ULTRAFAST DYNAMICS AND STRUCTURE AT AQUEOUS INTERFACES BY 2ND-HARMONIC GENERATION

Femtosecond time-resolved second harmonic generation studies of the ba rrierless isomerization of an organic dye, malachite green (MG), have been carried out at several aqueous interfaces. A comparison of the dy namics at the air/aqueous, alkane/aqueous and silica/aqueous interface s, indicates increased friction and increased water structure at the a queous interfaces relative to bulk water, in support of molecular simu lations, with the silica/aqueous interface being tile most structured. The dynamics are slower at all of these interfaces than in bulk water , by a factor of three to five in the case of the air/aqueous and alka ne/aqueous interfaces, and almost an order of magnitude in the case of the silica/aqueous interface. These investigations also indicate that the generally accepted isomerization model of twisting of the three a romatic rings about the central carbon atom requires modification in t hat the synchronous twisting of all three aromatic rings is not necess ary for rapid internal conversion from the excited to ground electroni c state. In contrast to MG, the dynamics of the activated photoisomeri zation of the cyanine dye, 3,3'-diethyloxadicarbocyanine iodide (DODCI ), is faster at the air/aqueous interface than in bulk aqueous solutio n. The different dynamics of MG and DODCI suggest that the interface f riction must be described in terms of the orientation and solvent stru cture in the vicinity of the chromophores involved in the isomerizatio n process.