A direct measurement of intermolecular solvation dynamics using time-resolved THz spectroscopy (TRTS)

Low frequency intermolecular solvent dynamics in the vicinity of a photoexcited dye molecule have been probed directly with time-resolved THz pulse spectroscopy. Unlike typical time-resolved laser studies, the spectrum of both the solvent and the solute is monitored. We have investigated the dynamics of TBNC (2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine) in chloroform, benzene, toluene, and pyridine. The far-infrared (FIR) spectrum from 7 cm(-1) to 60 cm(-1) was measured as a function of delay time after photoexcitation with an 800 nm pulse. The temporal resolution was approximately 200 fs. In all three solvents, a low frequency feature in the vicinity of 12 to 15 cm(-1) appears within a few hundred fs after photoexcitation. A higher frequency absorption near 50 to 60 cm(-1) occurs 250 to 350 fs later. A mechanism for this observed behavior is proposed.