Time spectral narrowing

All OFDs reported in the literature suffer from spectral interferences, long response times, and narrow dynamic responses. Many of these obstacles exist as a result of limitations due to the properties of UY/visible fluorescent dyes. These dyes typically absorb and fluoresce between 300 and 650 nm, a region susceptible to extensive interference, especially from biomolecules (Figure 7.1). The fluorescence of sample impurities combined with the inner effect of the matrix and polymer support greatly increase the signal interference of the analysis. 

For 3T, a broad A band was observed that becomes spectrally narrower within the first picosecond. The maximum of A1 lies at X = 600 nm. Additionally, small positive values of AD at very small delay times, indicating a further transient absorption A0, were found. The transient behavior of 4T showed that the maximum of A was found at about X = 770 nm. It was found that the whole absorption band A decays simultaneously and single exponentially with x = 530 ps. Positive AD values due to an absorption A0 seem to appear weak at zero decay. 

Deviations from predicted relaxation behavior have been observed for large proteins (3 -7 ), polymers (8y9j and highly associated small molecules (10). Particularly prominent are observations of Ti field dependences and low NOE s within the so-called "extreme spectral narrowing region," where single correlation time models predict field independence of Tp and full NOE s. 

If dipolar relaxation is the main relaxation mechanism, the ratio of15N and 13C relaxation times under the conditions of extreme spectral narrowing provides information on the relative motions of these substituents. 

In the TRPL measurements performed at 2K the excitation was made by a Ti sapphire laser system, with spectrally narrow (< 1 meV) 2 ps pulses. The emitted light was dispersed by a subtractive double-grating monochromator and detected with a multi-channel plate photomultiplier in the photon-counting mode with a time resolution of 20 ps. 

The first exponential factor describes the spectral narrowing of the gain profile with increasing time t due to saturation and laser mode competition, and the second factor can be recognized as the Beer-Lambert absorption law for the transmitted laser power in the th mode with the effective absorption length Leff = ct. In practice, effective absorption lengths up to 70,000 km have been realized [15]. The spectral width of the laser output becomes narrower with increasing time, but the absorption dips become more pronounced (Fig. 1.15). 

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]. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

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