Figure 5.

Self-absorption in fluorescence spectra of LHC-II crystals. (A) Schematic diagram of the experiment showing the two directions in which fluorescence spectra were collected, with the detector in two alternative positions at 90° or 270° in the plane defined by the incident laser beam and the rotation axis of the crystal. (B) Fluorescence emission spectra of a crystal in two opposite directions, as defined in (A), with the exciting laser focused on the crystal surface facing the detector at the 90° position. (C) Spectra recorded of the same crystal in two opposite directions as in (B), but with the laser focus near the surface that faces the detector at the 270° position. (D) Absorption spectrum of a LHC-II crystal at 100 K. Individual absorption bands cannot be identified due to the high Chl concentration of 150 mg/ml. The sharp drop in absorption between 680 and 700 nm is evident. (E, F) Detailed drawings of the experiments in (B, C). Depending on the position of the detector (90° or 270°), or on the position of the crystal with respect to the laser focal spot, the 680 nm emission is either reabsorbed or not.