Chlorophyll a (Chl a) belongs to the most important and most investigated molecules in the field of photosynthesis. The Q-band absorption is central for energy transfer in photosystems and the relative orientation of the Qy transitions of interacting chlorophylls governs the energy transfer. Chl a was well investigated, but a quantitative separation of Qx and Qy contributions to the Q-band of the Chl a absorption spectrum is still missing. We use femtosecond Vis-pump – IR-probe anisotropy excitation spectroscopy to disentangle the overlapping electronic Qx and Qy contributions quantitatively. In an anisotropy excitation spectrum we trace the dichroic ratio of a single vibration, i.e. the keto C[double bond, length as m-dash]O stretching vibration at 1690 cm−1, as a function of excitation wavelength. The change in dichroic ratio reflects altering Qy and Qx contributions. We identified Qx00 (0–0 transition of Qx) and Qx01 transition at (636 ± 1) nm and (607 ± 2) nm, respectively, and the Qy01 and Qy02 at (650 ± 6) nm, and (619 ± 3) nm, respectively. We find that Qx absorption, contributes to 50% to 72% at 636 nm and 49% to 71% at 606 nm to the Chl a absorption at room temperature. The Q band was well modelled by a single vibronic progression for the Qx and Qy transition of (700 ± 100) cm−1, and the energy gap between Qx00 and Qy00 was found to be (820 ± 60) cm−1. This precise description of the hexa-coordinated Chl a absorption spectrum will foster more accurate calculations on energy transfer processes in photosystems, and advance the detailed understanding of the intricate interaction of chlorophyll molecules with the solvent.
