Resonant vibrational excitation and de-excitation of N2(v) by low-energy electrons

We have calculated cross sections and rate coefficients for low-energy electron impact excitation of the nitrogen molecule from vibrationally excited levels N2(v) 1-8. Calculations are performed in the 2Pig shape resonance energy region, from 0 to 5 eV. The cross sections are determined by using our recent integral cross section measurements of the ground level vibrational excitation and the most recent cross sections for elastic electron scattering, applying the principle of detailed balance. The rate coefficient calculations are performed for the Maxwellian electron energy distribution. By using extended Monte Carlo simulations, the electron energy distribution functions (EEDF) and the rate coefficients are also determined for the nonequilibrium conditions, in the presence of the homogeneous external electric field for the typical, moderate values of the electric field over gas number density ratios, E/N.     

Rate coefficients for resonant vibrational excitation of CO

Rate coefficients for vibrational excitation of the carbon-monoxide molecule, via the ²Π shape resonance in the energy region from 0 eV to 5 eV have been calculated. Calculations are performed for a Maxwellian electron energy distribution by using our recent experimentally measured differential cross sections for excitation of the first 10 vibrational levels. By using extended Monte Carlo simulations the electron energy distribution functions (EEDFs) and rate coefficients are determined in non-equilibrium conditions, in the presence of a homogeneous electric field. Calculations are performed for typical, moderate values of the electric field over gas number density ratios, E/N. A difference between Maxwellian and non-equilibrium rate coefficients was found due to a specific shape of the electron energy distribution function under the considered conditions.     

Excitation of Electronic States of CO in Radio-Frequency Electric Field by Electron Impact

Electron impact excitation rate coefficients for singlet and triplet electronic states of the carbon monoxide molecule have been calculated under non-equilibrium conditions in the presence of radio-frequency electric field. A Monte Carlo simulation of electron transport has been performed in order to determine non-equilibrium electron energy distribution functions within one period of applied electric field. By using these distribution functions and corresponding cross sections, the excitation rate coefficients have been calculated for all electronic states of CO in the frequency range from 13.56 up to 500 MHz, at reduced root mean square electric field values ranging from 200 to 700 Td. We expect these rates to be valuable for modeling radio-frequency CO plasmas since excited neutrals exhibit greater chemical reactivity than neutrals in ground electronic state, hence altering many properties of plasma.     

Excitation of Electronic States of N<Subscript>2</Subscript> in Radio-Frequency Electric Field by Electron Impact

Excitation of electronic states of the N₂ molecule by electron impact is recognized as an essential process in nitrogen plasmas that strongly impacts their chemical reactivity and other properties. Many surface and coating technologies are based on radio-frequency plasma discharges in nitrogen. In this paper the electron impact excitation rate coefficients for singlet and triplet electronic states of the N₂ molecule have been calculated in non-equilibrium conditions in the presence of a radio-frequency electric field. A Monte Carlo simulation has been performed in order to determine non-equilibrium electron energy distribution functions within one period of the electric field. By using these distribution functions, the excitation rate coefficients have been obtained in the frequency range from 13.56 up to 500 MHz, at reduced electric field values from 200 to 700 Td.     

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.     

Cross sections and rate coefficients for electronic excitation of the triplet states of the hydrogen molecule in different vibrational levels

Cross sections for the excitation of the triplet state of H₂ from different vibrational levels of the ground electronic state have been calculated by using the Gryzinski approximation. The results for the ground vibrational level are in satisfactory agreement with the corresponding values obtained by the quantum mechanical close coupling method. The calculated cross sections have been used to generate rate coefficients for the excitation of the triplet states by using a self-consistent electron energy distribution function, obtained by numerical integration of the Boltzmann equation. The results show a strong increase of the different rate coefficients with increasing the vibrational quantum number.     

Dissociative Excitation of Nickel Quintet States upon Electron Collisions with NiCl2Molecules

Dissociative excitation of nickel atom quintet states upon electron collisions with nickel dichloride molecules was studied by means of the extended crossed beams technique combined with optical spectroscopy. Forty excitation cross sections were measured at an electron energy of 100 eV. Two optical excitation functions were detected in the electron energy range 0–100 eV. Possible reaction channels at low electron energies were analyzed. Cross sections for dissociative excitation were compared to those for direct excitation upon electron collisions with nickel atoms.     

Systematic trends in the radial matrix elements of E1 transitions in Li,F, Na and Cu isoelectronic sequences

Collision cross sections are calculated using the R-matrix method for excitations between the three lowest LS states for Na-like Cu ion. The complex resonance structures are investigated. The collision rate coefficients have been calculated assuming a Maxwellian distribution of electron-impact energies. The results of the collision cross sections are in good agreement with those of the other theory.     

Formation of excited tin ion in collisions of slow electrons with SnCl2 molecules

Formation of excited tin ions in collisions of slow electrons with tin dichloride molecules was studied experimentally. Fifty excitation cross sections for SnII at an electron energy 100 eV were measured. Six optical functions for dissociative excitation were detected in the electron energy range 0–100 eV. Cross sections for direct and dissociative excitation of several lines were compared. Dissociative excitation in the spectral bands of two transitions in SnCl was also studied.     

Cross sections for low-energy electron scattering from adenine in the condensed phase

Measurements of the vibrational and electronic excitation of a sub-monolayer up to a monolayer film of adenine were performed with a high resolution electron energy-loss (HREEL) spectrometer. The integral cross sections (over the half-space angle) for excitation of the normal vibrational modes of the ground electronic state and electronically excited states are calculated from the measured reflectivity EEL spectra. Most cross sections for vibrational excitation are of the order of 10(-17) cm(2), the largest being the out-of-plane wagging of the amino-group and the six-member ring deformations. A wide resonance feature appears in the incident energy dependence of the vibrational cross sections at 3-5 eV, while a weak shoulder is present in this dependence for combined ring deformations and bending of hydrogen atoms. For the five excited electronic states, at 4.7, 5.0, 5.5, 6.1 and 6.6 eV, the cross sections are of the order of 10(-18) cm(2), except in the case of the state at the energy of 6.1 eV, for which it is two to three times higher.     

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.     

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.     

Rotational excitations in para-H2+para-H2 collisions: full- and reduced-dimensional quantum wave packet studies comparing different potential energy surfaces

We study the process of rotational excitation in the collisions of para-H(2) with para-H(2) by propagating wave packets with the multiconfiguration time-dependent Hartree (MCTDH) algorithm. Transition probabilities are then calculated by the method of Tannor and Weeks based on time-correlation functions. Calculations were carried out up to a total angular momentum of J=70 to compute integral cross sections up to 1.2 eV in collision energy and thermal rate coefficients from 100 to 3000 K. The process is studied on the full-dimensional potential energy surface of Boothroyd-Martin-Keogh-Peterson (BMKP) as well as on the rigid rotor surface of Diep and Johnson. We test the validity of the rigid rotor approximation by also considering two rigid rotor restrictions of the BMKP potential energy surface (PES). Additionally, we investigate a variant of the BMKP PES suggested by Pogrebnya and Clary [Chem. Phys. Lett. 363, 523 (2002)] with reduced anisotropy. We compare our results with previous theoretical data for the cross sections and with experimental data for the rate coefficients at low temperatures.     

Electron scattering in Pt(PF3)4: elastic scattering, vibrational, and electronic excitation

Experimental absolute differential cross sections for elastic scattering, and for vibrational and electronic excitation of Pt(PF(3))(4) by low-energy electrons are presented. The elastic cross sections have a deep angle-dependent Ramsauer-Townsend minimum (E(min) = 0.26 eV at θ = 135°). The angular distributions of the elastic cross section at and above 6.5 eV show an unusually narrow peak at an angle which decreases with increasing energy (it is at 40° at 20 eV). Wavy structure is observed at higher angles at 15 and 20 eV. Vibrational excitation cross sections reveal five shape resonances, at 0.84, 1.75, 3.3, 6.6, and 8.5 eV. The angular distributions of the vibrational cross sections have a strong forward peak and are nearly isotropic above about 60°. Electronically excited states are characterized by electron energy-loss spectra. They show a number of unstructured bands, the lowest at 5.8 eV. They are assigned to Rydberg states converging to the 1st and 2nd ionization energies. The cross sections for electronic excitation have very high forward peaks, reaching the value of 50 A?(2) at 50 eV and 0° scattering angle. Purity of the sample was monitored by the very low frequency (26 meV) Pt-P stretch vibration in the energy-loss spectra.     

Dissociative Excitation Processes upon Collisions of Slow Electrons with MgBr2 Molecules

Dissociative excitation of MgI, MgII, and MgBr upon e–MgBr₂ collisions was experimentally studied. The excitation cross sections of the spectral lines of MgI and MgII, as well as two sets of bands of the MgBr molecule were measured at an incident electron energy of 100 eV. In the electron energy range from the excitation threshold to 100 eV, ten optical excitation functions with a complex structure were recorded. The principal pathways of dissociative excitation of MgI, MgII, and MgBr by slow electrons are discussed.     

Photoionization of the 2s, 2p and 3s subshells of Mg+

We present calculations of total and partial photoionisation cross sections for the 2s, 2p and 3s subshells of Mg⁺ from their ionisation thresholds up to a photon energy of 220 eV. Photoelectron angular distribution parameters are also calculated for the 2p subshell. Oscillator strengths are given for discrete structure, and calculated energy levels compared with experimental values. The significance of electron correlations and two electron excitation for this atomic ion is discussed.     

Dissociative excitation and fragmentation of S8 by electron impact

The vacuum-ultraviolet emission spectrum from 136 nm to 168 nm following the dissociative excitation of a predominantly S(8) target by electron impact at 100 eV incident energy was measured. The relative cross sections for the dominant multiplets at 138.9, 142.9, 147.9, and 166.7 nm are presented. Excitation functions are shown for electron-impact energies from below threshold to 360 eV for the two most prominent emissions at 142.5 nm and 147.4 nm. Five thresholds are clearly apparent in both excitation functions. For the four highest energy channels, the energy separation between the adjacent thresholds is approximately constant and the cross sections reduce regularly as the threshold energies increase. We suggest possible fragmentation pathways of the dissociating S(8) molecule that reproduce the energies of our observed thresholds.     

Absolute electronic excitation cross sections for low-energy electron (5-12 eV) scattering from condensed thymine

The absolute cross sections for electronic excitations of thymine by electron impact between 5 and 12 eV are determined by means of electron-energy loss (EEL) spectroscopy for the molecule deposited at submonolayer coverage on an inert Ar substrate. The lowest EEL features at 3.7 and 4.0 eV are attributed to the excitation of the triplet 1 3A'(pi --> pi*) and 1 3A'(n --> pi*) valence states of the molecule. The higher EEL features located at 4.9, 6.3, 7.3, and 9 eV with a weak shoulder around 6 eV are ascribed mostly to triplet valence (pi --> pi*) excitation manifold of the molecule. The energy dependence of the cross section for both the lowest triplet valence excitations shows essentially a peak at about 5 eV reaching a value of 2.9 x 10(-17) cm2. The cross sections for the higher EEL features are generally characterized by a common broad maximum around 8 eV. The latter reaches a value of 1.36 x 10(-16) cm2 for the combined 6 and 6.3 eV excitation region. The maxima in the present cross sections are found to correspond to the resonances that have been reported at about the same energies in the O- yield from electron impact on thymine in the gas phase.     

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%.     

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.