First-principles simulation of an ejected electron produced by monochromatic deposition energy to water at the femtosecond order.

This study uses a time-dependent first-principles simulation code to investigate the transient dynamics of an ejected electron produced in the monochromatic deposition energy from 11 to 19 eV in water. The energy deposition forms a three-body single spur comprising a hydroxyl radical (OH˙), hydronium ion (H3O+), and hydrated electron (eaq-). The earliest formation involves electron thermalization and delocalization dominated by the molecular excitation of water. Our simulation results show that the transient electron dynamics primarily depends on the amount of deposition energy to water; the thermalization time varies from 200 to 500 fs, and the delocalization varies from 3 to 10 nm in this energy range. These features are crucial for determining the earliest single-spur formation and facilitating a sequential simulation from an energy deposition to a chemical reaction in water photolysis or radiolysis. The spur radius obtained from the simulation correlates reasonably with the experimental-based estimations. Our results should provide universalistic insights for analysing ultrafast phenomena dominated by the molecular excitation of water in the femtosecond order.

Initial yield of hydrated electron production from water radiolysis based on first-principles calculation.

Many scientific insights into water radiolysis have been applied for developing life science, including radiation-induced phenomena, such as DNA damage and mutation induction or carcinogenesis. However, the generation mechanism of free radicals due to radiolysis remains to be fully understood. Consequently, we have encountered a crucial problem in that the initial yields connecting radiation physics to chemistry must be parameterized. We have been challenged in the development of a simulation tool that can unravel the initial free radical yields, from physical interaction by radiation. The presented code enables the first-principles calculation of low energy secondary electrons resulting from the ionization, in which the secondary electron dynamics are simulated while considering dominant collision and polarization effects in water. In this study, using this code, we predicted the yield ratio between ionization and electronic excitation from a delocalization distribution of secondary electrons. The simulation result presented a theoretical initial yield of hydrated electrons. In radiation physics, the initial yield predicted from parameter analysis of radiolysis experiments in radiation chemistry was successfully reproduced. Our simulation code helps realize a reasonable spatiotemporal connection from radiation physics to chemistry, which would contribute to providing new scientific insights for precise understanding of underlying mechanisms of DNA damage induction.

Fast Heavy-Ion-Induced Anion-Molecule Reactions on the Methanol Droplet Surface.

To gain insight into complex ion-molecule reactions induced by MeV-energy heavy ion irradiation of condensed matter, we performed a mass spectrometric study of secondary ions emitted from methanol microdroplet surfaces by 2.0 MeV C2+. We observed positive and negative secondary ions, including fragments, clusters, and reaction products. We found that a wider variety of negative ions than positive ions (such as C2H-, C2HO-, C2H5O-, and C2H3O2-) were formed. We performed measurements for deuterated methanol (CH3OD) droplets to identify the hydrogen elimination site of the intermediates involved in the reactions and to reveal the mechanism that generates various negative reaction product ions. Comparing the results of CH3OD with CH3OH droplets, we propose that the primary formation mechanism is association reactions of anion and neutral fragments, such as CH3O- + CO → C2H3O2-. Quantum chemical calculations confirmed that the reactions can proceed with no barrier. This study provides new insights into the importance of rapid anion-molecule reactions among fragments as the mechanism that generates complex molecular species in fast heavy-ion-induced reactions.

Secondary electron-induced biomolecular fragmentation in fast heavy-ion irradiation of microdroplets of glycine solution.

The influence of secondary electrons on radiation damage of biomolecules in water was studied by fast heavy-ion irradiation of biomolecular solutions. Water microdroplets containing the amino acid glycine under vacuum were irradiated by fast carbon projectiles with energies of 0.8-8.0 MeV. A variety of fragments from the droplets were observed by time-of-flight secondary-ion mass spectrometry: methylene amine cation and formate anion originating from the cleavage of C-Cα bonds, cyanide anion generated by cleavage of multiple bonds, and protonated and deprotonated glycine. The dependence of the yield of each fragment on projectile energy was examined; different behavior was observed for positive and negative fragments. Considering that biomolecular fragmentation may be induced by secondary electrons ejected from the water molecules surrounding biomolecules, we calculated the cross section for ejection of secondary electrons from liquid water. We found that the formation of both positive and negative glycine fragment ions correlated with the predicted emission of secondary electrons at different projectile energies. The formation of [Gly-H]- fragments, typical for gas phase dissociative electron attachment to amino acids, is shown to be caused by electrons from the low-energy part of the secondary electron distribution.

Dissociation of biomolecules in liquid environments during fast heavy-ion irradiation.

The effect of aqueous environment on fast heavy-ion radiation damage of biomolecules was studied by comparative experiments using liquid- and gas-phase amino acid targets. Three types of amino acids with different chemical structures were used: glycine, proline, and hydroxyproline. Ion-induced reaction products were analyzed by time-of-flight secondary-ion mass spectrometry. The results showed that fragments from the amino acids resulting from the C-Cα bond cleavage were the major products for both types of targets. For liquid-phase targets, specific products originating from chemical reactions in solutions were observed. Interestingly, multiple dissociated atomic fragments were negligible for the liquid-phase targets. We found that the ratio of multifragment to total fragment ion yields was approximately half of that for gas-phase targets. This finding agreed with the results of other studies on biomolecular cluster targets. It is concluded that the suppression of molecular multifragmentation is caused by the energy dispersion to numerous water molecules surrounding the biomolecular solutes.