# Investigations of the Energy Transfer
## 3 Results
### Ultrafast Dynamics in Phyococyanin
#### 3.1.2 Transient Absorption  
Time-resolved absorption spectroscopy was further employed to investigate the ultrafast dynamics in PC. Figure 4a depicts the difference absorption signal as a function of the probed wavelength and time delay between pump and probe pulses. Bleaching of the ground state occurred immediately after excitation, as well as positive absorption changes at the wings of the absorption band (Figure 4b). The bleaching signal recovered with increasing the delay between pump and probe pulses and showed a red-shift of a few nm as the ground state was repopulated. This effect was observed in the 570-660 nm spectral region and the kinetic curves decayed toward zero on a rather long time scale. In the 500-530 and 660-710 nm spectral ranges, positive absorption changes were observed on both sides of the bleaching band. Both of them decayed on a similar time scale as the bleaching band.  
The data were evaluated using global analysis and a sequential decay model with three compartments. Thus, the three-dimensional surface data was decomposed into three simple time-dependencies of compartment populations and three evolution-associated difference spectra assigned to each compartment (Figure 4c). We obtained an ultra-short decay component of around 1 ps, a middle component of 64 ps, and a long one that was kept constant at the value obtained from the TCSPC experiment of 1.34 ns. The EADS allow visualizing the amplitude evolution as a function of wavelength, as well as the spectral dynamics of the excited states of PC. The spectra associated to the different evolutionary steps looked similar to one another, differing only in their amplitude and presenting a small red-shift of 4 nm from the fastest decay to the longest one. The first associated  
Figure 4: **(a)** 3D transient absorption data carpet obtained from phycocyanin (PC) pumped at 580 nm; **(b)** Transient absorption spectra of PC showing bleach recovery of the ground state as a function of time delay between pump and probe pulses; the UV-VIS absorption spectrum is also given (dashed line); **(c)** The resulting evolution-associated difference spectra obtained with global analysis using three compartments; **(d)** Examples of kinetic traces together with the fits.  
spectrum presented a minimum at 630 nm, which corresponds well with the 629 nm band assigned to PC resolved in the absorption spectrum of the phycobiliproteins from A. marina [37], as well as the band resolved in the second-derivative absorption spectrum of PC presented in Figure S1 that is assigned to the ground state bleach of PC. The two positive bands located around 571 and 675 nm correspond to excited state absorption, while the negative band at longer wavelengths could indicate stimulated emission. We assume that the ultrafast 850 fs component represents energy transfer between the \(\alpha\)-84 and \(\beta\)-84 chromophores. A similar value (of approximately 500 fs) was reported for PC isolated from Mastigocladus laminosus and A. marina, respectively, using time-resolved absorption [38; 39]. The ground state bleaching observed in the second and third EADS shifted to 632 and 634 nm, respectively, and it can be assigned to spectrally different forms of \(\beta\)-84 chromophores. This suggests that the excitation energy is transferred among spectrally different forms of the \(\beta\)-84 chromophores in 64 ps, which relax afterwards back to the ground state in 1.34 ns.