Two Color Spectroscopy of Large Molecules with 20 fs Optical Pulses

The photophysics of large molecules has been described empirically by Kasha’s Rule that states the following: for any manifold of electronic states the first excited state of a given multiplicity will be the fluorescing state. For instance, numerous laser dyes show strong fluorescence emission from the S1 state with a spectrum that remains unchanged for any excitation wavelength in the range of the absorption spectrum. Thus, higher lying electronic states relax predominantly by radiationless processes to the bottom of the S1 state. This relaxation process is depicted schematically in the inset of Fig. 1 and occurs on a time scale much shorter than the S1 lifetime which has values between hundreds of picoseconds and several nanoseconds.