Phonon excitations, experimental technique

Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains . Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals. Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals. The dynamics of exciton trapping in p-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy. Ps time resolved spectroscopy was the experimental technique used in this work. 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

Many of the CC theoretical predictions, such as control of atomic and molecular processes via N versus M photon transitions [137], have been tested and demonstrated experimentally [138-146]. CC methods have also proved to be valid in the context of solid-state systems. In particular, it was shown that excitation by N and M multiphoton processes, having opposite parities, leads to symmetry breaking and the generation of DC electric currents [147-151]. These predictions have been confirmed experimentally in a number of semiconductors [152-155]. Similar techniques were shown to lead to the control of phonon emission [156] or injection of spin currents [157]. 

Relaxations of solvent-chromophore interactions can be studied experimentally by hole-burning spectroscopy, time-resolved pump-probe measurements, and photon-echo techniques that we discuss in the next chapter. If the temperature is low enough to freeze out pure dephasing, and a spectrally narrow laser is used to bum a hole in the absorption spectmm (Sect. 4.11), the zero-phonon hole should have the Lorentzian lineshape determined by the homogeneous lifetime of the excited state. The hole width increases with increasing temperature as the pure dephasing associated with tP comes into play [36, 37]. 

Infrared and Raman spectroscopies are extremely effective probes of the metal-insulator transitions in transition metal oxides. Infrared spectroscopy provides a measure of the optical spectral weight, ( n/m ), which is the natural order parameter for the Mott transition [5] as it approaches a zero value for either of the two paths by which a Mott transition is achieved, a diminution of the carrier density n or a divergence of the carrier effective mass m. On the other hand, Raman scattering is useful for simultaneously studying the evolution of electronic, phonon, and spin excitations, and most particularly the carrier scattering rate, through various metal-insulator transitions. Below, we provide some of the salient experimental details regarding these techniques. 

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