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Physical Review A

Collision broadening of the Sr-ion resonance line from the line core to the near-wing region

Hisatomo Harima, K. Tachibana, Y. Urano

Published Jan 1, 1987• 2 citations
Publisher

Abstract

Profiles of the singly ionized strontium resonance line at 421.6 nm (4p ${\mathrm{}}^{2}$${\mathrm{P}}_{1/2}$--4s ${\mathrm{}}^{2}$${\mathrm{S}}_{1/2}$) which are collisionally broadened by argon have been measured from the line core to near-wing region (within the distance of \ensuremath{\Delta}\ensuremath{\nu}\ensuremath{\lesssim}50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the line center). The observed broadening rate of the core is (1.87\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ at 690 K. A clearly asymmetric profile has been observed in the wing region. These results are discussed by considering semiclassical impact and quasistatic theories combined with theoretical interaction potentials for the ${\mathrm{Sr}}^{+}$-Ar pair which were previously reported by the present authors.

DOI
10.1103/physreva.35.109

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Physical Review Letters•Jul 1, 1985•19 citations
Profiles of the singly ionized strontium resonance line at 421.6 nm (4p {\mathrm{}}^{2}$${\mathrm{P}}_{1/2}--4s {\mathrm{}}^{2}$${\mathrm{S}}_{1/2}) which are collisionally broadened by argon have been measured from the line core to near-wing region (within the distance of \ensuremath{\Delta}\ensuremath{\nu}\ensuremath{\lesssim}50 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1 from the line center). The observed broadening rate of the core is (1.87\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}10\ensuremath−20{10}^{\mathrm{\ensuremath{-}}20}10\ensuremath−20 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1/cm\ensuremath−3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}cm\ensuremath−3 at 690 K. A clearly asymmetric profile has been observed in the wing region. These results are discussed by considering semiclassical impact and quasistatic theories combined with theoretical interaction potentials for the Sr+{\mathrm{Sr}}^{+}Sr+-Ar pair which were previously reported by the present authors.
The close-coupled theory of collisions in a radiation field is used to calculate the absorption profile for the Sr $^{1}\ensuremath{\leftarrow}^{1}$S resonance line broadened by collisions with Ar. The calculations predict the polarization ratios of Sr 1P^{1}\mathrm{P}1P fluorescence following line wing excitation by either linear or circular polarized light. Ab initio calculations were used to obtain the ground and excited SrAr molecular-potential curves, which were adjusted to give improved agreement with experiment. The radiative-scattering theory gives a unified description of the absorption coefficient and polarization redistribution from the small detuning impact limit region to the far spectral wings. The cross sections for elastic and inelastic depolarizing collisions of Sr 1P1^{1}\mathrm{P}_{1}1P1​+Ar were also calculated. The calculated absorption coefficient, impact-broadening rate, linear- and circular-polarization ratios, and depolarization rate coefficients are for the most part in good agreement with experiment.
The spectral lineshapes of the Sr (51P1–51P0) resonance transition perturbed by rare gases have been examined with a high-resolution laser-induced fluorescence method. In He and Ne, the collisional broadened spectral profiles are found to be symmetrical over the entire studied pressure range (0–600 Torr). The resulting line shapes are accurately represented by a simple Voigt profile expression. The Ar, Kr, and Xe perturbing gases, however, produce anomalous asymmetrical line broadening. To our knowledge, this represents the first measurement of asymmetrical broadening in the near wings of the emission profiles of the Sr-rare gas systems. Quantitative comparisons of the measured and theoretical line asymmetries suggest that atomic interactions in the Sr-rare gas systems are not entirely due to van der Waals (R-6)-type interactions. The line asymmetries could be caused by the correlations between thermal motions of the Sr atoms and collisional interactions.
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.
Absorption profiles of cesium resonance lines at 8521 and 8944 \AA{} have been measured for the wavelength range from 0.5 to 30 \AA{} from the line centers using a tungsten ribbon light source, cell lengths LLL from 0.31 to 6.1 cm, and Cs densities NNN from 2 \ifmmode\times\else\texttimes\fi{} 1014{10}^{14}1014 to 2 \ifmmode\times\else\texttimes\fi{} 1016{10}^{16}1016 ${\mathrm{cm}}^{\ensuremath{-}3}$. The wavelength dependence of the absorption coefficients are consistent with a Lorentzian form and values of $\frac{\ensuremath{\gamma}f}{N}=1.08\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ and 0.30 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$ are obtained for 8521 and 8944 \AA{} lines. Here $\ensuremath{\gamma}$ is the collisional (or Lorentzian) half-width and fff is the oscillator strength. The values of $\frac{\ensuremath{\gamma}f}{N}$ deduced from total fractional absorption data agree with those determined from the line profiles. Our results agree well (to within 15%) with those of Gregory and with theory, but disagree with more recent data. A source of error in some previous determinations of half-widths is the fact that even when the calculated absorption is very large, the measured transmission at the line center is significant; about 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ compared to \ensuremath{\sim} ${10}^{\ensuremath{-}100,000}$ predicted from our measurements in the wings of the lines for $N=1.14\ifmmode\times\else\texttimes\fi{}{10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and L=5.9L=5.9L=5.9 cm. This effect is shown to result from transmission through the monochromator at wavelengths in the far wings of the instrument function.
For the first time a transition from a second- to first-order dipole-dipole interaction (van der Waals to resonance interaction) has been observed in a study of the excited EuSr molecule which dissociates into europium 4${\mathrm{f}}^{7}$5s6p 8{\mathrm{}}^{8}8P9/2{\mathrm{P}}_{9/2}P9/2​ and strontium 5s2{\mathrm{s}}^{2}s2 1{\mathrm{}}^{1}1P1{\mathrm{P}}_{1}P1​ atoms. The transition was verified by measurement of the wavelength dependence in the quasistatic wings of the cw-laser--excited 4595.3-A\r{} line in Eu broadened by Sr ground-state atoms.