Unraveling the Relaxation Dynamics of Uracil: Insights from Time-Resolved X-ray Photoelectron Spectroscopy

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Abstract
We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase using time-resolved core-level photoelectron spectroscopy together with high-level calculations. The dynamics was investigated by trajectory surface hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics probed on the oxygen, nitrogen, and carbon sites by core electron spectroscopy. We find that the main de-excitation channel of the initially excited S2(ππ*) state involves internal conversion to the S1(nπ*) state with a time constant of 17 ± 4 fs, while a portion of S2(ππ*) population returns directly to the ground state by internal conversion. We find no evidence that the S1(nπ*) state decays to the ground state; instead, it decays to triplet states with a time constant of 1.6 ± 0.4 ps. Oscillations of the S1(nπ*) state O 1s intensity as a function of time correlate with those of calculated C4═O8 and C5═C6 bond lengths, which undergo a sudden expansion following the initial π → π* excitation. Our calculations support our interpretation of the data and provide detailed insight into the relaxation processes of uracil.
Funding Information
  • Novo Nordisk Fonden (NNF22OC0078009)
  • H2020 Marie Sklodowska-Curie Actions (101027796)
  • Ministero dell Universita e della Ricerca (2022K5W59T DEMIST CUP B53D2301378000, P20224AWLB HAPPY CUP B53D23025210001)
  • Hrvatska Zaklada za Znanost (HRZZ- IP-2022-10-4658, HRZZ-IP-2020-02-9932)

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