Phonon side hole

An experimental PHB hole profile generally consists of three parts a sharp zero-phonon hole, a small, broad hole of the phonon side band on the higher energy side, and a broad hole, called pseudo-phonon side hole, on the lower energy side of the laser frequency (Table 2.12 ). The pseudo-phonon side hole results from the overlap of the zero-phonon holes which exhibit phonon side bands at the laser frequency. 

Figure 11 shows a saturated hole proffle burned to estimate the low-energy excitation mode, E of TPP/PnPMA. A deep zero-phonon hole is created at 644.8 nm. In addition, there are two broad holes at bodi sides of the zero-phonon hole. The broad hole at the shorter wavelength is called a phonon side hole and the one at the longer wavelength is called a pseudo-phonon side hole . The phonon side hole consists of a phonon side band of the zero-phonon hole, and the p udo-phonon side hole is made of the zero-phonon line of the reacted molecules which have been excited via phonon side band. 

The first term describes the so-called zero phonon hole (ZPH). Its halfwidth is the double homogeneous halfwidth of ZPL. The second and third terms describe the so-called phonon side hole (PSH). However, in contrast to PSB from FLN spectra the PSH is shifted to the red and blue sides with respect to ZPH (Fig. 14a). The fourth term describes a structureless phon whidi can be ignored if we are interest in the PSB shape. However, the jdion term should be taken into account if we determine the Debye-Waller factor with the help trf HB spectra. This situation resembles that existing in FLN spectra. 

Fig. 14a, b. Zero-phonon hole and phonon-side holes at a small and b large intensities of burning light... 

Second, apart from the sharp zero-phonon hole in Fig. 10, there is a broad side hole whose maximum is shifted to the blue compared to the laser frequency. The side hole represents molecular and lattice vibrational transitions which are simultaneously bleached with the pure electronic line. From the relative magnitude of the area under the zero-phonon hole and the side hole, we see that the coupling to vibrational transitions is rather strong, as is expected for states with a considerable amount of charge transfer character. 

At room temperature, non-photochemical spectral holes usually are filled in by flucmations of the surroundings on the picosecond time scale. This process, termed spectral dijfusion, can be studied by picosecond pump-probe techniques. At temperatures below 4 K, non-photochemical spectral holes can persist almost indefinitely and can be measured with a conventional spectrophotometer. The shape of the hole depends on the lifetime of the excited state and the coupling of the electronic excitation to vibrational modes of the solvent, both of which depend in turn on the excitation wavelength. Excitation on the far-red edge of the absorption band populates mainly the lowest vibrational level of the excited state, which has a relatively long lifetime, and the resulting zero-phonon hole is correspondingly sharp (Fig. 4.22A). The zero-phonon hole typically is accompanied by one or more phonon side bands that reflect vibrational excitation of the solvent in concert with electronic excitation of the chromophore. The side bands are broader than the zero-... 

However, on the one hand, low ESR signals alone are a weak argument for the assumption of hole bipolarons. On the other hand, several experimental results are in contradiction of this model. For example, (a) the electrical conductivity of boron carbide is maximum at the minimum concentration of BnC icosahedra in the homogeneity range (b) polaron-type effects are restricted to one electron per icosahedron and no corresponding electron-phonon interaction with holes, in particular not with hole pairs in icosahedra, has been proved experimentally (c) the distortion of the icosahedra in boron carbide depends to only a small degree on electron-phonon interaction and (d) the electronic transport in boron-rich solids is due to classical band-type conduction and hopping processes side by side. Hence, the hole bipolaron theory for boron-rich solids can hardly be maintained. 

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