Absolute Differential Cross Sections Research Articles

Low-energy elastic electron interactions with pyrimidine

We present results of measurements and calculations of elastic electron scattering from pyrimidine in the energy range 3–50 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique. The measured cross sections are compared with results of calculations using the well-known Schwinger variational technique and an independent-atom model. Agreement between the measured differential cross sections and the results of the Schwinger calculations is good at lower energies but less satisfactory at higher energies where inelastic channels that should be open are kept closed in the calculations.

Influence of sulphur atom on the qualitative behavior of electron impact total cross sections of some sulphur containing molecules

Absolute elastic differential cross sections (DCS) have been measured experimentally for several sulphur containing molecules such as SF6, CS2, DMS and DMSO using a novel high resolution electron spectrometer. The measured DCS’s were extrapolated in the forward and backward angles using a tenth order legendre polynomial based least squares fitting technique. Total cross sections (TCS) were determined by integrating the DCS over all angles for each incident energy. The influence of the sulphur atom on the molecular TCS is studied by comparing the qualitative behavior of electron impact cross sections for the sulphur containing molecules against the TCS of the sulphur atom at each incident energy. Theoretical calculations based on independent atom model are also presented as a guide to understand the nature of the electron-molecule collision processes measured in this study.

NUCLEAR STRUCTURE OF 97Mo FROM THE (d, p) REACTION

The reaction 96 Mo (d, p)97 Mo has been studied at 12 MeV using the tandem Van de Graaff accelerator and a multi-channel magnetic spectrograph at the Atomic Weapon Research Establishment, Aldermaston, England. Angular distributions of protons are measured at 12 different angles from 5° to 87.5° at an interval of 7.5° and the reaction products are detected in nuclear emulsion plates. Thirty levels in the energy range from 0.000 to 2.458 MeV have been observed and absolute differential cross-sections for these levels have been measured. The data are analyzed in terms of the distorted-wave Born approximation (DWBA) theory of the direct reactions, and spins, parities and spectroscopic factors are deduced for various levels. Ambiguity in the spin assignments of d5/2 and d3/2 which is allowed in ln = 2(d, p) transition is removed by using the corresponding L-value of the 95 Mo (t, p)97 Mo reaction at Et = 12 MeV . Determined value of the sum of spectroscopic factors for transfers of d5/2 neutrons suggests configuration mixing in the ground state of 96 Mo . The properties of the levels in 97 Mo are compared with previous experimental results and theoretical predictions.

Absolute differential cross sections for electron excitation of silver at small scattering angles

We present results of our experimental and theoretical investigations of the electron excitation of the ground 4d105s state of silver. Differential cross sections (DCSs) for the excitation of the first combined resonant 4d105p state (two fine-structure levels with total angular momentum J=1/2 and 3/2 which cannot be distinguished in the present experiment) were measured at electron-impact energies (E0) of 10, 20, 40, 60, 80 and 100eV and for a range of scattering angles (θ) from 3° up to 15°. Absolute DCSs were obtained by the normalization of relative differential cross sections to the optical oscillator strengths. The relativistic distorted wave (RDW) method was used to calculate DCSs and generalized oscillator strengths for each level separately and the combined results are compared with the measurements.

Elastic electron scattering from formamide molecule

Elastic electron scattering from gaseous formamide (H2NCHO) has been investigated. Absolute elastic differential cross sections (DCSs) were determined both experimentally and theoretically. The measurements were performed using a cross beam technique, for the incident energies of 100, 150 and 300eV and scattering angles from 20° to 110°. Relative elastic DCSs were measured as a function of the angle and the absolute DCSs were determined using the relative flow method. The calculations of electron interaction cross sections are based on a corrected form of the independent-atom method, known as the SCAR (screen corrected additivity rule) procedure and using an improved quasifree absorption model. Calculated results agree very well with the experiment.

Vibrational excitation functions for inelastic and superelastic electron scattering from the ground electronic state in hot N2O

Inelastic and superelastic vibrational excitation functions are extracted from measured energy-loss spectra. The energy range of the incident electrons is 0.65–4.55 eV, with electrons scattered by 90° by nitrous oxide (N2O) gas at 300 K and at 700 K being detailed. Based on the known vibrational populations and the principle of detailed balance, the intensities of the inelastic and superelastic peaks are decomposed into the contributions from different vibrational transitions involving up to the second bending overtone (030) in the ground electronic state. The 2Π resonance peak around 2.3 eV is found to shift to lower energies for the higher bending states. This is interpreted as an effect of the bond-angle dependence of the 2A′ resonance that the 2Π resonance in the linear N2O molecule evolves into. Absolute differential cross sections for the same inelastic and superelastic transitions at 2 eV, within the 2Π resonance region, and at scattered electron angles in the range 45–120° are also determined in the same way. Even via a common resonance, the angular distributions exhibit quite different profiles depending on the vibrational transition.

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 Å(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.

Double Differential Cross Sections and Generalized Oscillator Strength Distributions of Ammonia

The absolute double differential cross section (DDCS), the generalized oscillator strength distribution (GOSD), and the ionization efficiency of ammonia (NH3) were investigated from the threshold to 40 eV under the condition of 200 and 400 eV incident electron energies and 6 and 8 degree scattering angles using electron energy-loss spectroscopy and electron- ion coincidence techniques. To determine the absolute values, we used a mixture of helium (He) and NH3 and normalized the measured relative DDCS spectrum by the differential cross section for 21P excitation of He. Our results are in close agreement with previous dipole (e, e) spectroscopy, although the incident electron energy is lower. The ionization efficiency curve obtained from coincidence measurements indicated the existence of doubly excited states that cause neutral dissociation.

A new experimental approach to investigate intermediate-energy proton elastic scattering and breakup reactions on exotic nuclei in inverse kinematics

We report on a new experimental setup for the investigation of proton induced reactions on exotic nuclei in inverse kinematics. It was installed and successfully used in the scattering study of intermediate-energy protons ( ∼ 700 MeV / u ) on neutron-rich nuclei 6,8He produced with the FRagment Separator (FRS) at the GSI facility, Darmstadt. A new pure liquid-hydrogen target was successfully adapted allowing to obtain low-background data as compared to commonly used (CH 2) n targets. Absolute differential cross-sections for elastic (p, 6,8He) scattering were obtained at angles corresponding to 0.05 ⩽ | t | ⩽ 0.125 (GeV/ c) 2 of the four-momentum transfer squared (− t) leading to the first diffraction minimum and spanning more than two orders of magnitude. They complement an earlier measurement at smaller angles 0.0025 ⩽ | t | ⩽ 0.05 (GeV/ c) 2 performed at the same energy with an active target. Both data sets agree well in the overlap region t ≈ 0.05 (GeV/ c) 2 and enable a refined and comparative study of the radial distribution of the nuclear matter density.

Cross sections for the three-body photodisintegration ofHe3atEγ=12.8, 13.5, and 14.7 MeV

The absolute differential cross sections for the $^{3}\mathrm{He}$($\ensuremath{\gamma},n)\mathit{pp}$ reaction were measured as a function of outgoing neutron scattering angle and energy at the incident $\ensuremath{\gamma}$-ray energies of 12.8, 13.5, and 14.7 MeV to within a precision better than $\ifmmode\pm\else\textpm\fi{}$6%. Both the absolute cross sections and the neutron energy distributions at each incident $\ensuremath{\gamma}$-ray energy agree with the state-of-the-art theoretical results when the Coulomb interaction in the final state is included.

Electron impact excitation of the 3s3p 3P state of magnesium from the ground state

Differential cross sections (DCSs) for electron-impact excitation of the 3s3p 3P state of magnesium from the ground state have been measured at incident electron energies of 10, 15, 20, 40 and 60 eV. The energy-loss (ΔE = 0–5 eV) spectra have been recorded at separate scattering angles (θ) up to 150°. The absolute DCSs were determined by using (3s3p 3P)-to-resonance (3s3p 1P) intensity ratios and the reported DCSs of the resonance state (Filipović et al 2006a J. Phys. B: At. Mol. Opt. Phys. 39 2583; Filipović et al 2006b Int. J. Mass. Spectrom. 251 66). The absolute DCSs of the 3s3p 3P state were extrapolated to θ = 0° and 180° and then integrated to obtain integral cross sections. The results have been analysed and compared with available experimental data and theoretical calculations.

Electron interaction cross sections for a low-temperature ‘plasma-like’ gas mixture

Electron interaction cross sections for species found in low-temperature industrial plasma environments are a crucial component to allow for accurate modeling of those plasmas. However, such data are still rarely reported in the literature, due to the complexities of working with such highly reactive species in a laboratory. Here, absolute differential cross section measurements for a ‘plasma-like’ gas mixture, containing CF3I, CF3, I2, I and C2F6, are reported.

Absolute differential and total cross sections for neutral fragments from dissociative collisions of triatomic hydrogen like ions on He

Neutral fragment products from dissociative collisions of triatomic hydrogen like ions incident on He atoms were studied. Absolute differential and total cross sections are reported here in the energy range of 1.00–5.00 keV and scattering angles between −5.0° and 5.0°. The differential cross sections show decreasing behaviour with a slight structure around 2.0°. The total cross sections for all triatomic molecular ions studied in this work are found to be comparable for the same velocity (E/M). The measured cross sections are between 0.7 × 10−17 cm2 and 0.9 × 10−16 cm2. The present results for the neutral total cross section correlate very well with previously measured total ions cross section for H+3, D+3 and HD+2 on He.

Electron-capture cross sections forLi+colliding on H andH2in the low-keV energy region

Absolute total and differential cross sections have been measured for neutral Li formation in single collisions of Li{sup +} on H{sub 2} from 1 to 5 keV. Total cross sections were obtained by direct integration of measured differential cross sections and are in good agreement with earlier cross-sectional measurements. A comparison of the differential cross sections plotted as a function of the reduced variables {theta}sin{theta}d{sigma}/d{Omega} vs E{theta} is also presented with a comparison of the maximum of the curves of the present data. Theoretically, we report state to state and summed charge transfer at collision energies ranging from 0.1 to 25 keV/amu. Cross sections were calculated using a nonadiabatic time-dependent direct approach for study of ion-atom and ion-molecule interaction processes. We verify that the main electron-capture channel is in the n=2 state of Li with a large probability for electron capture for impact parameters below 2 a.u. for collisions on both H and H{sub 2} in the so-called rotational region of the collision. In the case of atomic hydrogen, the electron-capture probability into the 2s and 2p states shows an oscillating behavior that smooths out when summed. For the molecular case, we find that for projectile energies larger than 1more » keV, ionization effects become important. Our results show good agreement when compared to other available experimental and theoretical data.« less

Measurement of then-pelastic scattering angular distribution atEn=14.9MeV

The relative cross sections for scattering of neutrons by protons have been measured at ${E}_{n}=10 \mathrm{MeV}$ for center-of-mass neutron scattering angles from $60\ifmmode^\circ\else\textdegree\fi{}$ to 180\ifmmode^\circ\else\textdegree\fi{}. Absolute differential cross section values were obtained by normalizing the angle-integrated relative angular distribution to the $n\ensuremath{-}p$ total cross section. The angular distribution exhibits a backward enhancement consistent with an exchange component of the $n\ensuremath{-}p$ interaction at this energy. The relative shape of the angular distribution is in good agreement with the prediction of the charge-dependent Bonn and Nijmegen potential models and with the Arndt phase-shift analysis. Better agreement is found with the evaluated nuclear data files (ENDF)/B-V than with the ENDF/B-VI evaluation.

Electron collisions with CO: Elastic and vibrational excitation cross sections

Absolute differential elastic and vibrational excitation cross sections up to $v=11$ were measured for CO in scattering angle ranges extending to ${180}^{\ifmmode^\circ\else\textdegree\fi{}}$ at energies between 0.2 and 5 eV (and an elastic measurement at 10 eV). The lowest angles were ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ for inelastic scattering and between ${5}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${20}^{\ifmmode^\circ\else\textdegree\fi{}}$ for elastic scattering, depending on energy. Integral cross sections were derived by integrating under the angular distributions and compared with previous beam and swarm measurements. The sum of the integral cross sections agrees very well with the available transmission measurements of the grand total cross section, thus validating the present measurements. The present elastic differential and integral cross sections are in excellent agreement with the best available measurement [Gibson et al., J. Phys. B 29, 3197 (1996)], but the $v=1$ inelastic cross section is about $25%$ higher. This could have consequences for simulations of cometary and planetary atmospheres.

Substitution effects in elastic electron collisions with CH3X (X=F, Cl, Br, I) molecules

We report absolute elastic differential, integral, and momentum transfer cross sections for electron interactions with the series of molecules CH(3)X (X=F, Cl, Br, I). The incident electron energy range is 50-200 eV, while the scattered electron angular range for the differential measurements is 15 degrees-150 degrees. In all cases the absolute scale of the differential cross sections was set using the relative flow method with helium as the reference species. Substitution effects on these cross sections, as we progress along the halomethane series CH(3)F, CH(3)Cl, CH(3)Br, and CH(3)I, are investigated as a part of this study. In addition, atomic-like behavior in these scattering systems is also considered by comparing these halomethane elastic cross sections to results from other workers for the corresponding noble gases Ne, Ar, Kr, and Xe, respectively. Finally we report results for calculations of elastic differential and integral cross sections for electrons scattering from each of the CH(3)X species, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements was found to be quite remarkable in each case.

Few-body Studies at the High Intensityγ-Ray Source (HIγS)

The HIγS facility is making it possible to perform studies of few body systems at a new level of accuracy and precision. A study of the photodisintegration of the deuteron using 100% linearly polarized beams at 14 and 16 MeV has determined the splittings of the three p-wave amplitudes involved in this process for the first time. These results show that the relativistic contributions, which when included in the theory lead to a positive value of the GDH integrand above 8 MeV, are valid. The near threshold data on the photodisintegration of the deuteron provide results which are used to extract the forward spin-polarizability of the deuteron for the first time. The experimental value is in good agreement with a recent effective field theory calculation. Measurements of the absolute differential cross section of the 3He(γ,n)pp reaction have been completed at three γ-ray energies. The measurements were made at incident γ-ray energies of 12.8, 13.5, and 14.7 MeV. It has been found that the shape of the outgoing neutron energy distribution at a given scattering angle at 12.8 MeV disagrees with current theoretical predictions. At these energies, the shape is consistent with a phase-space-only shape. At the higher energies, the measurements agree with theory.

The electronic states of pyrimidine studied by VUV photoabsorption and electron energy-loss spectroscopy

The electronic state spectroscopy of pyrimidine C(4)H(4)N(2) has been investigated using both high resolution VUV photoabsorption in the energy range 3.7 to 10.8 eV (335 to 115 nm) and lower resolution electron energy loss in the range 2 to 15 eV. The low energy absorption band, assigned to the (pi*) <-- 7b(2)(n(N)) (1(1)B(1)<-- 1(1)A(1)) transition, at 3.85(4) eV and the vibrational progressions superimposed upon it have been observed for the first time, due to the availability of a high-resolution photon beam (0.075 nm), corresponding to 3 meV at the midpoint of the energy range studied. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the lowest (1)B(1) state. The 2(1)B(1) state is proposed to have its origin at 7.026 eV according to the vibrational excitation reported in this energy region (7.8-8.4 eV). New experimental evidence of 4(1)A(1) state with a maximum cross section at 8.800 eV is supported by previous ab initio quantum chemical calculations. Rydberg series have been assigned converging to the three lowest ionisation energy limits, 9.32 eV ((2)B(2)), 10.41 eV ((2)B(1)) and 11.1 eV ((2)A(1) + (2)A(2)) with new members reported for the first time and classified according to the magnitude of the quantum defects (delta). Additionally, the absolute differential cross section for inelastic electron scattering has been measured for the most intense band from 6.9 to 7.8 eV assigned to (1)pipi* (3(1)A(1) + 2(1)B(2)).

Differential cross sections for the electron-impact near-threshold electronic excitation of argon

Absolute accurate differential cross section data are presented for the excitation of the 3p54s state in argon by electron impact. The study focuses on the near-threshold region, where previous studies have revealed persistent disparities between measurement and theory. The time-of-flight (TOF) technique is employed, allowing scattered electrons to be measured over a broad range of energies with constant transmission, thereby eliminating a potential source of error in relating relative intensities of elastic and inelastic transitions inherent to other techniques. The experimental results are compared to new relativistic distorted-wave (RDW) calculations as well as to previous experimental and theoretical studies.