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Physical Review

Phenomenon of the Metastable State Coherently Excited by Electron Impact

T. Hadeishi

Published Oct 5, 1967• 8 citations
Publisher

Abstract

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 $^{3}P_{2}$ 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 ${H}_{0}$ and the linearly polarized monitoring light is perpendicular to the electron beam.

DOI
10.1103/physrev.162.16

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The Metastable State in Mercury Vapor

Harold W. Webb

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T. Hadeishi, Orilla A. McHarris, William A. Nierenberg

The metastable state ${(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}$ 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 ${(2^{2}P_{\frac{3}{2}} 3^{2}S_{\frac{1}{2}})}_{2}$ 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 $6^{3}P_{2}$ 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 $\frac{{({g}_{J})}_{\mathrm{Ne}}}{{({g}_{J})}_{\mathrm{Hg}}}$ was 1.008\ifmmode\pm\else\textpm\fi{}0.024. This method could be applied to other atoms having a metastable state with $J>0$.

Physical Review•May 17, 1965•9 citations
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.
A brief survey of the experiment on electronic excitation of atoms, carried on at the Atomic Beam Group of the Lawrence Radiation Laboratory, is made.By means of alignment of excited atoms by electron-impact excitation, we first observed rf paramagneticresonance reorientation of Hg and neon metastable states.Later on, the method was extended to observe the coherence property of excited atoms associated with the electron-impact excitation.The coherent excitation of nondegenerate levels of excited atoms results in intensity beats either in the fluorescent decay due to spontaneous radiation or in the resonance absorption by a long-lived metastable state.
Optical pumping experiments have been performed on helium 4 atoms in the metastable level 2 3 S 1 with the object of studying the modulation of light in absorption in the longitudinal and transverse beams. Magnetic resonance curves were obtained similar to those found in emission in double-resonance experiments. With a particular geometrical arrangement employing circular polarizers, the Bloch magnetic resonance curves were obtained. Modulation at four times the applied frequency was obtained with a linear polarizer in the pumping beam. This modulation is ascribed to the circulation of coherence in the optical pumping cycle. From the behaviour of the resonance signals as a function of the frequency and amplitude of the stimulating field, it is inferred that a substantial part of the signal arises from coherence which has survived two complete cycles. The observations are satisfactorily explained by the theory developed in the previous paper.
Frequency-modulated light was generated by the application of a magnetic field oscillating at frequencies in the range 1-10 MHz to mercury vapour undergoing resonance fluorescence. The characteristics of the fluorescent light have been studied. When the primary illumination in the plane perpendicular to the field is anisotropic the sigma components are coherently excited. The light emitted along the field is then polarization-modulated. It becomes intensity-modulated if it is passed through a linear analyser. The light emitted obliquely to the magnetic field is intensity-modulated without passing through an analyser. The amplitudes of these modulations show resonance behaviour under the application of an additional static field parallel to the oscillating field. The polarization-modulated light was used to excite resonance fluorescence in a second cell of mercury vapour. The fluorescent light is intensity-modulated. The amplitude of modulation shows resonance behaviour when harmonics of the driving frequency coincide with Zeeman intervals in the excited atoms. These resonances have been explored in detail. They are satisfactorily interpreted by a theory given in an accompanying paper.
Metastable state produced in mercury vapor at low pressures by electron impacts of 4.9 volts.---The persistence of the "radiation" produced by 4.9 volt impacts in mercury vapor was measured in a four electrode tube of the type usually employed in resonance potential measurements, consisting of an equipotential cathode of simple design, an inner grid GGG for controlling the velocity of the impacting electrons, an outer photo-electric grid HHH and a photo-electric plate PPP. A.c. and d.c. voltages were applied to the two grids in such a way that the electrons had sufficient velocity to excite the radiation only in alternate half-cycles and that the photo-electric current to the electrometer alternated in direction simultaneously. As a result of the persistence of the radiation there resulted a decrease in the current with increasing frequency, reaching a minimum at 1800 cycles for a distance between grids of 17 mm and at 3800 cycles for a distance of 8.5 mm. These results did not vary much with pressure,.003 to.032 mm. They show a lapse of time between the excitation and the arrival of the first radiation at the plate PPP of about 1/3600 and 1/7600 sec. respectively. A mathematical discussion shows that the diffusion of the radiation by repeated emission and re-absorption (the "imprisonment" of radiation theory) cannot account for these results. A calculation, based on the assumption that the excited atoms remain in a metastable state and carry the energy of excitation to the photo-electric surfaces and there give it up, gives results in very close agreement with the observations. The conclusion is that a metastable state is formed by the atoms excited by the 4.9 volt electron impacts and that these are the effective ones in producing the photo-electric response in the tubes. In these experiments the 2p22{p}_{2}2p2​ state is the only one excited by the impacts in appreciable amount. How these results can be reconciled with the Bohr theory, according to which the 2p22{p}_{2}2p2​ state is not metastable, is not clear.
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.