Attachment Cross Sections Research Articles

Hysteresis in radio frequency capacitively coupled CF4 plasmas

Based on experiments and simulations, various plasma parameters are found to undergo a hysteresis as a function of the driving voltage amplitude in capacitively coupled CF4 discharges. Phase Resolved Optical Emission Spectroscopy reveals that the discharge operates in a hybrid combination of the drift-ambipolar and α-mode at low voltage. In this mode, the electric field and mean electron energy are high in the electronegative plasma bulk region. As the cross section for electron attachment is appreciable only at high electron energies, this mode results in strong negative ion production and keeps the electron density low as well as the mode of plasma operation stable, when the voltage is increased moderately. Increasing the driving voltage amplitude further ultimately induces a mode transition into a pure α-mode, once the electron density increases strongly. Decreasing the voltage again results in a reverse mode transition at a lower voltage compared to the previous mode transition, because the electron density is now initially high in the bulk and, thus, the bulk electric field and mean electron energy are low resulting in inefficient generation of negative ions via electron attachment. This keeps the electron density high even at lower driving voltages. This effect leads to the emergence of two steady states of plasma operation within a certain voltage range. The different electron energy distribution functions in these two states result in markedly different generation and density profiles of F atoms, with higher values occurring in the increasing voltage branch of the hysteresis. The ion flux and mean energy at the electrodes also differ. The voltage range, where the hysteresis occurs, is affected by the ion induced secondary electron coefficient (γ). A larger value of γ results in a shift of the hysteresis voltage range towards lower values.

Prediction of a complete and self-consistent collision cross-section set of C5F10O by data-driven method

The lack of a complete and consistent set of electron–molecule collision cross sections for the new eco-friendly gas C5F10O hinders the study of its microscopic discharge mechanisms. In this paper, we first calculate the elastic collision and electronic excitation cross-sections of C5F10O based on R-matrix theory and estimate its total attachment cross-sections through pulsed Townsend measurement. By combining cross sections from other literature, we are able to compile an initial set of collision cross-sections for C5F10O. However, this initial set remains incomplete and requires refinement. Therefore, this work establishes a neutral network to solve the inverse swarm problem of deriving cross-sections from swarm parameters. The inversion model is trained using a substantial amount of collision cross-sections from the LXCat project. Using the model and the initial set for C5F10O, a complete and self-consistent collision cross-section set for the C5F10O gas is predicted for the first time, based on the measured electron swarm parameters of C5F10O/Ar mixtures. The proposed method is capable of predicting C5F10O’s unknown vibrational excitation cross-section without any prior knowledge, thus enhancing its completeness. The refined set can reproduce the electron swarm parameters within an acceptable range of uncertainty, thus verifying its self-consistency. The set will be made available in the LXCat database, which is expected to be significant for fundamental studies of its discharge mechanisms as well as applications of C5F10O.

Integral Electron Scattering Cross Sections from N2O for Impact Energies Ranging from 1 to 1000 eV.

Accurate total cross sections (TCS), within 5%, for electron scattering by N2O molecules have been measured with a magnetically confined electron transmission apparatus for impact energies ranging from 1 to 200 eV. For higher energies, these measurements have been complemented with our independent atom-based screening corrected additivity rule, including interference (IAM-SCAR + I) method to determine a complete reference TCS data set in the energy range (1-1000 eV). After a critical discussion that includes our calculated integral elastic and ionization cross sections and the theoretical and experimental data available in the literature, a complete set of integral elastic and inelastic (rotational, vibrational, and electronic excitation, ionization and electron attachment) cross sections, consistent with the reference TCS data, have been derived. This update on the N2O collisional database may help to improve the accuracy of radiation-induced transport models.

Electron Impact Cross Sections and Transport Studies of C3F6O

Electron impact scattering from C3F6O is studied in this work. The R-matrix method was used for the calculations of elastic, momentum transfer, and excitation cross sections. The attachment cross section was obtained through a parametric estimator based on the R-matrix outputs. The Binary-Encounter-Bethe (BEB) method was used for computing the ionization cross section. The obtained cross section set was used for the transport studies using the BOLSIG+ code, which is a two-term Boltzmann equation solver. The present calculation was performed for steady-state Townsend experimental conditions for E/N, covering a range of 100–1000 Td. The critical dielectric strength of pure C3F6O was found to be 475 Td, which is much greater than that of SF6 (355 Td). The effect of the addition of different buffer gases, such as CO2, N2, and O2, was also examined. For the C3F6O–CO2, C3F6O–N2, and C3F6O–O2 mixtures with 65%, 55%, and 60% C3F6O, respectively, the critical dielectric strength was determined to be essentially the same as that of pure SF6. The presence of synergism was confirmed for these gas mixtures. We further derived the Paschen curve using a fitting method with the transport parameters as the basic inputs. The minimum breakdown voltage of C3F6O accounted for only 55% of that of SF6. The buffer gas mixture improved the condition; however, the performance of CO2 and O2 mixtures was not satisfactory. The addition of N2 as the buffer gas significantly improved the breakdown property of the gas. The mixture of ≥99% of N2 or ≤1% of C3F6O gave a better breakdown characteristic than SF6. Any proportion ≥90% of N2 or ≤10% of C3F6O was suitable in the higher pressure ranges. The present work demonstrates the potential of C3F6O as a substitute gas for SF6 with a negligible environmental threat.

Low energy electron induced chemistry for e-bromine molecule scattering

The present study focuses on electron collision with bromine molecule, employing ab-initio scattering calculations. Using the R-matrix method, we calculated elastic and inelastic cross-sections using two different scattering models: static exchange with polarization and close-coupling. Our findings reveal total three low energy resonances for e-Br2 scattering which are primarily formed through transient negative ions. This observation is supported by dissociative electron attachment cross-section analysis, potential energy curves, and molecular orbital diagrams. Out of these three, first is at zero-energy, the electron attachment is recognized as shape-resonance and further two π* resonances as core-excited resonances. Present computation becomes more important as many cross-sections are reported as bench mark results in absence of any reported data. For instance: elastic, differential, momentum transfer, excitation and total cross-section. Comparing our results with sparse existing literature, we find good agreement.

Theoretical Investigation of Electron Impact Scattering on Imidazole.

This study presents the results of electron scattering calculations on a biologically important molecule, imidazole, using the UK molecular R-matrix method. The R-matrix calculations are performed using SE, SEP, and CC models, and the resonance detected in the present SEP model is found to be in better agreement with available experimental data than previous theoretical data. The study also reports an inelastic scattering cross section, which comprises dissociative electron attachment (DEA), excitation, and ionization cross section, for the first time. The total scattering cross sections are also reported for the first time. We confirm the presence of the two well-known π* shape resonances predicted earlier experimentally. Due to the scarcity of total scattering cross section (TCS) data for imidazole, we have compared the TCS of imidazole with its isoelectronic target isoxazole and drawn important conclusions. A comparison among the resonances of imidazole with those of isoxazole helps us to conclude that electron attachment to π* molecular orbitals is a general feature displayed by these five-membered heterocyclic compounds. The comprehensive electron scattering studies presented in this work are expected to provide a deeper understanding of electron-induced biochemical processes and fill gaps in the available data. Furthermore, this study is anticipated to inspire further investigations on imidazole and other five-membered heterocyclic ring molecules, which have significant applications in medicine.

Fragmentation dynamics and absolute dissociative electron attachment cross sections in the low energy electron collision with ethanol.

Dissociative electron attachment (DEA) to ethanol has been probed to study fragmentation dynamics using Time-of-Flight (ToF) mass spectrometric technique. Several fragment ions, namely, H-, O-, OH-, C2H3O- and C2H5O- have been observed. Extra effort has been made to detect low mass ions (here, H-). Absolute DEA cross sections for the formation of O- and OH- have been measured for the first time using relative flow technique (RFT). The threshold energy of different dissociation channels has been calculated using density functional theory (DFT) method. By combining the experimental and theoretical data, we found evidence of hydrogen migration in the production of O and C2H3O- ions.

Dissociative Electron Attachment Cross Sections for Ni(CO)4, Co(CO)3NO, Cr(CO)6

Ni(CO)4, Cr(CO)6, Co(CO)3NO are some of the most common precursors used for focused electron beam induced deposition. Some of the compounds, even though extensively used have high requirements when it comes to handling being, explosives, highly flammable and with high toxicity levels, as is the case of Ni(CO)4. We are employing simulations to determine values hard to determine experimentally, and compare them with DFT calculations and experimental data where available. The use of Quantemol-N cross section simulations for dissociative electron attachment (DEA) at low electron energy in the range of 0–20 eV, gives valuable information on the fragmentation of the molecules, based on their bond dissociation energies, electron affinities and incident electron energies. The values obtained for the cross sections are 0.12 × 10−18 cm2 for Ni(CO)4, 4.5 × 10−16 cm2 for Co(CO)3NO DEA cross-sections and 4.3 × 10−15 cm2 for Cr(CO)6.

Velocity map imaging and cross sections of Fe(CO)5 for FEBIP applications

The present paper intends to be a new study of a widely used precursor in nanostructure deposition and FEBID processes with focus on its fragmentation at collisions with low energy electrons. Newer developments in nanotechnology with applications to focused electron beam-induced deposition (FEBID) and extreme ultraviolet lithography (EUVL) based on irradiation-induced chemistry come with advances in the size of the nanostructures at the surface and their flexibility in creating highly complex 3D structures. The deformation in the main structures of the FEBID process characterized by elongation, reduction in diameter of the main structure and the deposition of additional thin layers around the structure, on the substrate, are results of the secondary electrons effect, colliding with energies lower than 20 eV. Fe(CO)5 is one of the most used compounds in FEBID processes as it has a high vaporization pressure and has been shown to provide high-purity deposits (over 90%). This paper combines experiment and simulations to study electron scattering from Fe(CO)5, using Quantemol-N simulations with mass spectroscopy techniques to present the fragmentation pathways and channel distributions for each of the resulting negative ions at low electron energies, while experimental data on dissociative electron attachment make use of the velocity-sliced map imaging (VMI) technique to determine the anions at the incident electron energies. The Quantemol-N simulation package as a standalone is used to study collision processes of low-energy electrons with Fe(CO)5 molecules including elastic, electronic excitation, and dissociative electron attachment (DEA) cross sections for a wide range of process in nuclear industry, medical research and quantum chemistry.Graphical abstract

Coherent Preparation of Highly Vibrating and Rotating D2 Molecules.

Highly vibrationally and rotationally excited hydrogen molecules are of immense interest for understanding and modeling the physics and chemistry of the cold interstellar medium. Using a sequence of two Stark-induced adiabatic Raman passages, we demonstrate the preparation of rotationally excited D2 molecules in the fourth excited vibrational level within its ground electronic state. The nearly complete population transfer to the target state is confirmed by observing both the threshold behavior as a function of the laser power and the depletion of the intermediate level. The vibrational excitation reported here opens new possibilities in the study of the much debated four-center reaction between a pair of hydrogen molecules. Additionally, these rovibrationally excited molecules could be potentially used to generate the high-intensity D- ion beams considered essential for D-T thermonuclear fusion by enhancing the cross section for dissociative electron attachment by 5 orders of magnitude compared to that of the ground state.

Resonant electron capture by 5-Br-2'-deoxyuridine.

The results of the study of resonant electron capture by molecules of 5-Br-2'-deoxyuridine (BrdUrd) over the range of electron energies from near zero to 14eV are described. In the thermal energy range, long-lived molecular negative ions, unstable with respect to autoneutralization and dehalogenation, have been registered. Examination of the kinetics of these decay processes led us to the conclusion that the most probable structure for molecular negative ions is that with an extended C-Br bond, which was predicted earlier using quantum-chemical calculations. Estimates have shown that the BrdUrd molecule owns a significant electronic affinity of 0.93-1.38eV. The most intense fragmentation channel leads to the abundant formation of Br- ions. The dissociative electron attachment cross section for Br- ions formation was estimated to amount to no less than 1.65 × 10-15cm2, indirectly implying a fairly intense formation of complementary highly reactive deoxyuridine-5-yl particles. These particles are known to be responsible for the radiosensitizing properties of BrdUrd.

Dissociative electron attachment to 5-bromo-uracil: non-adiabatic dynamics on complex-valued potential energy surfaces.

Electron induced dissociation reactions are relevant to many fields, ranging from prebiotic chemistry to cancer treatments. However, the simulation of dissociation electron attachment (DEA) dynamics is very challenging because the auto-ionization widths of the transient negative ions must be accounted for. We propose an adaptation of the ab initio multiple spawning (AIMS) method for complex-valued potential energy surfaces, along the lines of recent developments based on surface hopping dynamics. Our approach combines models for the energy dependence of the auto-ionization widths, obtained from scattering calculations, with survival probabilities computed for the trajectory basis functions employed in the AIMS dynamics. The method is applied to simulate the DEA dynamics of 5-bromo-uracil in full dimensionality, i.e., taking all the vibrational modes into consideration. The propagation starts on the resonance state and describes the formation of Br- anions mediated by non-adiabatic couplings. The potential energies, gradients and non-adiabatic couplings were computed with the fractional-occupancy molecular orbital complete-active-space configuration-interaction method, and the calculated DEA cross section are consistent with the observed DEA intensities.

Electron Scattering Cross-Section Calculations for Atomic and Molecular Iodine

Cross sections for electron scattering from atomic and molecular iodine are calculated based on the R-matrix (close-coupling) method. Elastic and electronic excitation cross sections are presented for both I and I2. The dissociative electron attachment and vibrational excitation cross sections of the iodine molecule are obtained using the local complex potential approximation. Ionization cross sections are also computed for I2 using the BEB model.

Formation of negative-ion resonance and dissociative attachment in collisions of NO2 with electrons

The process of electron attachment to the NO2 molecule is investigated theoretically using an approach based on a study by O’Malley (1966 Phys. Rev. 150 14). The approach combines the normal mode approximation for representation of vibrational dynamics of NO2 and one-dimensional treatment, along each normal mode, of the attachment process as in O’Malley’s theory, such that only a modest computational effort is required to compute the attachment cross section. Taking into account the survival probability of the formed resonant state of , the cross section for dissociative electron attachment to NO2 is also estimated. To compare with available experimental data, the theoretical cross section is convoluted with energy distribution of NO2–e− collisions with uncertainties reported in experimental studies. Peak values of the convoluted theoretical cross section are found to be about a factor of 2–10 larger than the experimental results.

Toward a complete and comprehensive cross section database for electron scattering from NO using machine learning

We review experimental and theoretical cross sections for electron scattering in nitric oxide (NO) and form a comprehensive set of plausible cross sections. To assess the accuracy and self-consistency of our set, we also review electron swarm transport coefficients in pure NO and admixtures of NO in Ar, for which we perform a multi-term Boltzmann equation analysis. We address observed discrepancies with these experimental measurements by training an artificial neural network to solve the inverse problem of unfolding the underlying electron-NO cross sections while using our initial cross section set as a base for this refinement. In this way, we refine a suitable quasielastic momentum transfer cross section, a dissociative electron attachment cross section, and a neutral dissociation cross section. We confirm that the resulting refined cross section set has an improved agreement with the experimental swarm data over that achieved with our initial set. We also use our refined database to calculate electron transport coefficients in NO, across a large range of density-reduced electric fields from 0.003 to 10000 Td.

Kinetic energy released in the vibrational autodetachment of sulfur hexafluoride anion.

The kinetic energy release distribution (KERD) in the vibrational autodetachment (VAD) from sulfur hexafluoride anion SF6 - has been measured in a velocity map imaging spectrometer for delays in the range of a few tens of microseconds. The experimental KERD is analyzed within the framework of the detailed-balance: first using the standard Langevin model and subsequently using a more refined and realistic model based on the experimental attachment cross section. A discussion on the processes involved in the attachment and the VAD is presented based on an empirical fit of the attachment cross section. The lifetime derived from the model is in good agreement with the experimental time window, strengthening this theoretical approach for this model system.

Thymidine and stavudine molecules in reactions with low-energy electrons

The negative ion formation and fragmentation processes in thymidine and stavudine were studied by means of resonant electron capture mass spectrometry in the electron energy range 0–14 eV. Stavudine is a nucleoside analog of thymidine, that differs from the latter by the presence of double bond in the deoxyribose moiety. In the energy range below 5 eV transient negative ions have been found to arise via shape resonances resulting in electron attachment into the empty π*-orbitals. The main fragmentation channels for molecular ions are the H-atom loss and N-glycosidic bond cleavage between sugar and a nucleobase moieties. The dissociative electron attachment cross sections for the fragment negative ions were measured. The structures for some observed anions and neutral counterparts were suggested basing on the thermochemical approach for dissociative reactions energy balance under the support of quantum chemical calculations. The processes of ion formation in thymidine were found to be similar to those revealed previously for deoxyuridine nucleoside. In contrast, very strong dissociative channel was found for stavudine yielding [M–R]– ions (where R is a sugar moiety) in the energy range <3 eV. The efficient fragmentation found for stavudine in the low energy electron attachment reactions may have implications for the design of nucleoside radiosensitizers for cancer therapy.

Resonant collisions of electrons with O2 via the lowest-lying 2Πg state of O2−

We report the results of an ab initio study of resonant vibrational excitation of molecular oxygen by electron impact at low energies where the lowest-lying ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{g}$ resonant state of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ dominantly contributes to the cross sections. The contribution of this resonance to the dissociative electron attachment cross section is also determined and the origin of its unusual oscillations is discussed. Calculations were performed within the nonlocal resonance model describing the nuclear dynamics of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ after electron capture into the resonant state. The model was constructed using potential-energy curves obtained with standard quantum-chemical methods and eigenphase sums from the fixed-nuclei $R$-matrix calculations of electron scattering off ${\mathrm{O}}_{2}$. The effect of the spin-orbit interaction is taken into account when determining the potential-energy curves of the molecular negative ion. The vibrational excitation cross sections are compared with other available theoretical and experimental results.

Взаимодействие низкоэнергетических электронов с молекулами D-рибозы

Abstract– The interactions of low-energy electrons (&lt;20 eV) with D-ribose molecules, namely, electron scattering and dissociative attachment, are studied. The results of these studies showed that the fragmentation of D-ribose molecules occurs effectively even at an electron energy close to zero. as well as in the energy range 5.50–9.50 eV. In the total cross section of electron scattering by molecules, resonance features at energies of 5.00–9.00 eV in the region of formation of ionic fragments C3H4O2–, C2H3O2–, OH–, associated with the destruction of molecular heterocycles, were experimentally discovered for the first time. The correlation of the features observed in the scattering and dissociative electron attachment cross sections is analyzed.

Absolute dissociative electron attachment cross-section measurement of difluoromethane

Dissociative electron attachment (DEA) and ion-pair dissociation (IPD) processes of Difluoromethane (CH$_2$F$_2$) have been studied in the incident electron energy range 0 to 45 eV. Three different fragment anions (F$^-$, CHF$^-$ and F$_2^-$) are detected in the DEA range and two anions (F$^-$ and CHF$^-$) are detected in IPD range. Absolute cross-section of the F$^-$ fragment ion is measured for the first time. Three different resonances for both F$^-$ and CHF$^-$ ions and one single resonance peak for the F$_2^-$ ions are observed. Constant increase in ion counts above 8 eV incident electron energy indicates the involvement of IPD process. From the experimental observation, it is speculated that near 11 eV incident electron energy both DEA and IPD processes occur simultaneously.