Table 1.

Intensity dependence of switching rates between states A and B

Excitation wavelength, nm	Fluence, μJ/cm2 per pulse	Absorption probability per pulse	kA→B, s−1	kB→A, s−1
515	26.9	0.32	0.94	0.50
	11.4	0.14	0.077	0.33
	4.1	0.049	0.011	0.31
840	18.4	0.41	0.086	0.26
	8.1	0.18	0.11	0.27
	2.4	0.054	0.016	0.098
	1.3	0.029	0.0079	0.26
	0.51	0.011	0.0068	0.11

The switching rates k_A→B and k_B→A were determined for carotenoid and B850 excitation. The rates (k_i→j) are calculated by dividing the number of events (i→j) by the total time in state i. The orders-of-magnitude increase in k_A→B upon orders-of-magnitude increase in excitation fluence suggests this process is photoactivated. In contrast, k_B→A remains the same order of magnitude, suggesting a thermal process. The linear dependence on excitation fluence, as well as the per pulse absorption probabilities for a single LH2 complex (23, 39), shows that the dynamics of states A and B are not a result of nonlinear effects.