Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline

Ultrafast vibrational excitation and energy redistribution in the electronic ground state after internal conversion of 4-nitroaniline (PNA) and isotope labeled analogs is investigated by picosecond anti-Stokes resonance Raman spectroscopy. In PNA, PNA-di-15N and PNA-D4, anti-Stokes Raman lines of overtones and/or combination bands of out-of-plane vibrations display risetimes close to the decay time of the electronically excited state of about 0.5 ps and pronounced excess populations. Compared to such fast dynamics, the strongly Raman active totally-symmetric modes show a considerably slower picosecond rise time. Our results indicate primary excitation of out-of-plane vibrations by internal conversion and secondary excitation of strongly Raman active vibrations by redistribution of the vibrational energy.