Chong Fang

In this dissertation, my research work on excited state dynamics of small organic molecules in solutions is presented. Using femtosecond stimulated Raman spectroscopy (FSRS) as the main experimental tool, the excited state hydrogen-bonding dynamics (ESHBD) of Coumarin 102, a common laser dye molecule, in ethanol solution is examined with <150 fs time resolution. The dynamics extracted from our FSRS spectra brings new insights into the currently controversial picture of ESHBD of Coumarin 102, i.e. whether the hydrogen bond formed between the carbonyl oxygen of Coumarin 102 and a nearby hydrogen donor is strengthened or cleaved upon 400 nm photoexcitation. The dynamics of the spectator mode, i.e. the C=O stretching mode, is examined within a 600 ps time window. The excited state dynamics in the first few picoseconds after photoexcitation unequivocally supports a picture of excited state reaction pathway bifurcation, i.e. only a portion of the molecules in the initial excited state undergo hydrogen bond cleavage occurring within 160 ± 50 fs after 400 nm photoexcitation. In the first 200 fs after excitation, clear vibrational peaks of the hydrogen-bonded C=O stretching mode are observed for the first time, confirming that the ultrafast excited state hydrogen bond cleavage occurs after rather than during photoexcitation. Dynamics extracted from FSRS data on a longer timescale, when combined wit