Probing solvent dependent femtosecond transient coherent oscillations to reveal interfacial dynamics

Abstract

Probing transient states in molecules having vibronic transitions with femtosecond laser pulses often results in coherent oscillations either in the ground state, the excited states, or both. We find such coherent oscillations are highly solvent-dependent and provide a holistic overview of the pump-probe experiments for ultrafast dye dynamics at interfaces. For molecules dissolved in single solvents, modulations in oscillations occur due to transitions in the sub-vibrational levels of the electronic state. For binary solvents, in particular, these modulations are strongly sensitive to solvent compositions. The changes induced by various solvent compositions are drastic enough to act as a control parameter for dynamical control processes. We demonstrate an end-to-end understanding of ground-state coherent oscillations, vibrational cooling, ground-state recovery processes, and excited-state dynamics through a series of experiments. We further present a methodology for establishing such control using near-infrared dyes to measure the oscillations with femtosecond pump-probe techniques. In the case of immiscible binary solvents, the same method allows us to investigate the liquid-liquid interface. Our control methodology is validated by an experiment using a cyanine dye dissolved in dimethyl sulfoxide, interfaced with neat diethyl-ether. The dye dynamics are retarded on moving from the bulk dye solution towards the interface with the neat diethyl-ether. When sampled along the direction of the vector pointing from the bulk towards the near interface, monotonically decreasing time constants are obtained. This result strongly suggests the importance of microheterogeneity in interfacial dynamics.