Appearance Potential Measurements Research Articles

Electron-stimulated processes at solid surfaces

Surface analytic techniques that use incident electrons as the initial stimuli have, in many cases, been shown to be destructive. Examples of such destructive processes include the decomposition of chemisorbed species, stimulated surface reactions with gas-phase components, and the desorption of chemisorbed atoms. Since such processes reflect chemical changes in the surface under investigation, studies of these processes can yield useful surface information. In a review of some electron-stimulated surface reactions, the types of surface-chemistry information to be expected is discussed. Recent results of an electron-stimulated-desorption (ESD) investigation of the interaction of oxygen with Ni, W, and Mo surfaces are presented. Measurements of threshold values and of the energy distribution of ions desorbed for excitation energies near and well above threshold were made using potential-modulation techniques. A comparison is presented between these ESD results and soft-x-ray appearance potential measurements made on the same chemisorptive systems.

Studies in mass spectrometry. III—formation of a common intermediate in the primary fragmentation process of 1‐phenylcyclobutene and of 2‐phenyl‐1,3‐butadiene

AbstractThe mass spectra of the title compounds show peaks corresponding to the primary fragmentation of H, CH3 and (H2 + C2H2). Kinetic study suggests that the molecular ion of 1‐phenylcyclobutene and 2‐phenyl‐1,3‐butadiene loses H and CH3 through formation of a common intermediate. This is confirmed by appearance potential and ionisation potential measurements, fragmentation of deuterium labelled compounds and the corrected Z values.

Metastable ion studies. III—composite metastable peaks in fragmentations of some small hydrocarbon cations

AbstractComposite metastable peaks are generated in the unimolecular fragmentations (i) [C3H5]+ → [C3H3]+ + H2 (flat‐top upon flat‐top) and (ii) [C4H9]+ → [C3H5]+ + CH4 (flat‐top and gaussian). The measurement of appearance potentials and kinetic energy releases lead us to conclude, in agreement with earlier proposals, that in (i) the components can arise from the generation of the isomeric cyclopropenium and propargyl daughter cations. In (ii) the components are proposed to arise from the fragmentation of tert‐ and sec‐butyl cations yielding allyl as the common daughter ion.The composite peak observed in the fragmentation (iii) [C3H4]+· → [C3H3]+ + H· is shown to be present only if the decomposing molecular ion is large enough to also produce [C6H8]2+ ions. The second component in (iii) then arises from the reaction [C6H8]2+ → [C6H6]2+ + H2.

The measurement of appearance potentials of gaseous ions

The measurement of appearance potentials of gaseous ions

The structure of the [C2H5O]+ ion in the mass spectrum of diethyl ether

AbstractThe structure of the [C2H5O]+ ion in the spectrum of diethyl ether was examined by use of deuterated ether, CH3CD2OCH2CH3. The results show that, at all electron energies from threshold to 70 eV, the predominant ion is a rearrangement ion, probably protonated acetaldehyde, with very little of the [C2H5O]+ being formed by direct carbon‐oxygen bond cleavage. Appearance potential measurements made on the m/e 45, m/e 46 and m/e 47 ions in the deuterated ether show that the threshold structure of the rearrangement ion is protonated acetaldehyde.

Combined deuterium labeling and appearance potential measurements to uncover competing reaction mechanisms in the electron‐impact‐induced loss of water from cyclohexanol

AbstractThe appearance potentials for loss of H2O and DOH from the constitutional isomers 3,5‐dideuterocyclohexanol and 4,4‐ dideuterocyclohexannol show, in agreement with proposals based on stereoselectivites, that 1,3 loss of water is energetically more costly than 1,4 loss of water.

Thermochemical properties of gaseous ZrO and ZrO2

The dissociation energy of gaseous ZrO, D0° (ZrO), and the heat of atomization of gaseous ZrO2 have been measured using a high temperature mass spectrometer. A number of equilibria were studied and the following results were derived: ΔfH (ZrO,g,298.15 K) =83±12 kJ/mol, D0° (ZrO) =761±15 kJ/mol) (or 7.89±0.15 eV), ΔfH (ZrO2,g,298.15K) =−298±30 kJ/mol, and D0° (ZrO2) =1386±30 kJ/mol (or 14.4±0.3 eV). The ionization potentials of ZrO and ZrO2 were derived from the measured appearance potentials. In the case of ZrO it was necessary to make corrections for the thermal populations of low-lying electronic states. The derived values are I.P.(ZrO) =6.0±0.2 eV, I.P.(ZrO2) =9.4±0.2 eV. These measurements are discussed in relation to other studies. Uncertainties in the derived thermochemical quantities owing to uncertainties in the spectral data are discussed. The chemical bonds in ZrO and ZrO2 are briefly discussed.

Appearance potential measurements using ion cyclotron resonance spectrometry

The errors introduced by the kinetic shift make it desirable to develop methods for utilizing ion cyclotron resonance (ICR) ionization efficiency curves to obtain accurate thresholds for ionic fragmentation processes. The inverse convolution procedure of Vogt and Pascual [9] is tested and found to remove the effects of the high energy tail of the Maxwellian part of the electron energy distribution. New procedures for smoothing the ionization efficiency data and identifying the actual energy threshold are described and tested. The heat of formation of C 3H 5 + from propylene and cyclopropane was found to be 225± 2 kcal mol −1, in excellent agreement with other recent work.

Dissociative ionization of H 2, N 2 and CO by electron impact — measurement of kinetic energy, angular distributions, and appearance potentials

For investigating dissociative ionization processes induced by electron impact, an apparatus was constructed which enables the kinetic energy, angular distributions and appearance potentials of the observed ions to be measured. Special ion optics were designed in order to avoid kinetic energy discrimination effects. Kinetic energy and angular distributions of H +(H 2), N +(N 2), O +(CO) and C +(CO) ions and appearance potentials of O +(CO) and C +(CO) ions of given kinetic energies were measured. Reaction models are proposed and discussed in terms of potential energy curves by correlating them with the kinetic energy distributions and appearance potentials. In contrast to H +(H 2) and N +(N 2), the O +(CO) and C +(CO) angular distributions measured here cannot be interpreted by existing theories.

A simple method of obtaining excitation functions in digital form: application to electron impact dissociative ionization of CO 2

A method for obtaining excitation functions in digital form is described which is applicable to measurements of ion appearance potentials by electron impact in mass spectrometry. The method employs a sampling time-to-pulse height converter and synchronously pulsed operational amplifier. Excitation functions for the ions O + and C + produced by electron impact on a CO 2 gas beam are presented. An appearance potential of 19.4±0.3 eV was obtained for O +, probably corresponding to the threshold for excitation of the predissociation C 2Σ g + state of CO 2 +. The observed C + onset of 29.8±0.3 eV corresponds to total dissociation of the CO 2 molecule by the process e+CO 2 → C + + O+O+2 e.

Partitioning of reverse activation energy between kinetic and internal energy in reactions of some simple organic ions

Nine unimolecular reactions, in which simple organic cations lose molecular hydrogen, have been studied. For each reaction, the structures of the products are either unambiguous, or may be plausibly inferred. From appearance potential measurements, and metastable peak widths, it is possible to evaluate the forward and reverse activation energies, and the partitioning of reverse activation energy between kinetic and internal energy.

C6F5X Bond Dissociation Energies: Determination from Appearance Potential Measurements and Correlation with Thermochemical Data

Appearance potentials for C6F5+ from C6F5X (X = F, Cl, Br, I) have been determined relative to argon (15.76 eV) by the extrapolated voltage difference method. The respective appearance potential values are 16.86, 15.85, 14.93, and 13.21 eV with an estimated precision of ± 0.05 eV. Combining differences in these values with the kinetic values of D[C6F5—I] = 66.2 kcal mol−1 leads to D[C6F5&([a-z]+);F] = 150.4 ± 2.0 kcal mol−1, D[C6F5&([a-z]+);Cl] = 127.1 &([a-z]+); 2.0 kcal mol−1, and D[C6F&([a-z]+);Br] = 105.9 ± 2.0 kcal mol−1Comparison with D[C6F5—X] (X = F, Cl, I) from thermochemistry (1) indicates that the combined excitation energies, kinetic energies, and chemical shifts for the process C6F5X → C6F5+ + X are constant within experimental error.

ChemInform Abstract: ENERGY PARTITIONING BY MASS SPECTROMETRY, CHLOROALKANES AND CHLOROALKENES

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2‐elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3‐elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2‐elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four‐centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2‐elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3‐dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.

Ionization efficiency curves and fragmentations of deuterated ethylenes by electron impact

Fragmentation processes of CH 2CD 2 and cis-CHDCHD have been studied by monoenergetic electron impact. Ionization efficiency curves for the total and fragment ions have been measured from 10.00 to 20.50 eV, and for CH 2 +, CD 2 + and CHD + from 17.00 to 23.20 eV. The observed ionization and appearance potentials for fragment ions agree with those determined by photoionization. No appreciable isotopic effects have been observed in the ionization and appearance potential measurements. Energy dependence of the relative intensities of fragment ions are examined near their appearance thresholds, and mass spectra are observed for ions with one carbon atom as well as those with two carbon atoms. It has been found that the C 2H 2 + ions are formed through a scrambling process of hydrogens, whereas the CH 2 + ions are produced directly from the parent ions.

Electron energy dependence of the kinetic energy and angular distributions of O+ from dissociative ionization of O2

The electron energy dependence of the kinetic energy and angular distributions of O+ have been measured for the process of dissociative ionization of O2. The kinetic energy distributions for energies above 0.5 eV are compared with other dissociative ionization measurements as well as results of dissociative photoionization and dissociative excitation by electrons leading to production of Rydberg excited neutral fragments. Angular distributions are interpreted in terms of symmetries of possible molecular ion states contributing to dissociation, and the results near threshold are compared with recent measurements of O− from dissociative ion pair formation in O2. Information about the states of excitation of the dissociation fragments as a function of ion energy is obtained from appearance potential measurements. The results indicate that predissociation may be important and that dissociative ionization of O2 is in general a complicated process involving many O2+ states.

Energy partitioning by mass spectrometry: Chloroalkanes and chloroalkenes

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2-elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3-elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2-elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four-centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2-elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3-dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.

Thermochemistry and energy partitioning in the charge separation reactions of doubly charged triatomic ions

Ion kinetic energy and appearance potential measurements have been used to study the unimolecular charge separation reactions of the triatomic ions CO22+, COS2+, CS22+, N2O2+, NO22+ ·, and SO22+. The experimentally determined metastable peak shapes have been compared with calculated peak shapes including calculations that determine both the average kinetic energy release and the distribution about this average. The product ion states have been assigned from the thermochemical data, and in all the reactions total spin and angular momentum are found to be conserved. There is evidence that the metastable peaks are associated with maxima on single potential surfaces rather than with a slow predissociation. For CO2++ and COS++ almost all the available energy is partitioned into translational energy of product separation.

Mass spectrometry of onium compounds. XXIII—ionisation potentials in the structural assignment of [M – N2] ions from diazo‐oxides

AbstractThe ionisation potential shows that fulven‐6one is formed pyrolytically from o‐benzo‐diazo‐oxide. Appearance potential measurements show increasing pyrolytic contribution to the [M – N2] species with increase in ion source temperature.

Interpretation of the mass spectra of substituted chromium and tungsten carbonyls by means of appearance potential measurements

The 50 eV mass spectra of M(CO) 5PR 3 complexes (M = Cr and W; R = OCH 3, OC 2H 5, n-C 4H 9 and C 6H 5 have been determined. The chromium derivatives and the P(OCH 3) 3, P(OC 2H 5) 3 and P(C 6H 5) 3 tungsten derivatives show ions predominantly formed by CO elimination. This behaviour is related to the higher activation energy for the rupture of P—R compared with that of MCO bonds. The spectra of the P(n-C 4H 9) 3 and P(O-n-C 4H 9) 3 tungsten derivatives, on the other hand, are dominated by ions obtained by partial fragmentation of the heteroligands. The heptacoordination capability of tungsten, the length of the R chain, and energetic factors are responsible for the appearance of these new fragmentation pathways.

ChemInform Abstract: ELECTRON IMPACT STUDIES ON TRIPHENYL DERIVATIVES OF THE GROUP IIIB METALS

Only Ph3Al and Ph3Ga of the Group IIIB metal triphenyls produce ions derived from the dimers Ph6M2 under electron impact. Fragmentation behaviour reflects the progressive weakening of the Ph–M bond as the series is descended. Appearance potential measurements are reported for the major metal-containing ions and D(Ph2M+–Ph) evaluated.