Vibrational relaxation following ultrafast internal conversion: comparing IR and Raman probing

Femtosecond IR and Raman techniques are compared with respect to their abilities to monitor vibrational relaxation following ultrafast internal conversion. The study was performed on para-nitroaniline which undergoes sub-picosecond internal conversion to the ground state populating a variety of vibrational modes. The NO2 stretch vibration at 1315 cm−1 responds to the excitation by a strong red shift, visible in transient IR and Raman experiments. It is due to the off-diagonal anharmonic coupling with other modes. The population of the NO2 stretch mode itself can only be observed in the (anti-Stokes) Raman spectrum, whereas IR spectroscopy is superior in detecting the coupling induced shift.