Appearance potentials and transition probabilities for electron impact ionization of CO and CO+

A method 1 for the computerized acquisition of ionization efficiency data followed by mathematical analysis using the energy-distribution difference technique,2 which is a considerable improvement on conventional mass spectrometric techniques, has been applied to the determination of the ionization and appearance potentials of a variety of compounds and processes. Comparisons of the electronimpact ionization potential data with photoelectron spectroscopic measurements show that it is possible to measure adiabatic ionization potentials by electron impact in favourable cases and indicates a reason for the consistently high values found by other methods. The accuracy of measurement of appearance potentials approaches that of photoionization methods and in one case evidence for a preionization process is found. A high activation energy process is shown to occur without the expected large kinetic shift in appearance potential, suggesting that this computerized method is very sensitive. The measurements also yield a heat of formation and ionization potential for the "phenyl" fragment ion from monosubstituted benzenes.

Dissociative photoionization of ClN 3 using high-resolution synchrotron radiation: The N–Cl bond energy in ClN 3

Relative cross sections for electron-impact ionization of the rare-gas Van der Waals dimers ${\mathrm{Ar}}_{2}$, ArKr, ${\mathrm{Kr}}_{2}$, KrXe, and ${\mathrm{Xe}}_{2}$ have been measured with a mass spectrometer in the electron energy range from threshold to 180 eV. The following appearance potentials for direct ionization from the corresponding neutral dimers have been found: ${\mathrm{Ar}}_{2}^{+}$: 15.2\ifmmode\pm\else\textpm\fi{}0.2 eV, Ar${\mathrm{Kr}}^{+}$: 14.0\ifmmode\pm\else\textpm\fi{}0.2 eV, ${\mathrm{Kr}}_{2}^{+}$: 13.45\ifmmode\pm\else\textpm\fi{}0.3 eV, Kr${\mathrm{Xe}}^{+}$: 12.2\ifmmode\pm\else\textpm\fi{}0.2 eV, and ${\mathrm{Xe}}_{2}^{+}$: 11.75\ifmmode\pm\else\textpm\fi{}0.3 eV. These values agree with those obtained in recent photoelectron studies. The appearance potential of ${\mathrm{Ar}}_{2}^{+}$ formation via associative ionization of argon atoms is found to be 14.6\ifmmode\pm\else\textpm\fi{}0.2 eV.

The appearance potentials for the [R'CO2H2]+ ion produced in the fragmentation process \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm R}^{\rm '} {\rm CO}_{\rm 2} {\rm R}} \right]_{}^{_.^ + } $\end{document} → [R'CO2H2]++[R2H] have been measured using mono‐energetic electron impact techniques for ethyl, n‐propyl, and i‐propyl formates and acetates. The results indicate that at the threshold the product ion has the protonated acid structure with the hydrogen on the carbonyl and not the hydroxyl group, and that the neutral product for the propyl esters is the allyl radical and not the cyclopropyl radical. For the propyl formates and acetates the appearance potential of the [R'CO2H2]+ ion is identical with the adiabatic ionization potential of the parent ester (measured by photoelectron spectroscopy) indicating that fragmentation occurs for ground state molecular ions. A two‐step mechanism is proposed to rationalize the results.

The appearance potentials of CN+ in the mass spectra of HCN and (CN)2 have been remeasured and found to be 19.36±0.2 and 20.31±0.2 ev, respectively. Combination of the appropriate thermochemical data with the difference between these energies yields, D(NC–CN) = 6.88−2E(Z) ev where Z is the state of the CN radical either X2Σ+ or A2Πi, that accompanies CN+ from (CN)2, and E(X2Σ+) = 0.00 ev and E(A2Πi) = 1.13 ev. The appearance potential of Cl+ in the mass spectrum of ClCN has also been measured and found to be 17.81±0.2 ev, which value gives D(NC–CN) = 6.92−2E(Y) when combined with the spectroscopic ionization potential of the chlorine atom and the necessary thermochemical data. Here Y is the state of the CN radical that accompanies Cl+ from ClCN. In order that the two determinations of D(NC–CN) be consistent it is necessary that Y = Z and then the two permissible values of D(NC–CN) are 6.90±0.2 ev (Y = Z = X2Σ+) or 4.64±0.2 ev (Y = Z = A2Πi). These results do not permit a choice between White's determination, 6.3±0.2 ev, and Pauling's estimate, 3.9±0.6 ev, but do serve to eliminate from further consideration the determination by Kistiakowsky and Gershinowitz, 3.3 ev. Neither value is compatible with the 5.2 ev suggested by Glockler. However, electron impact data on ethylene and methylene radical give 5.6 ev as the upper limit to energy of dissociation of ethylene into two methylene radicals, and this can be taken as an upper limit to the dissociation energy of cyanogen, suggesting the lower of the two possible values of D(NC–CN) to be correct. Corresponding to this lower value of D(NC–CN), 4.64±0.2 ev, the dissociation energies of H–CN and Cl–CN are 4.80±0.1 and 3.66±0.1 ev, respectively, and the ionization potential of CN is 14.55±0.2 ev. The specific intensities of the various ions in the mass spectra of (CN)2, HCN, and ClCN characteristic of 75-volt ionizing electrons are given with reference to that of A+ in the argon mass spectrum.

The energy dependence of ionization efficiency by electron impact for polyatomic molecules is influenced by the entrance of decomposition processes. The observed yield of parent ion drops sharply at the energy onset of a high-yield decomposition process. Ionization efficiency curves for products from other decomposition processes are similarly effected. These effects are particularly apparent for decomposition processes with energy thresholds some volts above the parent ion appearance potential, and this is illustrated by data for the ionization efficiencies from the molecules NH3, C2H4, and C2F4. Perturbation from such competitive processes can result in erroneously high measured appearance potentials for the product ions.

NEGATIVE ION FORMATION AND ELECTRIC BREAKDOWN IN SOME HALOGENATED GASES

Apparently competing cleavage and rearrangement reactions in a series of molecular ions have been studied by ionization and appearance potential methods, and by determination of the electron energy dependence of both normal and metastable daughter ion peak intensities. The processes investigated were (i) [M  CH3] vs. [M  CH2O] in anisole; (ii) [M  OC6H5] vs. [M  CO] in phenyl ether; (iii) [M  NO2] vs. [M  NO] in nitrobenzene; (iv) [M  C3H7] vs. [M  C2H4] in butyrophenone: (v) [M  C3H7] vs. [M  C3H6] in n‐butylbenzene; (vi) [M  CH2OH] vs. [M  CH2O] in 2‐phenylethanol; (vii) [M  CH3CO2] vs. [M  CH2CO] in benzyl acetate; and (viii) [M  C4H9O] vs. [M  C4H7] in n‐butylbenzoate. The results are interpreted in terms of k vs. E curves with very different frequency factors for the two reaction types. Appearance potentials of metastable ions for the rearrangement reactions have also been measured.

The absolute cross-sections, Q, for the charge exchange in the gas phase between twenty eight primary positive ions of known electron recombination energy, E, and three isomeric esters, ethyl acetate, methyl proprionate and propyl formate, have been measured. A plot of Q against E for a particular product ion constitutes a charge-exchange mass spectrum, c.e.m.s., from which the product ion's appearance potential is directly obtained. The appearance potentials of the three esters and eleven of their principle ionic fragments are reported.

The dissociative ionization of nitrogen

The 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.

Appearance energy and electron impact ionisation cross-section of C 70

Collisionally activated dissociations (CADs) of various pyridine‐ring substituted N‐benzylpyridinium cations in the gas phase form the substituted pyridine and benzyl carbocation. Appearance energies (AEs) are estimated quantitatively from the appearance thresholds of the corresponding fragment ion from the laser‐desorbed pyridinium cations. MO calculations predict that such unimolecular dissociation processes lead initially to ion‐molecule pairs (IMPs): some such IMPs are of significantly lower energy than the fully dissociated products. The AEs have also been compared with the energy differences [ΔΔHf=ΔHf(Py) + ΔHf(R+) — ΔHf(Py+R)] determined by AM1 calculations: in many cases these are quite close to the calculated energy differences; discrepancies are discussed.

Much effort has been recently devoted to the experimental determination of absolute partial and total electron impact ionization cross sections of molecules and radicals due to the ever increasing importance of these cross sections in many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have been recently developed in order to allow the analytic quantitative description of these cross section functions for modeling codes. This review summarizes recent experimental and theoretical progress in this area of electron impact ionization including also a short description of (i) recent advances in the field of electron/ion collisions (i.e., kinetic energy release distributions in dissociative excitation reactions of molecular ions) and (ii) a novel approach to the determination of electron induced appearance energies.

Soft-x-ray appearance-potential spectroscopy has been used to determine the binding energies of the ${N}_{4}\ensuremath{-}{N}_{7}$, ${O}_{1}\ensuremath{-}{O}_{5}$, and ${P}_{1}\ensuremath{-}{P}_{3}$ levels of clean uranium surfaces. This is the first direct determination of most of these levels. The measured energies of the $N$ and $O$ levels are consistently a few electron volts lower than tabulated x-ray binding energies based on photoelectron spectroscopy of oxidized uranium. Exposure of clean uranium to oxygen, however, did not appreciably shift the appearance potentials. It is suggested that the discrepancy is in part a consequence of a "surface chemical shift." The density of $5f$ states above the Fermi energy was found to be double peaked with a peak separation of 2.5 eV.