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Le Journal de Physique Colloques

Magnetic circular polarization of the F center emission

H. Ohkura, Koichi Imanaka, K. Iwahaka, T. Wada, Masaji Tanaka

Published Jul 1, 1980• 0 citations
Publisher

Abstract

The magnetic property of the relaxed excited state (RES) of the F center has been manifested by the experimental and theoretical study of the magnetic circular polarization (ΔMCP) of the F center emission. The para- and diamagnetic components (Δp and Δd) of ΔMCP as well as the g-shift of the RES have been experimentally determined. From the comparison of these values with those computed values on the vibronic theory, the spin-orbit interaction constant, the orbital g-factor, and the spin polarization of the RES are determined. The tuning photon energy dependence of Δp is observed. The spin-memory-loss during radiationless decay is found to be significant. The temperature dependence of Δd is mentioned.

DOI
10.1051/jphyscol:1980611

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The magnetic property of the relaxed excited state (RES) of the F center has been manifested by the experimental and theoretical study of the magnetic circular polarization (ΔMCP) of the F center emission. The para- and diamagnetic components (Δp and Δd) of ΔMCP as well as the g-shift of the RES have been experimentally determined. From the comparison of these values with those computed values on the vibronic theory, the spin-orbit interaction constant, the orbital g-factor, and the spin polarization of the RES are determined. The tuning photon energy dependence of Δp is observed. The spin-memory-loss during radiationless decay is found to be significant. The temperature dependence of Δd is mentioned.
Magnetic circular polarization (MCP) of F centar emission has been measured for KCl, KBr, KI, RbCl, and RbBr. The optical pumping cycle is proposed to include three spin mixing parameters in absoption, radiationless decay ( ε r1 ), and emission processes. From the analysis of experimental data using formulae derived from rate equations for the cycle, para- and dia-magnetic components ( Δ p and Δ d ) of MCP, and the spin mixing parameters are determined. Pumping photon energy dependence of Δ p are observed. It shows that ε r1 for KBr depends on photon energy and predominates the net spin mixing parameter ( ε t ). Temperature dependence of Δ d are shown to be relevant to the spin orbit interaction in the relaxed excited state of F center. The source of the discrepancy found between our determined values of ε t and those by other schools is clarified.
The magnetic nature of the relaxed excited state (RES) of F centers and spin-flipping processes in the optical pumping cycle have been clarified by studying the magnetic circular polarization (MCP) of emission when the pumping light is modulated between right and left circular polarization at the frequency of (ω/2π). Paramagnetic component Δ p (ω) of MCP is found to be proportional to the spin polarization P * (ω) in the RES with a proportionality constant F p . From the analysis of the dependence of Δ p (ω) at particular frequencies on the pumping photon energy E p h , F p is estimated. Thus, the E p h -dependence of the net spin-mixing parameter ε t is determined. The values of F p and ε t are found to be smaller than those determined previously. The source of discrepancy is discussed. It is suggested that the spin-mixing in the non-radiative transition process is not neglected.
When the pumping light is modulated between right and left circular polarization, the magnetic circular polarization of F center luminescence observed after passing through a linear polarizer has been called “the anomalous effect”. We measured this effect for the F center in KI as a function of the magnetic field H and the photon energy. These experimental data are explained by deriving a form which includes the parameter ( ε I ) for the unbalancing of circular polarization in pumping light and the spin-lattice relaxation rate of the relaxed excited state ( T 1 * ) -1 . The H -dependence of ( T 1 * ) -1 is presented.
We have solved the rate equations of the optical pumping cycle of the FFF center in the case of two spin-mixing parameters, ${\ensuremath{\epsilon}}_{+}$ and ${\ensuremath{\epsilon}}_{\ensuremath{-}}$. The solutions and the experimental results for the magnetic circular dichroism of the FFF-center emission are in complete agreement, and have allowed for the first time the measurement of the "anisotropy" of the electron-spin memory, i.e., $\frac{({\ensuremath{\epsilon}}_{\ensuremath{-}}\ensuremath{-}{\ensuremath{\epsilon}}_{+})}{({\ensuremath{\epsilon}}_{\ensuremath{-}}+{\ensuremath{\epsilon}}_{+})}$. This quantity will be very useful in order to clarify the role of the various processes responsible for the loss of the spin memory in the optical cycle. Moreover, from the rate equations we have been able to find the value of the polarization in the relaxed excited state, which is essential to calculate the spin-orbit splitting in emission.
The alignment-orientation process is exactly treated for an alkali-like atom with purely electronic couplings. Analytic expressions for the circular polarization of the optical emission are given for specific types of anisotropic excitation in terms of the magnetic field and the spin-orbit parameter. No advanced formalism is used. It is shown how the hyperfine interaction with the electron orbital motion operates in a manner very similar to the purely electronic spin-orbit coupling, again leading to orbital orientation and to circularly polarized optical emission. Detailed computed results for alignment-orientation conversion for neutral helium are summarized, as are the results of experiments which directly exhibit circularly polarized emission due to the alignment-orientation effect in gases.
We studied alignment-to-orientation conversion caused by excited-state level crossings in a nonzero magnetic field of both atomic rubidium isotopes. Experimental measurements were performed on the transitions of the D2{D}_{2}D2​ line of rubidium. These measured signals were described by a theoretical model that takes into account all neighboring hyperfine transitions, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. In the experiments, laser-induced fluorescence components were observed at linearly polarized excitation and their difference was taken afterwards. By observing the two oppositely circularly polarized components, we were able to see structures not visible in the difference graphs, which give deeper insight into the processes responsible for these signals. We studied how these signals are dependent on intensity and how they are affected when the exciting laser is tuned to different hyperfine transitions. The comparison between experiment and theory was carried out fulfilling the nonlinear absorption conditions. The theoretical curves described the experimental measurements satisfactorily, reproducing even small features in the shapes of the curves.