Substrate dependence of electron‐stimulated O−yields from dissociative electron attachment to physisorbed O2

Oxidation-Induced Oxide Shell Rupture and Phase Separation in Eutectic Gallium-Indium Nanoparticles

Eutectic gallium-indium (EGaIn), a room-temperature liquid metal, has garnered significant attention for its applications in soft electronics, multifunctional materials, energy engineering and drug delivery. A key factor influencing these diverse applications is the spontaneous formation of a native passivating oxide shell that not only encapsulates the liquid metal but also alters the properties from the bulk counterpart. Using environmental scanning transmission electron microscopy, we report in situ observations of the oxidation of EGaIn nanoparticles by ambient air under high-energy electron beam irradiation. Our findings demonstrate that uneven oxide shell growth, driven by inward diffusion of adsorbed O species, creates unbalanced stresses. This compels the liquid metal to deform toward regions with slower oxide growth, resulting in shell rupture and allowing the liquid metal core to flow out. This process initiates top-down self-similar replication of the core-shell liquid metal nanoparticles, causing larger particles to break down into smaller particles. Additionally, internal oxidation triggers phase separation within the liquid core, ultimately leading to the pulverization of the liquid metal into finer solid particles rich in indium. These mechanistic insights into the oxidation behavior of the liquid metal hold practical implications for leveraging this process to reconfigure EGaIn nanoparticles for various applications.

Electron stimulated desorption from condensed pyrimidine and pyridazine

Low energy electron (LEE) interactions and the formation of transient negative ions play a dominant role in radiation-induced dissociation of condensed-phase biomolecules (e.g. in radiotherapy). Here we present data on the LEE-induced dissociation and desorption of the DNA/RNA-base and radiosensitizing agent analogues pyrimidine and pyridazine. Vapors of each molecule were condensed on either a Pt or Ar substrate to form a multilayer film or a submonolayer molecular target, respectively. These were irradiated with electrons of 0-80 eV and the desorbing anionic and cationic fragments analysed via time of flight mass spectrometry. The detected cations are the same species seen in gas-phase mass spectra, albeit of differing relative intensity. Anion yield functions exhibit strong maxima, indicating that transient negative ions contribute significantly, via dissociative electron attachment (DEA), to molecular dissociation below 20 eV. For both molecules, the <5 eV shape resonances, seen experimentally and predicted by theory, do not result in fragment desorption. The main anionic fragments are H^- and CN^- for both molecules, additionally the fragments C^-, CH^- C₂H^- and CHN^- desorb from pyrimidine and C^- and C₂H^- from pyridazine, with some resonances lying above the ionization limit. Pyrimidine shows higher anion desorption yields than pyridazine for all species except H^-. The anion signal also comprises dipolar dissociation (DD), investigated in both anionic and cationic yield functions. From analysis of anion and cation yields, fragmentation pathways are suggested. The direct ionization pathway provides information on the appearance energies for cations and their production processes in condensed phase.

O(-) from amorphous and crystalline CO2 ices

O(-) desorbed from amorphous and crystalline films of CO2 at 18 K under low energy electron impact is studied using time of flight mass spectrometry. The nature of the CO2 film is characterized by Fourier transform infrared spectrometry as a function of film thickness. It is found that the desorption rate from amorphous films is considerably larger than that from crystalline films. The desorption signal from the 4 eV resonance is found to be the dominant one as compared to that from the higher energy resonances, notably the one at 8 eV observed in the gas phase. This is explained in terms of the large enhancement in the dissociative electron attachment cross section for the 4 eV resonance in the condensed phase reported earlier using the charge trapping method.

Direct observation of ultrafast-electron-transfer reactions unravels high effectiveness of reductive DNA damage

Both water and electron-transfer reactions play important roles in chemistry, physics, biology, and the environment. Oxidative DNA damage is a well-known mechanism, whereas the relative role of reductive DNA damage is unknown. The prehydrated electron (e(pre)-), a novel species of electrons in water, is a fascinating species due to its fundamental importance in chemistry, biology, and the environment. e(pre)- is an ideal agent to observe reductive DNA damage. Here, we report both the first in situ femtosecond time-resolved laser spectroscopy measurements of ultrafast-electron-transfer (UET) reactions of e(pre)- with various scavengers (KNO(3), isopropanol, and dimethyl sulfoxide) and the first gel electrophoresis measurements of DNA strand breaks induced by e(pre)- and OH(•) radicals co-produced by two-UV-photon photolysis of water. We strikingly found that the yield of reductive DNA strand breaks induced by each e(pre)- is twice the yield of oxidative DNA strand breaks induced by each OH(•) radical. Our results not only unravel the long-standing mystery about the relative role of radicals in inducing DNA damage under ionizing radiation, but also challenge the conventional notion that oxidative damage is the main pathway for DNA damage. The results also show the potential of femtomedicine as a new transdisciplinary frontier and the broad significance of UET reactions of e(pre)- in many processes in chemistry, physics, biology, and the environment.

Effect of morphology of thin DNA films on the electron stimulated desorption of anions

We present a comparison between the electron stimulated desorption (ESD) of anions from DNA samples prepared by lyophilization (an example of poorly organized or nonuniform films) and molecular self-assembly (well-ordered films). The lyophilization (or freeze- drying) method is perhaps the most frequently employed technique for forming DNA films for studies of low-energy electron (LEE) interactions leading to DNA damage; however, this technique usually produces nonuniform films with considerable clustering which may affect DNA configuration and enhance sample charging when the film is irradiated. Our results confirm the general validity of ESD measurements obtained with lyophilized samples, but also reveal limitations of lyophilization for LEE studies on DNA films. Specifically we observe some modulation of structures, associated with dissociative electron attachment, in the anion yield functions from different types of DNA film, confirming that conformational factors play a role in the LEE induced damage to DNA.

Electron induced dissociation in condensed-phase nitromethane I: desorption of ionic fragments

Low energy electron induced dissociation of condensed nitromethane was investigated by measuring the electron stimulated desorption of anions and cations from multilayer films of CH(3)NO(2) and CD(3)NO(2), using a recently constructed, high sensitivity time of flight mass spectrometer. The desorbed yields were measured as a function of incident electron energy in the range between 1 to 20 eV and as function of coverage on Pt and Xe substrates. In anion desorption experiments, the following ions were observed: H(-) (D(-)), O(-), OH(-) (OD(-)), CN(-), NCO(-), NO(2)(-), CHNO(2)(-) (CDNO(2)(-)), CH(2)NO(2)(-) (CD(2)NO(2)(-)). Resonant structure seen in all anion yield functions, is attributed to dissociative electron attachment (DEA), though certain anion signals [e.g., OH(-) (OD(-)) and CH(2)NO(2)(-) (CD(2)NO(2)(-))] are likely the result of reactive scattering by O(-) ions. The dominant desorbed cation signals are CD(3)(+) and NO(+), and the appearance potentials of these species were measured to be 12.2 and 11.5 eV, respectively. The present measurements provide information on how the electron-induced dissociation processes of this proto-typical explosive molecule are modulated by the condensed environment and on how initial dissociation events occurring on a particular molecule, may induce further dissociation.

Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene

The electron stimulated desorption (ESD) of anions is used to explore the effects of electron irradiation on a thiophene film and we report measurements for electron impact on multilayer thiophene condensed on a polycrystalline platinum substrate. Below 22 eV and at low electron dose, desorbed anions include H- (the dominant signal) as well as S-, CH2-, SH- and SCH2-. Yield functions show that anions are desorbed both by dissociative electron attachment (DEA) with resonances observed at 9.5, 11, and 16 eV, and for energies >13 eV, by dipolar dissociation (DD). An increase in the S- signal from electron irradiated (beam-damaged) thiophene films and the appearance of a new DEA resonance in the S- yield function at 6 eV are linked to rupture of the thiophene ring and the formation of sulfur-terminated products within the film. The threshold energy for ring rupture is 5 eV. The desorption of new anions such as C4H3S- (Thiophene-H)- is also observed from electron irradiated films and these likely arise from the decomposition of large radiation product molecules synthesized in the film. The yield functions of H-, S-, SH-, (Thiophene-H)-, and (Thiophene+H)- anions from irradiated thiophene films that have been annealed to 300 K, each exhibit a single resonant feature centered around 5.1 eV, suggesting that all signals derive from DEA to the same molecular radiation product. In contrast, only H- and S- are observed to desorb from films of 2-2-bithiophene and no resonance is seen below approximately 10 eV in the anion yield functions. These data suggest that electron irradiation causes formation of ring-opened oligomers, and that closed-ring or 'classical" oligomers, (similar to bithiophene) if formed, contribute little to the ESD of anions.

Low energy electron and O– reactions in films of O2 coadsorbed with benzene or toluene

A detailed understanding of nascent reactive events leading to DNA damage is required to describe ionizing radiation effects on living cells. These early, sub-picosecond events involve mainly low energy (E < 20 eV) secondary electrons (SE), and low energy (E < 5 eV) secondary ion (and neutral) fragments; the latter are created either by the primary radiation, or by SE via dissociative electron attachment (DEA). While recent work has shown that SE initiate DNA strand break formation via DEA, the subsequent damage induced by the DEA ion fragments in DNA, or its basic components is unknown. Here, we report 0-20 eV electron impact measurements of anion desorption from condensed films containing O2 and either benzene (C6H6), or toluene (C6H5CH3); these molecules represent the most fundamental structural analogs of pyrimidine bases. Our experiments show that all of the observed OH- yields are the result of reactive scattering of 1-5 eV O- fragments produced initially by DEA to O2. These O- reactions involve hydrogen abstraction from benzene or toluene, and result in the formation of benzyl radicals, or toluene radicals centered on either the ring or exocyclic methyl group. O- scatters over nm distances comparable to DNA dimensions, and reactions involve a transient anion collision complex. Anion desorption is found to depend on both, the temperature of hydrocarbon film formation (morphology), and the order of overlayer adsorption, e.g. O2 on benzene, or benzene on O2. Our measurements support the notion that in irradiated DNA similar secondary-ion reactions can be initiated by the abundant secondary electrons, and may lead to clustered damage.

Intrinsic and extrinsic factors in anion electron-stimulated desorption: D− from deuterated hydrocarbons condensed on Kr and water ice films

The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process.

Enhancements in dissociative electron attachment to CF4, chlorofluorocarbons and hydrochlorofluorocarbons adsorbed on H2O ice

We report that the absolute cross sections for dissociative attachment of approximately 0 eV electrons to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are strongly enhanced by the presence of H2O ice. The absolute cross sections for CFCl3, CHF2Cl, and CH3CF2Cl on water ice are measured to be approximately 8.9 x 10(-14), approximately 5.1 x 10(-15), and approximately 4.9 x 10(-15) cm2 at approximately 0 eV, respectively. The former value is about 1 order of magnitude higher than that in the gas phase, while the latter two are 3-4 orders higher. In contrast, the resonances at electron energies > or = 2.0 eV are strongly suppressed either for CFCs and HCFCs or for CF4 adsorbed on H2O ice. The cross-section enhancement is interpreted to be due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of CFCs or HCFCs followed by its dissociation. This study indicates that electron-induced dissociation is a significant process leading to CFC and HCFC fragmentation on ice surfaces.

Dissociative electron attachment to DNA

Electron-stimulated desorption of anions from thin films of linear and supercoiled DNA is investigated in the range 3-20 eV. Resonant structures are observed with maxima at 9.4+/-0.3, 9.2+/-0.3, and 9.2+/-0.3 eV, respectively, in the yield dependence of H-, O-, and OH- on the incident electron energy. Their formation is attributed to dissociative electron attachment.

Single, double, and multiple double strand breaks induced in DNA by 3-100 eV electrons

Nonthermal secondary electrons with initial kinetic energies below 100 eV are an abundant transient species created in irradiated cells and thermalize within picoseconds through successive multiple energy loss events. Here we show that below 15 eV such low-energy electrons induce single (SSB) and double (DSB) strand breaks in plasmid DNA exclusively via formation and decay of molecular resonances involving DNA components (base, sugar, hydration water, etc.). Furthermore, the strand break quantum yields (per incident electron) due to resonances occur with intensities similar to those that appear between 25 and 100 eV electron energy, where nonresonant mechanisms related to excitation/ionizations/dissociations are shown to dominate the yields, although with some contribution from multiple scattering electron energy loss events. We also present the first measurements of the electron energy dependence of multiple double strand breaks (MDSB) induced in DNA by electrons with energies below 100 eV. Unlike the SSB and DSB yields, which remain relatively constant above 25 eV, the MDSB yields show a strong monotonic increase above 30 eV, however with intensities at least 1 order of magnitude smaller than the combined SSB and DSB yields. The observation of MDSB above 30 eV is attributed to strand break clusters (nano-tracks) involving multiple successive interactions of one single electron at sites that are distant in primary sequence along the DNA double strand, but are in close contact; such regions exist in supercoiled DNA (as well as cellular DNA) where the double helix crosses itself or is in close proximity to another part of the same DNA molecule.

Nanoscopic aspects of radiobiological damage: Fragmentation induced by secondary low-energy electrons

Low-energy electrons (LEEs) are produced in large quantities in any type of material irradiated by high-energy particles. In biological media, these electrons can fragment molecules and lead to the formation of highly reactive radicals and ions. The results of recent experiments performed on biomolecular films bombarded with LEEs under ultra-high vacuum conditions are reviewed in the present article. The major type of experiments, which measure fragments produced in such films as a function of incident electron energy (0.1-45 eV), are briefly described. Examples of the results obtained from DNA films are summarized along with those obtained from the fragmentation of elementary components of the DNA molecule (i.e., thin solid films of H(2)O, DNA bases, sugar analogs, and oligonucleotides) and proteins. By comparing the results of these different experiments, it is possible to determine fundamental mechanisms that are involved in the dissociation of biomolecules and the production of single- and double-strand breaks in DNA, and to show that base damage is dependent on the nature of the bases and on their sequence context. Below 15 eV, electron resonances (i.e., the formation of transient anions) play a dominant role in the fragmentation of all biomolecules investigated. These transient anions fragment molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment.

Cross sections for low-energy (10-50 eV) electron damage to DNA

We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10-50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10(-17) to 3 x 10(-15) cm(2). The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4 x 10(-15) cm(2) and 12 to 14 nm, respectively, over the range 10-50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electron's inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Low-energy (5-40 eV) electron-stimulated desorption of anions from physisorbed DNA bases

We present the results of experiments on anion desorption from the physisorbed DNA bases adenine, thymine, guanine and cytosine induced by the impact of low-energy (5-40 eV) electrons. Electron bombardment of DNA base films induces ring fragmentation and desorption of H(-), O(-), OH(-), CN(-), OCN(- ) and CH(2)(-) anions through either single or complex multibond dissociation. We designate the variation of the yield of an anion with electron energy as the yield function. Below 15 eV incident electron energy, bond cleavage is controlled mainly by dissociative electron attachment. Above 15 eV, the portion of a yield function that increases linearly is attributed to nonresonant processes, such as dipolar dissociation. A resonant structure is superimposed on this signal around 20 eV in the anion yield functions. This structure implicates dissociative electron attachment and/or resonant decay of the transient anion into the dipolar dissociation channel, with a minimal contribution from multiple inelastic electron scattering. The yields of all desorbing anions clearly show that electron resonances contribute to the damage of all DNA bases bombarded with 5-40 eV electrons. Comparison of the ion yields indicates that adenine is the least sensitive base to slow electron attack. Electron-irradiated guanine films exhibit the largest yields of desorbed anions.

Resonant formation of DNA strand breaks by low-energy (3 to 20 eV) electrons

Most of the energy deposited in cells by ionizing radiation is channeled into the production of abundant free secondary electrons with ballistic energies between 1 and 20 electron volts. Here it is shown that reactions of such electrons, even at energies well below ionization thresholds, induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonances localized on the DNA's basic components. This finding presents a fundamental challenge to the traditional notion that genotoxic damage by secondary electrons can only occur at energies above the onset of ionization, or upon solvation when they become a slowly reacting chemical species.