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

Formulation of the cosmic ray-driven electron-induced reaction mechanism for quantitative understanding of global ozone depletion

This paper formulates the cosmic ray-driven electron-induced reaction as a universal mechanism to provide a quantitative understanding of global ozone depletion. Based on a proposed electrostatic bonding mechanism for charge-induced adsorption of molecules on surfaces and on the measured dissociative electron transfer (DET) cross sections of ozone-depleting substances (ODSs) adsorbed on ice, an analytical equation is derived to give atmospheric chlorine atom concentration: [Formula: see text] where Φe is the prehydrated electron (epre-) flux produced by cosmic ray ionization on atmospheric particle surfaces, [Formula: see text] is the surface coverage of an ODS, and ki is the ODS's effective DET coefficient that is the product of the DET cross section, the lifetimes of surface-trapped epre- and Cl-, and the particle surface area density. With concentrations of ODSs as the sole variable, our calculated results of time-series ozone depletion rates in global regions in the 1960s, 1980s, and 2000s show generally good agreement with observations, particularly with ground-based ozonesonde data and satellite-measured data over Antarctica and with satellite data in a narrow altitude band at 13 to 20 km of the tropics. Good agreements with satellite data in the Arctic and midlatitudes are also found. A previously unreported effect of denitrification on ozone loss is found and expressed quantitatively. But this equation overestimates tropospheric ozone loss at northern midlatitudes and the Arctic, likely due to increased ozone production by the halogen chemistry in polluted regions. The results render confidence in applying the equation to achieve a quantitative understanding of global ozone depletion.

Microscopic Insight into Electron-Induced Dissociation of Aromatic Molecules on Ice

We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the π^{*} orbitals of the halobenzenes from where they are transferred to σ^{*} orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules.

Mass spectrometry of aerosol particle analogues in molecular beam experiments

Nanometer-size particles such as ultrafine aerosol particles, ice nanoparticles, water nanodroplets, etc, play an important, however, not yet fully understood role in the atmospheric chemistry and physics. These species are often composed of water with admixture of other atmospherically relevant molecules. To mimic and investigate such particles in laboratory experiments, mixed water clusters with atmospherically relevant molecules can be generated in molecular beams and studied by various mass spectrometric methods. The present review demonstrates that such experiments can provide unprecedented details of reaction mechanisms, and detailed insight into the photon-, electron-, and ion-induced processes relevant to the atmospheric chemistry. After a brief outline of the molecular beam preparation, cluster properties, and ionization methods, we focus on the mixed clusters with various atmospheric molecules, such as hydrated sulfuric acid and nitric acid clusters, N_x O_y and halogen-containing molecules with water. A special attention is paid to their reactivity and solvent effects of water molecules on the observed processes.

Temperature-dependent Shape Changes of Ice Nanoclusters on Ag(100)

We investigate the influence of the annealing temperature on the evolution of the ice nanoclusters' geometry by means of low-temperature scanning tunneling microscopy. The clusters, grown at 110 K on Ag(100), gradually increase in height and their shape becomes more compact during annealing at 120 K, 125 K, and 130 K. The increase in height indicates an upward mass transport and reflects a stronger water-water than water-surface bonding. The change in shape, quantitatively expressed as an increase in fractal dimension, is driven by a reduction of the total energy of the step edge. The significant changes in geometry induced by a relatively mild temperature increase underline the importance of temperature for the shape and all properties influenced by this shape of hydrogen-bonded clusters of water ice.

Mechanisms of Radiation-Induced Degradation of CFCl3 and CF2Cl2 in Noble-Gas Matrixes: An Evidence for "Hot" Ionic Channels in the Solid Phase

The X-ray-induced transformations of simple chlorofluorocarbons (CFCl₃ and CF₂Cl₂) in solid noble-gas matrixes (Ne, Ar, Kr, and Xe) at 7 K were studied in order to elucidate basic mechanisms of the radiation-chemical degradation with possible implications for stratospheric and extraterrestrial ice chemistry. The decomposition of parent molecules and formation of products were monitored by FTIR spectroscopy, and the identification was supported by ab initio calculations at the CCSD(T) level. It was shown that the ionic reaction channels were predominating in most cases (except for CF₂Cl₂/Xe system). The primary radical cations (CFCl₃^+• and CF₂Cl₂^+•) are either stabilized in matrixes or undergo fragmentation to yield the corresponding secondary cations (CFCl₂⁺, CCl₃⁺, CF₂Cl⁺) and halogen atoms. The probability of fragmentation through different channels demonstrates a remarkable matrix dependence, which was explained by the effect of excess energy resulting from the exothermic positive hole transfer from matrix atoms to freon molecules. A qualitative correlation between "hot" ionic fragmentation at low temperatures and gas-phase ion energetics was found. However, dissociative electron attachment leads to formation of neutral radicals (CFCl₂^• or CF₂Cl^•) and chloride anions. One more possible way of dissociative electron attachment in the case of CF₂Cl₂ is formation of CF₂^•• and Cl₂^-•. A general scheme of the radiation-induced processes is proposed.

The reaction of CF2Cl2 with gas-phase hydrated electrons

The reaction of dichlorodifluoromethane (CF2Cl2) with hydrated electrons (H2O)n(-) (n = 30-86) in the gas phase was studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The hydrated electron reacts with CF2Cl2, forming (H2O)mCl(-) with a rate constant of (8.6 ± 2.2) × 10(-10) cm(3) s(-1), corresponding to an efficiency of 57 ± 15%. The reaction enthalpy was determined using nanocalorimetry, revealing a strongly exothermic reaction with ΔHr(CF2Cl2, 298 K) = -208 ± 41 kJ mol(-1). The combination of the measured reaction enthalpy with thermochemical data from the condensed phase yields a C-Cl bond dissociation enthalpy (BDE) ΔHC-Cl(CF2Cl2, 298 K) = 355 ± 41 kJ mol(-1) that agrees within error limits with the predicted values from quantum chemical calculations and published BDEs.

Excitation and quenching mechanisms in the near-UV photodissociation of CH3Br and CH3Cl adsorbed on D2O or CH3OH on Cu(110)

Photochemical processes for CH3X (X = Cl, Br, I) adsorbed on top of thin films of D2O or CH3OH on a Cu(110) substrate is studied by time-of-flight mass spectrometry for a range of UV wavelengths (351-193 nm). Photodissociation via dissociative electron attachment by photoelectrons and by neutral photodissociation is identified and quantified based on the observed dynamics of the desorbing CH3 fragments. Photoelectron-driven dissociation of CH3X is found to be a maximum for monolayer quantities of the D2O or CH3OH on Cu(110), but with differing kinetic energy release on the two substrates. The dynamics of CH3Br and CH3Cl photodissociation qualitatively differ on CH3OH/Cu(110) as compared to D2O/Cu(110), which is ascribed to differing molecular structures for these systems. Evidence is presented for an efficient inter-molecular quenching mechanism for neutral photoexcitation of CH3Cl and CH3Br on the CH3OH/Cu(110) substrate.

Radiation Damage to DNA: The Indirect Effect of Low Energy Electrons

We report the effect of DNA hydration level on damage yields induced by soft X-rays and photo-emitted low energy electrons (LEEs) in thin films of plasmid DNA irradiated in N₂ at atmospheric pressure under different humidity levels. Contrary to a dilute solution of DNA, the number of H₂O molecules per nucleotide (Γ) in these films can be varied from Γ=2.5 to ~33, where Γ≤20 corresponds to layers of hydration and Γ=33 to an additional bulk-like water layer. Our results indicate that DNA damage induced by LEEs does not increase significantly until the second hydration shell is formed. However, this damage increases dramatically as DNA coverage approaches bulk-like hydration conditions. A number of phenomena are invoked to account for these behaviors including: dissociative electron transfer from water-interface electron traps to DNA bases, quenching of dissociative electron attachment to DNA and quenching of dissociative electronically excited states of H₂O in contact with DNA.

Cisplatin enhances the formation of DNA single- and double-strand breaks by hydrated electrons and hydroxyl radicals

The synergistic interaction of cisplatin with ionizing radiation is the clinical rationale for the treatment of several cancers including head and neck, cervical and lung cancer. The underlying molecular mechanism of the synergy has not yet been identified, although both DNA damage and repair processes are likely involved. Here, we investigate the indirect effect of γ rays on strand break formation in a supercoiled plasmid DNA (pGEM-3Zf-) covalently modified by cisplatin. The yields of single- and double-strand breaks were determined by irradiation of DNA and cisplatin/DNA samples with (60)Co γ rays under four different scavenging conditions to examine the involvement of hydrated electrons and hydroxyl radicals in inducing the DNA damage. At 5 mM tris in an N2 atmosphere, the presence of an average of two cisplatins per plasmid increased the yields of single- and double-strand breaks by factors of 1.9 and 2.2, respectively, relative to the irradiated unmodified DNA samples. Given that each plasmid of 3,200 base pairs contained an average of two cisplatins, this represents an increase in radiosensitivity of 3,200-fold on a per base pair basis. When hydrated electrons were scavenged by saturating the samples with N2O, these enhancement factors decreased to 1.5 and 1.2, respectively, for single- and double-strand breaks. When hydroxyl radicals were scavenged using 200 mM tris, the respective enhancement factors were 1.2 and 1.6 for single- and double-strand breaks, respectively. Furthermore, no enhancement in DNA damage by cisplatin was observed after scavenging both hydroxyl radicals and hydrated electrons. These findings show that hydrated electrons can induce both single- and double-strand breaks in the platinated DNA, but not in unmodified DNA. In addition, cisplatin modification is clearly an extremely efficient means of increasing the formation of both single- and double-strand breaks by the hydrated electrons and hydroxyl radicals created by ionizing radiation.

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.

First-principles study of fluoroform adsorption on a hexagonal ice (0001) surface: weak hydrogen bonds-strong structural effects

For isolated fluoroform (F(3)CH) molecules adsorbed on a hexagonal ice (0001) surface the properties of blue- and red-shifting hydrogen bonds were studied using static density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (CP-MD) simulations. A systematic search by starting from many initial configurations was performed to determine the lowest-energy structures of F(3)CH on the ice surface, and for the optimized geometries the vibrational frequencies were calculated. The local minima structures are analyzed in terms of their coordination to the surface, with special focus on identifying blue-shifting hydrogen bonds via their spectroscopic signature of an increased frequency of the C-H fundamental stretching vibration. Subsequently, by CP-MD simulations the stability of the lowest-energy configurations at finite temperatures was verified and possible transformation pathways connecting the local minima structures were explored.

Bond breaks of nucleotides by dissociative electron transfer of nonequilibrium prehydrated electrons: a new molecular mechanism for reductive DNA damage

DNA damage is a central mechanism in the pathogenesis and treatment of human diseases, notably cancer. Little is known about reductive DNA damage in causing genetic mutations during oncogenesis and killing cancer cells during radiotherapy. The prehydrated electron (e(-)(pre)) has the highest yield among all the radicals generated in cells during ionizing radiation and has subpicosecond lifetimes (10(-13) s) and energies below 0 eV, but its role in DNA damage is unknown. In this work, our real-time measurements by femtosecond time-resolved laser spectroscopy have revealed that while adenine and cytosine can effectively trap an e(-)(pre) to form stable anions, thymidine and especially guanine are highly susceptible to dissociative electron transfer of e(-)(pre), leading to bond dissociation in DNA. Our finding demonstrates a dissociative electron transfer pathway for reductive DNA damage that might be related to various diseases such as cancer and stroke. Moreover, this finding challenges the conventional notion that damage to the genome is mainly induced by the oxidizing OH* radical and might eventually lead to improved radiotherapy of cancer and radioprotection of humans.

Correlation between cosmic rays and ozone depletion

This Letter reports reliable satellite data in the period of 1980-2007 covering two full 11-yr cosmic ray (CR) cycles, clearly showing the correlation between CRs and ozone depletion, especially the polar ozone loss (hole) over Antarctica. The results provide strong evidence of the physical mechanism that the CR-driven electron-induced reaction of halogenated molecules plays the dominant role in causing the ozone hole. Moreover, this mechanism predicts one of the severest ozone losses in 2008-2009 and probably another large hole around 2019-2020, according to the 11-yr CR cycle.

Photoelectric emission from the alkali metal doped vacuum-ice interface

The photoelectron photoemission spectra and thresholds for low coverages of Li and K adsorbed on water-ice have been measured, compared with photoionization spectra of the gas-phase atoms, and modeled by quantum chemical calculations. For both alkali metals the threshold for photoemission is dramatically decreased and the cross section increased on adsorption to the water-ice surface. Quantum chemical calculations suggest that the initial state is formed by the metal atoms adsorbed into the water-ice surface, forming a state with a delocalized electron distribution. This state is metastable and decays on the hundreds of seconds time scale at 92 K. The decay is markedly faster for Li than for K, probably due to diffusion into the ice film.

Direct observation of the transition state of ultrafast electron transfer reaction of a radiosensitizing drug bromodeoxyuridine

Replacement of thymidine in DNA by bromodeoxyuridine (BrdU) has long been known to enhance DNA damage and cell death induced by ionizing/UV radiation, but the mechanism of action of BrdU at the molecular level is poor understood. Using time-resolved femtosecond laser spectroscopy, we obtain the real-time observation of the transition state of the ultrafast electron transfer (ET) reaction of BrdU with the precursor to the hydrated electron, which is a general product in ionizing/UV radiation. The results show that the ET reaction is completed within 0.2 picosecond (ps) after the electronic excitation, leading to the formation of a transition state BrdU*- with a lifetime of approximately 1.5 ps that then dissociates into Br- and a high reactive radical dU*. The present results can greatly enhance our understanding not only of the mechanism of BrdU as a radio-/photosensitizer but of the role of prehydrated electrons in electron-initiated processes in biological and environmental systems.

Reaction dynamics following electron capture of chlorofluorocarbon adsorbed on water cluster: A direct density functional theory molecular dynamics study

The electron capture dynamics of halocarbon and its water complex have been investigated by means of the full dimensional direct density functional theory molecular dynamics method in order to shed light on the mechanism of electron capture of a halocarbon adsorbed on the ice surface. The CF(2)Cl(2) molecule and a cyclic water trimer (H(2)O)(3) were used as halocarbon and water cluster, respectively. The dynamics calculation of CF(2)Cl(2) showed that both C-Cl bonds are largely elongated after the electron capture, while one of the Cl atoms is dissociated from CF(2)Cl(2) (-) as a Cl(-) ion. Almost all total available energy was transferred into the internal modes of the parent CF(2)Cl radical on the product state, while the relative translational energy of Cl(-) was significantly low due to the elongation of two C-Cl bonds. In the case of a halocarbon-water cluster system, the geometry optimization of neutral complex CF(2)Cl(2)(H(2)O)(3) showed that one of the Cl atoms interacts with n orbital of water molecules of trimer and the other Cl atom existed as a dangling Cl atom. After the electron capture, only one C-Cl bond (dangling Cl atom) was rapidly elongated, whereas the other C-Cl bond is silent during the reaction. The dangling Cl atom was directly dissociated from CF(2)Cl(2) (-)(H(2)O)(3) as Cl(-). The fast Cl(-) ion was generated from CF(2)Cl(2) (-)(H(2)O)(3) on the water cluster. The mechanism of the electron capture of halocarbon on water ice was discussed on the basis of the theoretical results.

Products and Reaction Sequences in Tetrahydrofuran Exposed to Low-Energy Electrons

Electron-stimulated reactions in solid films of tetrahydrofuran (THF), condensed on Kr spacers deposited on a Pt substrate, or directly onto the substrate, were induced and monitored simultaneously with use of high-resolution electron-energy-loss spectroscopy in the ranges of vibrational and electronic excitations. The spectra of the molecular films obtained after a certain time of exposure to electrons at incident energies of 14 and 15.5 eV were analyzed and different products were identified. Besides an aldehyde, which is the main product, olefins, conjugated olefins, as well as CO were identified. Closer investigation of the reactions of propionaldehyde, as a model aldehyde, demonstrates that CO appears in THF as a secondary product (i.e., from the intermediate aldehyde). On the basis of the cross sections for the formation of an aldehyde from THF, of CO from propionaldehyde, and for the loss of propionaldehyde under electron impact, the reaction sequences were evaluated with the help of a kinetic model. This analysis suggests that some CO could also be formed directly from THF (i.e., without involvement of an intermediate aldehyde).

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.