UV wavelength-dependent photoionization quantum yields for the dark 1 nπ* state of aqueous thymidine.

Despite the important role of the dark 1 nπ* state in the photostability of thymidine in aqueous solution, no detailed ultraviolet (UV) wavelength-dependent investigation of the 1 nπ* quantum yield (QY) in aqueous thymidine has been experimentally performed. Here, we investigate the wavelength-dependent photoemission spectra of aqueous thymidine from 266.7 to 240 nm using liquid-microjet photoelectron spectroscopy. Two observed ionization channels are assigned to resonant ionizations from 1 ππ* to the cationic ground state D 0 (π -1 ) and 1 nπ* to the cationic excited state D 1 (n -1 ). The weak 1 nπ* → D 1 ionization channel appears due to ultrafast 1 ππ* → 1 nπ* internal conversion within the pulse duration of ∼180 fs. The obtained 1 nπ* quantum yields exhibit a strong wavelength dependence, ranging from 0 to 0.27 ± 0.01, suggesting a hitherto uncharacterized 1 nπ* feature. The corresponding vertical ionization energies (VIEs) of D 0 and D 1 of aqueous thymidine are experimentally determined to be 8.47 ± 0.12 eV and 9.22 ± 0.29 eV, respectively. Our UV wavelength-dependent QYs might indicate that different structural critical points to connect the multidimensional 1 ππ*/ 1 nπ* conical intersection seam onto the multidimensional potential energy surface of the 1 ππ* state might exist and determine the relaxation processes of aqueous thymidine upon UV excitation.