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This roadmap reviews the new, highly interdisciplinary research field studying the behavior of condensed matter systems exposed to radiation. The Review highlights several recent advances in the field and provides a roadmap for the development of the field over the next decade. Condensed matter systems exposed to radiation can be inorganic, organic, or biological, finite or infinite, composed of different molecular species or materials, exist in different phases, and operate under different thermodynamic conditions. Many of the key phenomena related to the behavior of irradiated systems are very similar and can be understood based on the same fundamental theoretical principles and computational approaches. The multiscale nature of such phenomena requires the quantitative description of the radiation-induced effects occurring at different spatial and temporal scales, ranging from the atomic to the macroscopic, and the interlinks between such descriptions. The multiscale nature of the effects and the similarity of their manifestation in systems of different origins necessarily bring together different disciplines, such as physics, chemistry, biology, materials science, nanoscience, and biomedical research, demonstrating the numerous interlinks and commonalities between them. This research field is highly relevant to many novel and emerging technologies and medical applications.
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Trifluoroacetyl chloride, CF3COCl, is produced in the Earth's atmosphere by photooxidative degradation of hydrochlorofluorocarbons, and represents a potential source of highly reactive halogen radicals. Despite considerable insight into photochemistry of CF3COCl, its reactivity towards electrons has not been addressed so far. We investigate the electron ionization and attachment in isolated CF3COCl molecules and (CF3COCl)N, max. N ≥ 10, clusters using a molecular beam experiment in combination with quantum chemical calculations. The ionization of the molecule at 70 eV electron energy leads to strong fragmentation: weakening of the C-C bond yields the CF3+ and COCl+ ions, while the fission of the C-Cl bond produces the major CF3CO+ fragment ion. The cluster spectra are dominated by Mn·COCl+ and Mn·CF3CO+ ions (M = CF3COCl). The electron attachment at energies between 1.5 and 11 eV also leads to the dissociation of the molecule breaking either the C-Cl bond at low energies below 3 eV yielding mainly Cl- ions, or dissociating the C-C bond at higher energies above 4 eV leading mainly to CF3- ions. In the clusters, the intact Mn- ions are stabilized after electron attachment at low energies with contribution of Mn·Cl- fragment ions. At higher energies, the Mn·Cl- fragments dominate the spectra, and C-C bond dissociation occurs as well yielding Mn·CF3-. Interestingly, Mn·Cl2- ions appear in the spectra at higher energies. We briefly discuss possible atmospheric implications.
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Bond-breaking in CCl4via dissociative electron attachment (DEA) has been studied using a velocity map imaging (VMI) spectrometer. A number of effects related to the dissociation dynamics have been revealed. The near-zero eV s-wave electron attachment, which leads to the production of Cl- anions, is accompanied by a very efficient intramolecular vibrational redistribution. This is manifested by a small fraction of the excess energy being released in the form of the fragments' translation energy. A similar effect is observed for higher-lying electronic resonances with one exception: the resonance centered around 6.2 eV leads to the production of fast Cl2- fragments and their angular distribution is forward peaking. This behavior could not be explained with a single-electronic-state model in the axial recoil approximation and is most probably caused by bending dynamics initiated by a Jahn-Teller distortion of the transient anion. The CCl2- fragment has a reverse backward-peaking angular distribution, suggesting the presence of a long-distance electron hopping mechanism between the fragments.
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Details of electron-induced chemistry of methyl methacrylate (MMA) upon complexation are revealed by combining gas-phase 2D electron energy loss spectroscopy with electron attachment spectroscopy of isolated MMA and its clusters. We show that even though isolated MMA does not form stable parent anions, it efficiently thermalizes the incident electrons via intramolecular vibrational redistribution, leading to autodetachment of slow electrons. This autodetachment channel is reduced in clusters due to intermolecular energy transfer and stabilization of parent molecular anions. Bond breaking via dissociative electron attachment leads to an extensive range of anion products. The dominant OCH3- channel is accessible via core-excited resonances with threshold above 5 eV, despite the estimated thermodynamic threshold below 3 eV. This changes in clusters, where MnOCH3- anions are observed in a lower-lying resonance due to neutral dissociation of the 1(n, π*) state and electron self-scavenging. The present findings have implications for electron-induced chemistry in lithography with poly(methyl methacrylate).
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Irradiation-driven fragmentation and chemical transformations of molecular systems play a key role in nanofabrication processes where organometallic compounds break up due to the irradiation with focused particle beams. In this study, reactive molecular dynamics simulations have been performed to analyze the role of the molecular environment on the irradiation-induced fragmentation of molecular systems. As a case study, we consider the dissociative ionization of iron pentacarbonyl, Fe(CO)5, a widely used precursor molecule for focused electron beam-induced deposition. In connection to recent experiments, the irradiation-induced fragmentation dynamics of an isolated Fe(CO)5+ molecule is studied and compared with that of Fe(CO)5+ embedded into an argon cluster. The appearance energies of different fragments of isolated Fe(CO)5+ agree with the recent experimental data. For Fe(CO)5+ embedded into an argon cluster, the simulations reproduce the experimentally observed suppression of Fe(CO)5+ fragmentation and provide an atomistic-level understanding of this effect. Understanding irradiation-driven fragmentation patterns for molecular systems in environments facilitates the advancement of atomistic models of irradiation-induced chemistry processes involving complex molecular systems.
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C4F7N is a promising candidate for the replacement of sulfur hexafluoride as an insulating medium, and it is important to understand the chemical changes initiated in the molecule by collision with free electrons, specifically the formation of neutral fragments. The first step of neutral fragmentation is electronic excitation, yet neither the absorption spectrum in the vacuum ultraviolet (VUV) region nor the electron energy loss spectrum have previously been reported. Here, we experimentally probed the excited states by VUV photoabsorption spectroscopy and electron energy loss spectroscopy (EELS). We found that the distribution of states populated upon electron impact with low-energy electrons is significantly different from that following photoabsorption. This difference was confirmed and interpreted with ab initio modeling of both VUV and EELS spectra. We propose here a new computational protocol for the simulation of EELS spectra combining the Born approximation with approximate forms of correlated wave functions, which allows us to calculate the (usually very expensive) scattering cross sections at a cost similar to the calculation of oscillator strengths. Finally, we perform semi-classical non-adiabatic dynamics simulations to investigate the possible neutral fragments of the molecule formed through electron-induced neutral dissociation. We show that the product distribution is highly non-statistical.
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We report two-dimensional electron energy-loss spectra of CO_{2}. The high-resolution experiment reveals a counterintuitive fine structure at energy losses where CO_{2} states form a vibrational pseudocontinuum. Guided by the symmetry of the system, we constructed a four-dimensional nonlocal model for the vibronic dynamics involving two shape resonances (forming a Renner-Teller Π_{u} doublet at the equilibrium geometry) coupled to a virtual Σ_{g}^{+} state. The model elucidates the extremely non-Born-Oppenheimer dynamics of the coupled nuclear motion and explains the origin of the observed structures. It is a prototype of the vibronic coupling of metastable states in continuum.
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We report a combined experimental and theoretical investigation of electron-molecule interactions using pyrrole as a model system. Experimental two-dimensional electron energy loss spectra (EELS) encode information about the vibrational states of the molecule as well as the position and structure of electronic resonances. The calculations using complex-valued extensions of equation-of-motion coupled-cluster theory (based on non-Hermitian quantum mechanics) facilitate the assignment of all major EELS features. We confirm the two previously described π resonances at about 2.5 and 3.5 eV (the calculations place these two states at 2.92 and 3.53 eV vertically and 2.63 and 3.27 eV adiabatically). The calculations also predict a low-lying resonance at 0.46 eV, which has a mixed character-of a dipole-bound state and σ* type. This resonance becomes stabilized at one quanta of the NH excitation, giving rise to the sharp feature at 0.9 eV in the corresponding EELS. Calculations of Franck-Condon factors explain the observed variations in the vibrational excitation patterns. The ability of theory to describe EELS provides a concrete illustration of the utility of non-Hermitian quantum chemistry, which extends such important concepts as potential energy surfaces and molecular orbitals to states embedded in the continuum.
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It is well established that an isolated benzene radical anion is not electronically stable. In the present study, we experimentally show that electron attachment to benzene clusters leads to weak albeit unequivocal occurrence of a C6H6 - moiety. We propose here-based on electronic structure calculation-that this moiety actually corresponds to linear structures formed by the opening of the benzene ring via electron attachment. The cluster environment is essential in this process since it quenches the internal energy released upon ring opening, which in the gas phase leads to further dissociation of this anion.
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We probe resonances (transient anions) in nitrobenzene with the focus on the electron emission from these. Experimentally, we populate resonances in two ways: either by the impact of free electrons on the neutral molecule or by the photoexcitation of the bound molecular anion. These two excitation means lead to transient anions in different initial geometries. In both cases, the anions decay by electron emission and we record the electron spectra. Several types of emission are recognized, differing by the way in which the resulting molecule is vibrationally excited. In the excitation of specific vibrational modes, distinctly different modes are visible in electron collision and photodetachment experiments. The unspecific vibrational excitation, which leads to the emission of thermal electrons following the internal vibrational redistribution, shows similar features in both experiments. A model for the thermal emission based on a detailed balance principle agrees with the experimental findings very well. Finally, a similar behavior in the two experiments is also observed for a third type of electron emission, the vibrational autodetachment, which yields electrons with constant final energies over a broad range of excitation energies. The entrance channels for the vibrational autodetachment are examined in detail, and they point to a new mechanism involving a reverse valence to non-valence internal conversion.
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Pyruvic acid represents a key molecule in prebiotic chemistry and it has recently been proposed to be synthesized on interstellar ices. In order to probe the stability of pyruvic acid in the interstellar medium with respect to decomposition by slow electrons, we investigate the electron attachment to its homomolecular and heteromolecular clusters. Using mass spectrometry, we follow the changes in the fragmentation pattern and its dependence on the electron energy for various cluster sizes of pure and microhydrated pyruvic acid. The assignment of fragmentation reaction pathways is supported by ab initio calculations. The fragmentation degree dramatically decreases upon clustering. This decrease is even stronger in the heteromolecular clusters of pyruvic acid with water, where the non-dissociative attachment is by far the strongest channel. In the homomolecular clusters, the dissociative channel leading to dehydrogenation is active over a larger electron energy range than in the isolated molecules. To probe the role of the self-scavenging effects, we explore the excited states of pyruvic acid. This has been done both experimentally, by using electron energy loss spectroscopy, and theoretically, by photochemical calculations. Data on both optically-allowed and forbidden states allow for the explanation of processes emerging upon clustering.
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In this perspective, we review experiments with molecules picked up on large clusters in molecular beams with the focus on the processes in atmospheric and interstellar chemistry. First, we concentrate on the pickup itself, and we discuss the pickup cross sections. We measure the uptake of different atmospheric molecules on mixed nitric acid-water clusters and determine the accommodation coefficients relevant for aerosol formation in the Earth's atmosphere. Then the coagulation of the adsorbed molecules on the clusters is investigated. In the second part of this perspective, we review examples of different processes triggered by UV-photons or electrons in the clusters with embedded molecules. We start with the photodissociation of hydrogen halides and Freon CF2Cl2 on ice nanoparticles in connection with the polar stratospheric ozone depletion. Next, we mention reactions following the excitation and ionization of the molecules adsorbed on clusters. The first ionization-triggered reaction observed between two different molecules picked up on the cluster was the proton transfer between methanol and formic acid deposited on large argon clusters. Finally, negative ion reactions after slow electron attachment are illustrated by two examples: mixed nitric acid-water clusters, and hydrogen peroxide deposited on large ArN and (H2O)N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.
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Focused electron beam deposition often requires the use of purification techniques to increase the metal content of the respective deposit. One of the promising methods is adding H2O vapor as a reactive agent during the electron irradiation. However, various contrary effects of such addition have been reported depending on the experimental condition. We probe the elementary electron-induced processes that are operative in a heterogeneous system consisting of iron pentacarbonyl as an organometallic precursor and water. We use an electron beam of controlled energy that interacts with free mixed Fe(CO)5/H2O clusters. These mimic the heterogeneous system and, at the same time, allow direct mass spectrometric analysis of the reaction products. The anionic decomposition pathways are initiated by dissociative electron attachment (DEA), either to Fe(CO)5 or to H2O. The former one proceeds mainly at low electron energies (<3 eV). Comparison of nonhydrated and hydrated conditions reveals that the presence of water actually stabilizes the ligands against dissociation. The latter one proceeds at higher electron energies (>6 eV), where the DEA to H2O forms OH- in the first reaction step. This intermediate reacts with Fe(CO)5, leading to enhanced decomposition, with the desorption of up to three CO ligands. The present results demonstrate that the water action on Fe(CO)5 decomposition is sensitive to the involved electron energy range and depends on the hydration degree.
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Electronic resonances commonly decay via internal conversion to vibrationally hot anions and subsequent statistical electron emission. We observed vibrational structure in such an emission from the nitrobenzene anion, in both the 2D electron energy loss and 2D photoelectron spectroscopy of the neutral and anion, respectively. The emission peaks could be correlated with calculated nonadiabatic coupling elements for vibrational modes to the electronic continuum from a nonvalence dipole-bound state. This autodetachment mechanism via a dipole-bound state is likely to be a common feature in both electron and photoelectron spectroscopies.
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In a combined experimental and theoretical study we probe the transient anion states (resonances) in cyanogen. Experimentally, we utilize electron energy loss spectroscopy which reveals the resonance positions by monitoring the excitation functions for vibrationally inelastic electron scattering. Four resonances are visible in the spectra, centered around 0.36 eV, 4.1, 5.3 and 7.3 eV. Theoretically, we explore the resonant states by using the regularized analytical continuation method. A very good agreement with the experiment is obtained for low-lying resonances, however, the computational method becomes unstable for higher-lying states. The lowest shape resonance (2Πu) is independently explored by the complex adsorbing potential method. In the experiment, this resonance is manifested by a pronounced boomerang structure. We show that the naive picture of viewing NCCN as a pseudodihalogen and focusing only on the CC stretch is invalid.
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We probe the negative ion production upon the interaction of free electrons with gas-phase HNO3 and its mixed clusters with water. The electron-induced chemistry changes strongly with clustering, exhibiting significant electron energy dependence. For HNO3 hydrates, we identified three involved energy ranges with different behavior: low energies up to about 3.5 eV, an intermediate energy range around 6 eV, and a high energy range, approximately above 9 eV. The major difference is the degree to which the major gas-phase product, NO2-, is converted to NO3-. The latter is the dominant stratospheric anion. Its appearance due to the electron interaction with mixed HNO3/water ice particles thus strongly depends on the electron energy. We discuss the elementary processes and reaction pathways behind the anion conversion.
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Polar stratospheric clouds, which consist mainly of nitric acid containing ice particles, play a pivotal role in stratospheric chemistry. We investigate mixed nitric acid-water clusters (HNO3)m(H2O)n, m ≈ 1-6, n ≈ 1-15, in a laboratory molecular beam experiment using electron attachment and mass spectrometry and interpret our experiments using DFT calculations. The reactions are triggered by the attachment of free electrons (0-14 eV) which leads to subsequent intracluster ion-molecule reactions. In these reactions, the nitrate anion NO3- turns out to play the central role. This contradicts the electron attachment to the gas-phase HNO3 molecule, which leads almost exclusively to NO2-. The nitrate containing clusters are formed through at least three different reaction pathways and represent terminal product ions in the reaction cascade initiated by the electron attachment. Besides, the complex reaction pathways represent a new hitherto unrecognized source of atmospherically important OH and HONO molecules.
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Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH3Cl, CH3CH2CH2Cl, C6H5Cl, CH4, and C6H6) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters.
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We probe the separation of ligands from iron tetracarbonyl methyl acrylate (Fe(CO)4(C4H6O2) or Fe(CO)4MA) induced by the interaction with free electrons. The motivation comes from the possible use of this molecule as a nanofabrication precursor and from the corresponding need to understand its elementary reactions fundamental to the electron-induced deposition. We utilize two complementary electron collision setups and support the interpretation of data by quantum chemical calculations. This way, both the dissociative ionization and dissociative electron attachment fragmentation channels are characterized. Considerable differences in the degree of precursor fragmentation in these two channels are observed. Interesting differences also appear when this precursor is compared to structurally similar iron pentacarbonyl. The present findings shed light on the recent electron-induced chemistry of Fe(CO)4MA on a surface under ultrahigh vacuum.
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Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n â¼ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.