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Physical Review

Alignment of Ionic Ground State of Xe by Electron-Impact Ionization Observed through Exchange Collision with the Neutral Metastable State

T. Hadeishi, Chung-Heng Liu

Published Jun 5, 1968• 5 citations
Publisher

Abstract

The radio-frequency paramagnetic-resonance method was used to observe the exchange collisions between the singly ionized Xe ground state $^{2}P_{\frac{3}{2}}$ and the neutral metastable state $^{3}P_{2}$. The ionic ground-state and metastable-state atoms are both formed and aligned by unidirectional low-energy high-flux electron-beam impact. The magnetic resonances of even and odd ionic ground-state atoms were observed by monitoring changes in the transparency of the resonance radiation absorbed by the metastable-state atoms. Phenomenological theories are presented which explain the sign of the magnetic-resonance signal of the ionic ground state relative to that of the neutral metastable state, together with independent experimental evidence to support the theories. Conclusive experimental evidence is presented supporting our original assumptions that the ionic ground-state ${\mathrm{Xe}}^{+}(^{2}P_{\frac{3}{2}})$ is aligned by low-energy electron-impact ionization and that such alignment is detectable through the results of exchange collisions with the metastable state.

DOI
10.1103/physrev.170.145

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Physical Review•Oct 5, 1967•8 citations
The radio-frequency paramagnetic-resonance method was used to observe the exchange collisions between the singly ionized Xe ground state 2P32^{2}P_{\frac{3}{2}}2P23​​ and the neutral metastable state 3P2^{3}P_{2}3P2​. The ionic ground-state and metastable-state atoms are both formed and aligned by unidirectional low-energy high-flux electron-beam impact. The magnetic resonances of even and odd ionic ground-state atoms were observed by monitoring changes in the transparency of the resonance radiation absorbed by the metastable-state atoms. Phenomenological theories are presented which explain the sign of the magnetic-resonance signal of the ionic ground state relative to that of the neutral metastable state, together with independent experimental evidence to support the theories. Conclusive experimental evidence is presented supporting our original assumptions that the ionic ground-state Xe+(2P32){\mathrm{Xe}}^{+}(^{2}P_{\frac{3}{2}})Xe+(2P23​​) is aligned by low-energy electron-impact ionization and that such alignment is detectable through the results of exchange collisions with the metastable state.
The radiofreq'uency paragm.agnetic-re sonance m.ethod was used to observe the exchange collisions between th-e Xe ionic ground state 2P3/2 and the neutral metastable state 3 P2o The ionic ground-state and m.etastable-state atom.s are both form.edand aligned by unidirectional low-energy high-flux electron-beam.im.pact.The m.agnetic resonance of the ionic ground-state atom.s was observed by m.onitoring changes in the transparency of the resonance radiation absorbed by the m.etastable-state atom.s.The experim.ent is described in detail; along with a possible process for observing this resonance.Phenom.enologicaltheories are presented which m.ay explain the sign of the m.agnetic resonance signal of the ionic ground state com.pared with the m.etastable m.agnetic resonance.
We have observed spin-exchange collisions between the singly ionized 2P32^{2}P_{\frac{3}{2}}2P23​​ ground state of Xe and the 3P2^{3}P_{2}3P2​ metastable state of neutral Xe, both formed and aligned by electronic-impact excitation under space-charge neutralization. Extension to the rf spectroscopy of the ionized ground state of other noble-gas atoms seems promising.
The metastable state (22P3232S12)2{(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}(22P23​​32S21​​)2​ of neon was produced and aligned by electron impact. The diodestructure electron gun was operated at a high electron-current density under space-charge neutralization conditions. Radio-frequency resonance of the aligned metastable state (22P3232S12)2{(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}(22P23​​32S21​​)2​ was observed by monitoring the change of absorption of the $\ensuremath{\lambda}6143$-resonance radiation passing through the gap between the cathode and anode. The neon metastable-state resonance was compared with the mercury metastable-state 63P26^{3}P_{2}63P2​ radio-frequency resonance obtained by using the same gun for both, and observed by monitoring the absorption of the $\ensuremath{\lambda}5461$-resonance radiation passing through the same excitation region. The observed (gJ)Ne(gJ)Hg\frac{{({g}_{J})}_{\mathrm{Ne}}}{{({g}_{J})}_{\mathrm{Hg}}}(gJ​)Hg​(gJ​)Ne​​ was 1.008\ifmmode\pm\else\textpm\fi{}0.024. This method could be applied to other atoms having a metastable state with J>0J>0J>0.
The 6s2S126s^{2}S_{\frac{1}{2}}6s2S21​​ ground ionic state of even isotopes of mercury was polarized in a stepwise excitation process from the atomic $6{s}^{2}^{1}S_{0}$ ground state via the metastable 6s6p3P26s6p^{3}P_{2}6s6p3P2​ state. The latter was optically pumped with polarized 5461-\AA{} ($6s7s^{3}S_{1}\ensuremath{-}^{3}P_{2}$) radiation. Paramagnetic resonance, observed by monitoring the change in intensity of the relatively intense 2851-\AA{} ($5{d}^{9}6{s}^{2}^{2}D_{\frac{5}{2}}\ensuremath{-}5{d}^{10}6s^{2}S_{\frac{1}{2}}$) electric quadrupole resonance radiation, was used to determine the polarization of the even-isotope ground-state ion to be 15%. If one assumes conservation of spin angular momentum, this process should yield highly polarized electrons.
The 23S12^{3}S_{1}23S1​ (metastable) and 23P1,22^{3}P_{1,2}23P1,2​ excited-state atoms of He4{\mathrm{He}}^{4}He4 are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the 23S12^{3}S_{1}23S1​ level or the 22P1,22^{2}P_{1,2}22P1,2​ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the 23S1(F=32)2^{3}S_{1} (F=\frac{3}{2})23S1​(F=23​) and the 23P2(F=52)2^{3}P_{2} (F=\frac{5}{2})23P2​(F=25​) levels of He3{\mathrm{He}}^{3}He3 is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. The alignment of the 33P3^{3}P33P level of He4{\mathrm{He}}^{4}He4 is also observed by monitoring polarization changes of the 3889 \AA{} line.
Detailed information is given on an experiment which we have reported previously, studying intensity beats in the resonance absorption due to nondegenerate magnetic sublevels of the 3P2^{3}P_{2}3P2​ mercury metastable state coherently excited by electron impact. Rather straightforward theory is used to explain the experimental result. It is found that regardless of the exciting energy, as long as there is sufficient selective excitation of certain magnetic sublevels, the absorption beat frequency is modulated at twice the Larmor frequency when the incident electron beam is perpendicular to the externally applied field H0{H}_{0}H0​ and the linearly polarized monitoring light is perpendicular to the electron beam.