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OBSERVATION OF A COHERENT PRECESSION DURING THE EXCITATION OF ATOMIC LEVELS BY PULSED ELECTRON BOMBARDMENT

O. Nédélec, M.N. Deschizeaux, J.C. Pébay-Peyroula

Published Nov 18, 1963• 0 citations
OpenAlex

Abstract

An experiment is described in which a pulsating-intensity electron beam is used to excite the Zeeman levels of gas atoms. The light emitted from these excited atoms is found to have a time-dependent intensity. In particular, the intensity is found to be modulated with an angular frequency equal to twice the Larmor angular frequency. This modulation is attributed to coherent precession, and the theory of the effect is shown. Results are given for excited levels in Pb and Hg. (T.F.H.)

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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.

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

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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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Theory of modulation of light in a double resonance experiment

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Abstract A theory is formulated to describe the modulation which has been observed in fluorescent light from atoms subjected simultaneously to optical and radio-frequency radiation. The optical field stimulates one or more of a set of excited states of the atom, between which the radio-frequency field establishes coherence. This coherence is manifest in the fluorescent radiation. Interference between radiations of different frequency leads to modulation. General expressions are given for the intensity of the fluorescent light as a function of time. The Zeeman structure of the transition (63P1 — 6 1S0), A2537 A, in mercury is studied in detail. Modulation at frequencies, 1, 2, 3 and 4 times that of the radio-frequency field, w0, is predicted, and resonant effects at static magnetic fields, 0, 1/2, 1, 3/2, 2 and 3 times H0, the field for which w0 is the Larm or frequency. Resonances at fields other than H0 are due to excitation with light of mixed polarization. Most of the predicted effects have been found experimentally. A ‘frequency diagram’ is introduced and discussed, to represent the combined effects of static and radio-frequency magnetic fields. To each excited state belong a set of r frequencies, where r is the number of states linked by the radio-frequency perturbation. The 9 levels are drawn, as functions of H, for the states mJ = 0, ± 1, of 3P1. The resonances at fields other than H0 may be associated with intersections of frequency levels belonging to different mJ.

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Frequency- and polarization-modulated light in double resonance experiments on mercury vapour

G. D. Chapman, George William Series

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.

Journal of Physics B Atomic and Molecular Physics•Jan 1, 1970•11 citations
An experiment is described in which a pulsating-intensity electron beam is used to excite the Zeeman levels of gas atoms. The light emitted from these excited atoms is found to have a time-dependent intensity. In particular, the intensity is found to be modulated with an angular frequency equal to twice the Larmor angular frequency. This modulation is attributed to coherent precession, and the theory of the effect is shown. Results are given for excited levels in Pb and Hg. (T.F.H.)
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 type of double resonance experiment is reported in which resonance fluorescence from an atomic vapour is excited by modulated light. The amplitude of modulation of the fluorescent light undergoes changes which indicate an atomic resonance when the frequency of modulation is equal to the Larmor frequency of the atoms in an applied magnetic field, or equal to twice that frequency. The effects have been studied in the intercombination line, λ 3261 A, of cadmium. They are in good agreement with theoretical predictions. The method could be applied to the spectroscopy of excited atoms.
Abstract This work is an experimental study of the modulation which has been found in resonance radiation when the fluorescing vapour is subjected to static and radio-frequency magnetic fields, as in the double resonance experiment of Brossel & Bitter. The particular example chosen for study in this work also was the resonance line λ 2537 Å of mercury. The experimental observations are compared with the predictions of a theoretical treatment which has already been published. The phenomenon was studied under a variety of geometrical configurations. When the polarization of the exciting light allowed excitation to only one excited state (a component of a Zeeman multiplet), the depth of modulation was found to depend on the closeness to resonance of the frequency, ω0, of the radio-frequency field to the Larmor precessional frequency, ω = γH , in the static field H. When the polarization of the exciting light allowed excitation to more than one component of the multiplet, resonance effects were found in the depth of modulation and in the mean intensity of the fluorescent light at fields where the applied frequency was equal, not to the Larmor frequency alone, but to combinations of the Larmor frequency with the nutational frequency. From a quantum-mechanical point of view, these new phenomena are related to the interference effects which are found in resonance fluorescence when there is degeneracy between excited states: in the present case the degeneracy is induced by the radio-frequency field. The ease with which the geometrical conditions could be altered allowed the theory to be tested in considerable detail without quantitative assessment of the resonance line contours: nevertheless, a quantitative study was made of one particular feature. The predictions of the theory were confirmed at all points where they were tested.
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.
Abstract A theory is formulated to describe the modulation which has been observed in fluorescent light from atoms subjected simultaneously to optical and radio-frequency radiation. The optical field stimulates one or more of a set of excited states of the atom, between which the radio-frequency field establishes coherence. This coherence is manifest in the fluorescent radiation. Interference between radiations of different frequency leads to modulation. General expressions are given for the intensity of the fluorescent light as a function of time. The Zeeman structure of the transition (63P1 — 6 1S0), A2537 A, in mercury is studied in detail. Modulation at frequencies, 1, 2, 3 and 4 times that of the radio-frequency field, w0, is predicted, and resonant effects at static magnetic fields, 0, 1/2, 1, 3/2, 2 and 3 times H0, the field for which w0 is the Larm or frequency. Resonances at fields other than H0 are due to excitation with light of mixed polarization. Most of the predicted effects have been found experimentally. A ‘frequency diagram’ is introduced and discussed, to represent the combined effects of static and radio-frequency magnetic fields. To each excited state belong a set of r frequencies, where r is the number of states linked by the radio-frequency perturbation. The 9 levels are drawn, as functions of H, for the states mJ = 0, ± 1, of 3P1. The resonances at fields other than H0 may be associated with intersections of frequency levels belonging to different mJ.
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.