The absorption peak of chlorophyll a in the red region of the spectrum is approximately 662 nm. Chlorophyll a also has a strong absorption band in the blue-violet region, commonly reported near 430 nm, giving the pigment two major absorption regions.
The red band is especially important in photosynthesis because chlorophyll a captures those photons and transfers their energy through pigment-protein antenna complexes. The energy eventually reaches reaction-center chlorophyll molecules, including the P680 pair in photosystem II and the P700 pair in photosystem I.
A common mix-up is confusing chlorophyll a’s red absorption peak with the reaction-center labels P680 and P700. Those numbers describe the approximate wavelengths at which the reaction centers are most reactive, not the single absorption maximum of chlorophyll a in every sample. Reported absorption values can also shift slightly with solvent and molecular environment, often appearing near 662–665 nm.