Liquid water radiolysis induced by secondary electrons generated from MeV-energy carbon ions.

Fast ion beams induce damage to deoxyribonucleic acid (DNA) by chemical products, including secondary electrons, produced from interaction with liquid water in living cells. However, the production process of these chemical products in the Bragg peak region used in particle therapy is not fully understood. To investigate this process, we conducted experiments to evaluate the radiolytic yields produced when a liquid water jet in vacuum is irradiated with MeV-energy carbon beams. We used secondary ion mass spectrometry to measure the products, such as hydronium cations (H3O+) and hydroxyl anions (OH-), produced along with ·OH radicals, which are significant inducers of DNA damage formation. In addition, we simulated the ionization process in liquid water by incident ions and secondary electrons using a Monte Carlo code for radiation transport. Our results showed that secondary electrons, rather than incident ions, are the primary cause of ionization in water. We found that the production yield of H3O+ or OH- was linked to the frequency of ionization by secondary electrons in water, with these electrons having energies between 10.9 and 550 eV. These electrons are responsible for ionizing the outer-shell electrons of water molecules. Finally, we present that the elementary processes contribute to advancing radiation biophysics and biochemistry, which study the formation mechanism of DNA damage.

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.

Structure determination of molecules in an alignment laser field by femtosecond photoelectron diffraction using an X-ray free-electron laser.

We have successfully determined the internuclear distance of I2 molecules in an alignment laser field by applying our molecular structure determination methodology to an I 2p X-ray photoelectron diffraction profile observed with femtosecond X-ray free electron laser pulses. Using this methodology, we have found that the internuclear distance of the sample I2 molecules in an alignment Nd:YAG laser field of 6 × 1011 W/cm2 is elongated by from 0.18 to 0.30 Å "in average" relatively to the equilibrium internuclear distance of 2.666 Å. Thus, the present experiment constitutes a critical step towards the goal of femtosecond imaging of chemical reactions and opens a new direction for the study of ultrafast chemical reaction in the gas phase.

Photoelectron diffraction from laser-aligned molecules with X-ray free-electron laser pulses.

We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I2 molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I2 molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.

Photon-trap spectroscopy of mass-selected ions in an ion trap: optical absorption and magneto-optical effects.

A novel experimental technique has been developed to observe a trace of optical absorption of free mass-selected ions. The technique combines a linear radio-frequency ion trap with a high-finesse optical cavity to perform cavity ring-down spectroscopy (photon-trap spectroscopy for generality), where the storage lifetime of photons in the cavity provides a sensitivity high enough to probe the trapped ions. Absorption spectra of the manganese ion Mn(+) are presented, showing hyperfine structures for the (7)P(2,3,4)<--(7)S(3) transitions in the ultraviolet range. Implementation of a solenoidal magnet allows us to observe the Zeeman splitting and the Faraday rotation as well.