Negative Ion States Research Articles

Search for additional resonances in e--helium scattering below the first excitation threshold

Apart from the well known 2S negative ion state found in e--He scattering, additional p and d wave resonances below the first excitation threshold have been reported by several authors. A careful experimental investigation of the differential cross section does not confirm these observations. If such resonances exist, their width should be smaller than 10 mu eV.

Dissociative electron attachment in CO: angular distribution of the 0-ions

The angular distribution of O- ions produced by the principal process for dissociative attachment of electrons in CO has been studied in the energy range from 9.8 eV to 11.2 eV for scattering angles from 10 to 120 degrees. The angular distribution is observed to have a strong asymmetry with respect to the 90 degree scattering angle as well as only a weak dependence on the incident energy. The observations indicate that only one resonant state is responsible for the attachment process, and a preliminary comparison with theory would suggest Pi symmetry for the intermediate negative ion state.

Resonances near threshold in the electron impact excitation and polarization of the 61S0-63P1line of mercury

Data are presented for the positions and plarizations of resonances occurring within a few volts of threshold of the 2537 AA line of mercury, excited by electron impact. A 127 degrees electron monochromator was used in a standard crossed-beam apparatus, the results being obtained by measuring separately the I/sub /// and Iperpendicular to contributions to the excitation function. The normalization procedure and the checks made on the data are described. Several resonances are clearly observed; a comparison of the measured polarization of one of them with a theoretical value leads to a possible J value for the negative ion state being formed.

Variable ζ calculation for self‐consistent screening parameters in ab initio molecular theory

AbstractA resolution of Roothaan's HF–SCF–LCAO–MO equations is proposed in which atomic orbital exponents (ζ) are made dependent on the molecular charge distribution and included in the self‐consistent scheme. Screening parameters so obtained are self‐consistent with the molecular orbital coefficients and compare closely to optimum orbital exponents found by other methods.The technique is applied to the ground, lowest positive, and lowest negative ion states of the hydride series LiH, BH, and HF. Calculated potential curves are used to define purely theoretical values for the vertical and adiabatic ionization energies and electron affinities. Predictions are compared to experimental values where available.

Displacements of electron ejection energies in near-threshold excitation of autoionizing levels of helium by electron impact

The four lowest autoionizing levels in helium, namely (2s 2 ) 1 S, (2s2p) 3 P, (2p 2 ) 1 D and (2s2p) 1 P, have been studied near their thresholds for excitation by electron impact. Features in the spectra of electrons ejected from these states were seen to shift to steadily higher energies as the impact energy was lowered to within a few electron volts above threshold. This phenomenon is compared with a similar one which has been observed in the scattering of low energy helium ions from helium atoms. A qualitative explanation of these observations can be provided by considering a “post-collision interaction” between the scattered particle and the ejected electron. The limitations of this model are discussed. Structure in the spectra suggest that the situation is complicated by an interference phenomenon which cannot be explained in a classical treatment of the problem. The threshold electron excitation spectrum of these autoionizing levels has also been obtained, using an apparatus designed to detect very low energy electrons. As well as confirming the observations described above, this revealed the presence of one of the two negative ion states known to exist in this region. In an effort to obtain a complete picture of the processes taking place in helium at these energies, the negative ion states were also studied in direct ionization and in elastic scattering at various angles. The latter measurements confirmed the classifications of these states as (2s 2 2p) 2 P and (2s2p 2 ) 2 D, and we have obtained values for their energies of 57.22 ± 0.04 eV and 58.30 ± 0.04 eV respectively.

Spectroscopy of negative ion radiation emitted from a low current arc

A special low current arc-discharge is used, in which the radiation of the negative ions dominates. By the help of this negative ion radiation source, the electron affinity can be evaluated by measuring the longwave threshold of the affinity continuum with spectroscopic precision. Quantitative continuum and plasma temperature measurements allow the additional determination of detachment cross-sections. With this method, electron affinities and detachment cross-sections of the halogens and hydrogen have been measured. For iodine a shape-resonance and the evidence of a bound negative ion state was found.

Photo-ionization of aromatic hydrocarbon negative ions in ethers. The biphenyl anion

On the basis of Marcus' theory of electron transfer the mechanism and kinetics of electron ejection from lower excited states of aromatic hydrocarbon negative ions in liquid ethers are discussed.The application of the theory to the photo-ionization of biphenyl− in glymes by nanosecond ruby laser pulses studied by Makkes van der Deyl et al. leads to a free energy of electron ejection for the ground state ion, ΔG0e ≈ 12000 cm−1.

Matrix variational calculations of electron scattering by C, N, and O atoms

Total elastic cross sections for electron scattering by ground-state C, N, and O atoms have been computed by the matrix-variational method for energies up to the first $n=3$ excitation. Two levels of approximation are considered: single configuration (SC) and near-degenerate configuration interaction (CI). Both approximations neglect the principal effects of target-atom electric-dipole polarizability. The SC calculations are the variational equivalent of earlier close-coupling calculations including all states of the ground configuration $2{s}^{2}2{p}^{m}$. The CI calculations augment this with configurations $2s2{p}^{m+1}$ and $2{p}^{m+2}$. The short-range correlations introduced by the CI calculations are found to have dramatic effects on resonance structures found at low energies in SC calculations. In addition to the well-established negative-ion bound states ${\mathrm{C}}^{\ensuremath{-}}(^{4}S^{o})$ bound and ${\mathrm{O}}^{\ensuremath{-}}(^{2}P^{o})$, the CI calculations give qualitative predictions of the true location of other $2{s}^{2}2{p}^{m+1}$ states: ${\mathrm{C}}^{\ensuremath{-}}(^{2}D^{o})$ bound; ${\mathrm{C}}^{\ensuremath{-}}(^{2}P^{o})$ resonance near 1 eV; ${\mathrm{N}}^{\ensuremath{-}}(^{3}P)$ very near threshold, either weakly bound or a very narrow resonance; no ${\mathrm{O}}^{\ensuremath{-}}$ resonance. A method for precise location of these negative-ion states is proposed.

Study of the photodetachment ofC−andF−by many-body perturbation theory

Photodetachment cross sections of ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{F}}^{\ensuremath{-}}$ are calculated by means of the many-body perturbation theory with multiple basis sets. The ${V}^{N\ensuremath{-}1}$ basis set is defined to include the intrachannel interaction. The ${V}^{N\ensuremath{-}2}$ basis set is introduced to describe the correlation in the Hartree-Fock ground states of negative ions. The lowest-order cross sections agree very well with experiment. The first-order corrections are calculated in the case of ${\mathrm{F}}^{\ensuremath{-}}$, and a large cancellation is found between the initial-state correlation and the final-state interchannel interactions. The importance of the use of the energy relations in the lower-order calculations is pointed out.

ChemInform Abstract: ELECTRON ATTACHMENT TO CYCLIC ANHYDRIDES AND RELATED COMPOUNDS

Negative ion production cross section are sections are tabulated as a function of the bombarding electron energy for dissociative and nondissociative electron attachment to various cyclic anhydrides and related compounds. The unsaturated compounds, maleic anhydride (250 μsec), phthalic anhydride (313 μsec), pyromellitic anydride (8000 μsec), and maleimide (43 μsec) attach thermal electrons to form parent negative ions which are metastable with autodetachment lifetimes that decrease with an increase in the energy of the bombarding electron. The number in parentheses following each compound is the lifetime at peak cross section. Compound negative ion states are reported for the saturated compounds succinic anhydride (1.3 eV), cyclobutane‐dicarboxylic anhydride (1.3 eV), glutaric anhydride (0.6 eV), and succinimide (1.1 eV) for electron energies given in the parentheses. In most of the anhydrides the most abundant anions produced by dissociatve electron attachment are metastable RCO2‐* ions which result from...

Angular dependence of the elastic resonance scattering of slow electrons on argon and krypton

In an electron scattering experiment the angular dependence of the resonance doublets due to the 2P3/2 and 2P1/2 negative ion states has been studied for argon and krypton in the elastic channel. With an energy resolution of the spectrometer of slightly better than 30 meV (FWHM) it was possible to localize the energy positions of the 2P3/2 states to 11.07 eV for argon and 9.47 eV for krypton within 10 meV accuracy. The measured separation between the doublets was 172+or-5 meV for argon and 637+or-5 meV for krypton. A phase shift fit was performed on the angular dependences of the resonance effects and the potential scattering. Analysis of the results gave a natural width at half-height for the 2P3/2 resonances which lies between 3 and 4 meV in the argon case and is about 8 meV for krypton. For the best fit to the experimental data we found the following potential scattering phase shifts eta 0=4.99, eta 1=2.53, eta 2=1.17, and eta 3=0.10 for argon and eta 0=5.37, eta 1=2.54, eta 2=1.40, and eta 3=0.17 for krypton.

Characteristic energy losses by slow electron impact on thin-film alkanes at 77 °K

Characteristic energy losses by slow electron impact have been measured on thin films of several alkanes at 77 °K. The cross section for the lowest loss at [inverted lazy s] 0.6 eV increases tenfold in the series n -pentane to n -nonane with increasing carbon number. For a given carbon number it decreases markedly with branching. Losses at [inverted lazy s] 3, [inverted lazy s] 6, [inverted lazy s] 9, and [inverted lazy s] 22 eV are common to all alkanes with no strong dependence on molecular size or configuration. The lowest losses may be due to temporary negative ion states or to pre-existing trapping potentials. Losses at [inverted lazy s] 3 eV may be due to the highest temporary negative ions or to triplets. If the former is correct, then [inverted lazy s] 6 eV losses can be attributed to triplets. The [inverted lazy s] 9 eV losses are due to upper singlet states, and the [inverted lazy s] 22 eV losses may be due to collective excitations.

Threshold electron impact excitation of argon in the region of the M 1 edge

The threshold electron impact spectrum of argon has been obtained in the energy range 24–35 eV using the sulphur hexafluoride electron scavenging technique. Structure is observed which may be due to single excitation, double excitation and temporary negative ion states.

Electron attachment to cyclic anhydrides and related compounds

Negative ion production cross section are sections are tabulated as a function of the bombarding electron energy for dissociative and nondissociative electron attachment to various cyclic anhydrides and related compounds. The unsaturated compounds, maleic anhydride (250 μsec), phthalic anhydride (313 μsec), pyromellitic anydride (8000 μsec), and maleimide (43 μsec) attach thermal electrons to form parent negative ions which are metastable with autodetachment lifetimes that decrease with an increase in the energy of the bombarding electron. The number in parentheses following each compound is the lifetime at peak cross section. Compound negative ion states are reported for the saturated compounds succinic anhydride (1.3 eV), cyclobutane-dicarboxylic anhydride (1.3 eV), glutaric anhydride (0.6 eV), and succinimide (1.1 eV) for electron energies given in the parentheses. In most of the anhydrides the most abundant anions produced by dissociatve electron attachment are metastable RCO2-* ions which result from the ejection of neutral CO from the parent molecules. Experimental evidence shows that the RCO2- dissociates following autodetachment. The metastability of an observed CO2-* ion is attributed to poor Franck-Condon overlap between the potential energy surface of CO2 and CO2-. Negative ion mass spectra are included for acetic anhydride and acetyl peroxide.

ChemInform Abstract: CORRELATION EFFECTS IN THE NEUTRAL AND IONIZED GROUND STATES OF ACETYLENE

Correlation energies were calculated by “Effective Pair Correlation Energy” (EPCE) and minimal basis set configuration interaction (CI) methods for pairs of electrons in the occupied molecular orbitals for the (π4u) neutral ground state, (π3u) ground state of the positive ion, and (π4uπg) ground state of the hypothetical negative ion of acetylene.

Electron mobilities in liquid olefins: Structure effects

Electron mobilities (square centimeters per volt second) at 293°K and their activation energies (kilocalories per mole) in several liquid olefins are, respectively, cis-butene-2, 2.2 and 3.7; trans-butene-2, 0.029 and 5.4; 2, 3-dimethylbutene-2, 5.8 and 2.0; cyclohexene, 1.0 and 3.6. Mobilities in butene-1 and isobutene at 293°K are 0.064 and 1.44 cm2/v· sec, respectively. The mobilities in all of the liquids are independent of electric field strength in the range 1–14 kV/cm. Molecular dipole moments up to 0.5 D have negligible effects on the magnitudes of the mobilities. A plot of the thermal electron mobilities against epithermal electron ranges in the olefins falls on the same curve as that for electrons in saturated hydrocarbons. This implies that molecular negative ion states are not involved in the scattering processes that control the thermal electron mobilities, nor in those that limit the epithermal electron ranges. Attempts to measure electron mobilities in conjugated dienes failed, presumably due to capture of the thermalized electrons by the dienes to form molecular negative ions.

Electron impact studies of some cyclic hydrocarbons

The formation of negative ions by collisions of low-energy electrons with cycloöctatetraene, biphenylene, triphenylene, fluorene, indene, acenaphthene, acenaphthylene, and fluoranthene was examined in the gas phase under single collision conditions. Long-lived negative ions (τ > 1 μsec) were observed in the cases of cycloöctatetraene and acenaphthylene at approximately zero energy with mean lifetimes of 6.4 and 88 μsec, respectively. The energy breadth of the negative ion resonances for these ions was equal to the electron energy resolution, and thus the measured lifetimes are averaged over the width of the resonance. Fluoranthene captures slow electrons with a maximum in the cross section at 0.2 eV, and the width of the resonance is approximately 0.4 eV. The lifetime was found to decrease from 180 μsec at ∼ eV to 80 μsec at ∼ 1.0 eV. Energy-loss processes were observed using the SF 6 scavenger technique in biphenylene and triphenylene at electron energies of 2.15 and 2.35 eV, respectively, and are attributed to compound negative ion states.

Symmetry of negative ions of polar molecules

The spin and space symmetry of negative ions of polar molecules are studied. It was previously known that a dipole moment exceeding 1·625 debye is a sufficient condition for the existence of the corresponding ion. It is shown that such an ion has the same space symmetry in its electronic wavefunction as does the neutral molecule. Additional negative ion states of one or more other symmetries exist if the molecule has a symmetry axis or plane, and has a dipole moment exceeding 9·638 debye. The above results are proven in the approximation that the nuclei are held stationary.

Correlation effects in the neutral and ionized ground states of acetylene

Correlation energies were calculated by “Effective Pair Correlation Energy” (EPCE) and minimal basis set configuration interaction (CI) methods for pairs of electrons in the occupied molecular orbitals for the (π 4 ) neutral ground state, (π 3 ) ground state of the positive ion, and (π 4 π g ) ground state of the hypothetical negative ion of acetylene. The EPCE values allow detailed breakdown of the ionization potential and electron affinity (for the unstable negative ion). It is seen that the SCF values for the former can be modified by the EPCE values to give estimates close to the experimental quantity. The EPCE values are compared against the pair-wise correlation energies obtained by minimal basis set CI and the percentages of the latter as compared to the former are interpreted by considering the form of the available excited configurations used in constructing the correlated functions. The MBSCI calculation accounts for only 20–30 % of the EPCE correlation energy.

Electron impact ionization studies of ferrocene, cobaltocene, nickelocene, and magnesocene

Negative and positive ions of ferrocene, cobaltocene, nickelocene, and magnesocene produced by collisions with monoenergetic electrons have been studied. Appearance potentials relative to the ionization potential of krypton have been determined for the products (C10H10M)+, (C5H5M)+, and M+. The ionization potentials derived from the present electron impact studies are in good agreement with recent photoelectron values. In all cases cyclopentadienyl anions were produced at low electron energies; however, their cross sections for formation differed by orders of magnitude. The most striking feature of the negative ion studies was the observation of a long-lived parent negative ion of nickelocene at thermal electron energies, and a second broader compound negative ion resonance at approximately 1 eV. The autodetachment lifetime for the second resonance decreases from 60 μsec at 0.5 eV to 20 μsec at 1.5 eV. Cobaltocene also exhibits two long-lived negative ion states in the energy region below one electron volt which are barely resolvable. Ferrocene forms a short-lived (τ ≳ 2× 10−14 sec) compound negative ion state at ∼0.5 eV. The negative ion results can be qualitatively explained by assuming that the captured electron occupies one of the empty e1g or e2u orbitals of the parent molecule.