# Investigations of the Energy Transfer
## 3 Results
### Ultrafast Dynamics in Allophycocyanin
#### 3.2.2 Transient Absorption  
The TA results obtained using 580 nm pump pulses are given in the Figure 6 panel (a), showing the differential absorption signal as a function of the probed wavelength and delay time between pump and probe pulses. The blue regions are negative values normally associated with ground state bleaching, while the red ones are positive values typically assigned to excited states absorption. Representative TA spectra are shown in panel (b). At 260 fs after excitation, an immediate bleaching around 620 nm and two positive absorption bands in the 480-540 nm and 670-700 nm spectral regions were observed, as well as a stimulated emission negative band in the 700-760 nm region. At 400 fs following excitation, a second bleaching appeared around 655 nm that slowly shifted to longer wavelengths with increasing the delay time. The bleaching around 620 nm showed a rapid recovery in a few hundreds of fs followed by a slow, long recovery. The 650 nm bleaching, one the other hand, presented slower recovery, similar to the kinetics of the 480-540 nm and the 700-760 nm bands. The induced absorption in the 670-700 nm spectral region observed immediately after excitation persisted until 1-2 ps; then, it decreased and became negative (2-150 ps) and subsequently increased again approaching the zero line. The TA data were investigated with global analysis using three compartments and a sequential transfer model, and the results are presented in Figure 6c,d. The sequential model should mimic well the excitation transfer inside the APC trimer, and the results obtained here indicated the presence of  
Figure 6: (a) 3D transient absorption data carpet obtained from allophycocyanin (APC) pumped at 580 nm, (b) transient absorption spectra of APC 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 shown (dash line), (c) the evolution-associated difference spectra resulted following global analysis, and (d) examples of kinetic traces together with the obtained fits. both fast and slow EET processes. The kinetic components describing the kinetics of APC trimer had values of 370 fs, 44 ps, and 2 ns. Examples of kinetic traces recorded at various wavelengths with their fitted curves are given in Figure 6d. The fastest process identified in our TA experiment took place on a time scale of a few hundreds of fs (<500 fs) and showed an associated EADS with a minimum at 626 nm, two maxima at 677 and 522 nm, and another minimum at longer wavelengths, centered around 724 nm. At 620 nm, the absorption spectrum of APC presented a shoulder, which was considerably discussed upon in previous studies [36; 40] and attributed to the absorption of the higher energy chromophores (see Supplementary Material Figure S2). Therefore, the minimum observed in the associated evolutionary difference spectrum can be assigned to the ground-state bleaching of the high-energy chromophores in APC. This suggests that immediately after the ultrashort laser excitation, we observed the localization of the excitation energy on one of the two phycocyanobilin (PCB) chromophores within approximately 400 fs. The 44 ps component showed an associated spectrum with two minima at 625 and 656 nm, where the high energy, respectively the low-energy PCB chromophores absorb. Therefore, this component should reflect the energy transfer between pairs of low and high-energy PCB chromophores. Finally, the 2 ns EADS component exhibited a minimum at 660 nm, which was red-shifted from the previous EADS by a few nm. Additionally, a slower recovery was observed on the red side of the minimum and a broad shoulder around 615 nm. Given the minimum at 660 nm, which we believe reflects the stimulated emission of APC chromophores and the comparable value to the fluorescence lifetime identified using TCSPC, we assigned this kinetic component to the radiative and non-radiative relaxation of the system back to the ground state.