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Journal of Physics B Atomic and Molecular Physics

Blue asymmetry of potassium resonance lines broadened by caesium atoms

Robert Beuc, Mladen Movre, Cedomil Vadla

Published May 14, 1982• 8 citations
Publisher

Abstract

The authors have measured the absorption efficient in the quasi-static wings of the first potassium resonance lines in various K-Cs mixtures. At sufficiently high concentrations of caesium atoms both components of the potassium first resonance doublet have extended blue wings of the van der Waals type. The corresponding effective C 6 constants determined from the experimental data are (1.5+or-0.3)*10 -29 s -1 cm 6 and (2.4+or-0.5)*10 -29 s -1 cm 6 for the stronger and weaker components, respectively. The calculated values of the effective C 6 constants are 1.78*10 -29 s -1 cm 6 for the stronger component and 2.35*10 -29 s -1 cm 6 for the weaker component.

DOI
10.1088/0022-3700/15/9/010

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Journal of Physics B Atomic Molecular and Optical Physics•Oct 28, 1993•18 citations
The authors have measured the absorption efficient in the quasi-static wings of the first potassium resonance lines in various K-Cs mixtures. At sufficiently high concentrations of caesium atoms both components of the potassium first resonance doublet have extended blue wings of the van der Waals type. The corresponding effective C 6 constants determined from the experimental data are (1.5+or-0.3)10 -29 s -1 cm 6 and (2.4+or-0.5)10 -29 s -1 cm 6 for the stronger and weaker components, respectively. The calculated values of the effective C 6 constants are 1.7810 -29 s -1 cm 6 for the stronger component and 2.3510 -29 s -1 cm 6 for the weaker component.
The authors have measured the absorption coefficient in the impact region of the first potassium resonance lines broadened by rubidium atoms. For the broadening constants gamma K*-Rb they have obtained the values (6.2+or-0.5)*10 -9 s -1 cm 3 /atom and (7.8+or-0.5)*10 -9 s -1 cm 3 /atom for the stronger and weaker components, respectively. The theoretical values calculated from the Lindholm formula are 6.0*10 -9 s -1 cm 3 /atom for the stronger and 7.0*10 -9 s -1 cm 3 /atom for the weaker component. The corresponding theoretical effective C 6 constants are 21.6*10 -30 s -1 cm 6 and 15.3*10 -30 s -1 cm 6 , respectively.
In addition to previous absorption measurements of Cs resonance lines, new investigations on Na, K and Rb also showed definitely that beyond symmetrical Lorentzian central parts of the lines, the stronger components of the resonance lines have blue and the weaker components red asymmetry. This behaviour can be explained by a quasistatic treatment of the self-broadening mechanism taking interatomic interaction potentials into account. But with additional data it is now possible to give a different interpretation of the origin of observed asymmetries than suggested in the previous paper on Cs resonance lines (see abstr. A5740 of 1975). The asymmetries are shown to arise from the mutual repulsion of two potential curves of the same species.
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.
By direct measurement of the contours of the line wings, the resonance broadening of the sodium DDD lines in absorption was determined for vapor pressures of sodium ranging from ${10}^{\ensuremath{-}3}$ to 70 mm Hg with negligible amount of foreign gases. Over nearly the entire range, the half-intensity width varied linearly with the density of the absorbing atoms; i.e., $\frac{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2}}}{N}=0.80\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{sec}.}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}3}$---the constant being 1.6 times that predicted by theory but much less than the corresponding experimental results for potassium. Above 5 mm pressure, $\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2}}$ increased slightly faster than NNN; moreover, the contour for the absorption coefficient was more adequately represented by an inverse cube contour than by the inverse square. Somewhat in contradiction to theory, the relative width, $\frac{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2},8}}{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2},l}}$ was 1.12. No definite evidence of van der Waals force was observed at the higher pressures; likewise no appreciable shifts or asymmetries. The natural width measured was consistent with previous results.
The improved method for the determination of the atom number densities in pure and mixed alkali vapours has been proposed. In order to obtain the correct results one has to go beyond the simple Lorentzian shape of the quasistatic absorption profile. The method has been tested by the white-light absorption measurements in the pure K, Rb and Cs vapours. The statistical accuracy of the obtained data for the atom number density is +or-3%. The method has been applied for determining the atom number densities in the vapour over the Rb-Cs mixture. The obtained values have been used in the evaluation of the effective C 6 constants for Rb*-Cs interaction: C 6 eff =(4.9+or-0.3)* 10 -30 cm 6 s-1 for the D2 line. The theory predicts C 6 eff =4.95*10 -30 cm 6 s -1 for the D1 line and C 6 eff =2.37*10 -30 cm 6 s(-1) for the D2 line.