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Physical Review Letters

Mean-life Measurements in He I by the Hanle Effect on High-Velocity Atoms Excited in a Gaseous Target

M. Carré, Jean Desesquelles, Maurice Dufay, Michel Louis Gaillard

Published Nov 22, 1971• 19 citations
Publisher

Abstract

Mean lives of He I are deduced from the Hanle curves for high-velocity atoms excited in a gaseous target. For long---mean-life levels we observed oscillations in agreement with a partial precession of the dipoles around the magnetic field. For the $3p^{1}P$ level, the Hanle curve is not perturbed by self-absorption using this method, and the measured mean life (1.8\ifmmode\pm\else\textpm\fi{}0.1 nsec) is in good agreement with the theoretical value.

DOI
10.1103/physrevlett.27.1407

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The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the $2^{1}P$ state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of $2^{1}S$ metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the $2^{1}S$ atoms, which are excited to the $2^{1}P$ state. This state then primarily decays to the ground $1^{1}S$ state with the emission of resonance (584 \AA{}) radiation. The lifetime of the $2^{1}P$ state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.

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The oscillator strength for the resonance (11S-21P) transition in atomic helium has been determined from a measurement of the lifetime of the 2^ state.This life time was obtained using a modified zero field level crossing (Hanle) method.The scattering sample was a thermal velocity beam of 2^8 metastable atoms.Resonant 2 micron (218-2^2) radiation is absorbed by the 21S atoms, which are excited to the 2^P state.This state then primarily decays to the ground 1^8 state with the emission of resonance ($84A) radiation.The lifetime of the 21P state was determined from measurements of the magnetic field dependence of the 584Â intensity.This new technique eliminates the problem of constructing a vacuum ultra violet resonance lamp.The measured lifetime t(2 P) is (5.69 ±0.23) x Id"10 sec.The resulting absorption oscillator strength for the resonance transition is 0.270 ±0.011.This oscillator strength agrees with tie theoretical value.

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Lifetime of the First Excited Atomic States of Rb87

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The Hanle effect on the resonance lines of the ${\mathrm{Rb}}^{87}$ atom has been observed. The line shape of the signal for the $5^{2}S_{\frac{1}{2}}\ensuremath{\leftrightarrow}5^{2}P_{\frac{3}{2}}$ is found to be Lorentzian, while that for the $5^{2}S_{\frac{1}{2}}\ensuremath{\leftrightarrow}5^{2}P_{\frac{1}{2}}$ is dispersion-type. The half-width of these lines was observed to be 3.146\ifmmode\pm\else\textpm\fi{}0.13 G and 23.0 G\ifmmode\pm\else\textpm\fi{}5%, respectively. These values yield the lifetime of (2.71\ifmmode\pm\else\textpm\fi{}0.14) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$sec and (3.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$sec for the $5^{2}P_{\frac{3}{2}}$ and $5^{2}P_{\frac{1}{2}}$ states, respectively. The oscillator strengths for the corresponding transitions are found to be 0.673\ifmmode\pm\else\textpm\fi{}0.03 and 0.32\ifmmode\pm\else\textpm\fi{}0.03, respectively. When the temperature of the scattering cell was changed from 0 to 36\ifmmode^\circ\else\textdegree\fi{}C, an apparent increase of the linewidth at higher temperature was observed. This effect has been explained as a magnetic-field-dependent radiative transfer effect at optical depth of the order of the scattering cell dimensions. When the data were analyzed with such an effect included, the reduced Hanle signal showed no temperature dependence of the half-width.

Physical Review•Dec 5, 1967•17 citations
Mean lives of He I are deduced from the Hanle curves for high-velocity atoms excited in a gaseous target. For long---mean-life levels we observed oscillations in agreement with a partial precession of the dipoles around the magnetic field. For the 3p1P3p^{1}P3p1P level, the Hanle curve is not perturbed by self-absorption using this method, and the measured mean life (1.8\ifmmode\pm\else\textpm\fi{}0.1 nsec) is in good agreement with the theoretical value.
The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the 21P2^{1}P21P state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of 21S2^{1}S21S metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the 21S2^{1}S21S atoms, which are excited to the 21P2^{1}P21P state. This state then primarily decays to the ground 11S1^{1}S11S state with the emission of resonance (584 \AA{}) radiation. The lifetime of the 21P2^{1}P21P state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.
Hanle-effect measurements have been made on the (5p46s)4P52(5{p}^{4}6s)^{4}P_{\frac{5}{2}}(5p46s)4P25​​ state of iodine, which is connected to the ground state via the 1830-\AA{} line. The atomic iodine is produced by thermal dissociation of I2{\mathrm{I}}_{2}I2​, and is excited by unpolarized resonance radiation. Using the value of gJ{g}_{J}gJ​ obtained by Keiss and Corliss from high-field Zeeman splittings, we find a radiative life-time of 57.9 \ifmmode\pm\else\textpm\fi{} 1.4 nsec (all errors quoted are 67% confidence limits). Neither coherence narrowing nor self-broadening is observed, and we place an upper limit of 200 A˚2{\mathrm{\AA{}}}^{2}A˚2 on the latter process. Cross sections for broadening of the Hanle effect by the rare gases are found to be (in A˚2{\mathrm{\AA{}}}^{2}A˚2): He: 9.8 \ifmmode\pm\else\textpm\fi{} 1.1; Ne: 19.5 \ifmmode\pm\else\textpm\fi{} 1.6; Ar: 37.6 \ifmmode\pm\else\textpm\fi{} 3.1; Kr: 38.7 \ifmmode\pm\else\textpm\fi{} 3.8; Xe: 49.2 \ifmmode\pm\else\textpm\fi{} 3.1. The relative values of these cross sections are seen to be in agreement with the predictions of a van der Waals interaction.
The oscillator strength for the resonance (11S-21P) transition in atomic helium has been determined from a measurement of the lifetime of the 2^ state.This life time was obtained using a modified zero field level crossing (Hanle) method.The scattering sample was a thermal velocity beam of 2^8 metastable atoms.Resonant 2 micron (218-2^2) radiation is absorbed by the 21S atoms, which are excited to the 2^P state.This state then primarily decays to the ground 1^8 state with the emission of resonance ($84A) radiation.The lifetime of the 21P state was determined from measurements of the magnetic field dependence of the 584Â intensity.This new technique eliminates the problem of constructing a vacuum ultra violet resonance lamp.The measured lifetime t(2 P) is (5.69 ±0.23) x Id"10 sec.The resulting absorption oscillator strength for the resonance transition is 0.270 ±0.011.This oscillator strength agrees with tie theoretical value.
A semiclassical discussion is given of a simple case of the Hanle effect when the population of the level of interest varies with time because of decays to it from higher levels excited by collisions. It is found that the width of the intensity-vs-applied-field curve is no longer simply related to the lifetime of this single level, but depends on the relative populations and lifetimes of all other participating levels. It is shown by examples that, unless these effects are taken into account, the lifetimes deduced from the measurements can be greatly in error.
The lifetimes of the gallium 52S125^{2}S_{\frac{1}{2}}52S21​​ state, the indium 62S126^{2}S_{\frac{1}{2}}62S21​​ state, and the thallium 72S127^{2}S_{\frac{1}{2}}72S21​​ state were measured using the zero-field level-crossing (Hanle-effect) technique. The life-times obtained were (6.8\ifmmode\pm\else\textpm\fi{}0.3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$, (7.0\ifmmode\pm\else\textpm\fi{}0.3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$, and (7.45\ifmmode\pm\else\textpm\fi{}0.2) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ sec, respectively. Anomalous contributions to the level-crossing signals, from the wavelength dependence of the exciting light intensity and self-absorption by the fluorescing atomic beam, were investigated in detail.
The Hanle effect on the resonance lines of the Rb87{\mathrm{Rb}}^{87}Rb87 atom has been observed. The line shape of the signal for the $5^{2}S_{\frac{1}{2}}\ensuremath{\leftrightarrow}5^{2}P_{\frac{3}{2}}$ is found to be Lorentzian, while that for the $5^{2}S_{\frac{1}{2}}\ensuremath{\leftrightarrow}5^{2}P_{\frac{1}{2}}$ is dispersion-type. The half-width of these lines was observed to be 3.146\ifmmode\pm\else\textpm\fi{}0.13 G and 23.0 G\ifmmode\pm\else\textpm\fi{}5%, respectively. These values yield the lifetime of (2.71\ifmmode\pm\else\textpm\fi{}0.14) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$sec and (3.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$sec for the 52P325^{2}P_{\frac{3}{2}}52P23​​ and 52P125^{2}P_{\frac{1}{2}}52P21​​ states, respectively. The oscillator strengths for the corresponding transitions are found to be 0.673\ifmmode\pm\else\textpm\fi{}0.03 and 0.32\ifmmode\pm\else\textpm\fi{}0.03, respectively. When the temperature of the scattering cell was changed from 0 to 36\ifmmode^\circ\else\textdegree\fi{}C, an apparent increase of the linewidth at higher temperature was observed. This effect has been explained as a magnetic-field-dependent radiative transfer effect at optical depth of the order of the scattering cell dimensions. When the data were analyzed with such an effect included, the reduced Hanle signal showed no temperature dependence of the half-width.