| Abstract [eng] |
Optical two-dimensional (2D) coherent spectroscopy has been developed as a new method over the last 20 years. It probes the structure and dynamics of materials by exciting them with a sequence of phase-coherent pulses and recording their response as two or more delays are varied. It excels at determining if resonances are coupled, overcoming the effects of inhomogeneous broadening and disentangling congested resonances by spreading them in two dimensions. Objective of this work is to try the optimization of the exciton scattering method with the exciton overlap parameter. Likewise, attempt using the exciton scattering approach to model 2D spectra of the heliobacterial (hBc) reaction center. We have implemented the quasiparticle representation of the optical response as a computer package. The quasiparticle picture naturally emerges out of equations of motion for exciton variables, the nonlinear-exciton equations (NEE), that were derived and gradually developed at different levels. The nonlinear response is then attributed to exciton-exciton scattering. The scattering process let’s us simplify the NEE’s that are being solved and saves us tremendous computational time when applying to the 2D spectroscopy. In this work we used the simple J-aggregate system in order to test additional optimisations. We introduced the exciton overlap parameter. With it we could cut off some of the calculation cycles when excitonic overlaps are too low. When comparing the computational time between setting this parameter as 0 and setting it as 0.01 we saved considerable amount of time without affecting the spectres themselves. However, by increasing the parameter furthermore, the amount of time we win becomes negligible. And taking this parameter too big will start to affect the resulting 2D spectra. The exciton scattering approach was applied to model 2D spectra of a realistic system - the heliobacterial reaction center. The system chosen was, the hBc reaction center. The structure was characterized by Gisriel and coworkers. We used the excitonic model of Kimura Itoh for our calculations. While absorbtion spectrum coincided nicely with experimental data, the cross-peaks in the 2D spectrum do not compare to experiments. The homogenous broadening of the main peaks was too large and the cross-peaks in our calculations was hardly to be seen. In conclusion, usage of the exciton overlap parameter can optimize 2D spectra calculations for many pigment excitonic systems, while not loosing the precision of the results. Furthermore, our modelled diagonal peaks agreed nicely with the experiment. However, diagonal peak background covers the cross-peak amplitudes. Thus, Kimura and Itoh model is not viable to model cross-peaks of the 2D spectra. |
