Search

New measurements of differential cross-sections for excitation of the a″ 1Σ+g (v' = 0, 1) state in molecular nitrogen reveal a cusp-like angular distribution. This feature is distinctly observed for the first time in the present electron energy-loss experiment as a result of finer scattering angle grid and impact energy coverage than previous measurements. This feature is similar to that observed in atomic targets such as He, Hg and Ba. The observed phenomenon suggests an interference effect related to configuration–interaction coupling between lower and excited states that are of the same symmetry. It is hoped that the present work will stimulate theoretical investigations into the physics that governs this cusp-like behaviour.

New electron-impact differential cross-section (DCS) and DCS ratio measurements for the excitation of the four levels making up the 3p54s configuration of argon are reported at incident electron energies of 14, 15, 17.5, 20, 30, 50 and 100 eV. These cross-sections were obtained using a conventional high resolution electron spectrometer. Elastic electron scattering from argon was used as a calibration standard. Electron–helium DCSs were used to determine the instrumental transmission of the spectrometer. Further checks of the relative shape of these DCS measurements were made using the method of gas mixtures (Ne mixed with Ar). We also present results from new calculations of these DCSs using the R-matrix method, the unitarized first-order many-body theory, the semi-relativistic distorted-wave Born approximation, and the relativistic distorted-wave method. Comparison with available experimental DCSs and DCS ratios is also presented.

Differential and integral cross sections (ICSs) for excitation in atomic oxygen have been measured at electron-impact energies of 15, 17.5, 20, 22.5 and 27.5 eV. Differential measurements were conducted with a conventional electron energy-loss spectrometer and a microwave discharge source of atomic oxygen. Relative differential cross sections (DCSs) were determined between 15 and 30 eV impact energy. With the help of theoretical predictions of the shape of the DCS at large angles, measured results were extrapolated to 180° scattering angle and relative ICSs were deduced. The relative excitation function was normalized to the 30 eV impact energy ICS given by Kanik et al (2001 J. Phys. B: At. Mol. Opt. Phys. 34 2647). Normalization of the ICS values allowed the measured DCSs to be put on the absolute scale. Theoretical calculations of the DCSs were carried out using the R-matrix with the pseudostates approach. A total of 22 spectroscopic bound and autoionizing states, together with 19 pseudostates, were included in the close-coupling expansion. The pseudostates were chosen to simulate continuum target states. Theoretical results, along with other available experimental data, have been compared with the current experimental results.

We report electron-impact-induced emission cross sections for the 2p4 3P–3s 3So (130.4 nm), 2p4 3P–3d 3Do (102.7 nm), 2p4 3P–3s' 3Do (98.9 nm) and 2p4 3P–3s'' 3Po (87.8 nm) transitions of atomic oxygen. Measurements were made using a magnetically confined electron beam in collision with atomic oxygen produced by an extended microwave discharge source in a crossed-beam arrangement. A 0.2 m vacuum ultraviolet diffraction spectrometer was used to record the emission. The absolute excitation functions of these transitions were determined by normalizing to the O I (130.4 nm) cross section produced by dissociative excitation of O2 at 100 eV impact energy (Noren et al 2001a Geophys. Res. Lett. 28 1379). A 60-state model of the O I electron reaction structure has been used to extract the excitation cross sections from the experimental measurements. The model process establishes analytic collision strengths to the asymptotic limit.

Using energy loss spectroscopy, normalized differential and integral cross-sections for the electron impact excitation of the lowest bound excited states of H2 from the ground X 1Σg+ state have been measured at 17.5, 20 and 30 eV impact energies and for scattering angles from 5° up to 130°. These differential cross-sections (DCSs) are an effort to improve those taken previously (Khakoo M A and Trajmar S 1986b Phys. Rev. A 34 146). Here a more sophisticated electron spectrometer and a more accurate spectrum analysis code is used to unfold the energy loss spectra. The results are compared with earlier DCS measurements for the B 1Σu+, c 3Πu, a 3Σg+, C 1Πu states of H2 and with existing theoretical results for the B 1Σu+, c 3Πu, a 3Σg+, C 1Πu, E, F 1Σg+ and e 3Σu+ states.

We report absolute differential electron scattering cross sections (DCSs) of molecular oxygen (O2) at 20, 30, 50 and 100 eV impact energies (E0). DCSs were measured for inelastic features in the 6.5-12.5 energy-loss range including the Schumann-Runge (SR) continuum, the longest band (LB) and the second band (2B) along with the summed DCS for the third band (3B) plus the 10.76 and 11.03 eV energy-loss features. Measurements were performed at scattering angles of 0°, 5°, 10°, 15°, 20° and 25° using a conventional electrostatic electron energy-loss spectrometer. Inelastic electron energy-loss spectra of O2 were measured giving relative inelastic DCSs, which were then normalized via the relative flow technique using inelastic He and Ne transitions as standards. Past works (Trajmar et al 1972, Shyn et al 1994) have shown good relative agreement while displaying significant discrepancies in absolute terms. The current normalization technique represents an improvement over those employed in previous inelastic O2 DCS work. As such, the present results serve to firmly establish the absolute normalization of these DCSs. Furthermore, the current results are able to provide an additional test of the relative angular distributions of previous works throughout the important low-angle region where the DCSs undergo the most rapid change. This paper also represents the first measurements of inelastic O2 DCSs to extend right down to the forward-scattering limit.

We report both experimental and theoretical differential and integral excitation cross sections of atomic oxygen corresponding to the 2s22p4 3P→3s 3S (130.4 nm), 2s22p4 3P→3d 3D (102.7 nm), 2s22p4 3P→3s' 3D (98.9 nm) and 2s22p4 3P→3s'' 3P (87.8 nm) transitions at 30, 50, and 100 eV electron-impact energies. Experimental measurements have been made in the angular range from 0° to 25° with a conventional electrostatic electron energy-loss spectrometer. The atomic O differential cross sections (DCSs) were put on an absolute scale by normalization to the O2 DCS values of Johnson and Kanik (2001). Extrapolation of the measured results to larger angles was performed using theoretical calculations as a guide, and integral cross sections were derived. Theoretical calculations based on the R-matrix method, along with other available experimental data, have been compared with the current experimental results.

The optical excitation function of the O I 3So→ 3P transition (130.4 nm), produced by electron-impact excitation of atomic oxygen, has been measured over an extended energy range from threshold to 1.0 keV. Measurements were obtained in a crossed-beam experiment using both magnetically confined and electrostatically focused electrons in collision with atomic oxygen produced by a microwave discharge source. A 0.2 m vacuum ultraviolet monochromator system was used to measure the emitted O I radiation at 130.4 nm. The relative O I (130.4 nm) emission intensity corresponding to the 3So→ 3P transition was then put on the absolute scale by normalization to the O I (130.4 nm) cross section produced by dissociative excitation of O2 at 30 eV (Kanik et al 2000).

Electron-impact-induced emission spectra of Xe corresponding to the resonance transitions in the FUV spectral region from 114-165 nm at 15, 20 and 30 eV impact energies are reported. Absolute emission cross sections corresponding to the Xe I spectral features at 15, 20 and 30 eV impact energies and optical excitation functions for the two most prominent transitions at 119.2 and 147.0 nm in the 0-100 eV energy range were measured. In addition, the integral cross sections for direct electron-impact excitation of the corresponding levels in Xe, obtained by integration of the differential cross section measurements of Khakoo et al (Khakoo M A et al 1996 J. Phys. B: At. Mol. Opt. Phys. 29 3455, Khakoo M A et al 1996 J. Phys. B: At. Mol. Opt. Phys. 29 3477) are compared with the present results and the theoretical calculations of Csanak (Csanak G 1996 Private communication). A comparison of the measured emission and integrated direct excitation cross sections is made leading to indirect estimation of the cascade contributions to the emission cross sections.

This paper is the second part of two back-to-back papers concerning electron-impact excitation of Kr. This part concerns excitation of the individual levels that constitute the 4p55p, 4p54d and 4p56s configurations of Kr, at incident electron energies (E0) of 20.0 and 15.0 eV, for scattering angles (θ) ranging from 10° to 130°. As in part I, excitation cross sections are measured and calculated using the R-matrix (RM) and the unitarized first-order many-body theory (UFOMBT) methods. Comparisons between the present experimental and theoretical results, and also with other experimental results, are presented.

We report an experimental and theoretical investigation of low-energy electron scattering from 138Ba atoms prepared in the metastable 3D2 and 1D2 levels by laser-optical pumping. Differential cross sections were measured for excitation out of these metastable levels to a variety of higher-lying levels at electron-impact energies of 10, 20 and 36.7 eV. A comparison of experimental data with the results of theoretical approaches using the unitarized distorted-wave approximation and the convergent close-coupling scheme is given.

We present new experimental measurements and theoretical calculations of R-matrix and unitarized first-order many-body theory for electron-impact excitation of krypton. The usefulness of differential cross section ratios in providing sensitive tests of electron scattering models for the excitation of the configuration of the heavy rare gases is demonstrated. In addition to differential cross sections alone, these ratios provide interesting physical insights into the details of the collision process. Comparisons of the measured ratios with predictions from the present and other available calculations show some agreement, but also reveal that significant improvements of these models are required.

The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited 138Ba( ... 6s6p 1P1) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS-coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by 138Ba( ... 6s6p 1P1) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable 138Ba atoms were also obtained.

We have measured the differential and integral cross sections for electron impact excitation of the , , and states of CO in the 10-15 eV impact energy region. These measurements fill the gap which existed between the low-energy (threshold to about 12 eV) data of Zobel et al and the intermediate energy (20-50 eV) results of Middleton et al. Recent electron time-of-flight measurements are utilized to place the relative cross sections (deduced from energy-loss spectra) on the absolute scale. The cross sections are in excellent agreement with the results obtained by Zobel et al at the overlapping impact energies and always smoothly extrapolate to their low-energy results. Extrapolation to the intermediate energy domain and comparison with the results of Middleton et al is not so clear. Comparison with theoretical differential and integral cross sections of Sun et al and the integral cross sections of Morgan and Tennyson reveals angular and energy dependences which resemble those found for the experimental results but there are serious discrepances concerning the absolute magnitudes.

The results of a combined experimental and theoretical study of elastic electron scattering by laser-excited atoms are presented. From these studies, we extracted differential scattering cross sections and parameters for elastic scattering both by coherently prepared Ba atoms and by Ba atoms in an isotropic, incoherent state of the level. This is the first time that results of this type have been reported for elastic scattering by excited atoms. Good agreement was found between the limited experimental results and those obtained from our convergent close-coupling (CCC) theoretical calculations. It was demonstrated that elastic scattering can create alignment with significant probability.

Experimental and theoretical differential cross sections (DCSs) for electron impact excitation of the five lowest levels of xenon are reported. The experimental data were obtained at impact energies of 10, 15, 20 and 30 eV for scattering angles in the range of to . The scattering intensities for excitation of various levels with respect to that of the combined two lowest levels (representing the corresponding DCS ratios) were measured with conventional electrostatic electron energy-loss spectrometers. Absolute DCSs for excitation of the combined two lowest levels were derived from DCS ratios for excitation of the combined two lowest levels and elastic scattering and utilizing the elastic DCSs of Register et al. These DCS ratios were obtained from scattering intensity ratios measured with conventional electrostatic electron energy-loss spectrometers after correction for instrumental effects by utilizing electron time-of-flight measurements for determining the proper value of this ratio at scattering angle for each impact energy. The unitarized first-order many-body theory and the unitarized distorted-wave approximation were applied to obtain the theoretical differential cross sections for impact energies of 10, 15, 20, 30, 50 and 80 eV. The present experimental and theoretical results are compared with each other and with other available experimental and theoretical DCSs.

Experimental and theoretical differential cross sections for electron impact excitation of Xe are reported for the lowest 6th to 20th levels. These data are an extension of the work discussed in the preceding paper. The theoretical work is based on the unitarized first-order many-body theory and unitarized distorted-wave approximation. The experimental data span the same respective range of electron impact energies and scattering angles as the preceding paper. The present theoretical and experimental data are compared with each other and with other existing experimental data.

We have measured the extreme ultraviolet (EUV) spectrum of neon produced by electron excitation. The measurements were obtained under optically thin conditions, and at a spectral resolution of 0.5 nm full width at half maximum (FWHM). The most prominent features of the EUV spectrum between 45-80 nm are the resonance lines of Ne I at 73.6 and 74.4 nm and a multiplet of Ne II at 46.14 nm (the average value for the line center of the two closely spaced ion lines at 46.07 and 46.22 nm). Absolute emission cross sections of these lines at 300 eV were measured and compared to other previous measurements.

Total cross sections have been measured for electron scattering on Kr, O2 and CO in the 5-300 eV energy range utilizing a linear attenuation technique. The present results are found to be in good agreement with other experimental data for Kr and CO but deviate significantly from some of the previous measurements in the case of O2.

In this first part of a laboratory study of predissociation in CO the authors have measured in the electron-impact-induced fluorescence spectrum of CO in the wavelength range 40-125 nm at a spectral resolution of 0.5 nm. The experiment was performed in a crossed-beam configuration under optically thin conditions. Their spectral measurements at 200 eV electron impact, energy provide the emission cross sections of atomic dissociation fragments (C I, C II, O I, O II) and the vibrational transitions of the B 1 Sigma +, C 1 Sigma +, E 1 Pi -X 1 Sigma + band systems of CO. The measured emission cross sections of these molecular transitions will be needed for comparison with excitation cross sections from electron energy-loss spectra to determine predissociation yields.

Photodissociation cross sections and oscillator strengths for H2 from the X?1?+g (vi, Ji) levels to the continuum levels of the B?1?+u, C?1?u, B, D'?1?u and 5p??1?+u states have been calculated. The (vi, Ji) state-specific electron impact dissociation cross sections to the continuum levels of these states have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ? 5) np??1?+u and np??1?u continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B'?1?+u continuum is the dominant dissociation channel at room temperature, the C?1?u and B?1?+u continua become more important at high temperature (>5000 K). This work, along with the previous calculation of the B'?1?+u and D?1?u states by Liu et al (2009a J. Phys. B: At. Mol. Opt. Phys. 42 185203), provides the complete electron impact dissociation cross section of H2 through the singlet ungerade continua. Electron dissociation cross sections of the singlet ungerade continua are provided for the purpose of modelling atmospheric heating, analysis of occultation measurements and the hot atomic hydrogen plume observed at Saturn.

Relative electron-impact-induced emission cross sections for the a 1?g (v' = 3)?X 1?+g (v'' = 0) and a 1?g (v' = 2)?X 1?+g (v'' = 0) transitions are presented. Critical comparison is made with existing cross sections showing significant discrepancy with the widely accepted excitation function of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845?61) at energies below ~80 eV. A series of extensive measurements are presented that were performed to rule out any possible systematic or random errors in the present experimental apparatus and methodology. These efforts lead to the conclusion that the current measurements are robustly reproducible and, thus, should supplant the LBH cross-section shape of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845?61).

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-induced vacuum ultraviolet emissions are measured for NO 2 , a species important to discharge phenomena and ozone decomposition in the Earth's atmosphere. A calibrated spectrum for 100 eV incident electrons is presented, along with cross sections for strong emission features between 80 and 160 nm. The dominant NI (2p 34 S°-3s 4 P) emission at 120.1 nm is compared with that of N 2 and used to provide an absolute calibration for all measured cross sections. In addition, 10-300 eV excitation functions for the NI (120.1 nm) and OI (130.4 nm) emissions are presented and interpreted. Comparisons are made with similar measurements of related species, in particular N 2 O and NO. Of interest, it was found that, on average, the variation in the intensity of oxygen and nitrogen emissions could be reasonably explained by the difference in the number of constituent oxygen and nitrogen atoms in each target.

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.

Vacuum ultraviolet emissions following electron impact dissociative excitation of N2O have been studied over the wavelength range 80–180 nm and for electron energies from threshold up to 300 eV. Calibrated spectral data are presented at 100 eV incident energy. The dominant N I (2p3 4S°–3s 4P) multiplet at 120.0 nm has a measured maximum cross section of (2.2 ± 0.4) × 10−18 cm2 at 100 eV, which in turn was used to normalize 100 eV cross sections for all observed spectral features. In addition, excitation functions corresponding to the dominant emission features have been analysed in detail with particular emphasis on the near-threshold region. Possible dissociation channels are discussed.

Absolute elastic differential scattering cross sections (DCS) for electron impact on carbon dioxide in the electron impact range 20-100 eV and angular range 20-120' are reported. The results are obtained using a relative flow technique in which the DCS of CO2 is compared with that of helium at each energy and angle. The authors present both the absolute values of the DCS of CO2 and the ratios DCS(CO2)/DCS(He).