Simultaneous multi-photon ionization of aromatic molecules in polymer solids with ultrashort pulsed lasers

Radical cations of perylene, terthiophene, N,N,N′,N′-tetramethylbenzidine, N,N,N′,N′-tetramethyl-p-phenylenediamine, and N-ethylcarbazole were formed in poly(methyl methacrylate) solids by the direct excitation of each absorption band with a 355-nm picosecond (ps) laser and a 400-nm femtosecond (fs) laser and also by the indirect excitation with a 800-nm fs laser. The photoionization yields of perylene with ps and fs lasers were independent of the concentration of tetracyanobenzene, indicating that the relaxed singlet excited state of perylene is not involved in the photoionization. These results show that radical cations are formed through simultaneous multi-photon ionization enhanced by resonance with the lowest singlet excited state.