No abstract

Time-resolved photoelectron imaging has been utilized to probe the energetics and dynamics of the transient negative ion of the nucleobase uracil. This species was created through charge transfer from an iodide anion within a binary iodide-uracil complex using a UV pump pulse; the ensuing dynamics were followed by photodetachment with a near-IR probe pulse. The photoelectron spectra show two time-dependent features, one from probe-induced photodetachment of the transient anion state and another from very low energy electron signal attributed to autodetachment. The transient anion was observed to decay biexponentially with time constants of hundreds of femtoseconds and tens of picoseconds, depending on the excitation energy. These dynamics are interpreted in terms of autodetachment from the initially excited state and a second, longer-lived species relaxed by iodine loss. Hydrogen loss from the N1 position may also occur in parallel.T he observation that low-energy electrons can lead to DNA and RNA strand cleavage via temporary negative ion states 1 has motivated numerous studies of nucleic acid constituents. Gas-phase studies of DNA and RNA building blocks, including individual nucleobases, nucleosides, and nucleotides, have sought to provide insight into the mechanisms of this radiation damage. 2 Transient anion states of nucleobases have been posited to play a major role in DNA mutagenesis, perhaps via a charge-transfer process from an initially charged nucleobase moiety to a sugar-phosphate bond. 3,4 In this work, we explore the dynamics of transient anion states of uracil via time-resolved photoelectron (TRPE) imaging 5 of an iodide-uracil binary complex.The interaction of excess electrons with uracil and other nucleobases has been studied in the gas phase using low-energy electron scattering, 6,7 negative ion photoelectron spectroscopy (PES), 8−10 and Rydberg electron transfer (RET). 11 Total electron scattering cross section measurements showed structure below 2 eV associated with unoccupied π* orbitals of uracil, 6 while dissociative electron attachment (DEA) studies showed that hydrogen atom loss from the N1 position in the transient negative ion U* − occurs at collision energies as low as 0.7 eV. 6,7,12−14 The nature of the uracil anion has been directly probed in PES and RET studies. PES experiments have measured the binding energy of the dipole-bound species as ∼90 meV 8,9 and estimate that valence anions of uracil bind excess electrons by tens to hundreds of meV. 9−11 These species are clearly distinguishable in photoelectron (PE) spectra, as dipolebound states consist of narrow features with low electron binding energies, reflecting the similarity between the anion and neutral geometries, while valence-bound anions have characteristically broader features. 15 Only dipole-bound anions of uracil have been observed using conventional ion generation methods, 8−10 but the uracil anion can transform from a dipolebound state to a valence-bound state upon complexation with one Xe atom or water...

The energetics and dynamics of thymine and uracil transient negative ions were examined using femtosecond time-resolved photoelectron imaging. The vertical detachment energies (VDEs) of these systems were found to be 4.05 eV and 4.11 eV for iodide-thymine (I(-) x T) and iodide-uracil (I(-) x U) clusters, respectively. An ultraviolet pump pulse was used to promote intracluster charge transfer from iodide to the nucleobase. Subsequent electron detachment using an infrared probe pulse monitored the dynamics of the resulting transient negative ion. Photoelectron spectra reveal two primary features: a near-zero electron kinetic energy signal attributed to autodetachment and a transient feature representing photodetachment from the excited anion state. The transient state exhibits biexponential decay in both thymine and uracil complexes with short and long decay time constants ranging from 150-600 fs and 1-50 ps, respectively, depending on the excitation energy. However, both time constants are systematically shorter for I(-) x T. Vibrational autodetachment and iodine loss are identified as the primary decay mechanisms of the transient negative ions of thymine and uracil.

Electron attachment to uracil was investigated by applying time-resolved photoelectron imaging to iodide-uracil (I(-)U) complexes. In these studies, an ultraviolet pump pulse initiated charge transfer from the iodide to the uracil, and the resulting dynamics of the uracil temporary negative ion were probed. Five different excitation energies were used, 4.00 eV, 4.07 eV, 4.14 eV, 4.21 eV, and 4.66 eV. At the four lowest excitation energies, which lie near the vertical detachment energy of the I(-)U complex (4.11 eV), signatures of both the dipole bound (DB) as well as the valence bound (VB) anion of uracil were observed. In contrast, only the VB anion was observed at 4.66 eV, in agreement with previous experiments in this higher energy range. The early-time dynamics of both states were highly excitation energy dependent. The rise time of the DB anion signal was ∼250 fs at 4.00 eV and 4.07 eV, ∼120 fs at 4.14 eV and cross-correlation limited at 4.21 eV. The VB anion rise time also changed with excitation energy, ranging from 200 to 300 fs for excitation energies 4.00-4.21 eV, to a cross-correlation limited time at 4.66 eV. The results suggest that the DB state acts as a "doorway" state to the VB anion at 4.00-4.21 eV, while direct attachment to the VB anion occurs at 4.66 eV.

Decay dynamics of nascent dipole bound states of acetonitrile and nitromethane are examined using time-resolved photoelectron imaging of iodide-acetonitrile (I(-)·CH3CN) and iodide-nitromethane (I(-)·CH3NO2) complexes. Dipole-bound anions are created by UV-initiated electron transfer to the molecule of interest from the associated iodide ion at energies just below the vertical detachment energy of the halide-molecule complex. The acetonitrile anion is observed to decay biexponentially with time constants in the range of 4-900 ps. In contrast, the dipole bound state of nitromethane decays rapidly over 400 fs to form the valence bound anion. The nitromethane valence anion species then decays biexponentially with time constants of 2 ps and 1200 ps. The biexponential decay dynamics in acetonitrile are interpreted as iodine atom loss and autodetachment from the excited dipole-bound anion, followed by slower autodetachment of the relaxed metastable ion, while the dynamics of the nitromethane system suggest that a dipole-bound anion to valence anion transition proceeds via intramolecular vibrational energy redistribution to nitro group modes in the vicinity of the iodine atom.