All calculations were performed using the ORCA 12 5.0.2 software package. Ground state structures were optimized using the Gaussian electronic structure program implementation of the B3LYP (B3LYP G ) functional and excited state computations used the PBE0 functional, all utilizing the def2-TZVPD basis set and D3BJ atom-pairwise dispersion correction to the DFT energy with Becke-Johnson damping. All structures were optimized without any geometrical or symmetry constraints and were verified as true minima on the potential energy surface via frequency analysis, ensuring the Hessian matrix of energy second derivatives were without imaginary eigenvalues at the same level of theory. As previously described elsewhere, accelerating two electron integral computations, the resolution of the identity (RIJ) approximation was used for the Coulomb portion, while the chain of spheres exchange (COSX) algorithm was used for the exchange part of the electron-electron interaction. For ground state optimizations, the RIJCOSX approximation was turned off for the last steps of optimization. To model solvation in aqueous solution, all calculations took advantage of the default LR-CPCM model as well as the Gaussian charge scheme, where the number of Lebedev points were increased to 302. Additionally, calculations for the 6-OH photohydrate of cytosine, 6-hydroxycytosine, included explicit solvation using 5 water molecules in addition to the implicit (CPCM) solvation modelling. Visualization of the output geometries and their associated simulated absorption spectra utilized Avogadro, fitting each electronic transition with a Gaussian shape having a peak width of 20 nm. To eliminate the formation of hydrated electrons due to two-photon ionization of the solvent, one photon irradiation conditions were used as previously described. Following excitation of 5mCyd at 267 nm, a broad transient species is observed across the entire probe window region (Figure ), with maxima at approximately 320 and 575 nm. During the time delays between -0.24 and 0.45 ps, a decrease and blue shift in amplitude in the UV region is observed with a maximum at ca. 350 nm, while there is no change observed in the visible region (Figure ). This negative amplitude ΔA signal that grows in at ca. 350 nm corresponds very well with the steady-state fluorescence emission and therefore, is assigned to stimulated emission. Finally, as shown in Figure , between 0.45 and 24.6 ps, a uniform decay is observed across the entire probe region. Similarly, for Cyd, a broad transient species is observed following excitation with maxima at ca. 320 and 600 nm (Figure ). Between time delays of -0.15 and 4.61 ps, a decrease in amplitude is observed across the entire transient absorption spectrum to a small-amplitude signal with a maximum at ca. 370 nm (Figure ). This small-amplitude signal fully decays by a time delay of ca. 150 ps (Figure ). The full broadband transient absorption data for 5mCyd and Cyd were satisfactorily fitted with a two and three-component sequential kinetic model, respectively. The global, average lifetimes are reported in Table . Evolution associated difference spectra (EADS) and representative kinetic traces extracted from the global and target analyses are shown in Figures and, respectively. Figure shows the transient absorption spectra of phosphate buffer solution at pH 7.4 following laser excitation at 267 nm under the same high photon density conditions used in Figure . As described previously by Krul et al. , under this higher photon density conditions, a coherent absorption band is observed at probe wavelengths below ca. 450 nm within the cross correlation of the pump and probe beams. This is followed simultaneously by the observation of the hydrated electron band resulting from the twophoton absorption of the water solvent. The correction of the transient data for 5mCyd and Cyd for the latter process is done straightforwardly using the methodology originally developed by Kohler and coworkers. However, a correction procedure to eliminate the coherent signal from the solvent is not straightforward because it also contains the absorption of the hydrated electron band that expands all the way to 320 nm. A correction of the transient absorption spectra for the two-photon hydrated electron band using the standard method, leads to an over correction of the transient signal in the spectral region from ca. 320 to 450 nm. This is the reason for why it is important to use low photon density conditions when exciting at 267 nm (as shown in Figure and S5 above), if the goal is to report the broadband data of a DNA/RNA monomer (or any other chromophore) all the way from the UV to the visible. This is also the reason for why the transient data shown in Figures and of the main paper are "masked" or not shown below the probe wavelength of ca. 450 nm.