Design of electron energy analyzers for electron impact studies

The design and construction of an electron energy analyzer for the study of electron impact processes in atoms, molecules and solids is described. The analyzer incorporates a 180° hemispherical deflector and five-element entrance optics. Focusing characteristics and angular behavior of the analyzer have been investigated by using the electron-ray tracing simulation program, SIMION. The entrance lens system to the hemispherical deflector has been designed to have high collection efficiency for low-energy electrons. The fringing field correction has been done by tilting the input beam angle outward for real aperture configuration.     

Effect of structural factors on the disintegration of the molecular ions of nitrophenylisoxazoles during electron impact

The dissociative ionization of some nitrophenylisoxazoles was investigated. The effect of the energy of the ionizing electrons and the temperature of the inlet system on the elimination of NO by the molecular ion is examined. On the basis of a comparison of the intensities of the peaks of the (M-NO)⁺ ions, the presence of a correlation between the probability of detachment of NO from the molecular ion and the stability of the cyclic conjugated structures with localization of the charge on the oxygen atom is demonstrated.     

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.     

Dissociation of ethane by electron impact

The absolute total dissociation cross section for ethane is reported for electron energies between 10 and 600 eV. A maximum value of 7.6 × 10^?16 cm² occurs at 80 eV while the apparent threshold is ≈ 10 eV. Dissociative ionization is more probable than dissociation into neutral fragments at all energies except in the threshold region. The data indicates that fragmentation involving methane elimination (c^? + C₂ H₆ → e^? + CH₄ + CH₂) occurs in less than 2% of the dissociative events for 50 < E < 600 eV. Arguments are presented which suggest that some of the lower excited states of ethane are stable against dissociation.     

Threshold electron impact excitation of acetylene

Threshold electron energy loss spectra for acetylene are presented and assignments suggested for the excitation processes occuring.     

Extensive substituent scrambling in substituted diphenylacetylenes on electron impact

Extensive randomization of the substituents is found in mono- and disubstituted diphenylacetylenes under electron impact conditions. The study of diphenylacetylenes with a variety of substituents indicates that the electron-withdrawing groups favour the substituent scrambling, AH the substituted diphenylacetylenes give an ion at m/z 176, having a common ion-structure, arising as a result of the expulsion of a hydrogen radical and the substituent in monosubstituted diphenylacetylenes and the expulsion of both the substituents in disubstituted diphenylacetylenes. A ring-expanded structure is postulated for this common fragment.     

Secondary hydrogen-deuterium isotope effects on electron impact induced fragmentations

Secondary hydrogen-deuterium isotope effects have been observed in the mass spectra of cis-4-t-butylcyclohexyl iodide, 5-iodononane and 2-iodopropane. Under conditions which suppress competing and second generation fragmentations, β-deuterium substitution decreases the intensity ratio \documentclass{article}\pagestyle{empty}\begin{document}$ ([{\rm M} - {\rm I]}^{\rm + } /[{\rm M]}^{\mathop + \limits_ \cdot } ) $\end{document}, a result analogous to a normal isotope effect. The decrease is larger in the spectrum of cis-4-t-butylcyclohexyl iodide-trans-2-d than in the spectrum of the cis-2-d derivative. Since these effects parallel those in the better understood solvolysis reaction, both effects may have a common origin. In contrast, deuteration of more remote positions in cis-4-t-butylcyclohexyl iodide and 5-iodononane increases the indicated intensity ratio, an apparent inverse isotope effect. Although similar effects have been observed in solvolysis reactions, the mass spectral effect may be attributable to an increase in the nonfixed energy available for fragmentation. These results suggest that secondary isotope effects can be readily measured in certain cases, and that they may eventually become useful probes into the mechanisms of mass spectral fragmentations.     

Effect of fluoro substitution on the fragmentation of the K-shell excited/ionized pyridine studied by electron impact

Fragmentation of the pyridine ring followed by K-shell excitation/ionization has been studied with 2-fluoropyridine (2FPy) by electron impact. Ab initio molecular orbital (MO) calculations were also carried out to investigate the electronic states correlating with specific fragment ions. The fragment ions are produced characteristically at the N 1s edge, while the spectra observed at the F 1s and C 1s edges exhibit a small difference from that at the valence ionization. The production of the C(4)H(2)(+), C(4)H(3)(+) and C(4)H(2)F(+) ions indicates that the cleavage of the N-C6 and C2-C3 bonds or the N-C2 and C5-C6 bonds is likely to occur after the N 1s excitation/ionization. Ab initio MO calculations indicate that the former fission is likely to proceed through the n(N)(1)π(2)(1)π(3)(2) and n(N)(0)π(2)(2)π(3)(2) excited states of the parent molecular dication. On the other hand, the breakage of the N-C2 and C4-C5 bonds, which specifically proceeds at the N 1s edge for 2-methylpyridine, does not occur for 2FPy. The present calculation reveals that the products of this channel are unstable by the electronegativity of fluorine and that the relative energy of the Auger-final states of 2FPy is lowered by the reorganization and electron correlation effects.     

Dissociative excitation of water by electron impact

Absolute emission cross sections and threshold energies have been measured for radiation (1850–9000Å) from excited fragments (OH, O and H) produced by electron impact (0–1000 eV) on water vapour. The results are compared with previous experiments and the discrepancies are discussed. The measurements indicate that hydroxyl radicals excited in the A² ∑⁺ state originate from excitation of both singlet and triplet states of the water molecule. Excited atomic fragments arise partly from predissociation of Rydberg states of the water molecule converging to the third ionization potential.     

Dissociative excitation of benzene by electron impact

In the wavelength region 1850–9000 Å radiation from H, C and CH fragments is observed as a result of the dissociative excitation of benzene by electron impact (0–1000 eV). Emission cross sections and threshold energies have been determined for the Balmer series of the hydrogen atom and the A²Δ - X²Π emission of the CH fragment.     

Cyclization of dimethyl oxalate upon electron impact

Isotope labelling experiments and also consecutive fragmentation investigations of metastable ions, a novel technique in mechanistic studies, have been carried out to elucidate structure and genesis of the m/z 45 ions from dimethyl oxalate as well as dimethyl carbonate. It is shown that the formation of the m/z 45 ions, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{3}} \mathop {\rm{O}}\limits^{\rm{ + }} = {\rm{CH}}_{\rm{2}} $\end{document}, in the mass spectra of these compounds arises via single step processes. Mechanisms involving hydrogen transfer and subsequent formation of cyclic intermediates which then collapse to give \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{3}} \mathop {\rm{O}}\limits^{\rm{ + }} = {\rm{CH}}_{\rm{2}} $\end{document} directly from molecular ions are suggested. No evidence was found for a two-step fragmentation route to m/z 45 from the molecular ions of either dimethyl oxalate or dimethyl carbonate.     

Comparative dissociation of peptide polyanions by electron impact and photo-induced electron detachment

We compare product-ion mass spectra produced by electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) of multi-deprotonated peptides on a Fourier transform and a linear ion trap mass spectrometer, respectively. Both methods, EDD and EPD, involve the electron emission-induced formation of a radical oxidized species from a multi-deprotonated precursor peptide. Product-ion mass spectra display mainly fragment ions resulting from backbone cleavages of C_α-C bond ruptures yielding a and x ions. Fragment ions originating from N-C_α backbone bond cleavages are also observed, in particular by EPD. Although EDD and EPD methods involve the generation of a charge-reduced radical anion intermediate by electron emission, the product ion abundance distributions are drastically different. Both processes seem to be triggered by the location and the recombination of radicals (both neutral and cation radicals). Therefore, EPD product ions are predominantly formed near tryptophan and histidine residues, whereas in EDD the negative charge solvation sites on the backbone seem to be the most favorable for the nearby bond dissociation.     

Deuterium isotope effect in electron impact desorption of hydrogen on tungsten

The deuterium isotope effect of electron impact desorption cross sections has been measured for two adsorption states of hydrogen on tungsten with widely differing cross sections. The results of these measurements are in good agreement with values calculated according to the theory of this process proposed earlier.     

Characteristic ortho-methoxycarboxylate fragmentation of yerrinquinone and its derivatives on electron impact

Mass Spectrometry of a natural product, yerrinquinone, isolated from fungal-infested Diospyros montana, and isomeric 1-tetralonecarboxylates has revealed fragmentation pathways characteristic of a methoxy group flanked by a carboalkoxy and a carbonyl group.     

A study on electron impact mass spectrometry of epoxy methylated fullerenols

Mass spectra of the epoxy methylated[60]fullerenols were obtained by EI mass spectrometry using "desorption" or "in-beam" technique. The mass spectra have an intense molecular monocation peak M(+) and a weak dication peak M(++), revealing the stability of these products under the MS (EI) conditions. The remaining peaks correspond to the successive loss of methyl groups and oxygen atoms for which the pure fullerene represents a more stable product. The distinction between the multiply charged fullerene C(60)(z+) and their fragments with equal m/z was also studied.     

Tetraalkylammonium cations in electron impact mass spectrometry

Twenty-five tetraalkylammonium halide salts were investigated by means of electron impact mass spectrometry. In all cases, corresponding ammonium cations have been detected at low electron energy. Moreover, [C+M]⁺ have been detected (where C and M denote ammonium cation and molecule, respectively) for both symmetrical salts and iodine salts. Quaternary ammonium salts are considered to be more stable than believed previously.