Tamao Matsumoto

Time-Resolved Spectroscopic Study on the Type I Self-Trapped Excitons in Alkali Halide Crystals. I. Emission Spectra and Decay Behavior

Decay behaviors of Self-Trapped Exciton (STE) luminescence categorized into type I in seven alkali halides, i.e., the σ bands in NaCl, KBr, RbBr, KI and RbI, and the π bands in NaBr and NaI, are studied by using synchrotron radiation pulses from UVSOR under single-bunch operation. Counting photons over four orders of magnitude reveals that every σ band involves a phosphorescent component with a lifetime longer than 100 ns, in addition to the main fluorescent component. This evidences that the σ band originates from the lowest orbital state of the STE. Simultaneous detection through two different time windows discloses a split in the peak energy of the fluorescent and phosphorescent components. From this result, a slight difference in the location of the minima is suggested in the adiabatic energy surfaces for a nearly degenerate singlet-triplet STE pair with the on-center configuration.

Time-Resolved Spectroscopic Study on the Type I Self-Trapped Excitons in Alkali Halide Crystals: II. Excitation Spectra and Relaxation Processes

Excitation spectra for the fluorescent and phosphorescent components of type I bands in seven alkali halides, NaCl, NaBr, KBr, RbBr, NaI, KI and RbI, are measured in both energy ranges of the free exciton absorption and the band-to-band transition. Using SR pulses as the excitation light source, the two components are resolved by the method of simultaneous photon-counting through two independent time-windows. Excitation spectra for the phosphorescent bands of type II or III are also measured for comparison. From these spectra, total luminescence yield and the fraction of the type I band are determined as a function of excitation energy. On the basis of these results, the relaxation processes of free electron-hole pairs and free excitons, especially on the difference between them, are discussed.

Lattice Relaxation Effect Associated with Core Holes in Ionic Crystals Studied by Time-Resolved Luminescence Spectroscopy

Auger-decay-free core luminescence in RbF, CsF, CsCl, CsBr and BaF 2 has been studied under monochromatic vacuum ultraviolet light excitation using a time-resolved technique. Luminescence spectra associated with core holes are carefully separated from those originating in the valence-band excitation. Based on the spectral shape and energy range of the luminescence bands, a lattice relaxation effect following the core hole creation is discussed.

Self-Trapped Exciton Luminescence in KBr1-xIx and RbBr1-xIx Solid Solutions

Emission spectra and decay characteristics of luminescence in KBr 1- x I x and RbBr 1- x I x solid solutions have been investigated in the whole range of composition x . The emission band due to localized excitons at I - dimer centers in bromides, which consists of singlet and triplet components, is found to connect continuously with intrinsic σ band in pure iodides. The triplet component of the band is transformed into another emission with larger Stokes-shift in the intermediate region of x . Time-resolved studies on this emission in KBr 1- x I x have clarified that it consists of two triplet emission bands differing by ∼ 0.2 eV in their peak energies. The lower energy one corresponds to π band in KI. We demonstrate that the relaxed exciton configurations of different types are selectively realized depending on the local environment surrounding the I 2 - molecular core of the exciton. Discussion is given on the dynamical relaxation process of free excitons in iodides.

Parity-broken and -unbroken self-trapped excitons in alkali halides

Abslrmt: We review updated understandings of the electronic and atomic structures, relaxation dynamics and spin-multiplicity of self-trapped excitons (STEs) in alkali halide crystals, on the bases of recent experimental results of the STE luminescence. It is pointed out that seemingly complicated diversity in the features of STE luminescence and F-H pair formation can be explained from an unified viewpoint. That is, depending upon the kinds of crystals there can arise the “adiabatic instabilify”, which results in multiple local minima on the adiabatic potential ene~gy surfaces (APES’s) for the singlet and triplet STE states of the lowest orbital energy. A simple phenomenological model is proposed, which indicates that the electron-hole spin interaction plays an essential role in correlation with the electron-lattice and hole-lattice interactions. The shape of the APES’s, the configuration dependence of the electron-hole exchange energy, relaxation dynamics and the structure of STEs are discussed in relation to the bi-stability of paritybroken and -unbroken STEs.

Correlation between the Spin State and Structure of Self-Trapped Excitons in Alkali Halides

In order to clarify the origin of the preferential relaxation of singlet self-trapped excitons (STEs) into the on-center configuration in alkali halides, we have studied variation of exchange splitting energy along the relaxation path from the on-center to the off-center configurations using a simple phenomenological model. Estimated exchange energy is several tens of meV both at the on-center and off-center configurations, and takes its maximum in between. It provides a larger potential barrier intervening between the on- and off-center minima for the singlet STE than that for the triplet STE. This will inhibit the singlet STE to be relaxed into the off-center configuration.

Time-Resolved Luminescence Study of Relaxed Excitons in KBr:I and KCl:Br

Luminescence from relaxed excitons associated with guest halogen dimers was investigated for KBr:I and KCl:Br through time-resolved measurements of emission and excitation spectra using synchrotron radiation pulses from UVSOR under single-bunch operation. A weak slow decay component was found in addition to a main fast one in the 4.89 eV band in KCl:Br, in contrast to the 4.35 eV band of KBr:I which consists of a weak fast decay component and a main slow one. The fast and the slow decay components are assigned to transition from the singlet and the triplet relaxed excitons in the on-center configuration. It is suggested that the adiabatic potential energy surface for the triplet state, as well as that for the singlet state, must have a local minimum at the on-center configuration but that the triplet exciton in KCl:Br relaxes preferentially into the off-center configuration.

Time-resolved measurements of excitation spectra for intrinsic emission in alkali iodides

Abstract From time-resolved measurements of self-trapped exciton (STE) luminescence using synchroton radiation pulses, both singlet and triplet components of type-I luminescence from on-center STEs in KI and RbI were found to be depressed upon excitation of n = 1 free excitons, contrary to high efficiency of triplet luminescence of type II or III from off-center STEs.

Thermally activated population transfer between two different off-center exciton states in KCl:I

Temperature dependence of emission intensities and lifetimes of hetero-nuclear relaxed excitons [(ClI)-+e], in KCl:I has been studied in the range of 30 - 200 K. From population conversion between the type II and type III configurations observed below 100 K, the energy difference between the two configurations of 13 meV is obtained. An anomalous behavior observed above 100 K suggests that the singlet state above 74 meV from the triplet state of type III is thermally populated to serve as an additional decay channel effective at higher temperatures.