Conical intersection dynamics of pyrimidine nucleosides tracked with sub-20-fs UV pulses

When ultraviolet (UV) radiation is absorbed by DNA, the electronic energy acquired by the molecule is efficiently converted into vibrational energy on an ultrafast timescale, preventing photochemical reactions which might induce mutations or strand breaks. In those ultrafast processes, conical intersections (CIs) play a crucial role [1]. Over the past years many studies have been performed to shed light in the fate of photoexcitations in DNA, although most of them with limited (>100 fs) temporal resolution. In this work we combine transient absorption (TA) spectroscopy using sub-20 fs UV pump pulses [2, 3] and broad UV spectral coverage with TA spectroscopy simulations from first principles based on mixed quantum mechanics /molecular mechanics (QM/MM) framework at the dynamically correlated multiconfiguration wavefunction level (CASPT2) [4, 5] to study the ultrafast relaxation dynamics of pyrimidine nucleosides uridine (Urd) and 5-methyluridine (MTU)