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Journal of Physics B Atomic and Molecular Physics

Self-alignment and radiative lifetime measurements of some Ar II levels in a hollow-cathode discharge

Dimo Zhechev

Published Jan 14, 1985• 16 citations
Publisher

Abstract

The self-alignment of some Ar II levels belonging to 4p and 4d configurations excited in a hollow cathode discharge is investigated. The necessary degree of coherence in the 4d levels is obtained using high-frequency discharge excitation. The radiative lifetimes and the signal broadening constants for the argon and for charged particles are determined from the Hanle signals. The contributions of the optical and electron channels to the formation of self-alignment in the discharge are discussed.

DOI
10.1088/0022-3700/18/1/008

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The lifetime of the ${\mathrm{Ca}}^{+}$ $4^{2}P_{\frac{3}{2}}$ state and that of the ${\mathrm{Mg}}^{+}$ $3^{2}P_{\frac{3}{2}}$ state have been measured by the Hanle-effect method with optical excitation from the ground states of the ions. The lifetimes are, respectively, (6.72\ifmmode\pm\else\textpm\fi{}0.20)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ and (3.67\ifmmode\pm\else\textpm\fi{}0.18)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec. The ions were produced by introducing traces of calcium or magnesium into an argon discharge. "Alignment" depolarization cross sections $\ensuremath{\sigma}$ were obtained for the collisional depolarization of the following states of calcium and magnesium due to collisions with argon: $\ensuremath{\sigma}(\mathrm{Ca} 4^{1}P_{1} \mathrm{state})=(1.9\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}({\mathrm{Ca}}^{+} 4^{2}P_{\frac{3}{2}} \mathrm{state})=(1.4\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}(\mathrm{Mg} 3^{1}P_{1} \mathrm{state})=(1.9\ifmmode\pm\else\textpm\fi{}0.3){10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}({\mathrm{Mg}}^{+} 3^{2}P_{\frac{3}{2}} \mathrm{state})=(1.3\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2}.$The magnitudes of these cross sections are in accord with the theories of Byron and Foley and of Omont; we give a discussion of the extension of these theories to include ion-atom collisions.

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Oscillator Strengths of Ca II, Sr II, and Ba II

Alan C. Gallagher

The lifetimes of the $5p^{2}P_{\frac{3}{2}}$ and $5p^{2}P_{\frac{1}{2}}$ states of ${\mathrm{Sr}}^{+}$ and the $6p^{2}P_{\frac{3}{2}}$ and $6p^{2}P_{\frac{1}{2}}$ states of ${\mathrm{Ba}}^{+}$ have been measured by the Hanle-effect technique with optical excitation from the ion ground state. (The lifetimes of the ${\mathrm{Mg}}^{+}$ $3p^{2}P_{\frac{3}{2}}$ and ${\mathrm{Ca}}^{+}$ $4p^{2}P_{\frac{3}{2}}$ states were obtained by the same technique and reported previously.) These ${\mathrm{Ca}}^{+}$, ${\mathrm{Sr}}^{+}$, and ${\mathrm{Ba}}^{+}$ states each decay to a $^{2}D_{\frac{3}{2},\frac{5}{2}}$ doublet as well as to the ground $^{2}S_{\frac{1}{2}}$ state, and the branching ratios for these transitions have been measured. The ions were produced by introducing traces of the atomic vapor into an argon discharge. Consequently, the cross sections for collisional depolarization of these $^{2}P_{J}$ states due to collisions with argon were also obtained from the measurements. The experimental results are given in Tables I and II. A discussion of the ion-atom collisional depolarization cross sections is included.

Physical Review•May 5, 1967•217 citations
The self-alignment of some Ar II levels belonging to 4p and 4d configurations excited in a hollow cathode discharge is investigated. The necessary degree of coherence in the 4d levels is obtained using high-frequency discharge excitation. The radiative lifetimes and the signal broadening constants for the argon and for charged particles are determined from the Hanle signals. The contributions of the optical and electron channels to the formation of self-alignment in the discharge are discussed.
The degenerate Zeeman sublevels were found to contribute to the ionization in a gas discharge in accordance with their coherent state. The voltage across the discharge was monitored while the same ensemble of excited atoms was alternately self-aligned ↔ nonaligned or oriented ↔ aligned. In the latter case, the coherence was optically induced; the corresponding opto-galvanic signals, i.e. amplitude- and time-resolved ones, were compared. Each state of the ensemble of degenerate Zeeman sublevels, i.e. (self-) aligned, oriented or disordered was characterized by its own rate of ionization. Various hollow cathode discharge (HCD) media were studied, namely, Ne/As, Ne/Cu, Ne/Ni, Ne/Cd, Ne/Li and Ne/Si in the corresponding commercial HCD spectral lamps.
We have measured the atomic lifetime of the argon 2 p 4 level by the laser-induced-fluorescence spectroscopy to be 30.5 ns. This contradicts the recent measurement using the high-frequency plused electron-beam excitation. We conjecture that, in their experiment, the radiation trapping not only affects the decay of the upper-level population but also relaxes the alignment initially produced by the electron-collision excitation in the upper-level population. Thus, the directly observed decay of the emission-line intensity is different from the decay of the upper-level population. We have observed the radiative relaxation of alignment on the neon 2 p 2 level which is a complementary effect to coherence narrowing.
Alignment cascade phenomena have been studied in the neon 2p levels (Paschen notation) Hanle effect and double resonance techniques in a mild discharge. The levels are aligned directly by an optical excitation mechanism. Most 2p levels show evidence of alignment cascade from the 2s 4 , 3s 1 or 3d levels below 0.1 Torr; addition of argon impurities greatly enhances the cascade signal relative to the direct alignment signal. Data used previously in pressure broadening studies are shown to be free of cascade errors and are used to establish upper limits for the 2p level life times. A correction for coherence narrowing is estimated; this gives natural life times systematically less than those obtained by methods in which cascade effects are not carefully eliminated. In common with other Hanle effect measurements for these levels the values are marginally smaller than the threshold life times of Bennett and Kindlmann (1966).
The lifetime of the Ca+{\mathrm{Ca}}^{+}Ca+ 42P324^{2}P_{\frac{3}{2}}42P23​​ state and that of the Mg+{\mathrm{Mg}}^{+}Mg+ 32P323^{2}P_{\frac{3}{2}}32P23​​ state have been measured by the Hanle-effect method with optical excitation from the ground states of the ions. The lifetimes are, respectively, (6.72\ifmmode\pm\else\textpm\fi{}0.20)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ and (3.67\ifmmode\pm\else\textpm\fi{}0.18)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec. The ions were produced by introducing traces of calcium or magnesium into an argon discharge. "Alignment" depolarization cross sections $\ensuremath{\sigma}$ were obtained for the collisional depolarization of the following states of calcium and magnesium due to collisions with argon: $\ensuremath{\sigma}(\mathrm{Ca} 4^{1}P_{1} \mathrm{state})=(1.9\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}({\mathrm{Ca}}^{+} 4^{2}P_{\frac{3}{2}} \mathrm{state})=(1.4\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}(\mathrm{Mg} 3^{1}P_{1} \mathrm{state})=(1.9\ifmmode\pm\else\textpm\fi{}0.3){10}^{\ensuremath{-}14}{\mathrm{cm}}^{2};$ $\ensuremath{\sigma}({\mathrm{Mg}}^{+} 3^{2}P_{\frac{3}{2}} \mathrm{state})=(1.3\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{\mathrm{cm}}^{2}.$The magnitudes of these cross sections are in accord with the theories of Byron and Foley and of Omont; we give a discussion of the extension of these theories to include ion-atom collisions.
The lifetimes of the 5p2P325p^{2}P_{\frac{3}{2}}5p2P23​​ and 5p2P125p^{2}P_{\frac{1}{2}}5p2P21​​ states of Sr+{\mathrm{Sr}}^{+}Sr+ and the 6p2P326p^{2}P_{\frac{3}{2}}6p2P23​​ and 6p2P126p^{2}P_{\frac{1}{2}}6p2P21​​ states of Ba+{\mathrm{Ba}}^{+}Ba+ have been measured by the Hanle-effect technique with optical excitation from the ion ground state. (The lifetimes of the Mg+{\mathrm{Mg}}^{+}Mg+ 3p2P323p^{2}P_{\frac{3}{2}}3p2P23​​ and Ca+{\mathrm{Ca}}^{+}Ca+ 4p2P324p^{2}P_{\frac{3}{2}}4p2P23​​ states were obtained by the same technique and reported previously.) These Ca+{\mathrm{Ca}}^{+}Ca+, Sr+{\mathrm{Sr}}^{+}Sr+, and Ba+{\mathrm{Ba}}^{+}Ba+ states each decay to a 2D32,52^{2}D_{\frac{3}{2},\frac{5}{2}}2D23​,25​​ doublet as well as to the ground 2S12^{2}S_{\frac{1}{2}}2S21​​ state, and the branching ratios for these transitions have been measured. The ions were produced by introducing traces of the atomic vapor into an argon discharge. Consequently, the cross sections for collisional depolarization of these 2PJ^{2}P_{J}2PJ​ states due to collisions with argon were also obtained from the measurements. The experimental results are given in Tables I and II. A discussion of the ion-atom collisional depolarization cross sections is included.