# Investigations of the Energy Transfer
## 3 Results
### Excitation Energy Transfer in the Phycobilisome Antenna
#### 3.3.2 Transient Absorption  
Further, we applied time-resolved absorption spectroscopy to map the energy transfer processes within our PBS sample and identify the ultrafast decay times. Figure 8b shows the difference absorption spectra obtained at various delay times when the PBS was excited at 580 nm. A bleaching of the ground state centred at 624 nm appeared immediately following the laser excitation, within the first 400 fs. Concomitantly, two positive absorption bands were observed: a smaller one in the 470-550 nm and a stronger one in the 650-700 nm spectral range. The latter band was already reported in the scientific literature, and it was suggested that it could represent an excited state absorption of PC, since it emerged together with the bleaching band [43]. The former band decayed with dynamics similar to the bleaching band; however, the latter one decayed within 1-100 ps, after which it became negative and increased again approaching the zero line. A more recent study suggested that since the 650-700 nm positive band decays faster than the 470-550 nm one, it cannot be assigned to an excited state absorption and associated it instead to a temporary electrochromic response of phycocyanobilin chromophores that are in the ground state and in the proximity of the excited ones [41]. After reaching its maximum, the bleaching band started to decrease and its peak shifted to 633 nm until 100 ps delay time. Increasing  
Figure 7: (a) 3D TCSPC data carpet recorded from the phycobilisome excited at 580 nm. The white spectrum corresponds to the steady-state fluorescence emission spectrum. (b) The evolution-associated fluorescence spectra obtained from global fitting analysis. the pump-probe delay, the bleaching disappeared without any further shift within approximately 1000 ps. Additionally, at delays longer than 140 ps, the difference absorption spectra showed a rise in the bleaching band at 648 nm, close to where the absorption of APC is, followed by a negative band of lower intensity centered at 660 nm, which could be assigned to stimulated emission of APC. This band decreased without any wavelength shift in more than 1000 ps. In order to get more insight into the ultrafast dynamics and their spectral characteristics, we applied global analysis considering a sequential model with four compartments. The excitation energy was funneled from the high-energy components within the PBS to the lower energy ones and finally to the terminal emitter. The evolution-associated difference spectra and the time-dependent concentrations are presented in Figure 9b,c. The resulted lifetimes associated to the decay of each compartment were \(\tau_{1}=850\) fs, \(\tau_{2}=14\) ps, \(\tau_{3}=122\) ps, and \(\tau_{4}=1.15\) ns. The values obtained here compare well with the results reported previously [25; 26; 41], as we discuss further on. The EADS characteristic to the 850 fs component was described by a negative band centred at 619 nm and positive values above 647 nm and below 540 nm. The negative band at 619 nm indicates the ground-state bleaching of PC, especially the \(\alpha\)-84 chromophore, which should absorb at this wavelength [41]. We assign this component to the ultrafast equilibration of excitation energy between the sensitizing and fluorescing chromophores in PC. This value is comparable to the one obtained above for the energy transfer between the \(\alpha\)-84 and \(\beta\)-84 chromophores in PC using TA experiments (see Figure 3). A similar sub-picosecond value of 888 fs was reported before in the literature for the PBS extracted from T. vulcanus and the spectro-kinetic component presented a minimum at 620 nm. The decay time was assigned to ultrafast kinetics along the PC rod to the APC core mainly on the observation that the species-associated difference spectra showed positive values above 650 nm [25]. Another similar result to ours was recently published by [PERSON] et al.