Articles you may be interested inElectron stimulated desorption of anions from native and brominated single stranded oligonucleotide trimers J. Chem. Phys. 136, 075101 (2012); 10.1063/1.3685587 Comparative study of electron stimulated positive-ion desorption from LiCl and 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene Intrinsic and extrinsic factors in anion electron-stimulated desorption: D − from deuterated hydrocarbons condensed on Kr and water ice filmsWe present measurements of low energy ͑0-38 eV͒ electron stimulated desorption of H Ϫ from room temperature thin films of pure thymine ͑T͒ and uracil ͑U͒, condensed on polycrystalline Pt, and describe in detail the experimental methods required for such studies. The nominal film thicknesses are estimated to range from 0.08 to 2.7 monolayers; sublimation of the films at 69°C ͑T͒ and 82°C ͑U͒ onto the room temperature Pt substrate leads to nonuniform film growth, i.e., volumetric clustering, particularly in the submonolayer regime. H Ϫ formation by electron impact occurs via dissociative electron attachment ͑DEA͒ to the molecules, and results in strong desorption peaks near 8.6 eV for either molecule, whereas above 12-13 eV nonresonant dipolar dissociation dominates the desorption yields. Comparison of the present condensed phase results with gas phase measurements suggests that the desorbing H Ϫ produced at the DEA peak are mainly the result of CH bond cleavage, while near the desorption threshold of about 5 eV NH bond cleavage via DEA may also contribute to the H Ϫ signal. The present measurements suggest that localized resonances involving DNA bases, leading to the formation of anions and their associated neutral radical moieties, contribute to the resonant signature observed recently in the strand break yields of double stranded DNA irradiated with 3-20 eV electrons.

Monte Carlo simulations were performed to calculate the temperature dependence of the primary yields (g-values) of the radical and molecular products of the radiolysis of pure, deaerated liquid water by low linear-energy-transfer (LET) radiation. The early energy deposition was approximated by considering short segments (∼100 μm) of 300-MeV proton tracks (corresponding to an average LET of ∼0.3 keV/μm). The subsequent nonhomogeneous chemical evolution of the reactive species formed in these tracks was simulated by using the independent reaction times approximation, which has previously been used successfully to model the radiolysis of liquid water at ambient temperature under various conditions. Our calculated g-values for the radiolytic species:  , OH, H, H2, and H2O2, are presented as a function of temperature over the range 25−300 °C. They show an increase in g( ), g(OH), and [g(H) + g(H2)] and a decrease in g(H2O2) with increasing temperature, in agreement with existing experimental data. The sensitivity of the results to the values of reaction rate constants and to the temperature dependence of electron thermalization distances (r th) was also investigated. It was found that the best agreement with experiment occurs when the distances of electron thermalization decrease with increasing temperature, a result that is at variance with the predictions of previous modeling studies. Such a decrease in r th as the temperature increases could be linked to an increase in the scattering cross sections of subexcitation electrons that would account for the corresponding decrease in the degree of structural order of water molecules. Our simulations also suggest that the variations of the g-values with temperature, and especially that of g(H2), are better described if we account for the screening of the Coulomb forces between the two in the bimolecular self-reaction of the hydrated electron. Finally, the time-dependent yields of and OH are presented as functions of temperature, in the range 10-12−10-6 s. It was found that the temporal variation of g( ) at elevated temperatures is sensitive to the temperature dependence of r th, suggesting that measurements of the decay of hydrated electrons as functions of time and temperature could, in turn, provide information on the thermalization of subexcitation electrons. The good overall accord of our calculated results with the experimental data available from the literature demonstrates that Monte Carlo simulation methods offer a most promising avenue at present to further develop our understanding of temperature effects in the radiolysis of liquid water.

A combination of time-dependent density functional theory and Born-Oppenheimer molecular dynamics methods is used to investigate fragmentation of doubly charged gas-phase uracil in collisions with 100 keV protons. The results are in good agreement with ion-ion coincidence measurements. Orbitals of similar energy and/or localized in similar bonds lead to very different fragmentation patterns, thus showing the importance of intramolecular chemical environment. In general, the observed fragments do not correspond to the energetically most favorable dissociation path, which is due to dynamical effects occurring in the first few femtoseconds after electron removal. High-frequency electromagnetic radiation, energetic ions, and electrons can induce chemical changes that are lethal for living systems. For this reason, such radiation sources are often used in cancer therapies, which aim at damaging the DNA of malignant cells. In this field, the use of swift highly charged ions is very promising due to their ability to deposit energy and induce cellular death in very localized areas of deep tumors (a consequence of the wellknown localization of the Bragg peak [1]). These ions can produce DNA damage either directly, through ionization and excitation [1,2], or indirectly, through chemical reactions with the species produced in the aqueous environment. The early stages of damage, which occur during the first few femtoseconds after irradiation and lead to fragmentation of the biomolecule, are far from being understood. To unravel the mechanisms at this early stage, physicists have performed numerous experiments in gas phase (see, e.g., [3][4][5]), in which swift charged ionic projectiles impinge on DNA or RNA bases, sugars, nucleosides, or even biomolecular clusters. In contrast with experiments performed in solution or directly on living systems, gas-phase experiments provide direct and precise information on single collision events. Thus they allow one to unambiguously identify fragmentation channels associated with a given biomolecule and not with the environment. Such detailed information can be achieved by combining techniques that are state of the art in gas-phase chemistry, e.g., high resolution mass spectrometry, and in collision physics, such as multicoincidence detection techniques that provide the correlation between different charged fragments as well as their relative kinetic energies and momenta.Fragmentation results from relaxation of the excess electronic energy associated with vacancies created in the different electronic shells of the molecule. Since, in these collisions, electrons can be removed from many of these shells, experiments cannot tell us how fragmentation depends (i) on the shape or energy of the molecular orbital (MO) in which the electron vacancies are created and (ii) on the intramolecular environment, i.e., on the neighboring functional groups. This information can only be obtained from ab initio molecular dynamics (MD) calculations such as those based on time-dependent density functional the...

The early stages of the Coulomb explosion of a doubly ionized water molecule immersed in liquid water are investigated with time-dependent density functional theory molecular dynamics (TD-DFT MD) simulations. Our aim is to verify that the double ionization of one target water molecule leads to the formation of atomic oxygen as a direct consequence of the Coulomb explosion of the molecule. To that end, we used TD-DFT MD simulations in which effective molecular orbitals are propagated in time. These molecular orbitals are constructed as a unitary transformation of maximally localized Wannier orbitals, and the ionization process was obtained by removing two electrons from the molecular orbitals with symmetry 1B(1), 3A(1), 1B(2) and 2A(1) in turn. We show that the doubly charged H(2)O(2+) molecule explodes into its three atomic fragments in less than 4 fs, which leads to the formation of one isolated oxygen atom whatever the ionized molecular orbital. This process is followed by the ultrafast transfer of an electron to the ionized molecule in the first femtosecond. A faster dissociation pattern can be observed when the electrons are removed from the molecular orbitals of the innermost shell. A Bader analysis of the charges carried by the molecules during the dissociation trajectories is also reported.