Relative Cross Sections Research Articles

Measurements of differential cross sections for elastic electron scattering and electronic excitation of silver and lead atoms

This paper presents absolute differential cross sections (DCSs) and integrated cross sections (ICSs) for both elastic and inelastic electron scattering by lead and silver atoms in the energy range from 10 to 100 eV. The DCSs were measured as a function of scattering angle. Scattering angles are from 1° to 150° for the excitations of the unresolved 4d105p 2P1/2, 3/2 silver line and 6p7s 3P0,1 lead line, while for the elastic scattering they span from 10° to 150°. The measurements utilize crossed beam technique with effusive atomic beam being perpendicularly crossed by electron beam. Monoenergetic electron beam is obtained by means of hemispherical selector and it is focused by cylindrical electrostatic lenses while effusive atomic beam is formed by heating of Knudsen type oven. Absolute values for the resonance states are obtained by normalization of relative differential cross sections to the optical oscillator strengths, while the absolute values for the elastic scattering are obtained from the intensity ratios at particular scattering angles. Obtained absolute DCSs were extrapolated to 0° and 180° and numerically integrated to yield integral, momentum transfer and viscosity cross sections. The experimental results have been compared with the corresponding calculations.

Momentum spectroscopy of fragment ions of a multiply charged N2O molecule under impact of 10-keV electrons

The dissociative ionization of a N${}_{2}$O molecule is studied at an electron energy of 10 keV using the multiple-ion-coincidence imaging technique. The complete as well as the incomplete Coulomb explosion pathways for N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions are examined and identified. The precursor-specific relative partial ionization cross sections for resulting fragment ions are obtained. It is found that about 81.8$%$ of single ionization, 17.8$%$ of double ionization, and about 0.4$%$ of triple ionization of the parent molecule contribute to the total fragment ion yield. Furthermore, the relative ionic fractions for ion-pair and ion-triple formation are also determined. The kinetic energy release distributions for different coincidence channels are obtained and compared with the available data at lower energies of electron and photon impacts and with that of high-energy ion impact. From the angular correlation studies of fragment ions, it is inferred that states with bent geometries are involved for most of the fragmentation channels of N${}_{2}$O${}^{2+}$ and N${}_{2}$O${}^{3+}$ ions. The concerted and/or sequential nature of the six dissociation pathways is also assigned. No other experimental or theoretical data exist in the literature to compare with the results obtained at the considered impact energy.

New Precision Measurements of theU235(n,γ)Cross Section

The neutron capture cross section of (235)U was measured for the neutron incident energy region between 4 eV and 1 MeV at the DANCE facility at the Los Alamos Neutron Science Center with an unprecedented accuracy of 2-3% at 1 keV. The new methodology combined three independent measurements. In the main experiment, a thick actinide sample was used to determine neutron capture and neutron-induced fission rates simultaneously. In the second measurement, a fission tagging detector was used with a thin actinide sample and detailed characteristics of the prompt-fission gamma rays were obtained. In the third measurement, the neutron scattering background was characterized using a sample of (208)Pb. The relative capture cross section was obtained from the experiment with the thick (235)U sample using a ratio method after the subtraction of the fission and neutron scattering backgrounds. Our result indicates errors that are as large as 30% in the 0.5-2.5 keV region, in the current knowledge of neutron capture as embodied in major nuclear data evaluations. Future modifications of these databases using the improved precision data given herein will have significant impacts in neutronics calculations for a variety of nuclear technologies.

A theoretical examination of the GOS method for normalizing relative cross section data - the case of indium

We examine the use of the generalized oscillator strength method for normalizing relative differential cross section data by comparing them to our calculated results for the electron-impact excitation of the resonance transition in indium. We illustrate that inaccuracies in the data for the smallest scattering angles can lead to errors in the normalization process.

Elastic electron scattering from the DNA bases: cytosine and thymine

Relative elastic differential cross sections for elastic scattering from cytosine and thymine have been measured using the crossed beam method. The experimental data are compared with theoretical cross sections calculated by the screen corrected additivity rule method.

Fragmentation of acetonitrile in collisions with H− and O− ions

Relative cross sections for the production of negatively charged fragments have been determined as a function of ion impact energy in low-energy (0.5 - 5.5 keV) collisions of H− and O− with acetonitrile molecules. The most abundantly produced negative ions from fragmentation by H− and O− impact are CH3CN−, CH2CN− and CN−. Notably, the parent negative ion CH3CN− is produced abundantly.

Photoionization of singly and multiply charged tungsten ions

Relative cross sections for photoionization of Wq+ ions q = 1, ..., 5 were measured at energies up to 90 eV employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. While the cross sections for the lower charge states are dominated by broad resonance features in the range of 4f and 5p excitations, the cross section for W5+ shows much narrower peak features.

Electron transfer and ionization in collisions of He-like ions with Na(3s) and Na(3p)

Single electron transfer and ionization in collisions of He-like ions (N5+, O6+, Ne8+) and Na has been investigated both experimentally and theoretically at energies around the matching velocity of the valence electron (2 to 10 keV/amu). State selective cross sections and scattering angle distributions were obtained using recoil-ion momentum spectroscopy in combination with a magneto-optically cooled Na atom target. A strong dependence of the relative cross sections on the collision energy is observed. The results are compared with Classical-Trajectory Monte Carlo (CTMC) calculations and show an overall very good agreement.

Study of low energy ion-atom collisions using a magneto-optical trap

A magneto-optical trap (MOT) has been coupled to a recoil-ion momentum spectrometer to study the charge exchange process in Na+ +87Rb collisions in the keV impact energy range. The relative cross sections of the active charge exchange channels and the associated distributions in scattering angle were measured with a high precision. They provide a very detailed test of theoretical calculations.

Steady-State Phosphorylation of Light-Harvesting Complex II Proteins Preserves Photosystem I under Fluctuating White Light

According to the "state transitions" theory, the light-harvesting complex II (LHCII) phosphorylation in plant chloroplasts is essential to adjust the relative absorption cross section of photosystem II (PSII) and PSI upon changes in light quality. The role of LHCII phosphorylation upon changes in light intensity is less thoroughly investigated, particularly when changes in light intensity are too fast to allow the phosphorylation/dephosphorylation processes to occur. Here, we demonstrate that the Arabidopsis (Arabidopsis thaliana) stn7 (for state transition7) mutant, devoid of the STN7 kinase and LHCII phosphorylation, shows a growth penalty only under fluctuating white light due to a low amount of PSI. Under constant growth light conditions, stn7 acquires chloroplast redox homeostasis by increasing the relative amount of PSI centers. Thus, in plant chloroplasts, the steady-state LHCII phosphorylation plays a major role in preserving PSI upon rapid fluctuations in white light intensity. Such protection of PSI results from LHCII phosphorylation-dependent equal distribution of excitation energy to both PSII and PSI from the shared LHCII antenna and occurs in cooperation with nonphotochemical quenching and the proton gradient regulation5-dependent control of electron flow, which are likewise strictly regulated by white light intensity. LHCII phosphorylation is concluded to function both as a stabilizer (in time scales of seconds to minutes) and a dynamic regulator (in time scales from tens of minutes to hours and days) of redox homeostasis in chloroplasts, subject to modifications by both environmental and metabolic cues. Exceeding the capacity of LHCII phosphorylation/dephosphorylation to balance the distribution of excitation energy between PSII and PSI results in readjustment of photosystem stoichiometry.

One-neutron transfer study of135Te and137Xe by particle-γcoincidence spectroscopy: Theν1i13/2state atN=83

New information is reported on single-neutron states above the doubly closed-shell nucleus 132Sn. A radioactive ion beam of 134Te(N = 82) at 565 MeV and a stable ion beam of 136Xe(N = 82) at 560 MeV were used to study single-neutron states in the N = 83 nuclei 135Te and 137Xe, respectively, by (13C, 12C ) and (9Be, Be ) direct reactions in inverse kinematics. Particle- and particle- - coincidence measurements using HPGe and CsI arrays allowed determination of decay paths, high-precision level energies, multipolarities of transitions, and relative cross sections. One-neutron transfer with heavy ions is employed to gain selectivity to both low- and high-spin single-neutron states above the N = 82 shell closure. Results are presented for the 13/2+ states in the N = 83 nuclei 135Te and 137Xe at 2108.8(9) keV and 1752.6(3) keV, respectively, and for the 3 collective octupole state observed at 3749(5) keV in 134Te(N = 82) inelastic scattering, all previously unknown. While the 13/2+ state (or 1i13/2 centroid) in 133Sn(Z = 50, N = 83) remains unknown, the present results provide the best empirical measure of its energy available to date.

Low‐energy electron interactions with tungsten hexacarbonyl – W(CO)6

Low-energy secondary electrons are formed when energetic particles interact with matter. High-energy electrons or ions are used to form metallic structures from adsorbed organometallic molecules like W(CO)(6) on surfaces. We investigated low-energy electron attachment to W(CO)(6) in the gas phase to elucidate possible reactions during surface modification. Two crossed electron/molecular beam setups were utilised: (i) a high-resolution electron monochromator combined with a quadrupole mass spectrometer which was used for the measurement of relative cross sections as a function of the electron energy, and (ii) a double focusing mass spectrometer used for measurements of metastable decays of anions. The study was performed in the electron energy range between ~0 and 14 eV. W(CO)(6) efficiently decomposed upon attachment of a low-energy electron and no stable W(CO)(6)(-) anion was observed on mass spectrometric time scales. The transient negative ion formed lost instead sequentially CO ligands. The fragment anions W(CO)(5)(-), W(CO)(4)(-), W(CO)(3)(-), and W(CO)(2)(-) were observed. However, no W(-) was detectable. Dissociative electron attachment (DEA) to W(CO)(6) led to strong dissociation but a complete loss of all CO ligands was not observed in DEA. Deposit contaminations might be a direct result of DEA reactions close to the irradiation spot in beam deposition techniques.

Probing elastic and inelastic breakup contributions to intermediate-energy two-proton removal reactions

The two-proton removal reaction from ${}^{28}$Mg projectiles has been studied at 93 MeV/u. First coincidence measurements of the heavy ${}^{26}$Ne projectile residues, the removed protons, and other light charged particles enabled the relative cross sections from each of the three possible elastic and inelastic proton removal mechanisms to be determined. These more final-state-exclusive measurements are key for further interrogation of these reaction mechanisms and use of the reaction channel for quantitative spectroscopy of very neutron-rich nuclei. The relative and absolute yields of the three contributing mechanisms are compared to reaction model expectations based on the use of eikonal dynamics and $sd$-shell-model structure amplitudes.

Atomic-matter-wave diffraction evidenced in low-energy Na++Rb charge-exchange collisions

Single charge transfer in low-energy ${\mathrm{Na}}^{+}+{}^{87}\mathrm{Rb}(5s,5p)$ collisions is investigated using magneto-optically trapped Rb atoms and high-resolution recoil-ion momentum spectroscopy. The three-dimensional reconstruction of the recoil-ion momentum provides accurate relative cross sections for the active channels along with their associated distributions in projectile scattering angle. The measurements are accompanied by molecular close-coupling calculations. The predicted diffractionlike oscillations in angular distributions are clearly resolved by the experiment. An excellent agreement is found between the present molecular calculations and the experimental data.

An evolutionary attractor model for sapwood cross section in relation to leaf area

Sapwood cross-sectional area per unit leaf area (SA:LA) is an influential trait that plants coordinate with physical environment and with other traits. We develop theory for SA:LA and also for root surface area per leaf area (RA:LA) on the premise that plants maximizing the surplus of revenue over costs should have competitive advantage. SA:LA is predicted to increase in water-relations environments that reduce photosynthetic revenue, including low soil water potential, high water vapor pressure deficit (VPD), and low atmospheric CO2. Because sapwood has costs, SA:LA adjustment does not completely offset difficult water relations. Where sapwood costs are large, as in tall plants, optimal SA:LA may actually decline with (say) high VPD. Large soil-to-root resistance caps the benefits that can be obtained from increasing SA:LA. Where a plant can adjust water-absorbing surface area of root per leaf area (RA:LA) as well as SA:LA, optimal RA:SA is not affected by VPD, CO2 or plant height. If selection favours increased height more so than increased revenue-minus-cost, then height is predicted to rise substantially under improved water-relations environments such as high-CO2 atmospheres. Evolutionary-attractor theory for SA:LA and RA:LA complements models that take whole-plant conductivity per leaf area as a parameter.

Relative partial ionization cross sections of N2O under 10–25-keV electron impact

The relative partial ionization cross sections for the fragment ions produced in direct and dissociative ionization of a N${}_{2}$O molecule are measured for impact of 10--25-keV electrons by using an electron-ion-coincidence technique with a linear time-of-flight spectrometer. The six ionic fragments of N${}_{2}$O (N${}_{2}$O${}^{+}$, NO${}^{+}$, N${}_{2}$${}^{+}$, O${}^{+}$, N${}^{+}$, and N${}^{2+}$ $+$ O${}^{2+}$) are observed and identified. The impact energy dependence of the partial ionization cross sections for these ions is expressed relative to the cross section of N${}_{2}$O${}^{+}$ and is found to be nearly invariant. The relative ionic fractions for the produced ions of N${}_{2}$O are also obtained and compared with the earlier reported data available at lower energies of electron impact. It is found that the relative ionic fractions for singly charged fragments are almost energy independent. However, for the doubly charged fragment ions (N${}^{2+}$ $+$ O${}^{2+}$), the present data are found to be higher by almost a factor of four compared to the relative ionic fraction reported earlier at a very low impact energy.

Determination of the Σ(1385)0/Λ(1405) ratio in p+p collisions at 3.5 GeV

The aim of the present analysis is to determine the relative production cross sections of the $\Lambda$(1405) and $\Sigma(1385)^{0}$ resonances in p+p collisions at E$_{kin}$=3.5 GeV measured with HADES. Upper and lower limits have been determined for the ratio $\sigma_{(\Sigma(1385)^{0}+p+K^{+})}/\sigma_{(\Lambda(1405)+p+K^{+})}=0.76_{-0.26}^{+0.54}$. The knowledge of this ratio is an essential input for the analysis of the decay $\Lambda(1405)\rightarrow\Sigma^{\pm}\pi^{\mp}$, where an unambiguous separation of the $\Lambda$(1405) and $\Sigma(1385)^{0}$ signals is not possible.

A HIGH-RESOLUTION PHOTOIONIZATION AND PHOTOELECTRON STUDY OF58Ni USING A VACUUM ULTRAVIOLET LASER

In order to provide high-resolution spectroscopic data of nickel (58Ni) and its cation (58Ni+) for the assignment of vacuum ultraviolet (VUV) stellar spectra, we have obtained the photoionization efficiency (PIE) spectra of 58Ni by using a supersonically cooled laser ablation transition-metal beam source and a broadly tunable VUV laser in the range of 61,100-73,600 cm–1, covering the photoionization transitions: Ni+ (3d 9 2 D) ← Ni (3d 84s 2 3 D), Ni+(3d 9 2 D) ← Ni(3d 84s 2 3 F), and Ni+ (3d 84s 4 F) ← Ni(3d 84s 2 3 F). We have also measured the VUV laser pulsed-field-ionization-photoelectron (PFI-PE) spectra of 58Ni in these regions. The VUV-PFI-PE measurement has allowed the determination of a precise value of 61,619.89 ± 0.8 cm–1 (7.6399 ± 0.0001 eV) for the ionization energy (IE) of 58Ni. Due to the narrow VUV laser optical bandwidth of 0.4 cm–1 used in the present study, many complex autoionizing resonances exhibiting Fano line shape profiles are resolved in the PIE spectra. Four autoionizing Rydberg series originating from two-electron and one-electron excitations from the Ni(3d 84s 2 3 F 4) ground state to converge to the respective Ni+(2 D 3/2) and Ni+(4 FJ ) (J = 9/2, 7/2, and 5/2) ion states are identified. The Rydberg analysis, along with VUV-PFI-PE measurements, has yielded highly precise IE values for the formation of these excited ionic states from the Ni(3d 84s 2 3 F 4) ground state. The IE values, relative photoionization cross sections, and autoionizing Rydberg resonances observed in the present study are relevant to astrophysics by enhancing the atomic database of iron group transition metal atoms and for understanding the Ni and Ni+ contribution to the VUV opacity in the solar atmosphere.

Electron ionization of W(CO)6: Appearance energies

The electron ionization (EI) of W(CO)6 has been studied in detail using a high resolution crossed electron-molecular beams technique. We have detected singly (W+, CW+, (CO)xW+ (x=1–6), (CO)yCW+ (y=1–3), CO+) and doubly charge positive ions (W2+, CW2+, (CO)xW2+ (x=1–6) and (CO)yCW2+ (y=1–3)). The relative partial cross sections for formation of particular ions have been measured and the breakdown curves constructed. The ionization energies (IE) for single (8.47±0.06eV) and double ionization (22.90±0.08eV) of the molecule, as well as the appearance energies (AEs) for the dissociative ionization channels for singly and doubly charged ions have been estimated from experimental ion efficiency curves. On the basis of the IEs and AEs values we have calculated sequential bond dissociation energies (BDEs) of different bonds in the positive ions formed from W(CO)6.

Determination of m-Cresol and p-Cresol in Industrial Cresols by Raman Spectrometer

Adopting raman spectrometer to conduct qualitative and quantitative analysis of m-cresol and p-cresol, selecting the respective character peak of bencycloquidium after deformation vibration (m-cresol at 732 cm-1 and p-cresol at 841 cm-1), via experiment and calculations, the relative raman cross section of m-cresol and p-cresol can be concluded:(∂σ/∂Ω)732cm-1/(∂σ/∂Ω)1000cm-1 is 0.74 (height-fitting of the peak), and 0.61 (area-fitting of the peak). This method samples simply and rapidly, and has a short testing time, whose quantitative analysis result of m-/p-cresol is consistent with that of GC analysis.