Characterization of Neutral Radicals from a Dissociative Electron Attachment Process

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

Dissociative electron attachment to carbon tetrachloride probed by velocity map imaging

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.

Electron Energy Loss Processes in Methyl Methacrylate: Excitation and Bond Breaking

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 M_nOCH₃^- 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).

Excitation and fragmentation of the dielectric gas C4F7N: Electrons vs photons

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.

Processes Induced by Electrons at Sub-Ionization Energies Studied by the Correlated Ions-(Ions/Neutrals) Mass Spectrometry

Sub-ionization energy electrons play a substantial role in the early time of (radiation/photo-) chemistry by generating reactive ions and neutral radicals. As the ions can be easily identified by mass spectrometry methods, information on the neutral species produced in correlation relies mainly on theoretical calculations. Here we show that coupling a double counter-propagative electron beams with a dual (+/-) time-of-flight mass spectrometer is probably the most versatile instrument for studying processes induced by low energy electrons, by providing correlated information between (ion and ion) and (ion and neutral) species. We demonstrate the feasibility of this technique for the prototypical case of carbon tetrachloride, but this method is generally applicable as shown for nitromethane.

Observation of Transient Anions That Do Not Decay through Dissociative Electron Attachment: New Pathways for Radiosensitization

Low-energy electrons (LEEs) can very efficiently induce bond breaking via dissociative electron attachment (DEA). While DEA is ubiquitous, the importance of other reactions initiated by LEEs remains much more elusive. Here, we looked into this question by measuring highly accurate total cross sections for electron scattering from 1-methyl-5-nitroimidazole (1M5NI), a model radiosensitizer. The small uncertainty and high energy resolution allow us to identify many resonant features related to the formation of transient anions. In addition to novel insights about DEA reactions through the lower-lying resonances, our key finding is that the higher-lying resonances do not undergo DEA, implying alternative decay channels with significant cross sections. In particular, dissociation into two neutral fragments is probably involved in the case of 1M5NI. This finding has direct implications for the understanding of LEE-induced chemistry, particularly in the fundamental processes underlying the radiosensitization activity.

Fragmentation of propionitrile (CH3CH2CN) by low energy electrons

Propionitrile (CH₃CH₂CN, PN) is a molecule relevant for interstellar chemistry. There is credible evidence that anions, molecules, and radicals that may originate from PN could also be involved in the formation of more complex organic compounds. In the present investigation, dissociative electron attachment to CH₃CH₂CN has been studied in a crossed electron-molecular beam experiment in the electron energy range of about 0-15 eV. In the experiment, seven anionic species were detected: C₃H₄N^-, C₃H₃N^-, C₃H₂N^-, C₂H₂N^-, C₂HN^-, C₂N^-, and CN^-. The anion formation is most efficient for CN^- and anions originating from the dehydrogenation of the parent molecule. A discussion of possible reaction channels for all measured negative ions is provided. The experimental results are compared with calculations of thermochemical thresholds of the detected anions.

Dissociative electron attachment to amide bond containing molecules: N-ethylformamide and N-ethylacetamide

To advance our quest to understand the role of low energy electrons in biomolecular systems, we performed investigations on dissociative electron attachment (DEA) to gas-phase N-ethylformamide (NEF) and N-ethylacetamide (NEA) molecules. Both molecules contain the amide bond, which is the linkage between two consecutive amino acid residues in proteins. Thus, their electron-induced dissociation can imitate the resonant behavior of the DEA process in more complex biostructures. Our experimental results indicate that in these two molecules, the dissociation of the amide bond results in a double resonant structure with peaks at ∼5 eV and 9 eV. We also determined the energy position of resonant states for several negative ions, i.e., the other dissociation products from NEF and NEA. Our predictions of dissociation channels were supported by density functional theory calculations of the corresponding threshold energies. Our results and those previously reported for small amides and peptides imply the fundamental nature for breakage of the amide bond through the DEA process.

Formation of resonances and anionic fragments upon electron attachment to benzaldehyde

Benzaldehyde is a simple aromatic aldehyde and has a wide range of applications in the food, pharmaceutical, and chemical industries. The positive electron affinity of this compound suggests that low-energy electrons can be easily trapped by neutral benzaldehyde. In the present study, we investigated the formation of negative ions following electron attachment to benzaldehyde in the gas-phase. Calculations on elastic electron scattering from benzaldehyde indicate a π* valence bound state of the anion at -0.48 eV and three π* shape resonances (0.78, 2.48 and 5.51 eV). The excited state spectrum of the neutral benzaldehyde is also reported to complement our findings. Using mass spectrometry, we observed the formation of the intact anionic benzaldehyde at ∼0 eV. We ascribe the detection of the benzaldehyde anion to stabilization of the π* valence bound state upon dissociative electron attachment to a benzaldehyde dimer. In addition, we report the cross sections for nine fragment anions formed through electron attachment to benzaldehyde. Investigations carried out with partially deuterated benzaldehyde show that the hydrogen loss is site-selective with respect to the incident electron energy. In addition, we propose several dissociation pathways, backed up by quantum chemical calculations on their thermodynamic thresholds. The threshold calculations also support that the resonances formed at higher energies lead to fragment anions observable by mass spectrometry, whereas the resonances at low electron energies decay only by electron autodetachment.

Dissociation dynamics in low energy electron attachment to ammonia using velocity slice imaging

The complete dissociation dynamics of low energy electron attachment to the ammonia molecule has been studied using velocity slice imaging (VSI) spectrometry. One low energy resonant peak around 5.5 eV and a broad resonance around 10.5 eV incident electron energies have been observed. The resonant states mainly dissociate via H- and NH2- fragments, though for the upper resonant state, the signature of NH- fragments is also predicted due to a three-body dissociation process. Kinetic energy and angular distributions of the NH2- fragment anions are measured simultaneously around the two resonances. Based on our experimental observations, we conclude that a temporary negative ion (TNI) state with A1 symmetry is responsible for the lower resonance. Whereas, we find strong evidence for the existence of a TNI state having A1 symmetry at the 10.5 eV resonance for the first time.

Dipole-Supported Electronic Resonances Mediate Electron-Induced Amide Bond Cleavage

Dissociative electron attachment (DEA) plays a key role in radiation damage of biomolecules under high-energy radiation conditions. The initial step in DEA is often rationalized in terms of resonant electron capture into one of the metastable valence states of a molecule followed by its fragmentation. Our combined theoretical and experimental investigations indicate that the manifold of states responsible for electron capture in the DEA process can be dominated by core-excited (shake-up) dipole-supported resonances. Specifically, we present the results of experimental and computational studies of the gas-phase DEA to three prototypical peptide molecules, formamide, N-methylformamide (NMF), and N,N-dimethyl-formamide (DMF). In contrast to the case of electron capture by positively charged peptides in which amide bond rupture is rare compared to N─C_{α} bond cleavage, fragmentation of the amide bond was observed in each of these three molecules. The ion yield curves for ions resulting from this amide bond cleavage, such as NH_{2}^{-} for formamide, NHCH_{3}^{-} for NMF, and N(CH_{3})_{2}^{-} for DMF, showed a double-peak structure in the region between 5 and 8 eV. The peaks are assigned to Feshbach resonances including core-excited dipole-supported resonances populated upon electron attachment based on high-level electronic structure calculations. Moreover, the lower energy peak is attributed to formation of the core-excited resonance that correlates with the triplet state of the neutral molecule. The latter process highlights the role of optically spin-forbidden transitions promoted by electron impact in the DEA process.

Inducing chemical reactivity on specific sites of a molecule using the Coulomb interaction exerted by a low energy electron

Remarkable site-specificity in the resonant attachment of low energy electrons (LEEs) to molecular targets is proposed as an efficient method for inducing chemical reactivity on specific sites of molecules. The Coulomb interaction between the attached electron and the most polarizable molecular electrons localized on the attached site is the reason for site-specific chemical reactivity. The Coulombically induced site-specific chemical reactivity is best illustrated by the LEE induced chemical transformation of a weakly bound molecular complex into a strong covalent adduct. The chemical transformation occurs due to the Coulombically induced nucleophilicity on a specific molecular moiety in the complex. A simple strategy for inducing site-specific chemical reactivity using a LEE may find new avenues in chemical synthesis.

Electron interactions with the heteronuclear carbonyl precursor H2FeRu3(CO)13 and comparison with HFeCo3(CO)12: from fundamental gas phase and surface science studies to focused electron beam induced deposition

In the current contribution we present a comprehensive study on the heteronuclear carbonyl complex H2FeRu3(CO)13 covering its low energy electron induced fragmentation in the gas phase through dissociative electron attachment (DEA) and dissociative ionization (DI), its decomposition when adsorbed on a surface under controlled ultrahigh vacuum (UHV) conditions and exposed to irradiation with 500 eV electrons, and its performance in focused electron beam induced deposition (FEBID) at room temperature under HV conditions. The performance of this precursor in FEBID is poor, resulting in maximum metal content of 26 atom % under optimized conditions. Furthermore, the Ru/Fe ratio in the FEBID deposit (≈3.5) is higher than the 3:1 ratio predicted. This is somewhat surprising as in recent FEBID studies on a structurally similar bimetallic precursor, HFeCo3(CO)12, metal contents of about 80 atom % is achievable on a routine basis and the deposits are found to maintain the initial Co/Fe ratio. Low temperature (≈213 K) surface science studies on thin films of H2FeRu3(CO)13 demonstrate that electron stimulated decomposition leads to significant CO desorption (average of 8-9 CO groups per molecule) to form partially decarbonylated intermediates. However, once formed these intermediates are largely unaffected by either further electron irradiation or annealing to room temperature, with a predicted metal content similar to what is observed in FEBID. Furthermore, gas phase experiments indicate formation of Fe(CO)4 from H2FeRu3(CO)13 upon low energy electron interaction. This fragment could desorb at room temperature under high vacuum conditions, which may explain the slight increase in the Ru/Fe ratio of deposits in FEBID. With the combination of gas phase experiments, surface science studies and actual FEBID experiments, we can offer new insights into the low energy electron induced decomposition of this precursor and how this is reflected in the relatively poor performance of H2FeRu3(CO)13 as compared to the structurally similar HFeCo3(CO)12.

Dissociative electron attachment induced ring opening in five-membered heterocyclic compounds

Ring opening in five-membered rings induced by gentle impact of low energy electrons.