Direct Observation of Long-Lived Vibrational Hot Ground States by Ultrafast Spectroscopy and Visible/Infrared Double Excitation Fluorescence^†

It is widely believed that the relaxation-induced bleaching signals at nanoseconds observed in ultrafast infrared spectroscopic measurements are due to the local heat effect resulting from the thermalization of the infrared excitations. In this work, combining ultrafast IR pump/probe, 2D-IR, visible pump/IR probe, and ultrafast visible/IR double resonant fluorescence experiments, the vibrational hot ground states of fluorescein dianion in methanol solutions are found to be unexpectedly long, at the time scale of nanoseconds. This result indicates that the long-standing bleaching signal observed in the nonlinear IR experiments must have significant contributions from these hot ground states for the initial couple of ns. It is likely that a similar mechanism can also hold for other molecular systems. The hot ground states can last much longer than conventionally expected, which can potentially be applied to modify chemical reactions.