Electron Impact Dissociative Excitation Research Articles

Semiclassical theory of resonant dissociative excitation of molecular ions by electron impact

We have developed a semiclassical approach to the description of resonant dissociative excitation of a molecular ion induced by collisions with plasma free electrons and accompanied by the non-adiabatic transitions between its different electronic terms. It is based on the quasistatic treatment of nuclear particles relative motion in a molecular ion combined with the approximation of a quasicontinuum for rovibrational states. We have derived the semianalytic expressions for the Boltzmann-averaged cross sections, , and rate constants, α de(T, T e), of this process. The resulting expressions for and α de(T, T e) with explicit dependencies on the electron energy, ɛ, the gas, T, and electron, T e, temperatures are valid when the thermal energy k B T exceeds the value of the lowest vibrational quantum, ℏω e, of a molecular ion. The theory developed is applied to studying the resonant dissociative excitation of homo-nuclear , and ions as well as the hetero-nuclear rare gas ions RgXe+ (Rg = Ne, Ar, and Kr) with significantly different values of the dissociation energy. The efficiencies of the electron-impact dissociative excitation process and dissociative recombination leading to the population of the Rydberg states are compared. Our results of calculations are in good agreement with the available experimental data.

A photochemical model of ultraviolet atomic line emissions in the inner coma of comet 67P/Churyumov-Gerasimenko

Alice ultraviolet spectrometer onboard Rosetta space mission observed several spectroscopic emissions emanated from volatile species of comet 67P/Churyumov-Gerasimenko (hearafter 67P/C-G) during its entire escorting phase. The measured emission intensities, when the comet was at around 3 AU pre-perihelion, have been used to derive electron densities in the cometary coma assuming that H I and O I lines are solely produced by electron impact dissociative excitation of cometary parent species (Feldman et al., 2015). We have developed a photochemical model for comet 67P/C-G to study the atomic hydrogen (H I 1216, 1025, & 973 Å), oxygen (O I 1152, 1304, & 1356 Å), and carbon (C I 1561 & 1657 Å) line emissions by accounting for major production pathways. The developed model has been used to calculate the emission intensities of these lines as a function of nucleocentric projected distance and also along the nadir view by varying the input parameters, viz., neutral abundances and cross sections. We have quantified the percentage contributions of photon and electron impact dissociative excitation processes to the total intensity of the emission lines, which has an important relevance for the analysis of Alice observed spectra. It is found that in comet 67P/C-G, with a neutral gas production rate of about 1027 s−1 when it was at 1.56 AU from the Sun, photodissociative excitation processes are more significant compared to electron impact reactions in determining the atomic emission intensities. Based on our model calculations, we suggest that the observed atomic hydrogen, oxygen, and carbon emission intensities can be used to derive H2O, O2, and CO, abundances, respectively, rather than electron density in the coma of 67P/C-G, when the comet has a gas production rate of 1027 s−1 or more.

Electron-impact dissociative excitation cross sections for singlet states of molecular hydrogen

We report cross sections for electron-impact dissociative excitation of the $B{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, $C{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$, $D{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$, ${B}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, and $E,F{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ singlet states of molecular hydrogen from all ${v}_{i}=0--14$ vibrational levels of the ground $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ state. Calculations are performed using the adiabatic-nuclei convergent close-coupling method formulated in prolate spheroidal coordinates from threshold to 500 eV. Agreement with previous calculations varies with transition and impact energy, ranging from excellent to poor. Agreement with available experiment is generally good.

Low-energy electron-impact dissociative excitation of molecular hydrogen and its isotopologues

We apply the adiabatic nuclei convergent close-coupling method to electron-impact dissociative excitation of ${\mathrm{H}}_{2}$ in the low-energy regime. Differential and integrated cross sections are presented for excitation of the $b^{3}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ state, the primary pathway to dissociation of ${\mathrm{H}}_{2}$ at low energies. Agreement with experiment is satisfactory. Results are also presented for the isotopologues ${\mathrm{D}}_{2}$, ${\mathrm{T}}_{2}$, HD, HT, and DT, which show a pronounced isotope effect near threshold in both the differential and integrated cross sections.

FUV Spectral Signatures of Molecules and the Evolution of the Gaseous Coma of Comet 67P/Churyumov–Gerasimenko

Abstract The Alice far-ultraviolet imaging spectrograph onboard Rosetta observed emissions from atomic and molecular species from within the coma of comet 67P/Churyumov–Gerasimenko during the entire escort phase of the mission from 2014 August to 2016 September. The initial observations showed that emissions of atomic hydrogen and oxygen close to the surface were produced by energetic electron impact dissociation of H2O. Following delivery of the lander, Philae, on 2014 November 12, the trajectory of Rosetta shifted to near-terminator orbits that allowed for these emissions to be observed against the shadowed nucleus that, together with the compositional heterogeneity, enabled us to identify unique spectral signatures of dissociative electron impact excitation of H2O, CO2, and O2. CO emissions were found to be due to both electron and photoexcitation processes. Thus, we are able, from far-ultraviolet spectroscopy, to qualitatively study the evolution of the primary molecular constituents of the gaseous coma from start to finish of the escort phase. Our results show asymmetric outgassing of H2O and CO2 about perihelion, H2O dominant before and CO2 dominant after, consistent with the results from both the in situ and other remote sensing instruments on Rosetta.

CHANGES IN THE PHYSICAL ENVIRONMENT OF THE INNER COMA OF 67P/CHURYUMOV–GERASIMENKO WITH DECREASING HELIOCENTRIC DISTANCE

ABSTRACT The Wide Angle Camera of the OSIRIS instrument on board the Rosetta spacecraft is equipped with several narrow-band filters that are centered on the emission lines and bands of various fragment species. These are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67P with time and heliocentric distance, here between 2.6 and 1.3 au pre-perihelion. Our observations indicate that the emission observed in the OH, O i, CN, NH, and NH2 filters is mostly produced by dissociative electron impact excitation of different parent species. We conclude that CO2 rather than H2O is a significant source of the [O i] 630 nm emission. A strong plume-like feature observed in the CN and O i filters is present throughout our observations. This plume is not present in OH emission and indicates a local enhancement of the CO2/H2O ratio by as much as a factor of 3. We observed a sudden decrease in intensity levels after 2015 March, which we attribute to decreased electron temperatures in the first few kilometers above the surface of the nucleus.

VUV study of electron impact dissociative excitation of thymine

Dissociative excitation of thymine following electron impact was studied in the energy range up to 430 eV. Emissions in the vacuum ultra-violet spectral region below 150 nm were studied and found to be dominated by the hydrogen Lyman series. Emission cross section data reveal that Lyman-α excitation displays a broad maximum at an electron impact energy of 160 eV. The probability of extracting other excited atoms from the parent molecule is found to be insignificant. Possible excitation and dissociation mechanisms in the parent molecule are discussed.

Measurements of the number density of water molecules in plasma by using a combined spectral−probe method

A novel method for measuring the number density of water molecules in low-temperature plasma is developed. The absolute intensities of rotational lines of the (0,0) band of the OH(A2Σ–X2П) transition are used. Lines with sufficiently large rotational quantum numbers referring to the so-called “hot” group of molecules produced by electron-impact dissociative excitation of water molecules are chosen for measurements. The excitation rate of a process with a known cross section is determined by measuring the parameters of plasma electrons by means of the probe method. The measured number densities of molecules are compared with those in the initial plasma-forming mixture. The time evolution of the particle densities in plasma is investigated. The problems of the sensitivity and applicability of the absolute spectral method are considered.

Nitrogen oxide removal dynamic process through 15 Ns DBD technique

Nitrogen oxides exhaust gas assumes the important responsibility on air pollution by forming acid rain. This paper discusses the NO removal mechanism in 15 ns pulse dielectric barrier discharge (DBD) plasma through experimental and simulating method. Emission spectra collected from plasma are evaluated as sourced from N + and O (3 P ). The corresponding zero-dimensional model is established and verified through comparing the simulated concentration evolution and the experimental time-resolved spectra of N +. The electron impact ionization plays major role on NO removal and the produced NO + are further decomposed into N + and O (3 P ) through electron impact dissociative excitation rather than the usual reported dissociative recombination process. Simulation also indicates that the removal process can be accelerated by NO inputted at lower initial concentration or electrons streamed at higher concentration, due to the heightened electron impact probability on NO molecules. The repetitive pulse discharge is a benefit for improving the NO removal efficiency by effectively utilizing the radicals generated from the previous pulse under the condition that the pulse period should be shorter enough to ignore the spatial diffusion of radicals. Finally, slight attenuation on NO removal has been experimentally and simulatively observed after N 2 mixed, due to the competitive consumption of electrons.

Energy resolved actinometry for simultaneous measurement of atomic oxygen densities and local mean electron energies in radio-frequency driven plasmas

A diagnostic method for the simultaneous determination of atomic oxygen densities and mean electron energies is demonstrated for an atmospheric pressure radio-frequency plasma jet. The proposed method is based on phase resolved optical emission measurements of the direct and dissociative electron-impact excitation dynamics of three distinct emission lines, namely, Ar 750.4 nm, O 777.4 nm, and O 844.6 nm. The energy dependence of these lines serves as basis for analysis by taking into account two line ratios. In this frame, the method is highly adaptable with regard to pressure and gas composition. Results are benchmarked against independent numerical simulations and two-photon absorption laser-induced fluorescence experiments.

Plasmas sustained in bubbles in water: optical emission and excitation mechanisms

Plasmas in bubbles in water are being investigated for their ability to produce chemically reactive species for water purification and medical treatment. The gas forming the bubble is potentially a design parameter for water purification as the type and rate of production of active species may be controllable by the type of gas in the bubble. In this paper, we report on a computational investigation of the dynamics of plasmas in bubbles in water sustained in different gases. Images, optical spectra and plasma properties are discussed for plasmas in bubbles of N2, Ar and He in water, and compared to experiments. The differences in plasma dynamics and spatial distribution of the plasma (e.g., volume discharge or surface hugging) when using different gases depend in large part on the electron energy relaxation length, and the rate of diffusion of water vapour into the interior of the bubble. Electron impact dissociative excitation of water vapour, electron impact excitation of dissociation products and excitation transfer from the plasma excited injected bubble gases to water vapour all contribute to plasma emission. Variations in the contributions of these processes are responsible for differences in the observed optical spectra and differences in radical production.

Electron-impact dissociation of HeH+: absolute cross sections for the production of Heq+ (q = 1–2) fragments

Absolute cross sections have been measured for electron-impact dissociative excitation and ionization of the helium-hydride ion (HeH+) leading to the formation of helium ions Heq+ (q = 1–2). Measurements have been performed for electron energies ranging from the respective thresholds up to 2.5 keV. For He+ production, the maximum absolute cross section for dissociative excitation is found to be (2.5 ± 0.2) ×10−17 cm2 (around 40 eV), while for dissociative ionization, it is found to be (2.1 ± 0.1) ×10−17 cm2 (around 135 eV). For He2+ production, only the inclusive absolute cross section is presented, without attempting to isolate any distinct contribution, and it is measured to be (2.6 ± 0.2) ×10−19 cm2 (around the maximum, 195 eV). Appearance energies for He+ and He2+ are determined to be (11.2 ± 0.5) eV and (66.5 ± 1.0) eV, respectively. Kinetic energy release and angular distributions are obtained, at specific incident electron energies.

Energy distributions of neutrals and charged species in hollow cathode H2 discharges: a study of fast H atoms

A joint study of atomic and molecular emission spectroscopy, ion mass spectrometry and Langmuir probes is presented for a hollow cathode H2 discharge at pressures 0.7–20 Pa, rendering energy distributions of the different groups of particles that span five orders of magnitude. The H Balmer line profiles can be decomposed into three contributions (‘narrow peak’, ‘plateau’ and ‘far wings’) with very different widths. The narrow peak and the plateau are largely attributed to electron impact dissociative excitation and ionization of H2, and display Doppler temperatures of ∼0.3 eV and ∼6 eV, respectively. The energy-resolved mass spectra of H+ confirm the assignment of the plateau region. The far wings give rise to emissions extending to Doppler shifts of 0.5 nm, which correspond to translational energies of ∼300 eV. The dependence of the relative intensities of these components on pressure is studied. With a grounded grid before the observation window, a structure is induced in the blue-shifted wing. Assuming that the structure is solely caused by dissociative excitation of H2 upon collisions with the plasma ions (H+, and ), their relative concentrations can be estimated. These estimations are in very good agreement with the values derived from independent mass spectrometric measurements.

Electron-impact dissociative excitation of S2

Electron-impact dissociation of S2 has been studied by observation of the atomic emission features in the vacuum ultraviolet spectral region between 120 and 170 nm. The predominantly S2 targets were prepared by passing either S8 or CS2 through a microwave discharge. The excitation function of the S [3s23p4–3s23p3(2D°)4s, 3P-3D°] emission multiplet has been measured at 147.4 nm for electron-impact energies from below threshold to 370 eV. From appearance energy measurements in the near-threshold region, likely fragmentation channels are identified.

Electron-impact dissociative excitation and ionization of N2D+

Absolute cross sections for electron-impact dissociation of N${}_{2}$D${}^{+}$ producing N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragments were measured in the 5- to 100-eV range using a crossed electron-ion beams technique. In the 5- to 20-eV region, in which dissociative excitation (DE) is the principal contributing mechanism, N${}_{2}^{+}$ production dominates. The N${}_{2}^{+}$ + D dissociation channel shows a large resonant-like structure in the DE cross section, as observed previously in electron impact dissociation of triatomic dihydride species [M. Fogle, E. M. Bahati, M. E. Bannister, S. H. M. Deng, C. R. Vane, R. D. Thomas, and V. Zhaunerchyk, Phys. Rev. A 82, 042720 (2010)]. In the dissociative ionization (DI) region, 20- to 100-eV, N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragment production are comparable. The observance of the ND${}^{+}$ and N${}^{+}$ ion fragments indicate breaking of the $\mathrm{N}\phantom{\rule{-0.16em}{0ex}}\ensuremath{-}\phantom{\rule{-0.16em}{0ex}}\mathrm{N}$ bond along certain dissociation channels.

Studies of electron-impact dissociative excitation and ionization using the crossed electron-ion beams technique

Absolute cross sections for heavy ion fragment production due to electron-impact dissociation of XH2+ (X = B, C, N, O, F), N2D+ and O3+ have been investigated with the crossed electron-ion beams technique in the energy range 3 – 100 eV. This energy regime covers the dissociative excitation and ionization processes. Two and three-body dissociation channels are observed in these systems and marked resonant-like structure is also observed in the dissociative excitation channels of some systems. It is unclear if direct excitation or resonant capture processes lead to these enhanced dissociative excitation cross sections.

VUV fluorescence following electron-impact dissociative excitation of CS2

Electron-impact dissociation of CS${}_{2}$ has been studied by observation of the atomic spectral emission features in the range 115--170 nm. Absolute photoemission cross sections are presented over the complete wavelength range for an incident electron energy of 100 eV. As an example, the measured cross section of the strong C i emission at 165.7 nm, which is a prominent feature in many solar and other extraterrestrial spectra, is ($1.45\ifmmode\pm\else\textpm\fi{}0.19)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ cm${}^{2}$. Comparison with earlier cross-sectional measurements suggest that these were too high by a factor of more than three. Excitation functions of the dominant C i (156.1 nm) and S i (147.4 nm) emission lines have been measured for electron-impact energies from threshold to 360 eV. From appearance energy measurements in the near-threshold region, likely fragmentation channels are identified which involve both two-fragment breakup and total fragmentation of the parent CS${}_{2}$.

Excitation Mechanisms and Sheath Dynamics in Capacitively Coupled Radio‐Frequency Oxygen Plasmas

AbstractPhase resolved optical emission spectroscopy (PROES) was used to determine the spatio‐temporal behavior of excitation rates in rf sheaths of capacitively coupled plasmas at 13.56 MHz. The plasmas were ignited in pure oxygen at pressures from 20 to 100 Pa and rf powers from 10 to 100 W. The spatial and phase resolved excitation rates have shown four characteristic patterns, which differing in their spatial and temporal position.PIC‐MCC simulations of the oxygen capacitively coupled radio‐frequency discharge were used to get a detailed microscopic description of the dynamic processes in rf plasmas. The PIC‐MCC simulations reproduced the excitation patterns observed in experiment quite well and allowed to identify the underlying physics.Three excitation patterns appearing in front of the powered electrode were found tobe due to electron impact dissociative excitation of molecular Oxygen, whereas the fourth pattern very close to the powered electrode is attributed to collisions of the positive ions with the background gas (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron-impact ionization and dissociation of C2D+

Absolute cross sections for electron-impact single ionization, dissociative excitation and dissociative ionization of the ethynyl radical ion (C(2)D(+)) have been measured for electron energies ranging from the corresponding reaction thresholds to 2.5 keV. The animated crossed electron-ion beam experiment; is used and results have been obtained for the production of C(2)D(2+), C(2+), C(2)(+), CD(+), C(+) and D(+). The maximum of the cross section for single ionization is found to be (2.01 +/- 0.02) x 10(-17) cm(2), at the incident electron energy of 105 eV. Absolute total cross sections for the various singly charged fragments production are observed to decrease by a factor of almost three, from the largest cross-section measured for C(+), over C(2)(+) and CD(+) down to that of D(+) The maxima of the cross sections are obtained to be (14.5 +/- 0.5) x 10(-17) cm(2) for C(2)(+) (12.1 +/- 0.1) x 10(-17) cm(2) for CD(+) (27.7 +/- 0.2) x 10(-17) cm(2) for C(+) and (11.1 +/- 3.8) x 10(-17) cm(2) fo D(+). The smallest cross section is measured to be (1.50 +/- 0.04) x 10(-18) cm(2) for the production of the doubly charged ion C(2+). Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. Kinetic energy release distributions of dissociation fragments are seen to extend from 0 to 6 eV for the heaviest; fragment C(2)(+), up to 11.0 eV for CD(+) 14.2 eV for C(+) and 11.2 eV for D(+) products.

Diagnostic-based modeling on a micro-scale atmospheric-pressure plasma jet

Diagnostic-based modeling (DBM) actively combines complementary advantages of numerical plasma simulations and relatively simple optical emission spectroscopy (OES). DBM is applied to determine spatial absolute atomic oxygen ground-state density profiles in a micro atmospheric-pressure plasma jet operated in He–O2. A 1D fluid model with semi-kinetic treatment of the electrons yields detailed information on the electron dynamics and the corresponding spatio-temporal electron energy distribution function. Benchmarking this time- and space-resolved simulation with phase-resolved OES (PROES) allows subsequent derivation of effective excitation rates as the basis for DBM. The population dynamics of the upper O(3p3P) oxygen state (λ = 844 nm) is governed by direct electron impact excitation, dissociative excitation, radiation losses, and collisional induced quenching. Absolute values for atomic oxygen densities are obtained through tracer comparison with the upper Ar(2p1) state (λ = 750.4 nm). The resulting spatial profile for the absolute atomic oxygen density shows an excellent quantitative agreement to a density profile obtained by two-photon absorption laser-induced fluorescence spectroscopy.