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

摘要

Nonequilibrium vibrational excitatins of para-nitroaniline (PNA,4-nitroaniline)occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering.Vibratinal 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 claculations 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-plae mode giving rise to the overtone Raman band at this freqency acts both as coupling and acc epting mode in the internal conversion process.