Absolute Emission Cross Sections Research Articles

Double-differential neutron emission cross sections of 14-MeV neutron induced reactions on Na and Pb

The energy and angular distribution of secondary neutrons from the interaction of 14-MeV neutrons with sodium and natural lead were investigated. Absolute double-differential neutron emission cross sections were measured for reaction angles from 30\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} in steps of 15\ifmmode^\circ\else\textdegree\fi{} in the secondary neutron energy range 2--14 MeV using the time-of-flight facility at the Institut f\"ur Radiumforschung und Kernphysik, Vienna. Systematic uncertainties in these measurements could be reduced to less than 3--5% (depending on secondary neutron energy) resulting in total uncertainties of less than 5% for a large part of the studied range in angle and energy for Pb and of less than 8% for Na. These results agree with previous measurements within the relatively large uncertainties of the latter. The measured cross sections were compared to the results of statistical model calculations using the quantum mechanical theory of Feshbach, Kerman, and Koonin for preequilibrium emission and also with calculations based on the exciton model and the Kalbach-Mann systematics for the shape of the angular distributions. For compound nucleus decay, the Hauser-Feshbach model was used in both cases. Very good agreement was found with both predictions in the case of lead; in the case of sodium, noticeable discrepancies do show the limits of applicability of any statistical model for light nuclei.

Absolute emission cross sections of the Paschen-α line and production dynamics of the 4F state of the excited hydrogen atom in e-H2 collisions

Absolute emission cross sections of the Paschen-α line from H (4S,4P,4D,4F) produced in e-H2 collisions have been determined to be 4.31–10.1×10−20 cm2 at 20–100 eV by utilizing the Balmer-α emission cross sections as the standard; these values are about half those of the Balmer-β emission. From these values, the formation cross section of the 4F state has been first determined, together with the angular momentum distributions, which are the fraction of the formation cross sections of the 4S, 4P, 4D, and 4F state. The 4F state had appreciable cross sections at electron energies below 24 eV and was assigned to be produced from the singly-excited states of H2 . In the doubly-excited state channel opening above 27–32 eV, the 4P and 4D states are preferentially produced, and the 4F state is suppressed.

Visible light emission in collisions between O5+ ions and He atoms

Visible light emission spectra during collisions between O5+ ions and He atoms in the range of 200–800 nm at different projectile energies (4.06–5.31 keV-u−1) have been measured. Emission spectra show that there are three channels of excitation in the O5+-He collision system: (i) single-electron capture into excited states; (ii) double-electron capture into excited states; (iii) direct excitation of target atoms. There are transitions between states with comparably high quantum numbersnl. The absolute emission cross-sections of every spectral line are calculated. The relations of these cross-sections with the energy of the projectiles are studied.

Dissociative excitation of CH 4 by electron impact: Emission cross sections for the fragment species

Absolute emission cross sections for the hydrogen Lyman series, Balmer series CH(A 2 Δ → X 2 Π: λ = 420–440 nm), C(2p3s → 2p 2: λ = 165.7 nm) and C(2s2p 3 → 2s 22p 2: λ = 156.1 nm) produced by electron impact on CH 4 have been measured in the energy range from the apperance threshold to 1000 eV. Since all these appearance potentials rise above the first ionization potential (∼ 12.6 eV) of CH 4, super-excited states play important roles in producing excited fragment species. In the low energy region, a few appearance potentials are observed for these fragments, and dissociation channels are discussed. Fano-plot analysis shows that the excited hydrogen atoms are mainly produced via electronic states that have forbidden transitions to the CH 4 ground state.

Photoabsorption cross section and radiative processes of GeCl 4 in the valence region of 6–31 eV

Photoabsorption cross section and emission excitation functions of GeCl 4 have been measured in the valence region of 6–31 eV by using synchrotron radiation. The observed bands have been assigned to the (4s, 4d, and np) ← 3t 2, (4s and np) ← 1e, and (4s and 4p) ← 2t 2 Rydberg series where n = 4–6. Strong emissions have been observed for the C ̃ 2 T 2 → X ̃ 2 T 1 and C ̃ 2 T 2 → A ̃ 2 T 2 transitions of the GeCl 4 + ion and the GeCl 2( 3 B 1 → X ̃ 1 A 1) radical. Weak fluorescence from GeCl 2( A ̃ 1 B 1 → X ̃ 1 A 1) has also been observed. Absolute total emission cross section was obtained as 23.1 ± 0.8 Mb at hv = 21.22 eV and the ionization potential of the 2t 2 molecular orbital has been determined to be 14.65 ± 0.08 eV.

Far ultraviolet emission spectrum of xenon induced by electron impact at 100 eV

The FUV emission spectrum of xenon, which contains five emission lines at 117.04, 119.2, 125.1, 129.56 and 146.96 nm over the wavelength range 110–160 nm, produced by electron impact at 100 eV is presented. The measurements were obtained under near-optically-thin conditions at a spectral resolution of 1.0 nm FWHM. In addition, the absolute emission cross sections at 100 eV electron impact energy were obtained for these emission lines. Most prominent features of the FUV spectrum are the Xe I lines at 146.9 and 119.2 nm with the emission cross sections of 58.5 × 10 −18 cm 2 and 45.0 × 10 −18 cm 2, respectively, with an uncertainty of 27%.

Absolute and relative emission cross sections for electron impact on CH 3 F, CH 2 F 2 and CHF 3

Emission spectra following electron impact on CH3F, CH2F2 and CHF3 at various energies have been investigated in the spectral region from 200 nm to 700 nm. Emission thresholds and excitation functions for atomic and molecular fragments have been determined. Absolute emission cross sections were obtained for two band systems, A2 Δ → X2 Π, C2 Σ+ → X2 Π, observed in the emission spectrum of CH3F and for the H-Balmer radiation, Hα − Hγ, in the spectra of all compounds. The continuous emission between 200 nm and 400 nm in the spectra of CH2F2 and CHF3 has been examined systematically. It was found that CF2(A) is the main precursor in both spectra.

Electron-impact-induced emission cross sections of neon in the extreme ultraviolet

We have measured the extreme ultraviolet (EUV) spectrum of neon produced by electron impact 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 centre 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. The measured emission cross section values at 300 eV for the Ne I lines at 73.6 and 74.4 nm are found to be and respectively, and for the Ne II multiplet at 46.14 nm is found to be (sum of the 46.07 and 46.22 nm line cross sections) with an uncertainty of 41%. In addition, excitation functions were measured for the Ne I resonance lines (0 - 400 eV) and Ne II ion line (0 - 1 keV). The excitation functions for the Ne I resonance lines were corrected for polarization because these are strongly polarized.

Electron-impact emission cross sections of Ar

Electron impact emission cross sections of Ar were measured for the , and transitions from the threshold to 1000 eV by a polarization-free method. Two different methods were used in determining absolute emission cross sections for the first resonance lines. The first method is to measure the cascade cross sections to the 4s state by detecting visible to near-infrared radiation and the total cascade cross section was then combined with the level excitation cross section to the 4s state to give the emission cross section for the resonance line. The second method is to measure the vacuum ultraviolet sensitivity of the apparatus and an absolute emission cross section for the resonance line was obtained by comparing its spectral intensity with that of the Lyman- radiation produced from by electron impact. The two results showed good agreement within the experimental errors. The level excitation cross sections to the and states were obtained by subtracting the total cascade cross section to these levels from the corresponding emission cross sections.

Dissociative Excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by Controlled Electron Impact

AbstractThe optical emission spectra in the wavelength region 200–800 nm produced by electron impact on the silicon‐organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single‐collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH(A2Δ → X2Π) emission, the so‐called CH „4300 Å”︁ band, was found to have the largest emission cross section with values (at 100 eV) of 5.5 × 10−19 cm2 and 6.1 × 10−19 cm2 for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B2∑− → X2Π transition (the CH „3900 Å”︁ system) and the C2∑ → X2Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near‐threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single‐step dissociative excitation processes are unlikely to play a dominant role in low‐temperature processing plasmas containing TMS and HMDSO.

Far-Ultraviolet Emission Cross Sections of NE II and NE III Excited by Electron Impact

We have measured the electron-impact-induced fluorescence spectrum of neon in the wavelength range 120-270 nm at a spectral resolution of 0.43 nm (FWHM). The strongest lines observed in the far-ultraviolet (FUV) spectrum of neon are assigned to terms of the doublet system of Ne 2 (2s(sup 2) 2p(sup 4)nl and the triplet system of Ne 3 (2s(sup 2)2p(sup 3)3l). Our FUV spectral data, obtained at 300 eV electron-impact energy, provide absolute emission cross sections of these Ne 2 and Ne 3 lines, and are compared to previous measurements where available. In addition, the excitation function of the strongest Ne II line observed at 191.6 nm was measured from threshold to 1000 eV electron-impact energy.

Determination of absolute emission cross sections for electron-impact-induced line radiation in the vacuum ultraviolet

A method and an experimental setup have been developed for measuring absolute photoemission cross sections for electron-impact-induced line radiation in the vacuum ultraviolet (VUV). Unparalleled low uncertainties for the cross sections were achieved mainly from the use of the Berlin electron storage ring as a primary standard source in the VUV for the determination of the responsivity of the spectrometer-detector system used and from the use of a spinning rotor gauge as a secondary standard for the determination of the target gas density. As the first result we present a photoemission cross section for the Ar II 3s3p(6)(2)S(½)-3s(2)3p(5)(2)p?(3/2) transition at 91.98 nm for 2-keV electron-impact energy of 1.167 × 10(-18) cm(2) with a relative uncertainty of 4.4% (√3σ value). This low uncertainty demonstrates the suitability of the setup for further cross-section measurements.

Ultraviolet emission in dissociative excitation of N2 by He+ impact

Absolute emission cross sections for the excitation atomic and ionic lines of nitrogen are measured in the VUV region of 50 to 130 nm in the He+ projectile energy range 1-10 keV. Excitation of the intense lines are caused by formation of core-excited intermediate molecular states of N2+* in the charge exchange processes. In particular the most intense line N I (120.0 nm) can be excited by formation of a 2s sigma g hole in the ground state of the N2 molecule. This intermediate one-hole molecular state of N+2 can be formed by configuration interaction with 2 Sigma +g core-excited molecular Rydberg states.

Extreme ultraviolet emission spectrum of CO 2 induced by electron impact at 200 eV

We present the extreme ultraviolet (EUV) emission spectrum of CO 2 induced by electron impact at 200 eV. There are 36 spectral features which are identified with a resolution of 0.5 nm over the wavelength range of 40 to 125 nm. Absolute emission cross sections were obtained for each of these features. The EUV emission spectrum induced by electron impact consists of atomic multiplets of C I, II and O I, II, III as well as CO and CO + molecular band systems produced by dissociative excitation. The C I (119.4 nm) multiplet is the strongest feature of C I with a peak cross section of 3.61 × 10 −19 cm 2 at 200 eV. The strongest feature of O I in the EUV spectrum is the O I (99.0 nm) multiplet with a peak cross section of 3.59 × 10 −19 cm 2 at 200 eV.

Optical emissions from electron-impact-excited tetra-ethoxysilane

The optical emission spectrum in the spectral region 200–800 nm produced by electron impact on tetra-ethoxysilane (TEOS) under controlled single-collision conditions is analyzed. Absolute emission cross sections (at an impact energy of 100 eV) of 0.7±0.2×10−20 cm2 for the Si (3p4s 1P0→3p2 1S) line at 390.6 nm, 2.4±0.5×10−20 cm2 for the CH(A 2Δ→X 2Π) band centered around 430 nm, and cross sections below 0.5×10−20 cm2 for the hydrogen Balmer-α, -β, and -γ lines at, respectively, 656.4, 486.1, and 434.0 nm are measured. Comparatively high onset energies of 56.2±2.0 eV (Si) and 30.7±2.0 eV (CH) were found which when combined with the small cross sections indicate that the single-step dissociative excitation of TEOS by electron impact cannot be expected to play a major role in the collision processes that dominate the plasma chemistry of TEOS-containing processing plasmas.

Electron impact dissociation of H2O: emission cross sections for OH*, OH+*, H*, and H2O+* fragments

The optical emission spectrum in the near ultraviolet and visible following electron impact on H2O was studied in a crossed-beam and a static gas-target experiment. Emissions of H*, OH*, OH+*, and H2O+* fragments were detected and absolute emission cross sections for the different fragments were determined. A nonthermal rotational population was observed for the diatomic fragments which gives insight into the dissociation process. Further conclusions on the dissociation mechanism are possible based on appearance potentials and the shape of the emission cross sections as a function of impact energy.

Electron impact excitation of atomic oxygen

Absolute emission cross sections, for the VUV spectral features at 130.4, 102.7, 98.9 and 87.8 nm, have been measured for electron impact on atomic oxygen in the energy range from threshold to 100 eV. An RF discharge source containing NO was used to prepare a high density atomic oxygen beam. Significant differences were found with earlier measurements on these transitions. The data are compared with available theoretical results and with level excitation cross sections obtained using inelastic electron scattering techniques.

Absolute visible light emission cross sections for electron capture from Li atoms by slow, highly charged ions

Absolute cross sections for visible light emission subsequent to charge changing collisions (approximately 1-9 keV amu-1) of C6+ and Neq+ (q=6, 7, 8 and 9) with Li have been measured. The emission cross sections for different 6+ ions (C6+, N6+, O6+ and Ne6+) have been found to be of similar magnitude indicating that for these systems core effects are of minor importance. Comparison with calculations based on the classical overbarrier model and the assumption of conservation of classical angular momentum showed that the predicted, near statistical distribution is not in accordance with the actual final distribution over l states within a given n shell.

Electron impact dissociation of the NH3 molecule: Absolute and relative cross sections for NH*(c 1Π,A 3Π), NH+*(C 2Σ+, B 2Δ,A 2Σ−), and NH*2(Ã 2A1) fragment emissions

The emission spectrum between 2700 and 5000 Å following single electron impact on ammonia has been investigated. Emission bands of the electronically excited NH(c 1Π,A 3Π), NH+(C 2Σ+,B 2Δ,A 2Σ−), and NH2(Ã 2A1) fragments have been identified and absolute emission cross sections have been determined at 100 eV incident energy. Relative cross sections as a function of the impact energy reveal information about the excitation mechanisms of the parent molecule. Conclusions can be drawn on the dissociation process by a comparison of the observed appearance potentials with thermochemical thresholds and with the results of photodissociation experiments.

Heavy fragment emission in alpha-induced reactions on aluminum at 60 MeV

The production of intermediate-mass fragments ranging from Z=3 to 8 emitted in the reaction of 60 MeV alpha on an aluminum target has been experimentally studied. The energy spectra of the emitted fragments in the angular range of 20\ifmmode^\circ\else\textdegree\fi{}--130\ifmmode^\circ\else\textdegree\fi{} have been measured and the total production cross sections of each fragment have been estimated from the data. The double-differential cross-section data have been compared with the predictions made using (a) a phenomenological moving source model and (b) a binary fragmentation model. The absolute total emission cross sections for various fragments have been calculated assuming the statistical decay of a fully equilibrated compound nucleus and have been compared with the corresponding experimental estimates. It is found that the exit-channel deformation plays a significant role in the estimation of the Coulomb barrier of the separating nuclei. A theoretical procedure for the estimation of the exit-channel Coulomb barrier for the deformed system has been incorporated in the present calculation.