Electron-impact ionization and appearance potentials

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

An electron impact study of ionization and dissociation of trimethylstannanes

The photoionization of a series of ketones, acetone, 2-butanone, 2-pentanone, 3-methyl-2-butanone, 2-hexanone, and 4-methyl-2-pentanone was studied using mass spectrometric techniques. Adiabatic ionization potentials were determined. It was found that for the ketones the cross section above threshold is not constant, and a modified method of treatment of the results was developed in order to obtain information about fragmentation processes. The shifts in appearance potentials of fragments with increasing temperature were found to be close to the change in internally stored energy. Bond dissociation energies in ions were calculated. The adiabatic ionization potential of the acetyl radical was calculated to be 7.08 eV, which is considerably lower than reported by authors using electron impact where no effort was made to distinguish between adiabatic and vertical ionization potentials. A lower limit was calculated for the ionization potential of the n-propyl radical leading to a value of the adiabatic ionization potential of 7.2±0.2 eV.

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for mandelic acid (MA) and mandelic acid methyl ester (MAME) . An ionization energy of 8.55 eV was obtained for both compounds. For the formation of C 6 H 5 + ions, appearance energies of 14.25 eV or 14.40 eV were determined for MA or MAME, respectively. On the main fragmentation route from molecular ions to these C 6 H 5 + ions, an activation energy of (1.1±0.1)eV in excess of the calculated total reaction enthalpy was found for the elimination of CO from the prominent C 7 H 7 O + ions to C 6 H 7 + ions, involving the transfer of two H-atoms. The second (minor) fragmentation route via C 7 H 6 O + ions resembles the benzaldehyde decomposition pathway and the individual steps were found to be near to their calculated thermochemical reaction enthalpies. Gas-phase enthalpies of formation of (–4.652±0.3 eV) [≡ (–448.8±30) kJ mol −1 ] for MA and (–4.243±0.3) eV [≡ (–409.4±30) kJ mol −1 ] for MAME are derived, based on the appearance energy of their C 7 H 7 O + ions at m/z 107 and the formation enthalpy adopted from protonated benzaldehyde. The results are compared with data for benzylalcohol.

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.

Electron impact ionization of CO in a mass spectrometer at electron currents of up to 170 μA and pressures of up to 5 × 10-5 torr has been used to study electron collision with CO+ trapped in a space-charge well in the electron beam. The processes observed are as follows: Appearance potential (v) Transition probability (×10-4) (1) CO → CO+ (x 2Σ+) → C+ 14.0 1.5 (2) CO → CO+ (x 2Σ+) → O+ 14.0 0.43 (3b) CO+ (A 2IIi) → CO2+ 25 0 ± 0.4 17 (4b, c) CO+ (x 2Σ+) → CO+ (c 2Σ+) → CO2+ 15.9 ± 0.7 0.14 (5) CO → C+ + O- 20.5 ± 0.7 (6) CO → C+ + O(3P) 22 6 ± 0.7 345 (8) CO → O+ + C(?) 19 ± 1 145 (10) CO → CO2+ (41.8) 11 relative to CO → CO+ at 14.01 v with a transition probability of 1. From (3b) the vertical ionization potential for CO → CO+ (A 2IIi) is 16.8 ± 0.4 ev and the cross section for (3b) is 58 ± 30 A2 at 30 ev. From (4b, c) the vertical ionization potential for CO->CO+ (c 2Σ+) is either 25.9 ± 7 ev or 29.9 ± 7 ev, and the product of the cross sections for (4b) and (4c) is of the order of 40 A4.

The ionization potentials of methyl ethers of D‐homoequilenine, its 14ß‐isomer, D‐homoestrone and its 8°‐isomer, as well as appearance potentials of certain fragment ions have been determined by means of electron‐impact. The dissociation energies of the bonds which are split in the course of formation of these ions have been calculated.

For Abstract see ChemInform Abstract in Full Text.

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.

Electron impact ionisation energies of some halo-cyclotriphosphazenes

Photoionization efficiency (PIE) data for HgBr2+ and HgI2+ and their fragment ions have been obtained in the region 600–1350 Å using an oven-type supersonic beam source. The ionization energies (IE) for the X̃ 2Π3/2g states of HgBr2+ and HgI2+ were determined to be 10.560±0.003 and 9.5088±0.0022 eV, respectively. The analyses of the Rydberg series converging to the 2Π1/2g states of HgBr2+ and HgI2+ yield a value of 10.8846±0.0012 eV for the IE of the 2Π1/2g state of HgBr2+ and 10.1953±0.0025 eV for that of HgI2+. The major fragment ions from HgBr2 were identified to be HgBr+, Hg+, Br2+, and Br+ and those from HgI2 were found to be HgI+, I2+, and I+. The measured appearance energies for HgBr+ and HgI+ allow the calculation of the bond dissociation energies for HgBr+ and HgI+ to be 55±2 and 59±1 kcal/mol, respectively. Similar to the observation in the PIE curves for HgCl2+ and its fragment ions, the PIE spectra for HgBr2+, HgI2+, and their fragment ions are dominated by autoionization structures exhibiting asymmetric Beutler–Fano line profiles. The comparison of the PIE curves of HgCl2+, HgBr2+, HgI2+, and Hg+ from Hg confirms the previous conclusion that these autoionizing Rydberg series can be assigned to transitions [(5d)10σg2σu2πu4πg4] → [(5d)9σg2σu2πu4πg4 2D±5/2]np and [(5d)10σg2σu2πu4πg4] → [(5d)9σg2σu2πu4πg4 2D±3/2, ±1/2]np, n≥6, which are mainly associated with the mercury atom in HgX2, X=Cl, Br, and I.

The photoion yield curves of the free radicals AsH and AsH2, prepared by the reaction of H atoms with AsH3, have been measured. The adiabatic ionization potential of AsH (forming AsH+, X 2Π1/2) is 9.641±0.008 eV. Autoionizing Rydberg states are observed and analyzed to converge to an a 4Σ− state lying 1.94 eV higher in energy. The adiabatic ionization potential of AsH2 (forming AsH+2, X̃ 1A1) is 9.443±0.007 eV. The 3B1 state of AsH+2 is conservatively estimated from the spectrum to lie 0.60–1.46 eV higher in energy, with the lower figure expected to be close to the true value. In addition, the ion yield curves of AsH+3, AsH+2, and AsH+ from photoionization of AsH3 have been measured. From these measurements, the adiabatic ionization potential of AsH3 is 9.82±0.01 eV, the appearance potential of AsH+2 (+H) is 12.69±0.01 eV, and that of AsH+(+H2) is 11.295±0.05 eV. The latter two measurements, when combined with the corresponding ionization potentials, yield D0(H2As–H)=74.9±0.2 kcal/mol and D0(HAs–H)=66.5±0.2 kcal/mol. The value of D0(As–H), as deduced from these measurements, depends upon an accurate heat of atomization of AsH3, which in turn requires an accurate value for ΔH○f0 (As,g). An analysis of alternative values is presented, from which D0(As–H)=64.6±0.7 kcal/mol (2.80±0.03 eV) is obtained. When these stepwise bond energies, and earlier results on PHn and NHn, are compared with the semiempirical model of Goddard and Harding, the largest discrepancy occurs for NHn. An analysis of successive ionization potentials Pn, PnH, PnH2 (Pn=N, P, As), and also Ch, ChH, ChH2 (Ch=O, S, Se) based on the same philosophy again shows a large departure from prediction for the first row elements, but fair agreement for the second and third row hydrides. The deviation of the first row hydrides from the Goddard–Harding model is attributed to the substantial ionic character of these systems.

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).

The vacuum ultraviolet (VUV) photochemistry of trifluoromethanesulfonyl fluoride (CF3SO2F), a promising eco-friendly insulating gas to replace sulfur hexafluoride (SF6), was investigated in the 11-14 eV energy range by using synchrotron radiation photoionization mass spectrometry and high-level theoretical calculations. It was found that the CF3SO2F+ parent ion formed from VUV photoionization is not stable and dissociates into CF3+, SO2+, and SOF+ fragment ions. The vertical ionization energy (VIE) of the parent molecule to the repulsive X2A' cationic state was calculated to be 13.46 eV, while the adiabatic ionization energy (AIE) to the bound A2A″ state was determined to be 12.65 eV. Notably, the experimental appearance energies of the CF3+ and SO2+ fragments coincide with the AIE of the first excited state (A2A″), rather than the lower appearance energies predicted from ground-state potential energy surfaces. In addition, the dissociation mechanisms to produce CF3+ and SO2+ fragment ions were discussed in detail. For CF3+, photoexcitation populates the bound A2A″ state, which then crosses via a conical intersection to the repulsive X2A' state, leading to direct dissociation into CF3+ + FSO2. This predissociation pathway governs the experimental onset of the m/z = 69 signal. The m/z = 64 signal, corresponding to the SO2+ fragment, is less intense than the m/z = 69 signal because its formation involves a more complex, multistep pathway. This pathway needs isomerization and crossing another conical intersection, making it less favorable than the direct fragmentation that produces the CF3+ fragment.