Observation of molecular vibrations in real time

In a recent Letter Rosker, Wise, and Tang1 have reported ultrafast optical measurements on two large dye molecules, malachite green and nile blue 690. Using the transmission-correlation technique with 40-fs light pulses, they measured photoinduced changes in optical transmission and found a response consisting of damped oscillations superimposed on two exponential decays. They tentatively interpreted the oscillations as quantum beats between two eigenstates of the isolated molecule. In this Comment we report Raman and infrared measurements on the same molecules and discuss the implications these measurements have for the interpretation of the data of Rosker, Wise, and Tang.