Electron-impact Excitation Cross Sections Research Articles

Differential cross sections for electron-impact excitation of laser-excited 174Yb (…6s6p 3P1)

We have measured and calculated electron-impact excitation cross sections from the laser-excited Yb (…6s6p 3P1) level to the higher-lying Yb (…6s6p 1P1), (…6s5d 3D1, 2, 3) and (…6s7s 3S1) levels at 20 eV impact energy. Measurements were performed using a crossed-beam apparatus, with resonant laser radiation incident on the electron–atom interaction region, providing excited-state atomic targets. Calculations were performed in the relativistic convergent close-coupling (RCCC), convergent close-coupling (CCC), and relativistic distorted-wave (RDW) approximations. We present measured and calculated partial differential cross sections (), which relate to differential cross sections and electron-impact coherence parameters through the target state anisotropy introduced by the laser geometry. Using theoretical predictions of the h electron-impact coherence parameter, we present the differential cross section for the 3P1–3S1 excitation.

Generalized oscillator strengths and integral cross sections for the valence-shell excitations of oxygen studied by fast electron impact

The generalized oscillator strengths and differential cross sections for electron-impact excitation of the ${A}^{\ensuremath{'}} {}^{3}{\ensuremath{\Delta}}_{u}$, Schumann-Runge continuum, and $E {}^{3}{{\ensuremath{\Sigma}}_{u}}^{\ensuremath{-}}({\ensuremath{\nu}}^{\ensuremath{'}}=0,1)$ states of oxygen have been determined at an incident electron energy of 2.5 keV. Good agreement is found between the present generalized oscillator strengths of the Schumann-Runge continuum and $E {}^{3}{{\ensuremath{\Sigma}}_{u}}^{\ensuremath{-}}({\ensuremath{\nu}}^{\ensuremath{'}}=1)$ states and the previous ones measured at higher impact energies, which indicates that the first Born approximation is satisfied at an impact energy of 400 eV. However, the large difference between the present generalized oscillator strengths of the ${A}^{\ensuremath{'}}{{}^{3}\ensuremath{\Delta}}_{u}$ and $E {}^{3}{{\ensuremath{\Sigma}}_{u}}^{\ensuremath{-}}({\ensuremath{\nu}}^{\ensuremath{'}}=0)$ states and the previous ones is observed, and the possible reasons are discussed. The $\mathit{BE}$-scaled (binding energy $B$ and excitation energy $E$) or $\mathit{BEf}$-scaled ($B$, $E$, and $f$ values) integral cross sections for the ${A}^{\ensuremath{'}}{{}^{3}\ensuremath{\Delta}}_{u}$, Schumann-Runge continuum, and $E {}^{3}{{\ensuremath{\Sigma}}_{u}}^{\ensuremath{-}}({\ensuremath{\nu}}^{\ensuremath{'}}=0,1)$ states from its threshold to 5 keV were calculated based on the present generalized oscillator strengths. The present scaled integral cross sections are in good agreement with the previous ones measured at the moderate impact energies except for that of the ${A}^{\ensuremath{'}}{{}^{3}\ensuremath{\Delta}}_{u}$ state.

Optical emission measurements of electron energy distributions in low-pressure argon inductively coupled plasmas

Optical modeling of emissions from low-temperature plasmas provides a non-invasive technique to measure the electron energy distribution function (EEDF) of the plasma. While many models assume the EEDF has a Maxwell–Boltzmann distribution, the EEDFs of numerous plasma systems deviate significantly from the Maxwellian form. In this paper, we present an optical emission model for the Ar(3p54p → 3p54s) emission array which is capable of capturing details of non-Maxwellian distributions. Our model combines previously measured electron-impact excitation cross sections with Ar(3p54s) number density measurements and emission spectra. The model also includes corrections for radiation trapping of the Ar(3p54p → 3p54s) emission lines. Results obtained with this optical technique are compared with corresponding Langmuir probe measurements of the EEDF for Ar and Ar/N2 inductively coupled plasma systems operating under a wide variety of source conditions (1–25 mTorr, 20–1000 W, %N2 admixture). Both the optical emission method and probe measurements indicate the EEDF shapes are Maxwellian for low electron energies, but with depleted high energy tails.

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.

Excitation of the 3p 55p levels of argon from the 3p 54s metastables

Measurements have been reported recently of cross sections for electron-impact excitation of the argon 3p 55p manifold of levels from the 3p 54s metastable levels. We report results of calculations of the direct excitation cross sections at electron-impact energies from threshold to 500 eV for these transitions. The collision cross sections are calculated in the scaled plane-wave Born approximation (scaled PWB) developed by Kim. This scaling transforms the Born cross sections for dipole-allowed and spin-allowed excitations into reliable cross sections that compare as well with accurate measurements as does the sophisticated and more complex convergent close coupling method. We have used jj coupling in a single LS configuration Dirac-Fock calculation to describe the target wavefunctions. The optical emission cross section measurements, by Jung et al., include estimated contributions of 25% or more from cascading from higher excited levels and so are larger than direct excitation cross sections. Nevertheless, our cross sections agree reasonably well with the measurements for transitions where the core j value remains unchanged, except for the 1s 5–3p 8 excitation. The results do not agree well with the measurements when the core j value changes, as similarly seen in the results of recent relativistic distorted wave calculations by Sharma et al.

II. Electron-impact dissociative excitation of C2H 2 + and C2D 2 +

Absolute cross-sections for electron-impact dissociative excitation of C2H (2) (+) and C2D (2) (+) to C2D+, CH+, C+, C (2) (+) , H+ and CH (2) (+) singly-charged fragments have been estimated. The animated crossed-beams method is applied in the energy range from the corresponding reaction threshold up to 2.5 keV. Individual contributions for dissociative excitation are determined from the analysis of the total absolute cross-sections measured for each detected product. The dissociative excitation cross-sections are seen to decrease for more than an order of magnitude, from C2D+ (12.6 +/- 0.3) x 10(-17) cm(2) over CH+ (6.37 +/- 0.12) x 10(-17) cm(2), C+ (4.09 +/- 0.13) x 10(-17) cm(2), C (2) (+) (3.5 +/- 0.2) x 10(-17) cm(2) and H+ (1.57 +/- 0.11) x 10(-17) cm(2) to CH (2) (+) (3.6 +/- 0.5) x 10(-18) cm(2). Cross-section values and threshold energies are compared with the data which are available from the literature.

Relativistic R-matrix close-coupling method based on the effective many-body Hamiltonian: electron-impact excitation of electric dipole-allowed and spin-forbidden transitions of the S4 + ion

Electron-impact excitation cross sections of the electric dipole-allowed 3s2 1S0 → 3s3p 1Po1 and spin-forbidden 3s2 1S0 → 3s3p 3Po0, 1, 2 transitions in Mg-like sulfur (S4 +) are calculated using the effective many-body Hamiltonian R-matrix close-coupling method. The calculated near-threshold electron-impact excitation cross sections for the electric dipole-allowed 3s2 1S0 → 3s3p 1Po0 transitions are in good agreement with experimental results. The electron-impact excitation cross sections for the spin-forbidden 3s2 1S0 → 3s3p 3Po0, 1, 2 transitions have been computed to benchmark theoretical accuracy against future experimental studies.

Emission lines of Fe xi in the 257-407 Å wavelength region observed in solar spectra from EIS/Hinodeand SERTS

Theoretical emission-line ratios involving Fe xi transitions in the 257–407 Å wavelength range are derived using fully relativistic calculations of radiative rates and electron impact excitation cross-sections. These are subsequently compared with both long wavelength channel Extreme-Ultraviolet Imaging Spectrometer (EIS) spectra from the Hinode satellite (covering 245–291 Å) and first-order observations (∼235–449 Å) obtained by the Solar Extreme-ultraviolet Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 Å lines of Fe xi are detected in two EIS spectra, confirming earlier identifications of these features, and 276.36 Å is found to provide an electron density (Ne) diagnostic when ratioed against the 257.55 Å transition. Agreement between theory and observation is found to be generally good for the SERTS data sets, with discrepancies normally being due to known line blends, while the 257.55 Å feature is detected for the first time in SERTS spectra. The most useful Fe xi electron density diagnostic is found to be the 308.54/352.67 intensity ratio, which varies by a factor of 8.4 between Ne= 108 and 1011 cm−3, while showing little temperature sensitivity. However, the 349.04/352.67 ratio potentially provides a superior diagnostic, as it involves lines which are closer in wavelength, and varies by a factor of 14.7 between Ne= 108 and 1011 cm−3. Unfortunately, the 349.04 Å line is relatively weak, and also blended with the second-order Fe x 174.52 Å feature, unless the first-order instrument response is enhanced.

Precision calculation of low-energy electron-impact excitation cross sections of sodium

The collision cross sections of sodium from the ground state to the first four excited states at the incident energy ranging from 0 to 5.4 eV are calculated using the $R$-matrix method. The convergences of the cross sections are checked systematically by using four sets of high-quality target states, i.e., 5, 9, 14, and 19 physical target states. The influence of the Rydberg target states on the collision cross sections is also elucidated at higher incident energies; i.e., the amplitude of resonance structures will decrease with respect to the effective quantum number $\ensuremath{\upsilon}$ of the Rydberg target states. This result is very useful for the calculations of these cross sections at intermediate energy with finite target states by combining the partial-wave-expansion methods valid at low energy with the first Born approximation method valid at high energy, which would be of great importance in obtaining complete cross-section data for related scientific fields.

Theoretical study on electron impact excitation of highly charged Ne-like ions

The electron impact excitation （EIE） cross sections from the ground state to all of the 2s22p53 and 2s2p63（=s, p, d） states along the Ne-like isoelectronic sequence of ions （Z=50—57） have been calculated by using the multi-configuration Dirac-Fock package GRASP92 and the fully relativistic distorted-wave program REIE06. In the calculations, the relativistic effects and electron correlation effects are considered systematically. Based on those calculations, the EIE cross sections along the Ne-like isoelectronic sequence of ions for different incident electron energies are discussed, and some important conclusions are drawn. We also study the influence of the correlation effects on the values of the 3C/3D line-intensity ratio ［3C: （2p1/23d3/2）1→2s22p6 1S0, 3D: （2p3/23d5/2）1→2s22p6 1S0］ along the Ne-like sequence. A comparison is made between the present results and previous theoretical calculations and experimental results for the EIE cross sections in Ba46+ ions, and a good agreement is obtained.

Integral cross sections for electron-impact excitation of the C 3Πu, E 3Σ+g and a″ 1Σ+g states of N2

Absolute integral cross sections (ICSs) for electron-impact excitation of the C 3Πu, E 3Σ+g and a″ 1Σ+g states of N2 from the X 1Σ+g (v″ = 0) ground-state level are presented for electron-impact energies of 13, 15, 17.5, 20, 25, 30, 50 and 100 eV. ICSs are also presented for the individual vibrational levels of the C 3Πu state (v′ = 0, 1, 2, 3 and 4). The ICSs were derived from recent differential cross sections determined via energy-loss spectroscopy (Malone et al 2009 Phys. Rev. A 79 032704; Malone et al 2009 Phys. Rev. A 79 032705). The results are compared with existing measurements in the literature.

Electron impact excitation cross sections for argon of interest in plasma modeling

We have applied the relativistic distorted-wave (RDW) method to calculate electron impact excitation cross sections for a number of fine-structure transitions in argon atoms. These include excitation from the ground state to the 3p53d, 3p54d and 3p55s fine-structure levels, from the non-metastable levels of the 3p54s configuration to all ten fine-structure levels of the 3p54p configuration as well as from the 3p54p levels to the levels of the excited 3p53d, 3p54d, 3p55d configurations. Differential cross sections are reported for those transitions where experimental results are available for comparison.

Electron impact excitation cross sections of N2

Differential cross sections (DCSs) are presented for electron impact excitation of the C3Πu, E3Σ+g, and a′′ 1Σ+g states in N2 from the ground state. Vibrationally-resolved DCSs are also presented for excitation of the C 3Πν (ν′) state, where ν′ = 0–4. The DCSs were obtained from measurements of energy-loss spectra in the region of 10.75 eV to 12.75 eV measured at numerous incident energies between 13 eV and 100 eV, and for scattering angles ranging from 5° to 130°. Relative excitation probabilities for the vibrational levels of the C 3Πu state are shown to demonstrate non-Franck-Condon behavior for excitation energies less than approximately 50 eV. Integral cross sections are also presented for these excitation processes. These results are compared with existing measurements.

Nitrogen lasers, optical devices of variable gain coefficient: Theoretical considerations

Abstract Based on our previous measurements on the gain values by employing an oscillator amplifier (OSC–AMP) N2-laser system of variable AMP electrode lengths, a calculation has been made for evaluation of the gain coefficient using rate equations. It is shown both numerically and analytically that small signal gain, g0, is following our experimental observations of g 0 = m + n / l AMP , where m and n are some constants, and lAMP is the length of the amplifier. For simplifying the calculation in the OCS–AMP circuit, an experimental condition of imposing the OSC-open-circuit operation has been applied, where the voltage waveforms from the relevant sections of the Blumlein circuit have been used for evaluation of the circuit parameters. Due to the fact that during the past years different cross-sections for the electron-impact excitations from the ground to the upper, N2(C), and lower, N2(B), have been introduced, our experimental observations have also been applied to examine the effect of introduced electron impact-excitation cross-sections on the g 0 ( l AMP ) behavior.

The generalized oscillator strengths for Na: monopole, dipole and quadrupole transitions

The generalized oscillator strengths (GOSs) of Na from the ground state [2p63s]32S excited to [2p6(n + 1)s](n + 1)2S, [2p6np]n2P, [2p6nd]n2D(3 ⩽ n ⩽ ∞) and adjacent continuum states are systematically investigated by the R-matrix method within the first Born approximation. The calculated energy levels of Na+ and Na are in good agreement with the experimental values. The energy dependence of the GOS extrema for the monopole, dipole and quadrupole transitions is investigated. The GOS density surfaces of 2S, 2P and 2D channels are calculated and their accuracies are tested rigorously by the available experiments as well as by the comparisons between the calculations on the different bases and the sum rule. With the surfaces tested, the high-energy electron impact excitation cross sections of the infinite Rydberg states near the threshold can be readily obtained by interpolations. In addition, the GOSDs for the resonance transitions 2p63s → 2p53s2 and 2p63s → 2p53s3p are investigated.

Cross sections for electron impact excitation of the C Π1 and D Σ1+ electronic states in N2O

Differential and integral cross sections for electron-impact excitation of the dipole-allowed C (1)Pi and D (1)Sigma(+) electronic states of nitrous oxide have been measured. The differential cross sections were determined by analysis of normalized energy-loss spectra obtained using a crossed-beam apparatus at six electron energies in the range 15-200 eV. Integral cross sections were subsequently derived from these data. The present work was undertaken in order to check both the validity of the only other comprehensive experimental study into these excitation processes [Marinkovic et al., J. Phys. B 32, 1949 (1998)] and to extend the energy range of those data. Agreement with the earlier data, particularly at the lower common energies, was typically found to be fair. In addition, the BEf-scaling approach [Kim, J. Chem. Phys. 126, 064305 (2007)] is used to calculate integral cross sections for the C (1)Pi and D (1)Sigma(+) states, from their respective thresholds to 5000 eV. In general, good agreement is found between the experimental integral cross sections and those calculated within the BEf-scaling paradigm, the only exception being at the lowest energies of this study. Finally, optical oscillator strengths, also determined as a part of the present investigations, were found to be in fair accordance with previous corresponding determinations.

Electron-impact excitation and emission cross sections of the H and states and rotational dependence of photodissociation cross sections of the and continua

Rotational and vibrational dependence of photodissociation cross sections and oscillator strengths to the continuum levels of the H and states have been examined. The electron-impact excitation, dissociation and emission cross sections of the and band systems have been obtained for the first time over a wide energy range by using calculated continuum oscillator strengths along with previously published discrete transition probabilities and Lyman and Werner bands excitation functions. The present photodissociation cross section from the Ji = 0 level is in excellent agreement with that obtained by Glass-Maujean (1986 Phys. Rev. A 33 346–50). Photoexcitation from the state to the continuum is found to be weak. The present calculation shows significant contribution of quasi-resonance, which arises from transitions to the quasi-bound levels above the dissociation limit but stabilized by the centrifugal potential. The quasi-resonance is largely responsible for the significant rotational dependence of the continuum oscillator strength of the transition, which, in turn, leads to the noticeable temperature dependence of electron-impact excitation and emission cross sections. and electron-impact excitation, emission, and dissociation cross sections, important for modelling dayglow and auroral activity in the atmospheres of the outer planets, are presented.

Near-infrared collisional radiative model for Xe plasma electrostatic thrusters: the role of metastable atoms

Mestastable Xe atoms play an important role in the collisional radiative processes of dense xenon plasmas, including those of electric thrusters for space vehicles. Recent measurements and calculations of electron-excitation processes out of the 5p56s J = 2 metastable state (1s5 state in Paschen notation) have allowed for the development of a collisional radiative model for Xe near-infrared (NIR) emissions based on the population of the metastable level through 2p–1s5 radiative transitions, and based on depopulation through electron-impact excitation. A modified plasma radiative model incorporating newly computed electron-impact excitation cross sections using both relativistic distorted wave and semi-relativistic Breit–Pauli B-Spline R-matrix methods is presented. The model applies to optically thin, low-density regions of the thruster plasma and is most accurate at electron temperatures below 10 eV. The model is tested on laboratory spectral measurements of the D55 TAL and BHT-200 Hall thruster plasma NIR radiation. The metastable neutral fraction is determined to rise from 0.1 to slightly above 1% as the electron temperature increases from ∼2 to 10 eV, reaching a maximum around 15 eV. Electron temperatures derived with the modified model are approximately 20% lower than a previous version of the model that used an approximate approach to account for metastable population and line intensity enhancement.

Integral cross sections for electron-impact excitation of the4 P2state in copper

We report integral cross sections for electron-impact excitation of the $4\text{ }^{2}P$ state in copper for incident electrons with energies in the range from threshold to 100 eV. Measurements, based on an optical excitation function procedure, are compared with coupled-channel and coupled-channel-optical method calculations that we have also performed as a part of this study. Agreement between our measurements and theory was generally only modest. The present measurements are also found to be in quite good accord with the early near-threshold integral cross sections of Flynn et al. [C. Flynn, Z. Wei, and B. Stumpf, Phys. Rev. A 48, 1239 (1993)] and the higher energy measurements from Ismail and Teubner [M. Ismail and P. J. O. Teubner, J. Phys. B 28, 4149 (1995)]. Where possible, comparison of our data is also made with earlier theory.

Vibrational excitation of water by electron impact

Experimental and calculated differential cross sections (DCSs) for electron-impact excitation of the (010) bending mode and unresolved (100) symmetric and (001) antisymmetric stretching modes of water are presented. Measurements are reported at incident energies of 1--100 eV and scattering angles of $10\ifmmode^\circ\else\textdegree\fi{}--130\ifmmode^\circ\else\textdegree\fi{}$ and are normalized to the elastic-scattering DCSs for water determined earlier by our group. The calculated cross sections are obtained in the adiabatic approximation from fixed-nuclei, electronically elastic scattering calculations using the Schwinger multichannel method. The present results are compared to available experimental and theoretical data.