Absolute balmer line and NH(c Π→b Σ, 0-0) vibrational band emission cross sections from NH3 molecules excited by electron impact

The absolute Balmer line emission cross sections are determined in the processes of the electron impact dissociative excitation of ammonia. The optical excitation functions measured for these lines were investigated in the energy range 50–500 eV and normalized by the He benchmark procedure. The molecular continuum contribution has been eliminated from the obtained data. After that, the measured data have been corrected with the collection efficiency factor F to compensate the loss of optical signal due to non-thermal energies of the H excited fragments. The results for kinetic energy distribution functions for the ammonia molecules have been used for the F determination. The optical emission cross sections are determined with the accuracy of ± 15%. The cross sections for the NH(c ¹Π→b ¹Σ⁺, 0-0) vibrational band have also been determined with an accuracy of ± 25%.     

Dissociative excitation of some aliphatic hydrocarbons by electron impact

Cross sections and threshold energies are compared for radiation from fragments produced by electron impact on methane, ethylene, ethane and acetylene. Some previous measurements have been repeated. The emission cross sections for corresponding Balmer radiation are within 10% equal for these hydrocarbons. Also the thresholds for Balmer radiation lie close together. These results can be explained in a model where H fragments arise from Rydberg states excited by promotion of an inner valence electron to a non-bonding orbital. In this model a comparison between dissociative ionization yielding H⁺ and dissociative excitation yielding H⁺ is made. For radiation from molecular fragments it is shown that the CH(A² Δ-X²Π) emission cross sections are particularly high in the case of acetylene. The electron impact data appear to be consistent with photoabsorption data.     

Application of the “helium excitation standard” procedure to some hydrocarbon Hβ absolute emission cross section measurements

Absolute emission cross sections for H_β radiation are presented. The H_β emission is obtained by dissociative electron excitation of propane, propylene, n-butane and 1-butene under binary collision conditions. Optical excitation functions measured for these transitions are normalized by the “helium excitation standard” procedure, using the 4 ¹S-2 ¹P transition in He as a standard at an electron impact energy of 100 eV. The results obtained are determined with an accuracy of ± 15%. The investigated energy interval is 50–500 eV. The experimental results show that the emission cross sections for H_β radiation are not independent of the number of atoms in the parent molecule.     

Dissociative excitation of benzene by electron impact

In the wavelength region 1850–9000 Å radiation from H, C and CH fragments is observed as a result of the dissociative excitation of benzene by electron impact (0–1000 eV). Emission cross sections and threshold energies have been determined for the Balmer series of the hydrogen atom and the A²Δ - X²Π emission of the CH fragment.     

Dissociative Excitation of Nitromethane Induced by Electron Impact in the Ultraviolet – Visible Spectrum

The excitation of nitromethane (CH₃NO₂), which is an important propellant and prototypic molecule for large class of explosives, has been investigated by electron impact and subsequent emission of photons in the UV-VIS spectral region between 300 nm and 670 nm. Emission spectrum of nitromethane was recorded at an electron energy of 50 eV. New dissociative excitation channels were observed through the appearance of different CH, CN, NH, OH and NO bands, and the Balmer series of atomic hydrogen. In addition, relative emission cross sections were recorded for the transitions of selected fragments. The emission spectrum was captured with significantly higher resolution in comparison to previous studies.     

Dissociative excitation of the singly charged lead ion in electron collisions with PbCl2 molecules

Inelastic collisions of slow electrons with lead dichloride molecules yielding excited lead ions in a single encounter event were studied by the extended crossed-beam technique. At an incident electron energy of 100 eV, 67 cross sections for dissociative excitation of PbII spectral lines were measured. Three optical excitation functions were determined in the electron energy range 0–100 eV. The obtained results are compared with data on excitation cross sections of PbII in electron-atom collisions.     

Atomic and molecular emissions in laser-induced breakdown spectroscopy

This article summarizes measurements and analysis of hydrogen Balmer series atomic lines following laser-induced optical breakdown. Electron number density on the order of 1 × 10²⁵ m^− 3 can be measured using H_α Stark width and shift in the analysis of breakdown plasma in 1 to 1.3 × 10⁵ Pa, gaseous hydrogen. The H_β line can be utilized for electron number density up to 7 × 10²³ m^− 3. The historic significance is elaborated of accurate H_β measurements. Electron excitation temperature is inferred utilizing Boltzmann plot techniques that include H_γ atomic lines and further members of the Balmer series. Laser ablation of aluminum is discussed in view of limits of application of the Balmer series. H_β and H_γ lines show presence of molecular carbon in a 2.7 and 6.5 × 10⁵ Pa, expanding methane flow. Diagnostic of such diatomic emission spectra is discussed as well. Laser-induced breakdown spectroscopy historically embraces elemental analysis, or atomic spectroscopy, and to a lesser extent molecular spectroscopy. Yet occurrence of superposition spectra in the plasma decay due to recombination or due to onset of chemical reactions necessitates consideration of both atomic and molecular emissions following laser-induced optical breakdown. Molecular excitation temperature is determined using so-called modified Boltzmann plots and fitting of spectra from selected molecular transitions. The primary interest is micro-plasma characterization during the first few micro-seconds following optical breakdown, including shadowgraph visualizations.     

Excitation of analytes and enhancement of emission intensities in a d.c. plasma jet: a critical review leading to proposed mechanistic models

Experimental data and theoretical criteria are used to critically review existing models for analyte emission enhancement in the 3-electrode d.c. plasma (DCP). The analytical zone is characterized as a non-optically thin recombining plasma in partial thermodynamic equilibrium (PTE). Spectrochemical excitation the authors ascribe largely to: (1) argon resonance line radiative transport; (2) inversion of optically pumped argon states; (3) inversion of analyte populations by Franck-Condon collisions with argon; (4) energy cascading in analytes via a multitude of channels. Adding an easily ionized element (EIE): (1) induces additional resonance line radiative transfer; (2) raises electron densities in cooler, analyte-rich plasma margins; (3) locally increases argon optical absorption cross sections via Stark broadening; (4) redistributes ohmic heating. Coupling between the proposed mechanisms is non-linear. Relationships between radiative transfer and collisional redistribution and (1) background suppression by EIE and (2) analyte emission enhancement by helium are also examined. Similarities between DCP and inductively coupled plasma (ICP) excitation mechanisms are noted and practical implications are addressed.     

Experimental study of electron impact excitation of multiply charged ions

We report on measurements of angular differential cross sections for the excitation of multiply charged ions by electron impact. An ion beam is crossed by an electron beam; electrons which are inelastically scattered at different angles are identified by their energy loss due to the excitation process. Absolute excitation cross sections are obtained by comparing the signals of the elastic and the inelastic electron-ion scattering. Results obtained for the 3s→3p excitation of Ar⁷⁺ are discussed.     

Emission cross sections from fragments produced by electron impact on silane

Optical emission spectroscopy in the visible and near UV of a silane plasma was performed in a low pressure hot cathode glow discharge bounded into a magnetized multipolar wall. Emissions from Si, Si⁺, SiH, SiH⁺ and H are shown to originate from the dissociative excitation of silane molecules by electron impact. The absolute cross sections for the various photoemissive processes were measured in the 17–68 eV range. The relevance of optical emission spectroscopy to silane plasma diagnostics is discussed.     

Translational energy distribution of excited hydrogen atoms produced by 0–100 eV electron impact dissociation of SiH4 and GeH4

Translational energy distributions of H(n = 4) produced in e-SiH₄ and e-GeH₄ collisions have been obtained by differentiating the Doppler profile of Balmer H_β emission line. Results are similar for both molecules in contrast with CH₄. Two energy components centered at about 0.4 eV (respectively 0.3 eV) and 2.5 eV (respectively 2.0 eV) are attributed to different dissociative excitation processes of the parent molecule SiH₄ (respectively GeH₄). A third broad component at higher energy is also evidenced.     

Electron capture and target excitation in slow ion-alkali atom collisions

Experimental cross sections for single electron capture and target atom resonance line emission in impact of singly charged ions (He⁺, Ne⁺, Ar⁺, C⁺, N⁺, O⁺) on respectively Li(2s) and Na(3s) are presented and compared with semiempirical calculations of the Demkov-Olson type. It is shown that such calculations can still be useful despite involvement of metastable primary ion admixtures and several final states. In addition, observed undulations in the impact energy dependence of electron capture — and target excitation cross sections are discussed.     

Total cross-sections for electron scattering from iron at 10–5000 eV using a model optical potential

We report calculations on the total (elastic plus inelastic) electron-scattering cross sections in the energy range 10–5000 eV. A model complex optical potential, composed of static, exchange, polarisation and absorption terms, is employed to describe the collision system at each electron energy. The Iron atom is described by Dirac-Hartree-Fock-Slater self-consistent charge density. The complex phase shifts are computed in a variable phase approach. The absorption cross sections are compared with the experimental results. The experimental absorption cross sections are obtained by adding the experimental ionisation cross sections and available experimental excitation cross sections for electron impact of the allowed transitions a⁵ D → (x,y,z)⁵ D ⁰, (w,y,z)⁵ P ⁰. We have good qualitative agreement between our results and the experimental results available below 200 eV. The Born-Bethe parameters are also calculated. Elastic differential cross-sections with and without absorption are also reported at a few selected energies.     

Emission cross sections of balmer-α,β,γ radiation for electrons (0–2000 eV) on H2 and D2

Absolute emission cross sections (σ_em) of Balmer-α,β, γ radiation have been measured in the case of electrons (0–2000 eV) incident on H₂ and D₂. The H Balmer σ_em's are larger than the corresponding D Balmer σ_em's for every electron impact energy. It has been found that this isotope effect varies strongly as a function of the incident electron energy below 100 eV. The isotope effect will be interpreted using the H₂ and D₂ potential energy curves.     

Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV-Visible diode array and excitation emission matrix fluorescence

Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000 Da. Both the number-average (M_n) and weight-average (M_w) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M_n range of 0.8–1.1 kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S_R = S_275–295:S_350–400) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40 nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition.     

Fine structure excitation transfer between the lithiumD-lines by collisions with cesium atoms

Applying resonant Doppler-free 2-photon laser spectroscopy with thermionic diode detection, the cross sections for the excitation energy transfer of the collisional process⁷Li*(2P _1/2+Cs(6S _1/2)→⁷Li*(2P _3/2)+Cs(6S _1/2) have been measured. The experimental cross sections, σ^Li-Cs (1/2→3/2)=890 Ų and σ^Li-Cs (3/2→1/2)=430 Ų, are compared with theoretical data obtained by a sudden impact approximation approach taking into account the long-range interaction potentials only. The calculated cross sections show an excitation mixing process at large internuclear distances where Li-Cs dipole-dipole and dipole-quadrupole interaction forces are predominant.     

Absolute emission cross sections of fragments produced by 0–100 eV electron impact on NH3

The absolute emission cross sections (ECSs) of excited fragments produced by electron impact on NH₃ were investigated in an incident electron energy range of 0–100 eV. The ECS of the NH(c¹II-a¹Δ) transition was 20.4 × 10⁻¹⁹ cm² and that of the NH(A³II-X³Σ) transition was 28.1 × 10⁻¹⁹ cm² at an incident electron energy of 100 eV. The estimated uncertainties of these values were 16 and 20%, respectively. The ECSs of Balmer lines were also determined. The effective rotational temperature of the NH(c¹II, v = 0) state at the incident electron energy of 100 eV was obtained to be (1.24 ± 0.34) × 10³ K by observing the intensity destribution of the Q branch of the NH(c¹II, v = 0-a¹Σ, v″ = 0) emission band.     

Separated projectile and targetK x-ray production in symmetric heavy ion collisions as a function of the target thickness

Cross sections for projectile and targetK x-ray emission have been measured as a function of the target thickness for the symmetric systems Ni-Ni, Cu-Cu, Nb-Nb, Ag-Ag at energies between 75 and 105 MeV. The projectileK x-rays were separated from the target ones by using the Doppler shift. K_α and K_β energy shifts and K_β to K_α intensity ratios were also determined and used to calculateM- andL-ionization and the correspondingK-fluorescence yield for both collision partners. At non vanishing small target thicknesses, the targetK-vacancy production cross section is generally larger than that of the projectile. By analysing the target thickness dependence of the cross sections with a two component model which takes also into account the evolution of the projectileM-,L- andK-shell population inside the solid, targetK-vacancy sharing and cross sections forKK electron capture in symmetric systems could be determined. These results are in good agreement with molecular orbital model predictions.     

Collisional depolarization of excited114Cd 5s5p 3 P 1 atoms by collisions with molecular gases

Depolarization of excited¹¹⁴Cd 5s5p ³ P ₁ atoms induced by collisions with various molecular gases (N₂, H₂, D₂, CO, CO₂, CH₄, C₂H₆, C₂H₄) has been investigated using polarized fluorescence spectroscopy. After pulsed optical excitation of the Cd 5³ P ₁ level with appropriately polarized light the temporal behaviour of Zeeman quantum beats has been observed showing the influence of collisional destruction of orientation and alignment. By analyzing the signal curves at different molecular gas pressures the corresponding depolarization cross sections for¹¹⁴Cd atoms in the 5³ P ₁ state have been obtained. With regard to a test of a nuclear spin decoupling model for the collisions the cross sections were compared with previously measured hyperfine structure transfer cross sections of¹¹³Cd 5s5p ³ P ₁ atoms.     

Dissociative Excitation of Lead Atom 6pnd Levels in Electron Collisions with Lead Dichloride Molecules

Dissociation excitation of the 6pnd levels of the lead atom in e–PbCl₂ collisions resulting in the population of metastable PbI levels via cascade transitions was experimentally studied. Cross sections for the cascade population of the 6p2 levels at an incident electron energy of 100 eV were determined. The dissociative excitation cross sections measured were compared with the excitation cross sections in e–Pb collisions.