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Charge accommodation dynamics of cluster and molecular anions produced by photo-initiated intracluster charge transfer


Time-resolved photoelectron imaging spectroscopy is used to examine the dynamics of charge accommodation by solvent species and biomolecules upon photo-initiated intracluster charge transfer. Excitation of a charge transfer state of an iodide-complexed molecule or cluster with a UV pulse and subsequent interrogation by photodetachment with a lower energy probe enables detection of changes in photoelectron signals over hundreds of femtoseconds. Velocity map imaging detection permits simultaneous collection of electron kinetic energy (eKE) and photoelectron angular distributions that provide insight into the strength and structure of the association between the cluster or molecule and the excess electron.

Application of this methodology to iodide-containing clusters of small polar molecules such as water, methanol, and ethanol elucidates the stability and extent of intramolecular forces within a given cluster. In complexes of iodide with small solvent clusters (≤ 10 molecules), iodide is situated somewhat outside of the solvent network. Interaction of iodide-water clusters with a UV pulse to produce iodine and a free electron results in the partial solvation of the excess charge through hydrogen bonding interactions over hundreds of picoseconds before electron autodetachment. In contrast, methanol and ethanol cluster networks can only support the excess charge for tens of ps. Notably, stable bare water cluster anions have previously been measured with as few as two molecules, while upwards of seventy methanol molecules are necessary to stabilize an excess electron. Drawing an analogy between electron autodetachment and statistical unimolecular decay, an excited iodide-water cluster with a given number of water molecules might be expected to decay most rapidly given its significantly smaller density of states. The observation of the opposite pattern, as well as the similarity between iodide-methanol and -ethanol cluster anion lifetimes, suggests that energetics, rather than molecular structure, play a larger role in stabilizing an excess charge to autodetachment. Applying a thermionic emission model confirms this result.

The dynamics of charge accommodation are also examined for small biomolecules. Radiative damage to DNA caused by low energy electrons is thought to originate in the attachment of an electron to a nucleobase unit of a nucleotide in the DNA double helix. Previous experiments have examined binding motifs and fragmentation patterns of transient negative ions (TNIs) of nucleobases using Rydberg electron transfer from excited noble gas atoms or collision of the nucleobase with a beam of electrons of defined energy. Here, nascent TNIs of the nucleobase uracil are created by intracluster charge transfer from a complexed iodide ion and their decay examined with time-resolved photoelectron imaging. Anions created with several hundred meV of excess energy appear as valence anions and are observed to decay biexponentially with time constants of hundreds of fs and tens of ps by iodine atom loss and autodetachment. Repetition of these experiments with uracil molecules methylated at the N1, N3, or C5 positions results in a dramatic reduction of the longer time constant. The addition of the methyl group may hasten the intramolecular vibrational energy redistribution process preceding autodetachment.

Photoelectron spectroscopy of isolated nucleobase anions has measured only the dipole-bound state (DBS) of the anion consisting of an electron weakly associated with the molecular dipole moment and very delocalized over the molecular structure. Though the valence anion has not been directly measured, the DBS has been posited to serve as a `doorway' to the valence-bound state (VBS). Such a mechanism has also been proposed for nitromethane. In contrast, acetonitrile should only support a DB anion state. Examination of nascent acetonitrile and nitromethane anions excited near the vertical detachment energies of their corresponding iodide-molecule complexes indeed produces the DB acetonitrile anion, which then decays biexponentially with time constants of few and hundreds of ps by iodine atom loss and autodetachment. The nitromethane DB anion decays rapidly over hundreds of fs to form the valence anion, which decays biexponentially with time constants similar to those measured for the acetonitrile DB anion. This study marks the first direct observation of a transition from a dipole-bound anion to a valence anion and will inform future studies of iodide-nucleobase complexes.

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