Negative Ion States Research Articles

ChemInform Abstract: Influence of Alkyl Substituents on the π* Negative Ion States of Benzene and Its Derivatives. Electron Transmission Spectroscopy of p‐Alkylanilines

Abstract Electron affinities and electron attachment energies associated with the π* orbitals of several p-alkylanilines (alkyl = Me, Et, i-Pr, and t-Bu) have been determined in the gas phase by electron transmission spectroscopy and have been analyzed by ab initio molecular orbital calculations at the STO-3G and 6-31G levels of basis set. The results lead to the conclusions that (a) the 2A2 negative ion state in toluene lies below the 2B1 state, (b) the 2A2 state is stabilized in toluene relative to benzene by bonding overlap between the ortho carbon (C2(2pπ)) and the Me H(1s) orbitals in the LUMO, (c) the 2B1 state is stabilized on adding Me groups to toluene to form t-butylbenzene through reduced antibonding interactions between C1(2pπ) and Me C(2pπ) orbitals, and (d) the lowest resonance in t-butylbenzene is extensively overlapped with the second resonance and may involve strong mixing of the 2A2 and 2B1 states.

A reinvestigation of the S-/CS2dissociative attachment spectrum

An interpretation of the low-lying dissociative attachment bands in CS2 is attempted with the help of the combined application of fragment anion mass spectrometry with ion kinetic energy analysis, electron transmission spectroscopy and electron energy loss spectroscopy. Dissociative attachment bands at 3.35, 3.65, 6.2 and 7.8 eV were found, in agreement with earlier measurements. The ion kinetic energies were found to be nearly zero at all four bands. The onset of the 3.35 eV peak was found to be very gradual, with a weak signal extending down to 2.55 eV, which is in close proximity to the dissociation threshold at 2.41 eV. The transmission spectrum shows evidence of a 2 Sigma g shape resonance at 4.2 eV, in addition to the previously reported 2 Pi u resonance at 1.8 eV. The energy loss spectra reveal three low-lying structured valence bands with maxima at 3.31, 3.63 and 3.90 eV. The energy dependence of the excitation of these bands shows a stepwise onset and two broad bands at 4.8 and 5.6 eV, tentatively assigned as the ( pi )3( pi *)2 2 Phi and 2 Pi u core-excited shape resonances. Based on these experimental results it is suggested that the 3.35 and 3.65 eV S-/CS2 peaks are caused by dissociation or predissociation of negative-ion states in which an electron is loosely bound by the low-lying valence excited states of CS2, on potential energy curves that nearly coincide with those of the parent states in the Franck-Condon region.

Electron transmission study of the temporary negative ion states of selected benzenoid and conjugated aromatic hydrocarbons

Electron transmission spectroscopy is utilized to determine the vertical electron affinities and to characterize the temporary anion states of a series of alternant hydrocarbons including benzene, naphthalene, anthracene, tetracene, styrene, and cis- and trans-stilbene. The vibrational structure present in the low lying resonances is interpreted in light of the charge distributions of the temporarily occupied orbitals. The energies of the anion states are compared with the predictions of PPP, PPP-CI, and HAM calculations, the pairing theorem and with the results from optical absorption measurements on the ground state anions in rigid glasses.

Dissociative attachment and predissociation in ammonia

The dissociative attachment cross sections of NH3 and ND3 in the 5.0-6.0 eV region are found to consist of a series of overlapping peaks characteristic of the nu 2 (inversion mode) vibrational quanta. The total scattering cross section is also structured at these energies as observed by electron transmission spectroscopy. Comparison of the results with optical absorption and electron energy loss data for excitation of the lowest Rydberg state of NH3 suggests that fragment ions are produced through the negative-ion analogue of the predissociation process known to occur in this state. The authors assign the electronic state of the relevant temporary negative ion to the configuration ((. . .)(la2)1NH3+)(3sa'1)*(3sa'1)'2A2(D3h). The relative vibrational spacings in the cation, the Rydberg state of the neutral and the anion provide evidence for the nature of the electron correlation in the latter.

Collisions of Cs-with atoms and molecules

Presents the results for measurements of the absolute total cross sections for electron detachment sigmae(E) and charge transfer sigmai(E) in low-energy collisions of Cs-with He, Ne, Ar, Kr, Xe, D2, N2, O2, CO, CO2, SO2, N2O, CH4and SF6. In the case of the rare gases, the apparent energetic threshold for detachment is rather high (near E=50 eV); a similar observation has also been made for the Na-and K-projectiles. As before, a curve-crossing model well describes the observed sigmae(E). Structure in sigmae(E) for the CO2target has been attributed to charge transfer to a metastable state of CO2-(2A1). Similarly, in the case of N2O, both sigmae(E) and sigmai(E) exhibit behaviour which suggests that charge transfer to a temporary negative-ion state of that molecule is involved in the collisional dynamics. Charge transfer is also observed to be an important exit channel for other molecules; for example, the value of sigmai(E=30 eV) is about 500 a02for the SF6target.

Electroionization study of acetylene and fragment ions

A monoenergetic electron beam, produced by two 127° electrostatic selectors in series, was used to obtain electroionization efficiency curves of acetylene from threshold to 10 eV above threshold. Very high sensitivity measurements of high signal-to-noise ratio of the ionization and appearance energies of C 2H 2 +, C 2H +, C 2 +, CH 2 +, CH +, C + and C 2H 2+ are reported. Hot bands are detected below the C 2H 2 + threshold and features due primarily to autoionizing Rydberg series converging to vibrational excited states of this ion are observed immediately above threshold. A value of 11.92±0.09 eV is deduced for the ionization potential of C 2. The CH 2 ++C − ion pair formation is observed for the first time. All curves have broad structures above threshold except for C 2H 2+. Relatively narrow structures are observed between 12 and 16 eV in the C 2H 2 + data which are caused by negative ion and/or autoionization states.

Chemisorption of CO on Pd(100): An lcgto‐lsd cluster study

AbstractLCGTO‐LSD model potential calculations have been performed for CO interacting with two‐, four‐, and eight‐atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd8 + CO we obtain dC—O = 1.18 Å (1.13 ± 0.1 exp.), dPd—C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ωC—O = 1828 cm−1 (1895 exp.) and ωPd—CO = 454 cm−1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd2 + CO, Pd4 + CO, and Pd8 + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO‐derived levels are in excellent agreement with experiment (relative to ϵF: 4σ (‐11.0 eV, ‐11.2 exp.); 5σ (‐8.0, −8.2 exp.); 1π [−7.5 (b1), −7.3 (b2), −7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b1) and 2.7 eV (b2) above EF. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon‐energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).

Influence of alkyl substituents on the π * negative ion states of benzene and its derivatives : Electron transmission spectroscopy of p-alkylanilines

Electron affinities and electron attachment energies associated with the π * orbitals of several p-alkylanilines (alkyl = Me, Et, i-Pr, and t-Bu) have been determined in the gas phase by electron transmission spectroscopy and have been analyzed by ab initio molecular orbital calculations at the STO-3G and 6-31G levels of basis set. The results lead to the conclusions that (a) the 2A 2 negative ion state in toluene lies below the 2B 1 state, (b) the 2A 2 state is stabilized in toluene relative to benzene by bonding overlap between the ortho carbon (C 2(2p π)) and the Me H(1s) orbitals in the LUMO, (c) the 2B 1 state is stabilized on adding Me groups to toluene to form t-butylbenzene through reduced antibonding interactions between C 1(2p π) and Me C(2p π) orbitals, and (d) the lowest resonance in t-butylbenzene is extensively overlapped with the second resonance and may involve strong mixing of the 2A 2 and 2B 1 states.

Coupled cluster calculations with numerical orbitals for excited states of polar anions

A recently proposed technique, which combines numerical orbitals for diatomic systems with coupled cluster methods, has been applied to study excited states of negative ions of polar molecules. Computational strategy is presented and calculations of ground and excited state electron affinities are reported for the LiH and BeO molecules. LiH and BeO are found to have excited state electron affinities of 0.0028 and 0.0234 eV. Both excited state affinities should be amendable to experimental determination.

CI method for determining the location and width of resonances in electron-molecule collision processes

CI calculations are employed to evaluate the location and width of resonance states in electron-molecule collision processes. A diabatic negative-ion state is obtained by increasing the nuclear charge of the system and is interpreted as the compact component in Feshbach's projection operator formalism. Results are presented for the 2 Pi g state of N2-. Scattering cross sections for vibrational excitations to the lowest six energy levels are evaluated within the boomerang model.

Positron annihilation in the doubly excited states of positronium negative ions.

The two-photon-annihilation rates for some $^{1}\mathrm{S}^{\mathrm{e}}$ doubly excited states of positronium negative ions are calculated. Hylleraas-type wave functions are used to represent the system. The doubly excited wave functions are obtained by use of a stabilization method. In comparing the annihilation lifetimes with the autoionization lifetimes (related to the inverse of the resonance widths that were obtained previously by using the method of complex-coordinate rotation), it is concluded that the decay mode of these lower-lying doubly excited $^{1}\mathrm{S}^{\mathrm{e}}$ states is dominated by autoionization processes.

2 pi affinity level of adsorbed CO: Bonding and dispersion.

The inverse photoemission spectrum of CO adsorbed on the CU(100) surface reveals a split CO 2\ensuremath{\pi}--derived feature. For the ordered overlayers at high coverage, band dispersion of both components is clearly observed. We present evidence from cluster calculations that the splitting is caused by the occurrence of two classes of negative-ion final states. In the first (lower-energy) class, the added electron is in an orbital which is bonding with respect to Cu and CO and in the second class in an orbital which is antibonding.

Excitation and ionization at surfaces: CO on metals

We discuss the energetics, electronic structure, and stability of neutral excited and ionic states of adsorbates on metals using CO as a prototype. By ab initio cluster calculations we show that two types of negative ion states, bonding and antibonding, exist. We then show that the valence 4σ, 5σ→2π* excitations of chemisorbed CO are essentially unshifted from their free CO positions, while a charge-transfer excitation appears at ∼5–7 eV. Intra-adsorbate polarization effects are shown to be responsible for the spectral shifts of the corresponding core (C1s, O1s→2π*) excitations. The relation between screened positive ion states and neutral excited states is discussed. Finally, the connection between the above spectroscopic results and photon- or electron-stimulated desorption is examined.

Negative ion states of the halogens

Negative ion states of the halogens

Benzene chemisorption on palladium surfaces. II. Resonance electron scattering and overtone vibrations.

Inelastic electron scattering from the vibrational modes of benzene chemisorbed on Pd(100) and Pd(111) at room temperature show energy-dependent structure and mode selectivity. The results are characteristic of resonance scattering via temporary negative-ion states. We observe resonance behavior in the totally symmetric C-H stretching mode and its overtones, ${\ensuremath{\nu}}_{1}^{(n)}$ (n=1,2,3), near 2.5 eV impact energy, and associate it with a gas-phase benzene resonance at higher energy.

Spectroscopie d'électroionisation de HBr et DBr entre 11 et 25 eV

Electroionization spectra of HBr and DBr are measured and analyzed between the ionization threshold and 25 eV. Several negative-ion states having configurations (4pσ) (4pπ)4 5s2, 5p2, and 4d2, associated with Rydberg states converging to the 2Σ+ limit, have been identified. Other structures result from the excitation of the inner 4sσ electron. Finally, data on DBr are used to confirm the interpretation.

A study of the isomeric dependence of low-energy (<10 eV) electron attachment: Perfluoroalkanes

The attachment of low-energy (<10 eV) electrons to the open-chain i-C4F10, i-C5F12, neo-C5F12, and to the cyclic c-C4F8 and c-C5F10 perfluoroalkanes has been studied using a time-of-flight mass spectrometer (TOFMS) and a high pressure electron swarm technique as well as the TOFMS results reported earlier on c-C4F8 and i-C4F10. Both long-lived parent anions (autodetachment lifetimes τa >10−6 s) and dissociative attachment fragment anions were observed for these molecules in the mass spectrometric study. The relative cross sections for negative ion formation and the τa of the long-lived parent anions were measured as a function of electron energy in the mass spectrometric study. The absolute total electron attachment rate constants as a function of the mean electron energy have been measured in the swarm study and used to calculate the total electron attachment cross sections as a function of electron energy. Both studies have shown a strong dependence of dissociative and nondissociative electron attachment processes on the structure of the molecules studied. For neo-C5F12, the predominant anion is the fragment C4F−9. In contrast, for i-C4F10, i-C5F12, c-C4F8, and c-C5F10, the parent anions are stronger than any of the fragment anions. Energetic considerations have been employed to identify fragmentation mechanisms of the negative ion states (NISs) leading to the formation of the observed fragment anions and to deduce some thermochemical data. The positions of the NISs, the observed differences in the cross sections for parent and fragment anion formation, and the types of the fragment anions formed for the various molecules are discussed.

Excited states of HCl and DCl and their negative ions between 12.5 and 28 eV

The electron impact ionisation efficiency curves of HCl and DCl have been measured with a high signal-to-noise ratio, in the energy range immediately above the first ionisation threshold (12.75-26 eV). Many structures from HCl*, DCl*, HCl-* and DCl-* are seen. Vibrating Rydberg states related to the configurations (3p sigma )(3p pi )44s, 3d and their corresponding negative-ion states are observed. Furthermore the pair of states related to the (3s sigma )(3p sigma )2(3p pi )44s and 4p levels are reported.

Resonant negative ion states of H 2O as observed by measurement of the OH(A 2Σ → X 2Π) emission following near-threshold electron impact

The core-excited negative ion resonances in H 2O, first observed by Sanche and Schulz, have been identified in both the (0-0) and (1-0) bands of the OH(A 2Σ → X 2Π) emission following near-threshold electron impact. These observations allow limits to be placed on the energy of the predissociating state.

ChemInform Abstract: Temporary Negative Ion States in Hydrocarbons and Their Derivatives

ChemInform Abstract: Temporary Negative Ion States in Hydrocarbons and Their Derivatives