Abstract
We have measured the ultrafast dephasing times of vibrational quanta in functional groups covalently bound to graphene. It has been previously shown that electronic relaxation in graphene occurs on subpicosecond timescales, but this is the first venture into the vibrational dephasing times in functionalized graphene. We have employed time-resolved sum-frequency generation spectroscopy to observe the decay of the coherent signal arising due to vibrational excitation by delaying the upconversion pulse with respect to the infrared (IR) laser pulse. We observe decay times of 0.8 and 1.4 ps for vibrational stretches in phenyl groups attached to graphene or gold, respectively, but for the vibrations of the bond covalently connecting H atoms to graphene in hydrogenated graphene, we measure much faster decay times of 0.3 ps. This is likely due to coupling between the C–H stretch and either the two-dimensional (2D) mode of graphene or nonadiabatic coupling to electrons photoexcited by the upconversion laser pulse.