# Investigations of the Energy Transfer
## 3 Results
### Excitation Energy Transfer in the Phycobilisome Antenna  
#### 3.3.1 Time-Resolved Fluorescence  
The fluorescence kinetics of the PBS was recorded in the 645-700 nm spectral region after the system was pumped with 580 nm laser pulses. The resulted TCSPC data are presented in Figure 7. The fluorescence emission spectra showed a maximum around 660 nm with a red-shift toward 670 nm for emission spectra recorded at later time moments. The fluorescence kinetics was subject to global analysis fitting, assuming a simple sequential mode. We supposed that the excitation energy was transferred between different molecular species inside the PBS among the most energetically efficient route. The fluorescence kinetics of the PBS required two spectro-kinetic components for a satisfactory fit. We obtained a short component in the range of the IRF of our system and a long component of 1.77 ns. The resulted EAFS showed maxima at 655 nm and 668 nm, respectively. Additionally, the short EAFS decreased markedly until approximately 676 nm. The first component showed smaller amplitude compared to the second one, which could indicate smaller fluorescence yield induced by excitation energy transfer to the terminal emitter [41]. Moreover, the maximum of the short EAFS component was observed at 655 nm and matched the fluorescence emission maximum of PC; however, it also presented similarities to the short EAFS obtained in APC. Therefore, this kinetic component may be associated to PC or to a spectral form of PCB in APC that transfers the excitation energy to the terminal emitter in APC. The peaks correspond well with the species-associated spectra reported by [PERSON] et al. for the time-resolved emission spectra recorded from a wild-type PBS from Synechocystis. They reported peaks at 649 and 660 nm, which were assigned to PC 650 and APC660, respectively [42]. Additionally, [PERSON] et al. reported global fitting analysis of the time-resolved fluorescence data obtained for a wild-type PBS and obtained among faster kinetic components a decay time of 118 ps and one of 1.6 ns. The decay spectrum associated to the shorter kinetic component showed a positive peak at 645 nm with a negative one at 680 nm, similar to our short EAFS, and was assigned to the excitation energy transfer to APC, while the longer component peaked at 667 nm and was assigned to excited-state equilibrated PBSs [19].  
We performed a reconvolution analysis of the same data using the EasyTau2 software, which resulted in comparable decay components that were dependent on the pump and emission wavelengths. The mean value for the long component was 1.9 ns, while the one for the short component was 460 ps. The short component grew with increasing the detection wavelength from 640 to 665 nm and dominated the fit for this spectral region. Above 657 nm, the longer component dominated the fit. The calculated decay times together with the corresponding relative amplitudes are presented in Figure S3 (see Supplementary Material). The fact that the short component identified in our study varied with the emission wavelength may suggest that it encompasses several components and it is limited by the time-resolution of our system. However, the fluorescence emission of the latter component was detected, and its lifetime was calculated to 1.6-1.9 ns, depending on the pump pulse and confirmed by the TCSPC experiments on APC (see Figure 5).