Absolute electron-impact total ionization cross sections of chlorofluoromethanes

An experimental study is reported on the electron-impact total ionization cross sections (TICSs) of CCl4, CCl3F, CCl2F2, and CClF3 molecules. The kinetic energy of the colliding electrons was in the 10-85 eV range. TICSs were obtained as the sum of the partial ionization cross sections of all fragment ions, measured and identified in a linear double focusing time-of-flight mass spectrometer. The resulting TICS profiles--as a function of the electron-impact energy--have been compared both with those computed by ab initio and (semi)empirical methods and with the available experimental data. The computational methods used include the binary-encounter-Bethe (BEB) modified to include atoms with principal quantum numbers n> or =3, the Deutsch and M?rk (DM) formalism, and the modified additivity rule (MAR). It is concluded that both modified BEB and DM methods fit the experimental TICS for (CF4), CClF3, CCl2F2, CCl3F, and CCl4 to a high accuracy, in contrast with the poor accord of the MAR method. A discussion on the factors influencing the discrepancies of the fittings is presented.     

Measurement of Photoionization Cross Sections of the Excited States of Titanium

Resonance-enhanced multiphoton ionization of the titanium atoms has been investigated in the 293 321 nm wavelength. We couple a laser-ablated metal target into a molecular beam to produce free atoms. Ions produced from photoionization of the neutral atoms are monitored by a home-built time-of-flight mass spectrometer. Photoionization cross sections of the excited states of Ti I were deduced from the dependence of the ion signal intensity on the laser intensity for photon energies close to the ionization threshold. The values obtained range from 0.2 Mb to 6.0 Mb. No significant isotope-dependence was found from measurements of the photoionization cross sections of ^46Ti, ^47Ti, and ^48Ti.     

Scaled Born cross sections for excitations of H2 by electron impact

This article describes the scaling of plane-wave Born cross sections for the excitation of the H(2) molecule to four low-lying electronic states (B (1)Sigma(u) (+), C (1)Pi(u), B(') (1)Sigma(u) (+), and D (1)Pi(u)) by electron impact. The same BE and BEf scaling methods used on atoms were found to be equally effective for H(2) in converting Born cross sections into cross sections in good agreement with available experiments. These scaling methods are applicable only to dipole-allowed excitations. The possibility of using these scaling methods, as was done in atoms, to estimate the contribution of inner-shell excitations to the total ionization cross section via the excitation-autoionization mechanism is discussed, though this type of indirect ionization in molecules is not as common as in atoms.     

Total reactive cross section for K + Br2 in the energy range of 0–4 eV

3-D classical trajectory calculations were performed using diabatic as well as adiabatic potential energy surfaces. Non-adiabatic transitions were allowed and localized at the avoided crossing of the two adiabatic surfaces. The transition probability was calculated according to the Landau—Zener formalism. The total cross sections for the reaction K + Br₂ → KBr + Br were calculated and compared with experimental data. The total cross sections, calculated with the aid of adiabatic potential energy surfaces, were, contrary to those with diabatic surfaces, in very good agreement with the measured total reactive cross sections over the whole energy range of 0–4 eV.     

Scattering of N(2)O on electron impact over an extensive energy range (0.1 eV-2000 eV)

We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here.     

Nuclear shell model applied to metallic clusters

We apply the nuclear shell model to jellium clusters of up to twenty-one Na atoms. Binding energies, ionization potentials, and photoabsorption cross sections are calculated and compared with mean-field results.     

Resonant charge-transfer calculations using a pseudopotential approach and an atomic two-state expansion. (Na+,Na) and (K+,K) cross sections

Total resonant charge-transfer cross sections of (Na⁺,Na) and (K⁺,K) collisions are calculated using atomic wavefunctions and model potentials within the framework of the impact parameter formalism in complete analogy with an earlier successful calculation concerning Rb and Cs atoms. In the present calculations the agreement with experiment is very gratifying and better than in the previous calculation.     

New procedure calculation of K-shell ionization cross sections by proton impact

The database, which relies on different compilations available in the literature and on other experimental values extracted from papers published from 1992 till 2010, is used, within the individual treatment of the elements from beryllium (⁴Be) to uranium (⁹²U), to deduce the empirical cross sections. These experimental data can be presented in a single curve, depending on a scaling law extracted from studies in the most familiar theories of collision (PWBA and BEA). Then, a fourth order polynomial was used to fit very well the existing database of K-shell ionization cross sections by proton. This procedure generates a new set of parameters to calculate empirical cross sections. Following the present procedure, our results are compared with those obtained using the ECPSSR model where a discrepancy is observed in the low-proton energy regime.     

Calculation of cross sections for penning ionization: He(2S,2S) + H(1S)

Cross sections for ionization of hydrogen atoms by metastable helium atoms are calculated using the best published values for the complex potential. The present cross section for the singlet metastable is quite different from previous determinations. The validity of the JWKB approximation has been verified.     

Electron impact ionization of atomic hydrogen

The role of Coulomb-correlation in electron impact ionization of atomic hydrogen is investigated. Triple differential cross sections are calculated using the first order multiple scattering theory taking into account the propper asymptotic behaviour of the final state wavefunction. A semi-empirical procedure is outlined to choose the effective charges consistent with several physically required limits. A comparison with recent experimental data is made; the observed agreement strongly suggests the importance of asymptotic Coulomb-correlation in ionization even at high energies. The first order approximation used here for the hydrogen case is easily generalizable for ionization of several electron atoms at not too large scattering angles and not too low incident energies.     

Electron-impact ionization of the metastable Mg(...2p63s3p3P0,2) atoms

Electron-impact ionization of Mg atoms from metastable states was investigated. The method and tech-nique of crossed atomic and electron beam studies are described. The value of the total ionization cross section from the 3s3p ³P_j metastable states for 4... 21 eV incident electron energy was determined. It was found that ionization cross sections from the metastable and ground states differ considerably. This is mainly due to the different mechanisms of ion formation from the metastable and ground states. The results obtained are compared with those calculated in the classical mechanics binary approximation.     

Differential, partial and total electron impact ionization cross sections for SF(6)

Single and double differential ionization cross sections for the production of ions resulting from dissociative, single and double ionization of SF(6) by electron impact have been calculated using a semiempirical formulation based on the Jain-Khare approach. In addition, triple differential cross sections have been obtained for some of the doubly charged fragment ions at an incident electron energy of 100, 150, and 200 eV, respectively, and a fixed scattering angle of 30 degrees. As no previous data seem to exist for differential cross sections we have derived from these differential cross sections corresponding partial and total ionization cross sections from threshold up to 900 eV and compared those with the available theoretical and experimental data.     

Photoabsorption of Fe,Co, and Ni atoms embedded in Al-clusters near 3p-ionization threshold of impurity

Photoionization cross sections of Fe, Co, and Ni atoms inside Al jellium-clusters (containing up to 90 atoms) in the energy range near the 3p ionization threshold of the impurity atom are calculated using the time-dependent local spin-density approximation. The spectra are dominated by resonances and depend markedly on the cluster size. It is obtained that the resonances have an autoionization character and are caused by the interference between discrete electronic transitions from the 3p shell of the impurity atom to unfilled d shells of the jellium-cluster and ionization excitations ofd electrons. For comparison, within the same approach, photoionization cross sections of atomic Fe, Co, and Ni are computed.     

Calculation of Ionization,Excitation and Electron Capture Cross Sections for Be4++H(2s, 2p) Collisions

A computational study of Be⁴⁺+H(2s, 2p) collisions has been carried out employing the Classical Trajectory Monte Carlo (CTMC) method for the impact energy range from 20 keV/u to 1000 keV/u. The integral n partial cross sections for H(n) excitation and Be³⁺(n) electron capture and, the total ionization and electron capture cross sections are calculated and compared to recent semiclassical results. A general good agreement is observed for the n partial and total electron capture and ionization cross sections. The comparative study of the three inelastic processes show no significant differences between both excited targets.     

K-shell ionization induced by 30 MeV/u argon and neon beams

We have studiedK-shell ionization induced by 30 MeV/u Ne and Ar projectiles on target atoms with atomic numbers ranging from 27 to 90. X-ray production cross sections and energy shifts were measured with Si(Li) detectors. In most cases satisfactory agreement between measurements and theoretical direct ionization cross sections is obtained when the contribution of electron capture is included. The influence of multiple ionization on the fluorescence yield ω _K is discussed.     

Triple-differential cross sections of helium for (e, 2e) processes

The triple differential cross sections for electron impact ionization of helium in a symmetric coplanar energy-sharing geometry at incident energies from 45–500 eV and an angle of 45° are calculated by use of the modified BBK model. A comparison with other theoretical results has been performed. It has been found that the present results give an excellent agreement with the absolute experimental measurement for the energy range considered.     

S K-shell ionization cross sections by protons of 0.8 to 3.0 MeV

Thick targets containing 2, 4, 8 and 50% by weight of S in a Li₂B₄O₇ matrix and pure S have been irradiated by protons in the energy range 0.8 to 3.0 MeV. The S K-shell ionization cross sections have been measured and compared to available theoretical and experimental data.     

Quenching of metastable hydrogen atoms by low energy collisions with hydrogen molecules: comparison of experiments with theory

The experimental velocity dependence of the quenching cross section of metastable H(2S) atoms in low energy collisions with hydrogen molecules is compared with theoretical calculations using the formalism of Gersten. This formalism takes into account possible changes of the molecular rotational energy and is in good agreement with experiments at low energies where other theories fail. A dependence of this cross section on the temperature of the molecular gas is predicted which is due to the temperature dependent population of the molecular rotational levels.