Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source

Ghafur, Omair; Crane, Stuart W.; Ryszka, Michal; Bockova, Jana; Rebelo, Andre; Saalbach, Lisa; De Camillis, Simone; Greenwood, Jason B.; Eden, Samuel and Townsend, Dave (2018). Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source. The Journal of Chemical Physics, 149(3), article no. 034301.

DOI: https://doi.org/10.1063/1.5034419

Abstract

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility.

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About

  • Item ORO ID
  • 55834
  • Item Type
  • Journal Item
  • ISSN
  • 0021-9606
  • Project Funding Details
  • Funded Project NameProject IDFunding Body
    Irradiation of biomolecular clusters: model systems for the study of radiation damage in living materialEP/E039618/1EPSRC (Engineering and Physical Sciences Research Council)
    Electron attachment to biomolecular clusters: probing the role of multiple scattering in radio-sensitivity (SP-10-088-SE)EP/J002577/1EPSRC (Engineering and Physical Sciences Research Council)
    Absolute clustering effects on electron attachment (SP-13-004-SE)EP/L002191/1EPSRC (Engineering and Physical Sciences Research Council)
  • Academic Unit or School
  • Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
    Faculty of Science, Technology, Engineering and Mathematics (STEM)
  • Research Group
  • Physics
  • Depositing User
  • Sam Eden

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