Photophysical properties of the photosensitizer pheophorbide a studied at high photon flux densities.

The photophysical properties of pheophorbide a (Pheo a) under two-step laser activation were investigated. For the first time quantum yields of higher excited state formation were calculated. It was shown that the quantum yields of the formation of these states depend strongly on the pulse duration of the excitation source. The nonlinear properties of Pheo a are quite different in dependence on the excitation wavelength but the quantum yields of the higher excited state formation in both cases (lambda exc = 337 nm and lambda exc = 667 nm) rise up increasing the photon flux density of the laser light up to 10(26) phot cm-2 s-1. A further increase of the photon flux density has no effect on the quantum yields.