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Physical Review A

Column density and temperature effects on narrow resonance structures in atomic photoionization and photoabsorption

J. I. Lo, T. S. Yih, Yuxiang Luo, Hok‐Sum Fung, Y. Y. Lee, T. N. Chang

Published Jul 6, 2010• 1 citation
Publisher

Abstract

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.

DOI
10.1103/physreva.82.012504

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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−(1,0){}_{2}^{\ensuremath{-}}(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.
We report the observation of narrow peaks in Doppler-free spectra of thorium. The peaks are superimposed upon saturation-broadened line profiles in thorium atomic spectra by use of counterpropagating cw laser beams with resonance ionization mass spectrometry detection. At low laser powers the spectra show a conventional Lamb dip. As the laser power is increased, a narrow peak obscures the dip (for two of four transitions studied). The narrow peak is attributed to coherence effects at high laser powers. To confirm this assignment, we calculated the saturated absorption line shapes by using the theory of Haroche and Hartmann [ Phys. Rev. A6, 1280 ( 1972)]. The spectral simulations reproduce the narrow peaks and show the same dependence with laser power and transition strength as do the data. There are some quantitative discrepancies, particularly in the overall saturation broadening, but given that there were no adjustable parameters in the calculations, the agreement between theory and experiment is considered quite good. The Einstein A coefficients used in the calculations were determined from an analysis of the laser-power dependence of the saturation broadening for each transition.
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It is pointed out that the usual theories of pressure broadening disagree with direct experimental measurements of the widths of resonance-broadened alkali doublet lines, although the theories are at least qualitatively confirmed for the wings of resonance lines and for foreign-gas broadening. There are also two conflicting sets of experimental data (both in strong disagreement with theory). Some qualitative arguments are given for doubting the validity, near the center of a resonance line, of the "two-body" approximation generally used in theoretical treatments, and some formal objections to the usual procedure are also presented. The basic equation relating the susceptibility to a resolvent operator matrix element is derived from a more fundamental point of view than usual; the result contains in principle all effects due to translational motion, to the quantized nature of the radiation field, and to "spatial dispersion." In the limit of large absorber mass it is proved rigorously that the two-body approximation can never be valid near the center of a line, even for very low densities. By using different approximations for the wings and the line center, we obtain satisfactory agreement with experiments in both regions for the alkali doublet lines. We are also able to decide between the two conflicting experiments, favoring the square-root density dependence of the line-width observed by Lauriston and Welsh.
Experimental line profiles and integrated intensities for backscattered and forward-scattered resonance radiation using white-light excitation are compared with theory for Na densities between 1019{10}^{19}1019 and 1022{10}^{22}1022 atoms/m3{\mathrm{m}}^{3}m3. Absorption measurements are used to verify linewidth and line-wing asymmetry parameters and Na-vapor densities. The scattered spectral intensities are normalized to the incident flux using calibrated diffusers. Measurements of scattered intensities were also made as N2{\mathrm{N}}_{2}N2​ was added to the Na. The line profiles and integrated intensities were calculated using a model which assumes complete redistribution of the radiation scattered by an atom and takes into account both radiative and nonradiative transport of the resonance excitation. The theory includes the effects of quenching by Na2{\mathrm{Na}}_{2}Na2​ and by admixed N2{\mathrm{N}}_{2}N2​. With the white-light excitation used in these experiments the radiative transport is found to dominate the nonradiative transport except at wavelengths very near line center. The agreement between experimental and theoretical line profiles is good at high sodium densities (\ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} 1021{10}^{21}1021 atoms/m3{\mathrm{m}}^{3}m3) where instrumental resolution is not a serious limitation. The agreement between experimental and theoretical integrated intensities is within experimental error for Na densities of (1-10) \ifmmode\times\else\texttimes\fi{} 1019{10}^{19}1019 and (2-5) \ifmmode\times\else\texttimes\fi{} 1021{10}^{21}1021 atoms/m3{\mathrm{m}}^{3}m3. At the intermediate densities the approximately 20% discrepancy may be caused by the incomplete redistribution of radiation in the scattering process.
A ladder-type model of double optical resonance, extended by allowing some internal structure for the middle level, is considered and its classical analogue is given. Ionization and photoelectron energy spectra from such a model with singlet, doublet and triplet middle-level structures are evaluated numerically for a wide range of laser pulse intensities and durations. The topics of most interest are peak narrowing by extremely strong upper-level ionization damping, reversal of the peak-height asymmetry by increasing lower-resonance strength at non-zero upper-resonance detuning, and number-of-peak reduction and flattening of the spectrum by shortening of the atom-photon interaction time. When applied to natural barium, this model gives results in good agreement with experiment.