In earlier measurements of momentum distribution of annihilating e^+e^- pairs the intensity of o-Ps component seemed to be much higher than that expected from the lifetime data. In this paper we show that such a deviation disappears if one assumes that the pick-off component of angular correlation of annihilation radiation is narrower by 10% than the component related to free annihilation.
Electric field modulated absorption spectra of vapor deposited layers of phenothiazine, explained in terms of a quadratic Stark effect, indicate modification of intramolecular (Frenkel-type) excitons by charge transfer transitions throughout UVIS absorption spectrum in the range 250-450 nm. The charge transfer character of the excited states appears at energies characteristic of various oxidation products of phenothiazine as identified from a spectroscopic study of photochemical air oxidation, performed on solution and solid film samples of this compound. The charge transfer transitions are ascribed to electron transfer between the molecules of phenothiazine and their oxidized species created in the samples during preparation and handling procedures.
We present the first time-resolved pump-probe experiment performed at synchrotron SOLEIL at the CRISTAL diffraction beamline. The time-resolved setup will be used in the near future for sub-ns time resolution experiments. We studied spin state switching in a [TPA Fe(III) TCC] PF_6 single crystal as induced by a ≈6 ns laser pulse (1064 nm), by measuring the 3D diffraction volume of the 002 Bragg reflection as a function of time after excitation. The intensity profiles (rocking curves) are found to evolve at two characteristic timescales, namely hundreds of ns and few μs, exhibiting subtle profile changes and peak broadenings. Consistently with previous studies, we interpret these features as due to structural inhomogeneities related to laser-induced deformation wave propagation and heat diffusion, which both start from the absorbing sites.
The spin-crossover compound [(TPA)Fe(TCC)]PF_6 was experimentally investigated using time-resolved optical pump and supercontinuum probe technique. Changes of VIS optical density at thermal equilibrium of a crystal with different high-spin/low-spin fractions are compared to changes recorded picoseconds after femtosecond laser excitation. A white light snapshot of the high-spin state at 180 K is extracted from time-resolved spectra. Our results indicate that the temperature of the electronic state affects the absorption spectra of high- and low-spin states.
Spectral and temporal characteristics and their temperature dependence for the long-lived (phosphorescence and delayed fluorescence) emission of chemically mixed crystals of acridine II (host) with anthracene (guest) were studied under conditions of various spectral resolutions and different modes of excitation. The energy of 0-0 transitions of exciton band and of trap states have been determined and the nature and energy scheme of the triplet states of crystals have been established. The complex arrangement of host molecules into two different pairs in the crystal structure of acridine II is revealed in the energetic "doublet" structure of the trap states for both, the shallow and the deep traps (of acridine and of anthracene origin, respectively). Migration of triplet excitation energy in these chemically mixed crystals is controlled predominantly by the triplet-triplet annihilation of excitons (either heterogeneous or homogeneous) almost in the whole temperature range. The formation of triplet excimers of acridine was definitely ruled out.
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