Fano antiresonance

In the sixties Fano and Cooper (79) presented a theoretical explanation for the Beutler bands in the far-UV absorption spectra of rare gasses. Sturge, Guggenheim, and Pryce (80) showed that the Fano theory describing the band profile of a broad ionizing continuum in the vicinity of sharp intraatomic transitions could be applied to the situation of an impurity center for which a broad absorption band is overlapped by a sharp absorption line. This t5q)e of Fano antiresonance in solid state physics has been observed for two cP transition metal ions, viz., by Sturge et al. (80) and Cr by Lempicki et al. (.81). The explanation for the antiresonance observed for Cr " and is based on... 

Recently the observation of Fano antiresonance in the excitation spectra of the luminescence of Eu was reported (82). The two-photon absorption experiments by Downer et al. [37,38], for example, revealed the presence of sharp absorption lines due to transitions from the 87/2 ground state to the Pj, Ij and Dj states within the Af configuration of Eu ". These parity-forbidden transitions are overlapped by the broad 4/ 5d absorption bands of Eu. For this situation the appearance of Fano antiresonance in the vicinity of the sharp absorption lines is to be expected. 

The dips are ascribed to the presence of Fano antiresonance. Note that no dips due to Fano antiresonance are observed at the position of the Pj levels, located around 360 nm. These phenomena have been satisfactorily analyzed (82). 

Inside the band of two-particle states a channel opens up for polariton decay into two phonons. This process leads to a broadening of the polariton line (see, e.g. (59), (60)), and also a change in the polariton dispersion law. The two most important effects in the latter case are (a) interference of scattering by a polariton and two-particle states (this can lead to drops in intensity of the Fano antiresonance type see (22)), and (b) the presence of singularities in the density of two-particle states (those, in particular, that correspond to quasibiphonons). 

Since ordering of the TTF and the TCNQ chains occurs at different temperatures, and evidence that most of the conductivity above the 54°K transition is along the TCNQ chain (Cl), it appears unlikely that hybridized orbitals betweeen the chains are important (see also Tl, S3). The question of how much of the conductivity is due to collective modes remains open. If the CDW fluctuations are more like m.f.t. than ID, one must account for the large dip in a(uj) below 1000 cm on grounds other than the energy gap from pinned CDW s. A suggestion which perhaps deserves more careful study is whether or not a Fano antiresonance... 

Free carriers change Raman spectra, either by single particle contribution to the spectrum, or by phonon- plasmon interaction. In addition, interference of electronic transition continua with single phonon excitations may lead to Fano line shapes, as mentioned in the introduction. The Fano effect is encountered in p-doped Si crystals, as shown in Fig. 4.8-19. The shown lines correspond to the respective Raman active mode at 520 cm for crystals with 4 different carrier concentrations, excited with a red laser. The continuous line is calculated according to Eq. 4.8-6. Antiresonance on the low frequency side and line enhancement on the high frequency side are a consequence of the positive value of Q. A reverse type of behavior is possible in the case of a negative Q. 

The opposite case to a giant resonance, which exhausts the oscillator strength within its width, is an antiresonance, or a Fano resonance with q = 0. In principle, nothing prevents such a resonance from acting as the intruder. Double excitations appear above the first ionisation potentials of many-electron atoms, and are frequently observed as window resonances. An example where an antiresonance acts as the intruder [418] occurs in the spectrum Ar shown in of fig. 8.12. 

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