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Physical Review A

Doublet line-strength ratios for the principal series of cesium

Dragos Constantin Popescu, Iovitzu Popescu, J. Maurer, C. B. Collins, B. W. Johnson

Published Oct 1, 1975• 12 citations
Publisher

Abstract

Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 \AA{} to measure the relative absorption probabilities for transitions of the cesium-principal-series ($6^{2}S\ensuremath{-}n^{2}P_{\frac{1}{2},\frac{3}{2}}$) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of ${\mathrm{Cs}}_{2}$*, were examined over a range of cesium-atom concentration varying from 1.2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ to 1.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. From the corrected doublet line-strength ratios for $11\ensuremath{\le}n\ensuremath{\le}16$, the segment of the curve of Fano $X$ parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near $19^{2}P$.

DOI
10.1103/physreva.12.1425

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Journal of the Optical Society of America B•Mar 1, 1986•16 citations
Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 \AA{} to measure the relative absorption probabilities for transitions of the cesium-principal-series (62S\ensuremath−n2P12,326^{2}S\ensuremath{-}n^{2}P_{\frac{1}{2},\frac{3}{2}}62S\ensuremath−n2P21​,23​​) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of Cs2{\mathrm{Cs}}_{2}Cs2​*, were examined over a range of cesium-atom concentration varying from 1.2\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 to 1.5\ifmmode\times\else\texttimes\fi{}1015{10}^{15}1015 cm\ensuremath−3{\mathrm{cm}}^{\ensuremath{-}3}cm\ensuremath−3. From the corrected doublet line-strength ratios for 11\ensuremath≤n\ensuremath≤1611\ensuremath{\le}n\ensuremath{\le}1611\ensuremath≤n\ensuremath≤16, the segment of the curve of Fano XXX parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near 192P19^{2}P192P.
Space-charge amplification of the direct and indirect ionization resulting from photoabsorption in alkali-metal vapor is shown to provide a useful complement to the conventional techniques of absorption spectroscopy. Because of its inherently greater sensitivity, this technique is particularly useful in spectroscopic applications involving weak transitions such as those to highly excited states or absorptive transitions from excited states of low population. This paper reports the use of this technique to extend prior measurements of the oscillator strengths of the ($6^{2}S\ensuremath{\rightarrow}n ^{2}P$) series in Cs, the n2Dn^{2}Dn2D doublet separations, and the photoionization spectrum of Cs2{\mathrm{Cs}}_{2}Cs2​.
The doublet oscillator-strength ratios ρn of the cesium principal series (62S1/2−n2P3/2,1/2) have been measured for principal quantum number n = 7–21. Both emission and absorption spectroscopic measurements were performed with a 9.1-m focal-length spectrometer. The results show that ρn increases monotonically with n. Discrepancies between emission and absorption results are attributed to collisional mixing of the excited-state populations. Oscillator strengths of both fine structure lines were obtained from the absorption measurements. These agree substantially with similar work by Lorenzen and Niemax [ J. Phys. B11, L723 ( 1978)]. Fano X parameters derived from the measured results are found to decrease monotonically with n. The value of X, extrapolated to the ionization potential, is 2.05 ± 0.03, in good agreement with recent laser-excited atomic-beam results of Raimond et al. [ J. Phys. B11, L765 ( 1978)]. This makes unlikely the existence of a pole of ρn in the discrete spectrum, which requires that X have the value 2.
The use of in situ ion detection in alkali vapors coupled with multiphoton ionization provides a technique of highest sensitivity for the investigation of the spectroscopy of excimer states involving alkali atoms. This technique has been applied in this work to the molecular components of the spectrum of the $6S\ensuremath{-}7P$ absorption doublet of cesium. The dispersion curve was obtained with a tunable dye laser for the two-photon ionization of cesium over the 4500-4600-\AA{} wavelength range as a function of vapor pressure and source intensity. Strongly developed satellite bands were observed to be associated with the atomic lines $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{1}{2}}$ and $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{3}{2}}$, and were found to have well-defined band heads at 29.4 and 31.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ to longer wavelengths, respectively. Sensitivity was observed to be an order of magnitude in excess of that obtained from classical absorption techniques, and the observation of a quadratic dependence of the molecular components confirmed the multiphoton nature of the detection sequence.
16 lines of the principal series of Cs(6 2 s 1/2 to 7-16 2 p 3/2 and 6 2 s 1/2 to 7-12 2 p 1/2 ) were investigated in absorption. Cesium vapour density was varied from 10 13 up to 10 17 cm -3 . Red asymmetries of all lines (extended red wings) were detected. This can be understood if electron transitions in the long-range interaction region follow the selection rules of molecules. It was established that the red and the blue wings of the lines each can be fitted to lorentzian profiles in small wavelength intervals. With the help of quasistatic theory the interaction constants from the long-range interaction potential Delta omega =C 3 /r 3 multiplied by the oscillator strength were determined from measurements. For all lines the ratio of C 3 (red)/C 3 (blue) was found to equal 2 in accordance with earlier theoretical calculations. In the far line wing, deviation from a lorentzian profile occurs. This was interpreted as a consequence of electron transition in the short-range region of interatomic potentials.
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 62S126^{2}S_{\frac{1}{2}}62S21​​ level to resonant n2Dn^{2}Dn2D and n2Sn^{2}Sn2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9to13n=9\mathrm{to}13n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from n=11to14n=11\mathrm{to}14n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.
By pumping the 6s–7p line of neutral cesium at 4593 Å with a flash-pumped dye laser we obtained ~80% ionization in a heat-pipe-generated Cs vapor. An absorption spectrum of Cs+ was then obtained, showing the 5p6–5p5nd and ns Rydberg series both below and above the 5p5 2P3/2 threshold. Effects of channel mixing are seen in the broadening of the nd series members above threshold and in the anomolous intensity behavior below. This strong interaction is reflected in the Lu–Fano plot shown for these data.