Study of chlorophyll a (in vitro) flourescence induced by nitrogen-pulsed laser radiation

The solvent effect on the chlorophyll a (chl a) laser induced fluorescence at different pigment concentrations is reported, when nitrogen pulsed lasers are used as pumping sources. It comes out that the fluorescence radiation spectral distribution is strongly dependent on the pigment concentration and seems to be associated with chl a dimer formation. The self-absorption process was observed as function of pigment concentration and sampling set-up. Under intense nitrogen laser irradiation, gain, linewidth narrowing and lifetime shortening were observed as a result of stimulated emission of radiation which is characteristic to the (0,0) vibronic region of the chl a fluorescence band. Since the number of excitation photons exceeds with about two orders of magnitude the number of chl a molecules in the irradiated volume, a high density of excited singlet states is produced; possible mechanisms for energy losses that prevent significant increasing of S₁ population, besides fluorescence, lasing emission and intersystem crossing, are absorption from the S₁ excited state and exciplex formation.