Electron Attachment Spectroscopy Research Articles

Temporary Anion States of Pyrimidine and Halopyrimidines

The empty-level electronic structures of pyrimidine and its 2-chloro, 2-bromo, and 5-bromo derivatives have been studied with electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS) in the 0-5 eV energy range. The spectral features were assigned to the corresponding anion states with the support of theoretical calculations at the ab initio and density functional theory levels. The empty orbital energies obtained by simple Koopmans' theorem calculations, scaled with empirical equations, quantitatively reproduced the energies of vertical electron attachment to π* and σ* empty orbitals measured in the ET spectra and predicted vertical electron affinities close to zero for the three halo derivatives. The total anion currents of the halo derivatives, measured at the walls of the collision chamber as a function of the impact electron energy, presented intense maxima below 0.5 eV. The mass-selected spectra showed that, in this energy, range the total anion current is essentially due to halide fragment anions. The DEA cross sections of the bromo derivatives were found to be about six times larger than that of the chloro derivative. The absolute cross sections at incident electron energies close to zero were evaluated to be 10(-16)-10(-15) cm(2).

Spectroscopic states of PTCDA negative ions and their relation to the maxima of unoccupied state density in the conduction band

Electron attachment spectroscopy is employed to demonstrate that the scattering of slow (0–15 eV) electrons from perylenetetracarboxilic dianhydride (PTCDA) molecules in the gas phase leads to the resonant formation of molecular and fragment negative ions detected in the mass-spectrometric experiment. Depending on the electron energy, currents of anions have clearly manifested peaks at 0.14, 1.9, 3.0, 4.8, and 5.7 eV. In addition, resonant states are also detected at thermal energy (0 eV) of scattered electrons, as well as at 0.4 and 1.0 eV, as shoulders on experimental curves. The spectroscopic states of PTCDA anions at energies exceeding 0 eV are interpreted in terms of the formation of shape resonances on the basis of calculated values of energies of π*-type unoccupied molecular orbitals. It is found that the positions of unoccupied orbitals of an isolated PTCDA molecule correspond to the peaks in the density of states of the conduction band of PTCDA films provided that the energies of the orbitals are shifted by 1.4 eV. The latter value can be interpreted as the binding energy of a molecule in the film due to the polarization interaction with the surroundings.

Electron Attachment to Dye-Sensitized Solar Cell Components: Cyanoacetic Acid

The energies of electron attachment associated with temporary occupation of the lower-lying virtual orbitals of cyanoacetic acid (CAA), proposed as a possible component of dye-sensitized solar cells, and its derivative methyl cyanoacetate (MCA) are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The vertical and adiabatic electron affinities are also evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. Dissociative electron attachment spectroscopy (DEAS) is used to measure the total anion current as a function of the incident electron energy in the 0-4 eV energy range, and the negative fragments generated through the dissociative decay channels of the molecular anion are detected with a mass filter. In both compounds only two intense fragment anion currents are observed, that due to loss of a hydrogen atom from the molecular anion ([M - H](-)) and that due to formation of CN(-). In CAA the former signal displays a very sharp feature at 0.68 eV, assigned to a vibrational Feshbach resonance arising from coupling between a dipole bound anion state and a temporary σ* anion state.

Degradation of gas phase decabromodiphenyl ether by resonant interaction with low-energy electrons

Resonance electron attachment in a series of brominated phenyl ethers, including decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of electron transmission spectroscopy (ETS) and dissociative electron attachment spectroscopy (DEAS). Attachment of thermal electrons to DBDE leads to various dissociative decay channels of the temporary molecular anion. In contrast to other bromophenyl ethers, the bromide anion is not the most intense negative fragment. The neutral counterparts of the observed [Br(2)](-) and [C(6)Br(4)O](-) anion fragments are ascribed to the closed-shell species octabromodibenzofuran and hexabromobenzene, respectively, although their formation implies complex atomic rearrangements. Density functional theory calculations are employed to evaluate electron affinities, thermodynamic energy thresholds for production of the anion fragments observed in the DEA spectra and the proton affinities of the corresponding neutral radicals. Since DBDE is one of the most widespread organic pollutants, the present gas-phase DEA study can provide indications on the reaction mechanisms which occur in vivo and cause injuries to living cells.

Resonance capture of electrons by electroactive organic molecules

Bound states of electrons with electroactive aromatic molecules, promising for use in molecular electronics, are studied employing electron attachment spectroscopy Anions are produced in the gas phase through the capture of electrons with energies of up to 15 eV by molecules via the resonance mechanism. The possible pathways of fragmentation of molecular anions and the times of electron retention by the test molecules at various incident electron energies are measured. The resonance states identified are interpreted with the help of quantum-chemical calculations. The possibility of applying the results obtained to molecular electronics is discussed.

Molecular anion formation in 9,10-anthraquinone: Dependence of the electron detachment rate on temperature and incident electron energy

Attachment of low-energy electrons to gas phase 9,10-anthraquinone (AQ) was observed with electron transmission (ET) spectroscopy, and interpreted with the support of quantum chemical calculations. The ET spectrum displays three shape resonances at 0.45, 0.7, and 2.2 eV, associated with temporary electron capture into empty pi( *) molecular orbitals of AQ, the first two anion states being stable. According to TD-B3LYP calculations, the first pi-pi( *) core-excited resonance lies at about 1.8 eV, although no experimental evidence for this anion state was found. The long-lived parent molecular anion [AQ](-) was observed by means of Electron Attachment Spectroscopy (EAS) using two different mass spectrometers and also by measuring the total anion current at the collision chamber walls. The molecular anion current shows maxima at zero energy, around 0.6 eV and at 1.8 eV. Association of these maxima with the corresponding resonant anion states is discussed. The experimentally measured electron detachment times from [AQ](-) as a function of the incident electron energy and the temperature of the target molecule show a pronounced change of slope around 1.5 eV, regardless of the temperature. This unexpected behavior can be qualitatively reproduced within the framework of a multiexponential approach which describes the electron detachment event in terms of a redistribution of the anion excess energy, regardless of the initial mechanism of temporary anion formation.

Electron Attachment to Dye-Sensitized Solar Cell Components: Rhodanine and Rhodanine-3-acetic Acid

The energies of electron attachment associated with temporary occupation of π* and σ* virtual orbitals of the pentaheterocyclic rhodanine molecule are measured in the gas phase with electron transmission spectroscopy. The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies. The same computational procedure is applied to rhodanine-3-acetic acid, proposed as a possible component of dye-sensitized solar cells. In addition, the (positive) vertical and adiabatic electron affinities are evaluated at the B3LYP/6-31+G(d) level. The calculations also indicate a thermodynamic tendency toward dissociation of the ring H2C−S bond of the molecular anion. Dissociative electron attachment spectroscopy is used to measure the total anion current, as a function of the incident electron energy, and detect with a mass filter the negative fragments generated thr...

Spectroscopic and Theoretical Study of the Electronic Structure of Curcumin and Related Fragment Molecules

The low volatility and thermal instability made the photoelectron (PE), electron transmission (ET), and dissociative electron attachment (DEA) spectroscopy measurements on curcumin (a potent chemopreventive agent) unsuccessful. The filled and empty electronic structure of curcumin was therefore investigated by exploiting the PES, ETS, and DEAS results for representative fragment molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron attachment energies of curcumin was proposed. The pi frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and associated with a remarkably low ionization energy and a negative electron attachment energy (i.e., a largely positive electron affinity), diagnostic of a stable anion state not observable in ETS. The lowest energy electronic transitions of half-curcumin and curcumin and their color change by alkalization were interpreted with time-dependent density functional theory (DFT) calculations. For curcumin, it is shown that loss of a phenolic proton occurs in alkaline ethanolic solution.

Temporary anion states and dissociative electron attachment to nitrobenzene derivatives

The nitrobenzene derivatives 1,2-nitrotoluene, 1,3-nitrotoluene, 1,2-fluoronitrobenzene, 1,3-fluoronitrobenzene, 1,4-fluoronitrobenzene, 1,2-chloronitrobenzene, 1,3-chloronitrobenzene and 1,4-chloronitrobenzene are investigated by means of electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DEAS) and negative ion mass spectrometry (NIMS). The observed energies of electron attachment to π* MOs were interpreted with the support of the empty level structures of the neutral molecules supplied by HF/6-31G and B3LYP/6-31G* calculations. In the chlorine-substituted derivatives three main long-lived negative ions (NIs) (M −, Cl −, and NO 2 −) are observed, only M − and NO 2 − in the other investigated compounds. The detection of the metastable NIs corresponding to the process M* − → R − + (M–R) suggests that the molecular NIs formed near thermal electron energy dissociate on a microsecond timescale. Mean autodetachment lifetimes of the molecular NIs as a function of electron energy are evaluated.

Electron energy loss and dissociative electron attachment spectroscopy of methyl vinyl ether and related compounds

The triplet and singlet excited states of methyl vinyl ether have been characterized by electron energy loss spectroscopy and multireference density functional theory with configuration interaction (DFT/MRCI). The electron attachment energy was determined from the excitation functions for vibrational excitation to be 2.3 eV. Dissociative electron attachment spectra have been measured with a mass spectrometer having a trochoidal monochromator in the incident electron beam. Fragment anion bands peaking at 3 eV, 6.5 eV and 9.5 eV have been found in methyl vinyl ether. The first band is assigned to the 2(π∗) shape resonance, the second band to a resonance where an electron is loosely bound by dipole and polarization forces to the target molecule in its valence excited 1(π, π∗) state, and the third band to Feshbach resonances with occupation of Rydberg orbitals. CH 3O − and H 2CC − are formed at 3 eV, CH 3O − and HCC − at 6.5 and 9.5 eV. The fragmentation pattern of ethyl vinyl ether at 3 eV is similar, yielding ethanolate and vinylidene anions, but with the unexpected addition of the deprotonated acetaldehyde anion CH 2CHO −. The fragmentation patterns of methyl allyl ether and benzyl methyl ether are dominated by the methanolate anions with high intensity, indicating that dissociation of C–O bond in unsaturated ethers is much faster if this bond does not lie in the plane of the π system.

Empty level structure and dissociative electron attachment in gas-phase nitro derivatives

The gas-phase empty level structures of nitromethane, nitrobenzene, 2-nitroanisole and 2-nitrophenol are studied by means of electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DAS) and ab initio calculations, in the 0–6 eV energy range. Both the ETS and theoretical results indicate that the vertical electron affinity is positive in nitrobenzene, but negative in nitromethane. Consistently, the DA spectra of the nitrobenzenes display an intense molecular anion current at zero energy, while in nitromethane only the NO 2 − fragment (peaking at 0.6 eV) is observed. The most abundant anion is NO 2 −, also in the DA spectra of the nitrobenzenes, although in 2-nitrophenol the loss of the hydroxy radical from the molecular anion gives rise to a signal of the same intensity. Interestingly, the ET spectra of nitrobenzene and 2-nitrophenol display one and two narrow features, respectively, below 4 eV not associated with simple electron capture into π∗ orbitals. These signals are ascribed to core-excited resonances lying at exceptionally low energy. Comparison between the anion current measured with a quadrupole mass filter and that measured at the collision chamber suggests that the occurrence of intramolecular hydrogen bonding in 2-nitrophenol increases the lifetime of the molecular anion formed at zero energy.

Effect of intramolecular hydrogen bonding on the empty level structure of gas-phase salicylaldehyde:electron affinities and anion yield upon electron impact

The gas-phase empty level structures of γ-butyrolactone, α-hydroxy-γ-butyrolactone, benzaldehyde, 3-hydroxybenzaldehyde and 2-hydroxybenzaldehyde (salicylaldehyde) are studied by means of electron transmission spectroscopy (ETS) and ab initio 6-31G ∗∗ calculations. The most stable conformer of salicylaldehyde is characterised by intramolecular hydrogen bonding which causes relevant geometrical variations and, in particular, a lengthening of the CO double bond. The empty π ∗ molecular orbitals of salicylaldehyde with large contribution from the carbonyl group are significantly stabilised with respect to their counterparts in benzaldehyde and in the 3-hydroxy derivative. Dissociative electron attachment spectroscopy (DAS) did not reveal formation of negative fragments at low energy, that is, the presence of dissociative decay channels for the π ∗ resonances observed in the ET spectra. The total anion current measured at the walls of the collision chamber (close to the electron beam), as a function of the incident electron energy, displays signals peaking at zero energy and at about 0.8 eV in benzaldehyde and in the two hydroxy derivatives. However, the only signal detected using a quadrupole mass filter (which requires a lifetime ≥10 −6 s) is a zero energy peak in salicylaldehyde, due to the molecular anion.

Empty levels in germanium compounds studied by XAS, ISEELS, ETS, DEAS and ab initio calculations: GeH4, GeCl4 and Ge(CH3)3Cl

Abstract The lower-lying empty levels of gaseous GeH4, GeCl4 and Ge(CH3)3Cl have been studied using X-ray absorption spectroscopy (XAS), inner shell electron energy loss spectrocopy (ISEELS), electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DEAS) and ab initio calculations. The inner shell excitation spectra include Ge 2p in GeH4, GeCl4 and Ge(CH3)3Cl; Ge 3s, Ge 3p and Cl 1s and Cl 2p in GeCl4. The Ge 2p and Cl 1s excitation spectra were obtained in gas phase at room temperature with a direct ion-yield technique at the LURE SUPER ACO storage ring in Orsay. The excitation spectra generated from p core orbitals are very sensitive to the change of ligand and geometry. Ab initio calculations employing the 1h-1p CI scheme performed for the Ge 1s, Ge 2s, Ge 2p absorptions for GeH4 and GeCl4 allow to assign and reproduce the main experimental below edge features whereas the composition of empty orbitals are deduced from the calculated oscillator strengths. The continuum above edge features reveal the presence of double excitations and shape resonances. Moreover complementary experimental results are reported for GeCl4. The Ge 3s, ge 3p and Cl 2p inner shell electrn energy loss spectra recorded under electric dipole scattering conditions give a complete picture of empty levels of different symmetries. Furthermore the ET spectrum (0–7 eV) of GeCl4 shows two resonance states located at 1.7 and 5.6 eV whereas the DEA spectrum is dominated by formation of the GeCl3− anion at zero impact energy and formation of Cl− and GeCl2− anions around 5.6 eV.

CN − formation following electron attachment to cyanogen ((CN) 2)

Electron attachment spectroscopy is employed to study negative ion formation from cyanogen. CN − is generated from a resonance at 2.7 eV with thermal kinetic energy, while a broad resonance centered at 5.5 eV yields CN − ions with considerable excess kinetic energy (≈ 0.5 eV). The results are discussed in terms of a simple MO picture.

Electron capture in irradiated gases—recent developments

An electron-attachment spectroscopy technique is described which enables the mean energy of electrons to be varied in the range 0.04–0.5 eV by means of a radiofrequency voltage superimposed on the d.c. voltage of an electron-capture detector. Plotting the electron-capture coefficients or the mean electron-capture cross sections against the mean electron energies for each compound yields a characteristic curve which may be helpful for the identification of unknown substances. An increase of the sensitivity and the specificity of the electron-capture detector is also possible. The dependence of the electron-capture coefficient on the mean electron energy for eight substances is discussed.

Negative Ions in the Gas Phase Observed in Electron Transmission and Electron Attachment Spectroscopy

AbstractA new electron transmission spectrometer is applied to study the formation of negative ions in ethylene and the fluoroethylenes. It is found that the electron attachment energy in the fluoroethylenes observed in electron transmission completely coincides with the energy of the resonances associated with the formation of F− (and various other stable negative fragments) observed in dissociative electron attachment. This demonstrates that the combination of electron transmission and dissociative electron attachment spectroscopy yields information on the formation and decomposition of molecular negative ions in the gas phase.

Low energy electron impact on benzene and the fluorobenzenes. Formation and dissociation of negative ions

Electron attachment spectroscopy with nearly monoenergetic electrons is employed to study the formation and dissociation of negative ions in benzene and the fluorobenzenes. From all compounds, various negative ion fragments resulting from resonance dissociative electron attachment are observed. Electron affinities in the limit of the unknown excess energy are determined for the radicals C 6F 5 ·, C 6HF 4 ·, C 6H 2F 3 ·, and C 6H 2F 2 ·. Only in hexafluorobenzene is a long-lived parent molecular negative ion observed.

Formation and dissociation of the chloroethylene anions

Electron attachment spectroscopy is employed to study the formation of negative ions from the chloroethylenes. It is found that the resonances recently observed in the total electron scattering cross section are predominantly associated with the formation of Cl −. Only in tetrachloroethylene is a long-lived parent negative ion observed.

Electron attachment spectroscopy using a modified electron-capture detector

An electron attachment spectroscopy technique is described, which enables the mean energy of electrons to be varied in the range 0.04–0.5 eV by means of a radiofrequency voltage superimposed on the d.c. voltage of an electron-capture detector. Plotting the electron-capture coefficients against the mean electron energies for each compound yields a characteristic curve which may be helpful for the identification of unknown substances. This method also makes it possible to increase the sensitivity and the specificity of the electron-capture detector. The dependence of the electron-capture coefficient on the mean electron energy for eight compounds (CHCl 3, CCl 4, C2HCl 3, CCl 2F 2, C 6H 5Cl, O 2, SO 2, N 2O) is discussed.

Electron-attachment spectroscopy: formation and dissociation of negative ions in the fluorochloroethylenes

The formation and dissociation of negative ions in C 2Cl 4, C 2FCl 3, 1,1-C 2F 2Cl 2, 1,2-C 2F 2Cl 2 (isomeric mixture), C 2F 3Cl and C 2F 4 have been studied employing (dissociative) electron-attachment spectroscopy with nearly monoenergetic electrons in the energy range 0–15 eV. All six compounds show low-lying resonances (below 4eV) associated with various dissociation channels leading to one negative and one neutral fragment. It is found that the resonance energy increases if Cl is replaced by F. Electron affinities in the limit of the unknown excess energy are given for the radicals C 2Cl 3, C 2F 2Cl 2, C 2F 2Cl and C 2F 3. For C 2Cl 4 and C 2F 2Cl 2 long-lived parent molecular anions are observed. Only tetrafluoroethylene gives fragments by cleavage of the double bond, as a result of the perfluoro effect.