Electron Impact Experiments Research Articles

Proton production from H2by electron impact excitation of autoionising states near threshold

Energetic protons produced via the autoionising states of H2 have been studied in an electron impact experiment. A new method has been developed for determining the transmission properties of the proton energy analyser and has allowed the authors to derive the relative differential cross sections for proton production at the formation threshold of each of four autoionising states in the 23-29 eV energy range. Furthermore, a new process has been observed with a threshold at 22.3 eV which yields protons with energies around 1.3 eV.

Fourier analysis of spherically averaged momentum densities for some gaseous molecules

The spherically averaged autocorrelation function, B( r), of the position-space wavefunction, ψ( r ), is calculated by numerical Fourier transformation from spherically averaged momentum densities, ϱ( p), obtained from either theoretical wavefunctions or (e,2e) electron-impact ionization experiments. Inspection of B( r) for the π molecular orbitals of C 4 H 6 establishes that autocorrelation function differences, Δ B( r), can be qualitatively related to bond lengths and numbers of bonding interactions. Differences between B( r) functions obtained from different approximate wavefunctions for a given orbital can be qualitatively understood in terms of wavefunction difference,Δψ( r ), maps for these orbitals. Comparison of the B( r) function for the 1 a u orbital of C 4H 6 obtained from (e,2e) momentum densities with that obtained from an ab initio SCF MO wavefunction shows differences consistent with expected correlation effects. Thus, B( r) appears to be a useful quantity for relating spherically averaged momentum distributions to position-space wavefunction differences.

Limiting slope of the generalized oscillator strength vs momentum transfer curve

Although at the limit of zero momentum transfer, the generalized oscillator strength is equal to the Born value [E. N. Lassettre, A. Skerbele, and M. A. Dillon, J. Chem. Phys. 50, 1829 (1969)], it is found that the limiting slopes (df/dK)K=0 and (df/dK2)K=0 do not obey the Born approximation. For dipole allowed transitions (df/dK)K=0 is nonzero but finite and (df/dK2)K=0 becomes infinite at all finite incident energies. Also, the non-Born correction to f0 deduced from zero angle data is just as important as the next higher multiple term in the Born approximation. Thus in the analytic fit of experimental data it is imperative to include odd power terms in K even when the Born approximation is apparently obeyed if the purpose is to deduce the limiting slope as well as the intercept. However, if only the intercept is needed and the measured f’s shows no apparent energy dependence, it is still possible to extrapolate to f0 using a series even in K. The relevance of the present result to a number of recent electron impact experiments is discussed.

The identification of low energy K and Ca + autoionizing levels observed in electron impact experiments

The ejected electron spectra of K and Ca vapours following electron impact excitation of the 3p-subshell have been compared with Hartree–Fock calculations. Most K ɪ and Ca ɪɪ 3p 5 3d 4s levels have been identified, albeit tentatively in many cases. A detailed analysis of low energy K ɪ lines is presented, including some minor changes to earlier assignments of photo-absorption lines. A good general description of the low energy ejected electron spectrum of Ca has also been achieved, almost exclusively in terms of autoionization of Ca ɪɪ levels.

Ionization of N2O by low-energy electrons and near-thermal rare-gas ions

Emission spectra between 185 and 600 nm have been investigated following near-thermal charge exchange between He+ and N2O and ≤ 100 eV electron impact on N2O. The charge exchange produces N2O+Ã→X˜and N2+ B → X emission, but the two band systems account for at most 5% of all charge transfer products. These results and literature data on Ar+/N2O are discussed in the light of Franck—Condon and energy resonance criteria as applied to low-energy charge exchange. The electron-impact experiments revealed a weak (≈ 10−3) long-lived (≈ 50 × 10−6 s) component in the N2O+Ã→X˜emission.

A new interpretation of the Rb I 4p subshell excitation spectrum between 15 and 19 eV

The Rb I 4p subshell excitation spectrum, as observed in both photo-absorption and electron impact experiments, has been compared with Hartree-Fock calculations. Detailed assignments are proposed for more than half the 40 lowest energy (15–19 eV) resonances. Apart from the leading doublet, 4p 6 5s 2 S ½ –4p 5 5s 22 P ½, 1½ previous assignments are com­pletely revised. Assignments of the eight strongest lines, in addition to the leading doublet, are reasonably certain and a good general interpretation of the spectrum between 15 and 19 eV is achieved.

Inelastic scattering of positive ions and electrons from water: The 4–6 eV energy loss region

Inelastic scattering of positive ions and electrons by water molecules has been investigated with particular emphasis on the 4–6 eV energy loss region. Aside from vibrational excitation of the ground state, no inelastic process was observed below 6 eV with either H+ or H2+ projectile ions although with H2+ impact, singlet–triplet transitions above 6 eV were easily detected. Molecular beam target electron impact experiments using incident energies from 10 to 20 eV showed the previously-observed weak continuum energy loss in the 4–6 eV region. Time of flight measurements established that the continuum signal was from electrons produced in the collision region and not negative ions. An investigation of signals obtained from the apparatus when it was heavily contaminated with water showed a background continuum starting at 3.5 eV energy loss whose intensity varied directly with the recent history of exposure of the apparatus to water vapor and inversely with the collision region temperature. These results, taken together, suggest that there is no bound electronic state of water below 6 eV and that the apparent 4–6 eV energy loss process observed in electron impact experiments actually arises from one or both of two unrelated effects; background scattering of electrons from the water-contaminated apparatus and H− production by dissociative attachment when the incident energy is near 6.5 eV.

Beth cross sections for the sodium isoelectronic sequence

Bethe cross sections for the excitation of Na-like ions to the $3^{2}P_{\frac{1}{2}}$, $3^{2}P_{\frac{3}{2}}$, $4^{2}P_{\frac{1}{2}}$, and $4^{2}P_{\frac{3}{2}}$ states from the ground state are computed from the relativistic Hartree-Fock wave functions. As the nuclear charge is increased, excitation cross sections to the $^{2}P_{\frac{3}{2}}$ states are affected far more by relativistic effects than those to the $^{2}P_{\frac{1}{2}}$ states. The asymptotic ionization cross sections for the neutral Na atom and ${\mathrm{Fe}}^{15+}$ ion are computed from a sum rule, i.e., by subtracting the sum of discrete excitation cross sections from the total inelastic-scattering cross section obtained from the sum rule. The ionization cross sections thus obtained are ${\ensuremath{\sigma}}_{\mathrm{ion}}(\mathrm{Na})={\ensuremath{\beta}}^{\ensuremath{-}2}(2.51{\mathrm{ln}[\frac{{\ensuremath{\beta}}^{2}}{(1\ensuremath{-}{\ensuremath{\beta}}^{2})}]\ensuremath{-}{\ensuremath{\beta}}^{2}}+ 26.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20} {\mathrm{cm}}^{2},$ ${\ensuremath{\sigma}}_{\mathrm{ion}}({\mathrm{Fe}}^{15+})={\ensuremath{\beta}}^{\ensuremath{-}2}(0.209 {\mathrm{ln}[\frac{{\ensuremath{\beta}}^{2}}{(1\ensuremath{-}{\ensuremath{\beta}}^{2})}]\ensuremath{-}{\ensuremath{\beta}}^{2}}+1.64)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20} {\mathrm{cm}}^{2},$ where $\ensuremath{\beta}$ is the incident electron or proton speed divided by that of light. The cross section for Na is in agreement with that calculated by McGuire, but not with the electron-impact experiment by McFarland and Kinney. The ionization cross section for ${\mathrm{Fe}}^{15+}$ is a factor of 2 to 3 larger than the Coulomb-Born results by Rudge and Schwartz and by Bely in the asymptotic region.

L-changing electron impact cross sections for high Rydberg atoms with application to helium experiments

The impact parameter method is applied to high Rydberg atoms for estimating Δn=0 dipole transition cross sections. Values for high-l states are determined in large part by polarization and relativistic fine-structure contributions to energy levels. Computed cross sections for low-l helium atoms show good agreement with angular momentum transfer cross sections from electron impact experiments over the range 20<n<80.

Electronic energy levels of benzene below 7 eV

The electronic energy levels of benzene below 7 eV have been investigated in an electron impact experiment. Vibrational structure has been resolved for the first three singlet–triplet transitions and the second triplet state has been separated from the overlapping first singlet state. Energies of the vibrational bands of the observed singlet–triplet transitions are (strongest band underlined) S0→T1, 3.67, 3.79, 3.89, 3.98 eV; S0→T2, 4.63, 4.73, 4.85, 4.95 eV; S0→T3, 5.58, 5.69, 5.81 eV. The previous result of Lassettre et al. [E. N. Lassettre, A. Skerbele, M. Dillon, and K. Ross, J. Chem. Phys. 48, 5066 (1968)] which suggested that two electronic states are involved in the 6.2 eV region scattering has been confirmed. These results together with ultraviolet absorption data allow the construction of a rather detailed energy level diagram for benzene up to 7 eV.

The microchannel plate as detector for VUV photons and metastable atoms in the 1-0.1 Pa pressure region

The problem of detecting extremely low count rates of VUV photons or metastable atoms, at the maximum possible target gas density and opening angle, can conveniently be solved by using a double channel plate detector. In an electron-impact experiment, the first of two channel plates has been operated as a separative wall between the target gas region (0.1 Pa) and a high vacuum in order to achieve an opening angle of 0.8 sr.

Ejected-electron spectrum of Mg I and Mg II autoionising levels between 20 and 53 eV excited by low-energy electron impact on magnesium vapour

The ejected-electron spectrum of magnesium vapour has been observed at 75 degrees with respect to an incident electron beam of 62 to 400 eV incident energy. Ejected electrons corresponding to both Mg I autoionising levels and Auger transitions are reported. Comparison is made with photoabsorption data, ejected-electron spectra from ion impact and high-energy electron impact experiments and available theoretical calculations.

Ionization efficiency curves of ethane by electron impact

Ionization efficiency curves have been measured for ions from ethane by impact of a monoenergetic electron beam. The ethane parent ions and all the possible fragment ions including CH 4 + have been observed over wide ranges about 13 eV below and above their appearance thresholds. The curves have been processed by a computer and the total ionization efficiency curve was calculated. The total efficiency curve shows breaks corresponding to the ionization potentials which agree with those determined by photoelectron spectroscopy. The appearance potentials for fragment ions are in agreement with those determined by photoionization but do not always agree with those determined by previous electron impact experiments.

Ionization Efficiency Curves of Acetylene by Electron Impact

Ionization efficiency curves have been measured for the ions including CH2+ produced from acetylene by monoenergetic electron impact over ranges of about 10 eV near their appearance thresholds. The total ionization of ficiency curve shows breaks at the energies in agreement with the ionization potentials determined by photoelectron spectroscopy. Other breaks in the curve were found to correspond to autoionizing states. The appearance potential of C2H+ almost agrees with that by photoionization mass spectrometry. The potentials of other fragment ions are not always close to those by previous electron impact experiments.

The low lying states of ammonia; generalized valence bond and configuration interaction studies

The nine lower singlet and triplet states of ammonia were studied using ab initio generalized valence bond (GVB) and configuration interaction (CI) wavefunctions. The eight lowest (vertical) excited states were found to be Rydberg in nature. Vertical excitation energies were determined to be (in eV): 6.14 and 6.37 for n → 3s; 7.86 and 7.88 for n → 3p x,y , and 7.87 and 8.15 for n → 3p z (in each case the triplet energy is first). These results are in excellent agreement (0.06 eV average error) with the observed electron impact excitation energies (in eV), 6.39 (A 1A″ 2), 7.93 (B 1E″), and 8.26 C 1A′ 1). The small singlet triplet energy splittings are consistent with those expected for Rydberg-like excited states. We find no vertical transition consistent with the small peak at 4.4 eV apparently observed in threshold electron impact experiments.

Empirically adjusted ab initio calculations for the valence and rydberg excited states of formic acid

We report ab initio calculations on the valence and Rydberg excited states of formic acid which we find below 11.0 eV. Energies and wave functions have been obtained from a frozen core single excitation configuration mixing. Single excitations from the four highest MOs have been used and their energies shifted to match the computed ionization potentials with the experimental values. Our results support the assignment by Bell et al.of the three Rydberg series of formic acid as n 0 - ns, n p and n d, and suggest that 1 (n 0 -3s) is below 1 (Π 2 -Π * ). We find 1 (Π 2 -3s) near the expected position, but its intensity should be very low. We do not find low lying valence n 0 -σ * states, and suggest an alternative explanation for the broad band under C assigned by Robin as 1 (n 0 -σ * ). In agreement with a recent suggestion, we assign the bands at 9.91 and 11.08 eV as Π 2 -3pa″ and 4pa″. Our triplet state excitation energies stand as predictions for future electron impact experiments.

Bound and resonant molecular states formed from and dissociating to atoms and ions which do not exist

The importance of considering the existence of bound and resonant molecular states arising from and dissociating to atoms and ions which are thermodynamically unstable is discussed. Physical mechanisms can exist at finite internuclear separations which do not exist at infinite separations that can stabilize such states. As an example, it is shown that such states explain the unexpected minima occuring in the potential energy curves for the 1Πu and upper 1Σg+ states of Zn2. Such states may also be of fundamental importance in understanding experimental phenomena such as the dissociative attachment and vibrational excitation patterns observed in electron impact experiments on HCl.

Resonant vibrational excitation of the NO ground state by electron impact in the 0.1-3 eV energy range

The cross sections for differential elastic scattering and vibrational excitation of the NO ground state have been studied in an electron impact experiment in the 0.1-3 eV energy range. Strong resonant processes are observed. They are associated with the3Sigma-,1Delta and1Sigma+states of NO-. The vibrational levels of the3Sigma-state can be observed up to about 1.2 eV. Sudden increases are observed in the peak spacings and widths, and also rapid changes of the angular distribution of the ejected electrons, and these can be explained by the formation of the NO-1Delta state ( omegacapproximately=185 meV). In order to explain all the observed structure, the third low-lying1Sigma+state of NO-must be taken into account from 1.120 eV upwards. Interference effects between these three resonant states strongly affect the structure heights from one exit channel to the next.

Calculation of Xe II line strengths and radiative lifetimes in intermediate coupling

Line strengths for 5s2 5p4 6s−5s2 5p4 6p transition in xenon are calculated in intermediate coupling scheme. The parametric potential method is used in calculating the radial part of the wave functions while the method of least squares fit of energy levels is applied in obtaining the angular part of the wave functions. Lifetimes of the 6p states obtained using these methods show a good agreement with data from electron-impact experiments.

Threshold electron-impact excitation spectrum of pyridine

The electronic excitation spectrum of pyridine has been investigated in a low-energy electron-impact experiment. Assignments are suggested for the observed excitation processes.