Vertical excitation energies. Table lists vertical excitation energies calculated for both 7H and 9H tautomers of 2,6-dAP at the SCS-ADC(2) level of theory. In the case of these tautomers, the lowest energy excitation is dominated by the ππ* molecular orbital character and characterized by a relatively high oscillator strength. It is also worth adding that these optically bright ππ* excitations are energetically well separated from higher-energy excited states. This indicates that excitation of 2,6-dAP with low energy photons, e.g. near the 0-0 transition or lowest energy absorption maxima will likely entail photodeactivation mechanisms characteristic of ππ* excited states, e.g. ring puckering. Therefore, we anticipate that the photorelaxation of 2,6-dAP observed in our experimental setup is primarily associated with these excitations. According to our SCS-ADC(2) results, the S 1 (ππ*) state of 7H Both 7H and 9H tautomers are also characterized by low-lying S 2 (πσ* NH ) states for which the repulsive σ* orbital is centered around the N 7 -H or N 9 -H bond in the five-membered ring of 2,6-dAP. While these states could directly contribute to the photorelaxation of 2,6-dAP even at low excitation energies, photodissociation of N-H bonds is an ultrafast process occurring on the timescale of tens to hundreds of fs and, thus, is below the probe resolution of our experiments. Furthermore, in our experiment we only observed parent masses. The S 3 state of 7H 2,6-dAP is dominated by the nπ* electronic configuration, which is analogous to the nπ* state of 9H adenine. However, we expect this state to play a minor role in the photodynamics of 2,6-dAP excited near the 0-0 transition, owing to the excitation energy of this nπ* state being 5.19 eV. Similarly, the corresponding nπ* of the 9H tautomer of 2,6-dAP is even higher in energy (5.61 eV) and assigned to the S 4 state according to our calculations. Nevertheless, the nπ* states might play a role in the photodeactivation of 2,6-dAP at higher absorption energies, which were not considered in this work.