Picosecond characteristics, measurement

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

Above Tc the first component/Q< (t) relates to the structural relaxation while below Tc it measures the amount of structural arrest. The second part describes fast motional processes (that would take place in the picosecond range, not accessible by NSE) not related to transport phenomena, is the characteristic time of such fast microscopic dynamics. Concerning the structural relaxation, the following predictions are made ... 

As better and better methods for following fast reactions with precision were introduced and exploited, characteristic reaction times faster than a second— times measured in milhseconds (ms, 10 s), or microseconds (ps, 10 s), or nanoseconds (ns, 10 s) and then in picoseconds (ps, 10 s)—were measured through stopped-flow techniques (Chance, 1940), flash photolysis (Norrish and Porter, 1949), temperature-jump and related relaxation methods (Eigen, 1954), and then... 

The absorption spectrum of DAMS+ intercalated into CdS3 showed features that are vastly different from that of pure dye and these are characteristic of the J-aggregates a strong absorption at 600 nm and a broad shoulder around 530 nm. The SHG efficiency of the corresponding intercalation compound, measured on a picosecond Nd YAG pulsed laser operating in the fundamental mode (1064 nm), was found to be negligible. 

Because the picosecond continuum is generated through a highly nonlinear process, its detailed spatial, spectral, and intensity characteristics vary from shot to shot more severely than do the laser pulses used to generate it. In order to achieve a high degree of reliability in our spectral measurements, it is therefore necessary to obtain double beam spectra in which the data are corrected for continuum fluctuations for every shot. 

Photophysical characteristics of Pis (Scheme 12.1), especially the quantum yields of their dissociation O iss, are very important. Most of the photophysical data were measured by nanoseconds or picoseconds laser flash photolysis (LFP) or phosphorescence at low temperature. The properties of representative Pis are given in Table 12.1. 

The next chapter by Ohshima, Kajimoto and Fuke reviews some results obtained with linked systems, in which charge transfer is facilitated by torsional motion around a single bond. Picosecond time-resolved experiments allowed the direct measurement of the decay of the LE state and the rise of the CT state fluorescence. In some cases a third excited state, also characterized as a charge-transfer state, was observed. It was assigned to a charge-transfer state in which the two connected moieties (anthracene and aniline in this case) are twisted from the planar conformation characteristic of the ground state, but not to a fully perpendicular conformation. The photophysics of twisted intramolecular charge-transfer (TICT) states, were studied extensively in the gas phase and in solution (see the previous chapter by Herbich and Brutschy). 

In nanocomposite media, is worth about a few picoseconds (see 8.3.2.3 below). Eq. (30) then helps explaining the fact that, as noticed in the preceding section, xOl values measured with femtosecond pulses are smaller than those obtained with longer pulsewidths. However, dynamical thermal effects are likely to play a crucial role in the material nonlinear optical response, as will be shown in tire following. As their influence depends on the excitation temporal regime, the measurement analysis is not as simple as one could expect from the only characteristic time comparison of Eq. (30). We now go deeper into these thermal effects. 

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