Résumé
The two-photon excited fluorescence (TPEF) cross-section of Protoporphyrin IX (PpIX) in solution is determined at different excitation wavelengths. Maxima observed at 620 and 634 nm are associated with two different forms of PpIX already observed in one-photon excited fluorescence studies. In order to further provide insights into these two forms, first hyperpolarizabilities are also reported for these two forms with corresponding depolari-zation ratio further demonstrating the existence of specific symmetry differences. The static and the frequency dependent first hyperpolarizability were also calculated using Gaussian 09 and compared to the experimental data. This work shows that PpIX can exhibit two forms that might be targeted in applications like oncology clinical agents.