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EXPERIMENTS ON ELECTRONIC EXCITATION OF ATOMS PERFORMED AT LAWRENCE RADIATION LABORATORY, BERKELEY, CALIFORNIA, U.S.A.

T. Hadeishi

Published Jan 1, 1967• 0 citations
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Abstract

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.

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Physical Review A•Sep 1, 1991•13 citations
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.
The important role of the scientific school of Prof. I. Zapisochny in the study of electron-atom collisions and its determination of most important parameters in this field are emphasized. The processes of electron-impact excitation to and from the metastable states of rare gases atoms are examined. The methods of measurement of the cross sections for such processes are described and the influence of cascade population on the metastable cross sections discussed also. The cross section data are presented. Keywords: electron-atom collisions, metastable states,
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.
The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the n 1 P states of helium is given. The extension of the technique to the study of excitation of the n=2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.
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.
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The observation of radiation trapping effects in time-resolved, stepwise-excited, electron-photon coincidence signals is reported. Hg atoms were excited by electron impact to the 6 1^{1}1P1{\mathit{P}}_{1}P1​ state and then by laser excitation to the 6 1^{1}1D2{\mathit{D}}_{2}D2​ state. Coincidence signals were observed from the 6 1^{1}1D2{\mathit{D}}_{2}D2​--6 3^{3}3P1{\mathit{P}}_{1}P1​ (313-nm) transition. The decay time of these signals was found to depend upon radiation trapping of the electron-impact-excited 6 1^{1}1P1{\mathit{P}}_{1}P1​--6 1^{1}1S0{\mathit{S}}_{0}S0​ (185-nm) resonance line. An explanation of this effect is given in terms of the laser excitation coupling the 313- and 185-nm transitions.