The wavelength of visible light that causes the re-excitation of electrons in Photosystem I (PSI) is primarily in the range of 700 to 750 nanometers (nm). This corresponds to the red portion of the electromagnetic spectrum. PSI is a complex of pigments and proteins found in the thylakoid membrane of chloroplasts, and it plays a crucial role in the light-dependent reactions of photosynthesis.
In PSI, the primary electron donor is a chlorophyll molecule called P700. When P700 absorbs a photon of light with the appropriate energy, one of its electrons becomes excited and is raised to a higher energy level. This electron is then passed through a series of electron carriers within the PSI complex, eventually being used to reduce NADP+ to NADPH, which is an important energy carrier in photosynthesis.
The specific wavelength range of 700 to 750 nm is ideal for exciting P700 and initiating the flow of electrons in Photosystem I.