Picosecond time-resolved emission

Linearity. A common source of error for picosecond time-resolved emission spectrometers is a non-linear response of the detector to emission intensity. Streak camera temporal dispersors, for example, exhibit a limited dynamic range, which, in unfavorable cases, can lead to severe experimental artifacts (20-21). 

Due to low, dark current and rapid readout characteristics, their large dynamic range (>18 bit for the CCD) and the two-dimensional array feature, the CCD and LCI-CCD are more versatile detectors. These detectors and the MCP-SPD array detector are particularly useful for picosecond, time-resolved emission, absorption, and Raman spectroscopy, and for imaging applications where signal averaging is required. 

Figure 10.5 Picosecond time resolved emission after photodissociation of trans-stilbene-He dimers. Excitation at 198 cm is followed by both decay of the reagent (top trace) and the appearance of product stilbene (bottom trace). The reference trace at zero energy is of the dimer, which is characterized by an instrumentally limited rise time of 20 ps and the fluorescence decay (2.67 ns) of the dimer. At an excitation energy of 95 cm , the rate is monitored by the appearance of ground state stilbene and vibrationally excited stilbene. The excited product is formed more rapidly than can be measured, while the ground state product is formed slowly (45 ps). Furthermore, the rates are faster at 95 cm than they are at 198 cm " excitation energies. These findings show that this reaction is not statistical. Taken with some modification from Semmes et al. (1987).

Fig. 5. Picosecond time-resolved emission spectra of SHF in DMF in presence of y-CD.

Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. 

The relaxation time for this new dynamic equilibrium varies from femtoseconds to picoseconds. The fast reorientation of solvent molecules causes a fast solva-tochromic shift in the fluorescence band of the organic chromophores. Solvation dynamics is measured in terms of (8v (0) 8v (/)), where the fluctuating frequency v(t) is the difference in solvation energies between the two electronic states involved, i.e., v(t)= sE(t)/h [110]. In time-resolved emission spectroscopy the time dependence of the excited-state distribution is monitored via the frequency shift of the emission... 

S. Schneider, A. Mindl, G. Elfinger, and M. Melzig, Photochromism of spirooxazines. I. Investigation of the primary processes in the ring-opening reaction by picosecond time-resolved absorption and emission spectroscopy, Ber. Bunzenges Phys. Chem., 91, 1222-1224 (1987). 

Time-resolved emission spectroscopy has provided valuable information on the nature of excited states in polymers. Two distinct types of excimers have been observed in poly(Y-vinylcarbazole) using picosecond time-resolved fluorescence. The sandwich-type excimer emitting at 420 nm was formed in several nanoseconds, whereas a second excimer emitting at 375 nm was formed immediately after a lOps electron pulse. (Scheme 13). Similar observations were also... 

However, time-resolved optical spectroscopy is perhaps the premier method for learning about the dynamics of a complex system, especially on nanosecond or picosecond time scales. Some DNA dynamics data from NMR spectroscopy are presented in Table 4.3. Time-resolved emission decays, time-resolved fluorescence anisotropy, and time-resolved Stokes shifts measurements of probe molecules in DNA have been described (and see below) and fast components in the time decays assigned to various DNA motions. The dynamics as a function of sequence are incompletely mapped and provide an exciting area for future investigations. 

Gryczynski, Z and Bucci, E. 1998. Time resolved emissions in the picosecond range of single tryptophan recombinant myoglobins reveal the presence of long range heme protein interactions. Biophysical Chemistry 74, 187-196... 

The materials used are spectra grade and further purified by recrystallization Or passing through a column of activated alumina to eliminate impurities to the level necessary to avoid solvent or spurious fluorescence. The picosecond data are recorded either via a streak camera, emission, or by means of imaging devices for absorption. The data are analyzed and plotted by a microvax computer [3]. Typical time-resolved emission data are shown in Fig. 2 for bromoaryls which have been excited by a 266-nm,... 

---