Ground state bleach

The relaxation of Fe(II) protoporphyrin dimethyl ester in benzene is seen in Figure 1. It can be seen tnat in this complex which has no axial ligands and a high spin state, S=2 (14), there is an excited state absorption band which extends from 520 nm into the blue. The peak is not seen because of experimental cut-off in the spectrum at 450 nm. This difference spectrum also indicates an absorption band extending to the blue from 620 nm and interupted by the strong ground state bleaching at 570 nm. The peak of the excited state absorption band seems to be centered at about 585-590 nm. 

Deoxymyoglobin has an excited state spectrum without any strong ground state bleaching. Thus, the 450 absorption band and a broad band at 600-650 nm can be seen but the fused Q band in the ground state must be counterbalanced by a like absorption in the excited state so that no bleaching is seen in the 500-570 nm range. The decay from the excited state occurs with a 34 psec time constant in the 460 nm excited state absorption band but is delayed to 60 psec in the broad 600 nm absorption band as shown in Fig. 4. 

What can again be seen from work-up of the transient absorption spectra of this strained complex is the early appearance of a new photoproduct X in the dissociative pathway that is not present in the unstrained CO-complexes. The difference spectrum for the 0 ps delay shows only 30% ground state bleaching. Removing the remaining ground state and the intermediate B from this difference spectrum reveals the presence of the X characterized by a double peaked band. The smoothed transient absorption spectra and the spectra of the intermediates are shown in Figs. 5 and 6. The spectroscopic... 

Fig. 3. Pump-deplete-probe spectroscopy on lycopene in hexane, a) Experimental setup After excitation and depletion of Car S2 with a delay of r=50fs, a white-light probe pulse at delay tprob<.=2ps measures the transient absorption spectrum, b) Spectra without (solid curve) and with depletion pulse (dotted) and their difference (shaded area). Only the Car Si state is depleted the ground state bleach (S0-S2) and positive absorption feature on its low energy side (hotSo-S2) are unaffected.

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.

For the reference compound 4, two different parts in the transient spectrum were found (data not shown) a negative signal (480...600 nm) resulting from a combined ground state bleaching and excited state absorption, and second positive part (>600 nm) due to excited state absorption. Both decay on a ns time scale. Close inspection of the spectra reveils another process with a time constant of 10 ps, which in accordance with previous results [1] is attributed to vibrational/solvent relaxation. 

Figure 2 Transient IR spectrum (solid line) after IR excitation of the OH stretch (3550 cm-1) of dilute 1 1 hydrogen-bonded triethylsilanol-acetonitrile in CC14 at 295 K. The positive feature is the complex ground state bleach while the negative feature is the OH stretch (v = 1 2) excited state absorption. These spectra are...

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