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The excited-state dynamics of o-nitrophenol have been explored using trajectory surface hopping nonadiabatic dynamics combined with floating occupation molecular orbital complete active space configuration interaction. We focus on the effect of excitation energy on the subsequent dynamics. The absorption spectrum of o-nitrophenol has two peaks, centered at 3.9 eV (∼320 nm) and 5.1 eV (∼240 nm), and we performed dynamics starting from each of these peaks. The results show that even though the relaxation time constants are similar for the two excitation windows, the underlying dynamics are different. When exciting to the low energy peak, the dynamics are dominated by intramolecular proton transfer followed by internal conversion to the ground state, while exciting to the high-energy peak leads to fast internal conversion to the first excited state and slower decay to the ground state. In this case, intramolecular proton transfer does not occur as frequently, and many trajectories decay to the ground state through conical intersections without proton transfer. By calculating spin–orbit coupling values along the trajectories, we also show that intersystem crossing is possible. Based on the Landau–Zener probability formula, we estimate that there is about a 30%–40% probability that intersystem crossing will occur within 1 ps. 

Time-resolved photoionization measurements were performed on o-nitrophenol pumped with UV laser pulses at a central wavelength of 255 nm (4.9 eV) and probed with vacuum ultraviolet (VUV) pulses at 153 nm (8.1 eV). The photoelectron spectrum and time of flight mass spectrum for ions were recorded at each pump–probe delay. The measurements are interpreted with the aid of electronic structure calculations for both the neutral and ionic states. Evidence is found for the formation of a bicyclic intermediate followed by NO dissociation through a process of internal conversion and intersystem crossing. The combination of photoelectron and photoion spectroscopy, together with computational results, provides strong evidence of intersystem crossing that is difficult to establish with only a single technique.