Early time solvation dynamics of acetonitrile and methanol studied by broadband fluorescence upconversion and phase retrieval analysis

Abstract

Solvation dynamics of the small polar solvents acetonitrile and methanol were studied using broadband fluorescence upconversion techniques. Although broadband techniques were able to reproduce the solvent response functions of acetonitrile and methanol measured by traditional narrow bandwidth upconversion methods, they could not reproduce the entire time resolved fluorescence Stokes shift of Coumarin 153 in acetonitrile and methanol. One advantage of using broadband upconversion techniques is seen in the increased sensitivity to small oscillations in the fluorescence frequency that are predicted and observed at relaxation times of less than one picosecond. These oscillations, attributed to librational motion, were observed in the time resolved fluorescence of Coumarin 153 in acetonitrile using broadband fluorescence upconversion. Some evidence for the presence of librational motion was seen from the early time dynamic fluorescence of Coumarin 153 in methanol, but it was not as conclusive as that seen in acetonitrile. Another advantage is that the solvation dynamics were readily reproduced using data from an upconversion experiment without tuning of the sum frequency crystal. In addition to the inclusion of broadband detection, phase retrieval analysis techniques were adapted from optical physics to provide higher resolution dynamical information about the early time solvation dynamics of acetonitrile and methanol. The broadband nature of the Coumarin fluorescence posed an obstacle to directly calculating an instantaneous frequency from the retrieved temporal phase. To address this problem a time-windowed intensity weighted average frequency was defined using the first time derivative of the temporal phase. In turn, a solvent response function was calculated using this average frequency. Applying this analysis to the results from a single-angle broadband fluorescence upconversion experiment showed favorable results for producing a solvent response function for acetonitrile. The results from applying phase retrieval analysis to the methanol experimental data were less successful. The problem with application to the methanol data was attributed, in part, to the higher bandwidth of its fluorescence. The increased bandwidth and resulting decreased coherence time caused problems with the calculation of the time-windowed intensity weighted average frequency.