L}$, fluorescence at the resonance frequency ${\ensuremath{\omega}}_{0}$ of strontium, and a third component at $2{\ensuremath{\omega}}_{L}\ensuremath{-}{\ensuremath{\omega}}_{0}$. These three components have been studied as a function of the frequency and intensity of the laser and also as a function of the argon buffer gas pressure. While the Rayleigh emission was found to vary as ${\ensuremath{\Delta}}^{\ensuremath{-}2}$ ($\ensuremath{\Delta}={\ensuremath{\omega}}_{0}\ensuremath{-}{\ensuremath{\omega}}_{L}$), the fluorescence component, which was produced by Sr-Ar collisions, was found to be asymmetric with the sign of $\ensuremath{\Delta}$ as predicted by line-broadening theory. By measuring the ratio of the intensities of the fluorescence and Rayleigh components, we were able to measure directly the collisional redistribution function, important in the study of radiative transfer in stellar and planetary atmospheres. At high laser intensities all three components were found to saturate. The results were compared with the theoretical predictions of Mollow's steady-state theory. Theoretical fits for the high-intensity results were obtained when the collisional cross sections were taken to be considerably smaller than in our low-intensity measurements. We believe the discrepancy lies in the use of a steady-state theory for a transient experiment. Effects of radiative trapping and spatial averaging are also discussed.
The relative optical excitation functions and polarizations of the Sr resonance line (4607 \AA{}) and of the Sr+{\mathrm{Sr}}^{+}Sr+ resonance lines (4078 and 4216 \AA{}) have been measured, using crossed electron and strontium-atom beams, for electron energies from the excitation thresholds to 1497 eV. The electron-beam energy resolution was \ensuremath{\sim} 0.22 eV for energies below 13 eV, and the atom beam was optically thin. The excitation functions of the ionic lines were measured relative to that of the atomic resonance line at fixed energies near the maxima of the excitation functions. Using spontaneous-emission branching ratios, this yields the ratios of total excitation cross sections for the atomic and ionic resonance levels (including cascades). The Sr 51P5^{1}P51P excitation function has been normalized to Born theory taking account of cascade at 1497 eV, where the energy dependence of the excitation function has converged to the theoretical behavior. The resulting normalized cross sections have been compared with available theoretical calculations and other measurements. The 4607-\AA{} polarization function is consistent with the theoretical threshold limit within experimental uncertainity.