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Journal of Physics B Atomic and Molecular Physics

Observation of modulation of mercury-cadmium sensitized fluorescence under the influence of a rotating magnetic field

R S Baker, W Gough

Published Mar 11, 1975• 4 citations
Publisher

Abstract

Modulation has been observed in mercury-cadmium sensitized fluorescence under the simultaneous application of a static and a rotating magnetic field. The dependence of the depth of modulation at various frequencies on the static field has been investigated. There are discrepancies between theory and experiment which indicate an anisotropy in the observation of the various collisions. An attempt is made to explain the discrepancies for one particular case on a simple model of the collision, but this attempt is unsuccessful.

DOI
10.1088/0022-3700/8/4/014

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Proceedings of the Physical Society•Jan 1, 1967•29 citations
Modulation has been observed in mercury-cadmium sensitized fluorescence under the simultaneous application of a static and a rotating magnetic field. The dependence of the depth of modulation at various frequencies on the static field has been investigated. There are discrepancies between theory and experiment which indicate an anisotropy in the observation of the various collisions. An attempt is made to explain the discrepancies for one particular case on a simple model of the collision, but this attempt is unsuccessful.
It is predicted theoretically that mercury-cadmium sensitized fluorescence should be modulated in general, when a rotating magnetic field is applied, and a steady magnetic field perpendicular to the plane of the rotating field. The time dependence of the density matrix, expressed in terms of irreducible tensors, is calculated. The final expression for the intensity of the sensitized fluorescence, as a function of time, is shown to be similar to the corresponding one for resonance fluorescence (both cases referring to 'broad-band' excitation).
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.
Further experiments are reported in which resonance fluorescence of an atomic vapour is excited by modulated light. The system studied was the 3 P 1 level of cadmium excited by the λ 3261 λ intercombination line modulated at 462 kHz. Resonant changes in the amplitude of modulation of the fluorescent light have been observed in both static and time-dependent magnetic fields whenever the modulation frequency matches a frequency separation between the states of the excited atoms. Quantitative investigations of the shape and position of the observed resonances show good agreement between theory and experiment. The application of the technique to the spectroscopy of excited atoms is considered.
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.
By means of the selection rule Σ m J = constant along the axis of a collision between two atoms, it is shown that sensitized fluorescence should in some cases be partially polarized. The degree of polarization of the fluorescence of cadmium vapour sensitized by the presence of mercury vapour is calculated for certain states of polarization of the incident radiation. In the case studied experimentally the degree of polarization varies over a range of 2-5%. The theoretical value is 3/11, which is reduced in practice by various depolarizing effects. The possibility of transfer coherence between eigenstates from mercury to cadmium during a collision is considered.