Vibrational modes second singlet excited state

Other experimental techniques have been used to study the very fast relaxation of dye molecules in solution. Ricard and Ducuing studied rhodamine molecules in various solvents and observed vibrational rates ranging from 1 to 4 ps for the first excited singlet state. Their experiment consisted of two pulses with a variable delay time between them the first excites molecules into the excited state manifold and the second measures the time evolution of stimulated emission for different wavelengths. Ricard found a correlation between fast internal conversion and vibrational relaxation rates. Laubereau et al. found a relaxation time of 1.3 0.3 ps for coumarin 6 in CCI4. They used an infrared pulse to prepare a well-defined vibrational mode in the ground electronic state, and monitored the population evolution with a second pulse that excited the system to the lowest singlet excited state, followed by fluorescence detection. 

Fig. 1. Jablonski diagram showing absorption and subsequent photophysical modes of excited state decay. Abbreviations are as follows IC = internal conversion ISC = intersystem crossing VR = vibrational relaxation S = singlet and T = triplet. The subscripts (i.e., 0, 1, and 2) indicate ground, first, and second states, respectively, and the arrows in the boxes indicate electron spins.

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