Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline

Nonequilibrium vibrational excitations of para-nitroaniline (PNA, 4-nitroaniline) occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering. Vibrational excess populations with distinctly different picosecond rise and decay times are found for a number of modes with frequencies between 860 and 1510 cm(-1), including the overtone of a non-Raman active mode. A nonthermal distribution of vibrational populations exists up to about 6 ps after photoexcitation. The time-resolved experiments are complemented by steady-state infrared and Raman measurements as well as calculations based on density functional theory, providing a detailed analysis of the steady-state vibrational spectra of PNA and two of its isotopomers. A weakly Raman active vibration at about 1510 cm(-1) displays the fastest rise time and a pronounced excess population and-thus-represents the main accepting mode. We suggest that an out-of-plane mode giving rise to the overtone Raman band at this frequency acts both as coupling and accepting mode in the internal conversion process. (C) 2002 American Institute of Physics.