Abstract: The photoacoustic(PA)spectrum in amplitude and phase of chlorophyll a(chl a)has been used to study the energy level and relaxation processes(see Fig.1).At the two main absorption bands S₁ and S₂ of chl.a, the phase angles are 6.5°and 0°,which indicate that the relaxation processes of excited states are different:the S₁ band relaxation undergoes the lowest triplet state(T₀) while the S₂ band relaxation does not.According to the relaxation processes,a model and corresponding rate equations have been established (see Fig.2).The absorption spectrum of chl.a compared with the PAspectrum has also been reported(see Fig.3),and the fluorescence quantum yield (Q) has been deduced from the intensity ratio of absorption and PAsignal.The result Q=0.24 is obtained which is consistent with the original references,It is proved that the PAspectroscopy is a promising and convenient analytical technique in the energy levels and relaxation studies of biological substances.

Key words: PAS, Chlorophyll a, Relaxation process