Absorption spectra of the P2 and Pa overlaid with the laser spectrum (gray). (a) P2 in dark, in the Pr state; P2 with illumination of 690 nm LED light, in a mixture of Pr and Pfr states; laser spectrum. (b) Pa with illumination of 740 nm LED light, adopting the Pr state; Pa in dark, in a mixture of Pr and Pfr states; laser spectrum. To see this figure in color, go online.