Bleaching signal

The bleaching signal for Hbl-NO, Hbl-fhS, Hbl-N3 and metaquo Hbl were observed at 410 nm, 410 nm, 415 rnn and 407 nm, respectively, and similar to the ferrous complexes, the sharp rice in the probe transmission signal suggests that the ligand photodissociation occurs within the time resolution of our experiment. The transient spectra were well fitted with time constants of 600 fs and 3 ps. Similar time constants have been previously observed [9]. More than 90 % of the signals were recovered after 10 ps of photoexcitation. 

Fig. 2. Pump-probe transients for (A) M9, (B) Ml 1, (C) Ml3 and (D) Ml5 at different wavelengths corresponding to induced absorption from S and S and ground state bleach. For all carotenoids, the bleach signal corresponds to the shortest wavelength, the S signal is at the intermediate wavelength and the Si signal is the reddest.

Therefore the lack of an observable bleach can only be explained by the cancellation of all contributions to the pump-probe signal, which is the case for a perfect harmonic state. It can be shown that the anharmonicity of a vibrational exciton is a direct measure of its degree of delocalization [5]. Thus, we conclude that the free exciton state is almost perfectly delocalized at 90 K. As temperature increases, a bleach signal starts to be observed, pointing to a non-complete cancellation of the different contributions of the total pump-probe signal. Apparently, thermally induced disorder (Anderson localization) starts to localize the free exciton. The anharmonicity of the self-trapped state (1650 cm 1), on the other hand, originates from nonlinear interaction between the amide I mode and the phonon system of the crystal. It... 

In order to identify the origin of the instantaneous bleaching signal at 3215 cm-1 we compared transients at 3215 cm-1 after excitation at 1630 cm-1 and... 

Results for excitation of the AT DNA oligomer at 1630 cm-1, presented in Fig. 8b, show bleaching signals at the ground state absorption positions of the vc2=02 vibration (1716 cm-1), the vca=oa and nh2 vibrations (both... 

The parallel component of the probe transmission change is plotted in Fig. 17 in the range 2850-3650 cm 1 for different delay times (left-hand ordinate scales, experimental points, calculated solid curves). The transient spectrum during the excitation process, to = 0 ps, is depicted in Fig. 17a. A bleaching at the frequency position of 2974 cm-1 is shown because of the excitation of the CH vibration. The excess population of the upper level v = 1 can be directly monitored from the induced absorption around 2952 cm-1 and is attributed to excited-state absorption (width of 17 2 cm-1). The bleaching signal at lower frequencies indicates... 

Fortunately in a number of cases there is experimental information on these points from broad band pump/probe experiments when the anharmonicity A is larger than the linewidth but much smaller than the bandwidth 8(o of the laser. Then the 0-1 transition is seen as a bleaching signal and the 1-2 (66,67,71) as well as the 2-3 and often higher quantum number transitions (68,95) appear as new absorptions to an extent that depends on the pump intensity. A direct comparison of the total linewidths (1/T2) of these transitions, and the population relaxation times for the v = 1, v = 2 and perhaps higher levels can be obtained from such data. For N3 we found that ratio of the state to state relaxation from v = 2 to v = 1 was 1.8 times that for v = 1 to v = 0, not far from the harmonic value of 2 (50,95). However, the bandwidth of both transitions was roughly the same. 

Figure 1 shows the transient absorption spectrum of PB in methanol after the excitation at 640 nm. In the figure we set time zero as the rise of the pump pulse. The ground state hole created by the pump pulse is broadened mostly by 1 ps. Near the absorption maximum of the ground state, more than half of the bleach signal observed at 1 p>s is recovered until 2 ps, and the residual bleach is diminished with a decay constant of about 10 ps. The hole broadening... 

Figure 2 shows the transient absorption spectrum of PB in CI%H at 5.7 MPa. The pattern of the transient sp>ectrum is almost the same as those in methanol. The hole broadening occurs mostly within 0.8 ps, and the bleach is recovered with two different time constants in a similar manner as in the liquid solvents, although the recovery after 2 ps is much slower in the fluid near the critical density (about 40 ps). It is also to be noted that the bleach signal after 2 ps is narrow banded in comparison with the equilibrium absorption. This can be interpreted by the overlapping of the excited state absorption, and/or, the small inhomogeneity remained after 2 ps due to the long time density fluctuation. 

Figure 16. Time course of the transient absorbance changes measured upon laser excitation of cis-[Ru"(dcbpy)2(NCS)2] dye adsorbed on nanocrystalline Ti02 films. Bleaching signals were measured at A = 520 nm in anhydrous propylene carbonate, a) without electrolyte, and in the presence of b) 0.1 M TBAI, c) 0.1 M Lil, and d) 0.05 m Mgl2. Insert, dependence of the half-lifetime t /2 of the dye ground-state absorbance recovery upon the concentration of Li+ cations. Iodide concentration of [I ] = 0.1 M was kept constant while pLi = -log[Li+] was varied.

The ground state bleach signal decayed with the same lifetime as the singlet exciton population in the dilute case, but showed a doubling in magnitude over the first 10 ns in the concentrated solution, indicative of exciton fission from a localized state. 

Considering equilibrium (14), the reaction of interest is that of thiyl with thio-late. Rate constants for this process have been determined in some elegant work by Mezyk [12, 27, 38]. In combined pulse radiolysis/photolysis experiments he photodissociated the (RS SR)" radical anion and subsequently followed the recovery kinetics of the (RS SR)" bleaching signal. Some measured rate constants for the respective RS + RS" reactions are 4.8 x lO M s (cysteine). 

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