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

Electron stimulated desorption from condensed benzene

The electron induced dissociation of condensed benzene (C6H6) in thin films deposited on a Pt substrate is investigated by electron stimulated desorption (ESD) of anions and cations. The desorbed yields are recorded as a function of incident electron energy in the range of 10 to 950 eV for a fixed film thickness of 2 monolayers (ML) and for a fixed energy of 950 eV, as well as a function of film thickness from 0.5 to 8 monolayers (ML) for anions, and from 0.5 to 12ML for cations. Both energy and thickness dependencies are discussed in terms of the three main mechanisms yielding positively and/or negatively charged fragments: dissociative electron attachment (DEA), dipolar dissociation (DD) and dissociative ionization (DI) processes. At the probed energies, DD is the major mechanism, while DEA is predominantly induced by secondary electrons from the Pt substrate. Desorption of the parent positive ion is strongly suppressed. Similar qualitative behaviours are observed for the energy dependence of both anion and cation ESD yields, while some discrepancies exist in the thickness dependence, including a very significant systematic magnitude difference found between such ions formation. An estimation of the effective DD cross-section including the desorption probability is obtained. Feasible mechanisms behind the observed energy and thickness dependences for anion and cation yields are proposed. These results highlight the need for further investigations to better understand the underlying processes of electron induced dissociation in condensed matter.

Revealing the mechanism of damage to the carbonate ester in PADC polymeric nuclear track detector using low-energy electron stimulated desorption

We investigate the mechanism of damage to the carbonate ester chemical functions in Poly allyl diglycol carbonate (PADC) induced by low-energy electrons (LEEs) of <50 eV, which are major components of the initial secondary products of ionizing radiation. PADC is the world's most widely used polymeric nuclear track detector (PNTD) for swift ion detection. Using diethylene glycol monoethyl ether acetate as a surrogate for PADC, we have measured for irradiation with low-energy electrons (LEEs) of <50 eV, the electron stimulated desorption (ESD) signal of O- from 3-monolayer thick films of DGMEA by time-of-flight mass spectrometry. We find that for electron irradiation at energies >6-9 eV, the instantaneous ESD yield of O- increases with the cumulative number of incident electrons (i.e., fluence), indicating that the additional O- signal derives from an electron-induced DGMEA product. From comparison with ESD measurements from films of acetic acid and acetaldehyde, we identify that the additional desorbed O- signal derives from oxygen atoms originally adjacent to the carbonyl bond in DGMEA. Since LEEs are the predominant secondary particles produced by ionizing radiation, this finding helps to better understand the mechanism of damage to carbonate ester in PADC, which is a key step for latent track formation in PADC.

Chemical transformation of molecular ices containing N2O and C2D2 by low energy electrons: New chemical species of astronomical interest

We have employed electron stimulated desorption (ESD) and x-ray photoelectron spectroscopy (XPS) to study the chemical species generated from multilayer films of N₂O, C₂D₂, and mixtures thereof (i.e., N₂O/C₂D₂) by the impact of low energy electrons with energies between 30 and 70 eV. Our ESD results for pure films of N₂O show the production of numerous fragment cations and anions, and of larger molecular ions, of sufficient kinetic energy to escape into vacuum, which are likely formed by ion-molecule scattering in the film. Ion-molecule scattering is also responsible for the production of cations from C₂D₂ films that contain as many as six or seven carbon atoms. Many of the same anions and cations desorb from N₂O/C₂D₂ mixtures, as well as new species, which is the result of ion-molecule scattering in the film. Anion desorption signals further indicate the formation of C-N containing species within the irradiated films. XPS spectra of N1s, C1s, and O1s lines reveal the fragmentation of N-O bonds and gradual formation of molecules containing species containing O-C=O, C=O, and C-O functional groups. A comparison between ESD and XPS findings suggests that species observed in the ESD channel are primarily products of reactions taking place at the film-vacuum interface, while those observed in the XPS derive from reactions occurring within the solid.

Evolution of CH3NO2/Si interfacial chemistry under reaction conditions: a combined experimental and theoretical study

Dissociative adsorption of CH₃NO₂ onto a Si(100)-2 × 1 surface is studied using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations.

Exploding Nitromethane in Silico, in Real Time

Nitromethane (NM) is widely applied in chemical technology as a solvent for extraction, cleaning, and chemical synthesis. NM was considered safe for a long time, until a railroad tanker car exploded in 1958. We investigate the detonation kinetics and explosion reaction mechanisms in a variety of systems consisting of NM, molecular oxygen, and water vapor. Reactive molecular dynamics allows us to simulate reactions in time-domain, as they occur in real life. High polarity of the NM molecule is shown to play a key role, driving the first exothermic step of the reaction. Rapid temperature and pressure growth stimulate the subsequent reaction steps. Oxygen is important for faster oxidation, whereas its optimal concentration is in agreement with the proposed reaction mechanism. Addition of water (50 mol %) inhibits detonation; however, water does not prevent detonation entirely. The reported results provide important insights for improving applications of NM and preserving the safety of industrial processes.

Oxygen attachment on alkanethiolate SAMs induced by low-energy electron irradiation

Reactions of (18)O2 with self-assembled monolayer (SAM) films of 1-dodecanethiol, 1-octadecanethiol, 1-butanethiol, and benzyl mercaptan chemisorbed on gold were studied by the electron stimulated desorption (ESD) of anionic fragments over the incident electron energy range 2-20 eV. Dosing the SAMs with (18)O2 at 50 K results in the ESD of (18)O(-) and (18)OH(-). Electron irradiation of samples prior to (18)O2 deposition demonstrates that intensity of subsequent (18)O(-) and (18)OH(-) desorption signals increase with electron fluence and that in the absence of electron preirradiation, no (18)O(-) and (18)OH(-) ESD signals are observed, since oxygen is unable to bind to the SAMs. A minimum incident electron energy of 6-7 eV is required to initiate the binding of (18)O2 to the SAMs. O2 binding is proposed to proceed by the formation of CHx-1(•) radicals via resonant dissociative electron attachment and nonresonant C-H dissociation processes. The weaker signals of (18)O(-) and (18)OH(-) from short-chain SAMs are related to the latter's resistance to electron-induced damage, due to the charge-image dipole quenching and electron delocalization. Comparison between the present results and those for DNA oligonucleotides self-assembled on Au (Mirsaleh-Kohan, N. et al. J. Chem. Phys. 2012, 136, 235104) indicates that the oxygen binding mechanism is common to both systems.

Electron interaction with nitromethane embedded in helium droplets: attachment and ionization measurements

Results of a detailed study on electron interactions with nitromethane (CH(3)NO(2)) embedded in helium nanodroplets are reported. Anionic and cationic products formed are analysed by mass spectrometry. When the doped helium droplets are irradiated with low-energy electrons of about 2 eV kinetic energy, exclusively parent cluster anions (CH(3)NO(2))(n)(-) are formed. At 8.5 eV, three anion cluster series are observed, i.e., (CH(3)NO(2))(n)(-), [(CH(3)NO(2))(n)-H](-), and (CH(3)NO(2))(n)NO(2)(-), the latter being the most abundant. The results obtained for anions are compared with previous electron attachment studies with bare nitromethane and nitromethane condensed on a surface. The cation chemistry (induced by electron ionization of the helium matrix at 70 eV and subsequent charge transfer from He(+) to the dopant cluster) is dominated by production of methylated and protonated nitromethane clusters, (CH(3)NO(2))(n)CH(3)(+) and (CH(3)NO(2))(n)H(+).

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 the condensed-phase nitromethane: II. Desorption of neutral fragments

Low energy electron induced dissociation in multilayer films of nitromethane (CD3NO2) was investigated by high resolution electron energy loss spectroscopy (HREELS) and by the electron stimulated desorption (ESD) of neutral species. HREELS measurements show that the lowest electronic states of the condensed molecule are very similar to those seen in the gas phase. Desorbed neutrals were detected using combined non-resonant multi-photon ionization at 355 nm and time of flight mass spectrometry. The most intense signals detected were those of CD3 (+) and NO (+) and are attributed primarily to the desorption of CD3 and NO2 fragments following molecular dissociation via low-lying electronic excited states of nitromethane (the detected NO (+) being the result of the dissociative ionization of NO2). By varying the time delay between the incident electron pulse and the ionizing laser pulse, it is possible to measure the kinetic energy distributions of desorbing fragments. The kinetic energy distributions above ∼ 5 eV appear invariant with incident electron energy, indicating that the same desorption process (dissociation via low-lying electronic states) operates at all the studied incident energies. Nevertheless, measurements of neutral yields as functions of incident electron energy demonstrate that excitation of the dissociative electronic states also proceeds via previously identified transient negative ions. At energies less than ∼ 5 eV, contributions from dissociative electron attachment are also observed in the yield of CD3 and other neutral fragments.