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Proceedings of the Royal Society of London A Mathematical and Physical Sciences

Radiation width and resonance broadening in helium

Heinrich Gerhard Kuhn, Janet Maria Vaughan

Published Feb 11, 1964• 60 citations
Publisher

Abstract

Abstract By means of a direct recording Fabry-Perot interferometer, the contours of several singlet lines of helium were studied over a wide range of pressure and temperature. The source was a microwave discharge cooled with liquid nitrogen or liquid helium. Analysis of the tracings allowed the Doppler and Lorentz components to be separated quantitatively. A detailed study was carried out for the line λ 7281 Å (21P - 31S). By extrapolation of the Lorentz width to zero pressure the radiation width of the level 2 was determined and led to an ƒ-value 0·377 + 0·035 of the resonance line λ 584 Å (11S— 21P) The resonance broadening is found to be independent of the speed of the atoms, over a range of a factor five in the speed, in agreement with theories of optical collision by Weisskopf and others. The absolute value of the resonance broadening constant is compared with various theories. The strong line λ 6678 Å (21P -31D) showed self-absorption due to imprisonment of resonance radiation.

DOI
10.1098/rspa.1964.0024

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The normalized fluorescence spectrum of the Li 6708-\AA{} resonance line perturbed by the noble gases has been measured for 100 \AA{} either side of the resonance line, at a temperature of 400 \ifmmode\pm\else\textpm\fi{} 10\ifmmode^\circ\else\textdegree\fi{}C. The noble-gas pressure range of 100-1800 Torr was investigated. It was observed that the line wings scale linearly with this pressure for $2<|\ensuremath{\Delta}\ensuremath{\lambda}|<100$ \AA{}, and it was was demonstrated that a Lorentzianbroadened line core can be observed for this pressure range. The Li density was \ensuremath{\sim} ${10}^{10}$/${\mathrm{cm}}^{3}$ and was varied to establish the fluorescence spectrum in the limit of no radiation entrapment. The central region of the line has been measured with high resolution and compared to calculated profiles due to the various fine structure and isotopic line components, each broadened by a convolution of Lorentz, Doppler, and instrumental profiles. The shift and Lorentz broadening rates, as well as the wavelengths for transition to non-Lorentzian behavior, are reported. In contrast to the heavier alkali-metal-noble-gas cases, no distinctive far-wing features such as satellites or sharp inflections are observed. This is consistent with the much larger collision velocities for the Li case.

Physical Review A•Jul 1, 1975•39 citations
Abstract By means of a direct recording Fabry-Perot interferometer, the contours of several singlet lines of helium were studied over a wide range of pressure and temperature. The source was a microwave discharge cooled with liquid nitrogen or liquid helium. Analysis of the tracings allowed the Doppler and Lorentz components to be separated quantitatively. A detailed study was carried out for the line λ 7281 Å (21P - 31S). By extrapolation of the Lorentz width to zero pressure the radiation width of the level 2 was determined and led to an ƒ-value 0·377 + 0·035 of the resonance line λ 584 Å (11S— 21P) The resonance broadening is found to be independent of the speed of the atoms, over a range of a factor five in the speed, in agreement with theories of optical collision by Weisskopf and others. The absolute value of the resonance broadening constant is compared with various theories. The strong line λ 6678 Å (21P -31D) showed self-absorption due to imprisonment of resonance radiation.
Measurements of the self-induced pressure shift of the helium triplet line lambda 706.5 nm (1s2p 3 P 2.1,0 -1s3s 3 S 1 ) from a gas discharge have been carried out at temperatures of 77 K and 300 K using a scanning Fabry-Perot interferometer. The shift coefficient is positive (a blue shift) and at 77 K it is consistent with the results of a theoretical calculation carried out by Leo et al. (1992). At 300 K, the measured shift parameter is much greater than the theoretical value. As expected, the shifts are significantly larger in both cases than those of the corresponding singlet line, for which the broadening and shift are dominated by the resonance interaction. The authors have also compared their experimental results with predictions based on the Lennard-Jones potential.
The spectral profile of the strongly resonance-broadened transition lambda 585.2 nm of Ne I has been studied in absorption. A narrow-band tunable dye laser probed a weak discharge in natural neon cooled with liquid nitrogen. The Lorentzian width extrapolated to zero density was found to be 108+or-20 MHz, consistent with the value 114+or-4 MHz calculated from the natural widths of the levels. There is no indication of the excess width found in earlier emission experiments in helium and neon. A preliminary result of (1.7+or-0.2)10 -21 MHz atom -1 m 3 was obtained for the broadening constant of lambda 585.2 nm in absorption at 77K. This is consistent with the value found in emission.
\n\t\t\t\tA theoretical analysis of the line broadening of localized atomic transitions in liquid helium is presented. It is shown that accurate information can be derived on the long-range part of the He*-He interaction as well as on the local structure near the He* emitters. The analysis confirms that in corona discharges in liquid helium the emitting He* atoms reside in cavities and that for known He*-He interaction the size of the cavities can be deduced from the line profile. The He*-He interaction was calculated using the full configuration interaction (CI) method as implemented in the Molpro package. The widths of atomic lines due to fluorescent transitions between different excited states of helium atoms were calculated as a function of external pressure in the range from 0.1 and 3.5 MPa using the static approximation method, and the input from the results of the CI calculation and cavity diameters calculated using the bubble model. The calculated widths showed excellent agreement with experimental data of liquid helium excited by corona discharges. A second, analytical analysis using a power function to represent the He*-He interaction showed qualitative agreement with the experimental data.\n\t\t\t
The resonance broadening of the sodium D-lines in absorption was determined by measuring the contour of the line wings directly. The vapor pressures of sodium ranged from 10-3 to 75 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. Δν1/2/N = 0.78 x 10-7 sec-1cm3 --the constant being about twice that predicted by Houston, but much less than the experimental result by Hughes and LLoyd. Above 5 mm pressure, the half-intensity width appeared to increase slightly faster than N. Somewhat in contradiction to theory, the relative width, Δν1/2,s/Δν1/2,1' was 1.16. The natural width measured was, within experimental error, consistent with previous values. The inverse square contour for the absorption coefficient was satisfactory up to 5 mm Hg; beyond this pressure an inverse cube contour fitted better. No definite evidence of van der Waal force was observed at high pressures; likewise, no appreciable shifts and asymmetries. MgO windows were used and the temperature determination was made reliable.
Three-photon excitation and multiphoton ionization techniques have been used to measure self-broadening for several resonance lines in xenon and krypton. Tabulations of widths and shifts of the xenon lines at 146.9, 129.6, 119.2, and 117.0 nm were made for pressures between 50 and 900 Torr. Similar measurements were made for the krypton resonance line at 116.5 nm. For pressures less than 500 Torr the widths were found to be in reasonable agreement with theoretical results, which were extrapolated into regions of concentration where the criteria for their validity are at best questionable. We have also found the lines to be shifted to the high-energy side of the unperturbed line. For four of the five lines studied, the blue shifts increased nearly linearly with increasing pressure. In the four cases where the shift could be measured the ratio of shift to width was close to (1/2. Our experimental technique is very simple, but it has yielded what we believe to be the first resonance broadening data based on absorption under conditions where there is strong trapping of the resonance radiation.
The normalized fluorescence spectrum of the Li 6708-\AA{} resonance line perturbed by the noble gases has been measured for 100 \AA{} either side of the resonance line, at a temperature of 400 \ifmmode\pm\else\textpm\fi{} 10\ifmmode^\circ\else\textdegree\fi{}C. The noble-gas pressure range of 100-1800 Torr was investigated. It was observed that the line wings scale linearly with this pressure for $2<|\ensuremath{\Delta}\ensuremath{\lambda}|<100$ \AA{}, and it was was demonstrated that a Lorentzianbroadened line core can be observed for this pressure range. The Li density was \ensuremath{\sim} 1010{10}^{10}1010/cm3{\mathrm{cm}}^{3}cm3 and was varied to establish the fluorescence spectrum in the limit of no radiation entrapment. The central region of the line has been measured with high resolution and compared to calculated profiles due to the various fine structure and isotopic line components, each broadened by a convolution of Lorentz, Doppler, and instrumental profiles. The shift and Lorentz broadening rates, as well as the wavelengths for transition to non-Lorentzian behavior, are reported. In contrast to the heavier alkali-metal-noble-gas cases, no distinctive far-wing features such as satellites or sharp inflections are observed. This is consistent with the much larger collision velocities for the Li case.