Hiroshi Ohkura

Vibronic Theory of the Stark Effects on the Relaxed Excited State of the F Center

The \(\varGamma^{-}_{4}\)-mode phonon interaction in the relaxed excited state (RES) of the F center has been investigated through a theoretical study of the Stark effects on the RES, as an extension of the Kayanuma and Toyozawa theory. The general expressions of the temperature dependence of both the Stark polarization P ( T ) and the radiative life time τ( T ) are derived. They include two parameters; 2 s -2 p electronic energy difference Δ , and \(\varGamma^{-}_{4}\)-phonon coupling constant S 1 . These parameters are determined by theoretical curve-fitting of the experimental data of P ( T ) for potassium and rubidium halides. They give a consistent explanation of τ( T ) for KF, KCl, KBr, and RbCl, and also give the values of the other important parameters of the RES. It is concluded that, in the RES, Δ ≥0; this implies that the 2 s state lies either below the 2 p states or degenerates with them before introducing the electron-phonon coupling.

The Resonant Secondary Emission of the F Center in KCl at 80 K

The secondary emission RSE emitted after resonant excitation of F center in KCl has been measured over the whole Stokes range at 80 K. The degree of polarization P defined for the RSE intensities parallel and perpendicular to resonant light polarization is measured over the same range. The spectrum of P is divided into three successive regions; the depolarization range at the one-phonon Raman scattering, the near plateau range, and the depolarization range down to vanishing. The former two are found to depend on the resonant photon energy. The latter may be caused by the level crossing of 2 s - and 2 p -like electronic states during non-radiative process.

Magnetic Circular Polarization of F Center Emission and the Spin Mixing Processes in Optical Cycle

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 ( e 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 e r1 for KBr depends on photon energy and predominates the net spin mixing parameter ( e 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 e t and those by other schools is clarified.

Vibronic Theory on Magnetic Properties of the Relaxed Excited State of F Center

The magnetic properties of the relaxed excited state (RES) of the F center are investigated on the basis of the vibronic theory including the interaction with \(\varGamma_{4}^{-}\)-mode phonon; both the vibronic and the spin-orbit interactions are diagonalized. By analyzing the experimental results of the g -factor in the RES, the magnetic circular polarization in emission, and the temperature dependence of radiative lifetime of the RES, the vibronic parameter values and the spin-orbit interaction constant, λ, are determined consistently for KF, KCl and KBr. The results show that 1) λ-values in the RES are positive and have the same order of magnitude as those in the unrelaxed excited state; 2) the orbital g -factors are reduced appreciably; 3) the spin polarization in the RES, P * , is of the relatively small order of 10 -3 . An alternative method to estimate P * is proposed.

Vibronic Scheme of the Relaxed Excited State of the F Center

The vibronic scheme of the relaxed excited state (RES) of the F center which includes the \(2 \varGamma_{1}^{+}\), \(\varGamma_{3}^{+}\), and \(\varGamma_{4}^{-}\)-mode interactions has been developed. Formulae of the morphic effects have been derived. The vibronic parameter values for NaF, KF, KCl, KBr, RbF, RbCl, and RbBr are determined from the consistent analysis of the temperature dependence of the radiative lifetime and of the Stark polarization. Available data of the temperature dependence of the stress-induced polarization are consistently explained. Using parameter values, the dipole moment and stress coupling coefficient (\(\varGamma_{3}^{+}\)-mode) are determined; the red Stark shift is interpreted. By including the spin-orbit interaction, the forms of the magnetic effects are derived. The spin-orbit interaction constant, the orbital g -factor, and the spin polarization in the RES are determined from the analysis of experimental data.

Stark Effects in F Center Emission in KBr, KI, RbBr, and RbI Crystals

The Stark effects in F center emission have been investigated for KBr, KI, RbBr, and RbI crystals as a function of emission wave-length and polarization, electric field amplitude, and sample temperature, with using a PbS IR detector. Observed results are qualitatively similar to those measured by Bogan and Fitchen for several other alkali halides. The Stark polarization and the first moment change (a red shift) of the emission band are quadratic in electric field strength; their coefficients are tabulated. Two different theoretical forms of the temperature dependence of the Stark polarization P ( T ), which have been derived by Bogan and Fitchen, and Ham and Grevsmuhl, respectively, are found to be invalid to explain the experimental data consistently. A phenomenological form to describe P ( T ) is proposed by introducing a new parameter.

Relaxation of optically excited F centers

Abstract The relaxation of optically excited F centers was studied by resonant secondary radiation (RSR), which occurs successively as Raman scattering, hot luminescence and ordinary luminescence. The RSR and its linear polarization were measured as a function of excitation photon energy mainly at 77 K, for F centers in seven alkali halides. Dynamic processes, including these optical processes were analyzed in terms of a semi-classical model based on the Franck-Condon principle in an A1g anharmonic adiabatic potential derived from the Born-Mayer potential. It is proposed that, abruptly after the optical excitation, the F electron expands its orbital radius and changes its nature including decoupling from the T2g Jahn-Teller mode. This model works particularly for KCl, RbCl and NaCl. The relaxation of F centers in KBr and KI is different from that in KCl, RbCl and NaCl. The difference is related to the characteristic differences in lattice dynamics, namely predominant coupling with localized modes and the T2g frequency near that of the LO phonons. The relaxation of type II FA centers is attributed to two steps, one F-like and the other Li+-dominated.

Spin polarization in the relaxed excited state of the F center in case of saturated optical pumping

Abstract The dependence of the F center MCP on the polarization of pumping light is explained in terms of the spin polarization in the RES (relaxed excited state) which is derived by examining the spin-mixing processes; the spin-mixing parameter in the RES and that associated with the relaxation from the URES (unrelaxed excited state) to RES are evaluated. A new ESR method in the RES is proposed.

Magnetic Nature of the Relaxed Excited State of F Centers and Spin Flipping Processes in the Optical Pumping Cycle

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 e t is determined. The values of F p and e 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.

Anomalous Spin-Lattice Relaxation of the FA(Li) Centers in KCl

The spin-lattice relaxation time of the F A (Li) centers in KBr shows a remarkable temperature dependence with a distinguished isotope effect. It is found that the Murphys relaxation mechanism is the most suitable to explain these characteristics. Tunneling probability, W 0 , and tunneling splitting, Δ, are determined to be as 1.0×10 3 and 0.5×10 3 sec -1 , and 6.7 and 3.9 K, for the F A (Li 6 ) and F A (Li 7 ) centers, respectively.