The relaxation dynamics of an exciton in rubrene was investigated by femtosecond absorption spectroscopy. Exciton relaxation to a self-trapped state occurs via the coherent oscillation with 78 cm(-1) due to a coupled mode of molecular deformations with phenyl-side-group motions and molecular displacements. From the temperature dependence of the decay time of excitons, the energy necessary for an exciton to escape from a self-trapped state is evaluated to be ~35 meV (~400 K). As a result, a self-trapped exciton is stable at low temperatures. At room temperature, excitons can escape from a self-trapped state and, subsequently, they are dissociated to charged species. The exciton dissociation mechanism is discussed on the basis of the results.
1.r. spectral measurements were made to obtain frequency shifts ( AvOH) and integrated intensities (AOH) of the hydroxy stretching vibration for intramolecular hydrogen bonding in ortho-substituted phenols in dilute carbon tetrachloride solution. l H N.m.r. spectra were also measured to observe the chemical shift of the hydroxy proton.Two kinds of correlations were found between AvOH and AOH: one was a large increase of A,, with increasing nvOII, and the other a small increase of AOH. The former was found in phenols having a proton acceptor unconjugated with the benzene ring and the latter in those having a conjugated acceptor. The latter correlation was interpreted as resulting from a large delocalization effect of the OH bond electrons through the hydrogen bond system; this was supported by model calculations using the CND0/2 method to estimate&,.A linear correlation was also found between Avo, and As,,.An empirical equation linearly relating the hydrogen bond energy to AvOH is proposed. The calculated force constants, frequency shifts, and energies for the hydrogen bond systems were compared with the experimental data giving a fairly good result.
A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd2CuO4, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm−1) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling.
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