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

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-tau-pulse 2-T) pulse sequence, it was found that the shape of the echo at T = tau is influenced markedly by the delay t, for 0 < t < 40 ns. In addition a second echo signal was found at T = 2 tau for t = tau. 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 = tau) is ascribed to quantum beats; the second echo (T = 2 tau) is due to double-quantum coherence.