Figure 2.

(a) Excitation (blue curve) and emission (red curve) spectra of RS-GFP at 1.6 K before (—) and after (---) burning into I_0–0 at 499 ± 1 nm (right hole). Before burning, only the I-form is present. After burning, the vibronic bands of the B-form between 450 and 475 nm increase (blue, ---); thus, I→B. By burning into B_0–0 at 476 ± 1 nm, a hole is produced (Left), and the original spectrum recovers; thus, B→I. After burning into the I_0–0, the A-form is also photoinduced, which is reflected in the fluorescence spectrum. In the region from 400 to 490 nm there is no signal before burning (red, —), but there is fluorescence after burning (red, ---). (b) Excitation spectra of A with λ_det = 450 nm. Optical switching between A and I occurs. (1) The A-form has been produced by burning into I_0–0 at 499 ± 1 nm. (2) After burning a hole in the 0–0 transition of A at 434 ± 1 nm, the intensity of the whole A-spectrum decreases whereas that of the I-form increases (not shown). (3) After subsequent burning into I, the population of the A-form increases again.