[en] Highlights: • High-precision modeling of $Q_y$ band of chlorophylls a, d and f was performed. • Vertical excitation energies difference is the main reason in the band shifts. • Vibronic structures of $Q_y$ band of chlorophylls a, d and f are similar. The key components of the far-red-light forms of PSI and PSII are chlorophylls d and f which are able to absorb this light contrary to chlorophyll a. The aim of this work is to reveal the origin of their absorbance shift. XMCQDTP2/SA-CASSCF with large number of active valence orbitals results were found to be in a good agreement with the experimental data. The modeling of the main peak of $Q_y$ band has shown that absorbance shift is mainly due to the difference of vertical excitation energies, while the vibronic structure of this peak is quite similar for all three molecules.

[en] The electronic properties of light-harvesting complexes determine the efficiency of energy transfer in photosynthetic antennae. Ab initio calculations of the electronic properties of bacteriochlorophylls (composing the LH1 complex of the purple bacteria Thermochromatium tepidum) were performed. Based on these calculations, the excitonic Hamiltonian of a native cyclic complex and the Hamiltonians of open complexes with several removed bacteriochlorophylls were constructed. Absorption spectra calculated based on these Hamiltonians agree well with the experimental data. We found that the parameters of interaction between the neighboring bacteriochlorophylls are significantly larger than the empirical parameters suggested previously.

[en] We predict and investigate the emission of high-order harmonics by atoms that cross intense laser hot spots that last for a nanosecond or longer. An atom that moves through a nanometer-scale hot spot at characteristic thermal velocity can emit high-order harmonics in a similar fashion to an atom that is irradiated by a short-duration (picosecond-scale) laser pulse. We analyze the collective emission from a thermal gas and from a jet of atoms. In both cases, the line shape of a high-order harmonic exhibits a narrow spike with spectral width that is determined by the bandwidth of the driving laser. Finally, we discuss a scheme for producing long-duration laser hot spots with intensity in the range of the intensity threshold for high-harmonic generation. In the proposed scheme, the hot spot is produced by a long laser pulse that is consecutively coupled to a high-quality micro-resonator and a metallic nano-antenna. This system may be used for generating ultra-narrow bandwidth extreme-ultraviolet radiation through frequency up-conversion of a low-cost compact pump laser. (paper)