Electron spin echo detection of quantum beats and double-quantum coherence in spin-correlated radical pairs of protonated photosynthetic reaction centers

Abstract

The spin-correlated radical pair [P +.Q − A] in protonated zinc replaced reaction centers of Rb. sphaeroides R26 is studied using electron spin echo spectroscopy. Applying a (flash- t-pulse 1-τ-pulse 2- T) pulse sequence, it was found that the shape of the echo at T = τ is influenced markedly by the delay t, for 0 < t < 40 ns. In addition a second echo signal was found at T = 2 τ for t = τ. Both phenomena can be simulated well using the approach of Tang and Norris [Chem. Phys. Letters 233 (1995) 192]. The change in shape of the first echo ( T = τ) is ascribed to quantum beats; the second echo ( T = 2 τ) is due to double-quantum coherence.