Monitoring Nonadiabatic Dynamics of the RNA Base Uracil by UV Pump-IR Probe Spectroscopy

Resolving the excited-state dynamics of DNA and RNA nucleobases has attracted considerable attention. UV irradiation of the isolated nucleobases leads to the population of an electronic excited state, which is quenched by internal conversion mediated by conical intersections on an ultrafast time scale. We present nonadiabatic on-the-fly molecular dynamics simulations of the UV pump-IR probe signal of the pyrimidine nucleobase uracil using a novel semiclassical protocol that takes into account the path integral over the excited-state vibrational dynamics and properly describes the joint temporal and spectral resolution of the technique. Simulations of vibrational motions of carbonyl fingerprint modes in the electronically excited states reveal clear signatures of different relaxation pathways on a time scale of hundreds of femtoseconds, which arise from an ultrafast branching in the excited state. We show that the inherent temporal and spectral resolution of the technique is not purely instrumental but also depends on the vibrational fluctuation time scale.