Vernon H. Dibeler

Mass-Spectrometric Study of Photoionization. VI. O 2 , CO 2 , COS, and CS 2 †

Photoionization-efficiency curves are obtained for the molecule and fragment ions of the subject molecules in the wavelength region extending from onset of ionization to 600 A. The initial onset of O2+ is observed at 12.072 eV. Autoionization in the continuum is correlated with various progressions of Rydberg series. Curves for both O+ and O− ions formed by the ion–pair process from O2 are obtained and 1.48 eV is derived for the electron affinity of oxygen atoms. The shape of the CO2+ curve including structure ascribed to autoionization is discussed. The unresolved 2Пg-doublet threshold is observed at 13.77 eV with the first vibrational level at 13.93 eV. The onset of the O+-fragment ion indicates about 0.04 eV excess energy in the dissociation process. It is suggested that dissociative ionization occurs from the autoionizing Rydberg level just above the calculated threshold value. Partially resolved doublet components of COS+ are observed at 11.18 and 11.22 eV, respectively. Intense autoionization is observed. Various thermochemical values are calculated from the fragment-ion thresholds. The doublet components of the ion ground state of CS2+ are observed at 10.059 and 10.112 eV, respectively. Intense autoionization is observed at wavelengths which are in excellent agreement with known Rydberg levels for the molecule. Although the S+ ion is formed with excess energy, the CS+ ion gives a value of 11.71 eV for the ionization energy of the CS radical. This is in agreement with but more precise than a directly measured electron-impact value.

Photon impact studies of C2HCN and CH3CN in the vacuum ultraviolet; heats of formation of C2H and CH3CN

A photodissociation process to produce CN B2Σ from C2HCN and CH3CN has been studied as a function of incident wavelength. Threshold photon energies required for the production of CN B2Σ from C2HCN and CH3CN are 9.41 ± 0.04 and 8.52 ± 0.03 eV, respectively, from which D0 (C2H–CN) ≤ 6.21 ± 0.04 eV and D 0(CH3–CN) ≤ 5.32 ± 0.03 eV are obtained. The photoionization yield curves have been measured for the C2HCN+ and C2H+ ions. Threshold photon energies obtained for the production of CN B2Σ, C2HCN+, and C2H+ from C2HCN lead to the following thermochemical values; I.P.(C2HCN) = 11.64 ± 0.01 eV, I.P.(C2H) = 11.96 ± 0.05 eV, Δ H f0°(C2HCN) ≥ 85 ± 1 kcal mol−1 (355 ± 4 kJ mol−1). Δ H f0°(C2H) = 127 ± 1 kcal mol−1 (531 ± 4 kJ mol−1) and D0 (C2H–H) = 5.38 ± 0.05 eV. Δ H f0°(C2H) obtained is in good agreement with the recent value obtained directly from a study of the high temperature reactions of graphite with hydrocarbons. Δ H f0°(CH2CN) ≥ 14 ± 1 kcal mol−1 (59 ± 4 kJ mol−1) derived from D0 (CH3–CN) agrees within th...

Mass‐Spectrometric Study of Photoionization. XI. Hydrogen Sulfide and Sulfur Dioxide

Photoionization‐yield curves are obtained for the molecule and fragment ions of H2S and SO2 from onsets of ionization to 600 A. Structural features of the curves are discussed. Ionization thresholds are tabulated and heats of formation of ions and radicals are calculated. Derived quantities (in kilocalories per mole) include D0(HS–H) = 89.3, D0 (SH) = 83.2, ΔHf0°(S) = 65.0, D0(S2) = 99.4, and D0(OS–O) = 129.1. The ionization energy, I(SO) = 10.21 eV, is obtained by indirect means.

Experimental and Theoretical Studies of Photoionization‐Efficiency Curves for C2H2 and C2D2

Photoionization‐efficiency curves for C2H2 and C2D2 have been remeasured in the wavelength region from onset of ionization to 600 A, using a gold‐coated grating and the Hopfield continuum as a photon source. The curve shapes of the molecule ions near threshold are compared with calculations of the Franck—Condon factors. Nearly exact agreement between experiment and calculation is obtained when taking into account hot bands. A general discussion is given for the shape of the molecule‐ion curve and for that of the fragment ions, C2H+ and C2D+. The observed onsets of ionization for the latter ions are 17.22— and 17.34 eV, respectively.

Mass‐Spectrometric Study of Photoionization. III. Methane and Methane‐d4

Photoionization‐efficiency curves for the molecule and fragment ions of CH4 and CD4 are obtained in the wavelength region 1000 to 600 A. The electronic structure of the molecule ion is related qualitatively to the shape of the parent‐ionization‐efficiency curve and the implications of the complicated vibronic behavior of the CH4+ ion are discussed. A qualitative explanation is given for the observed isotope effect on the parent‐ionization‐efficiency curve and on the fragment‐ion threshold energies. New upper bounds for the ionization energies I(CH4) = 12.71 eV and I(CD4) = 12.87 eV are obtained. Other thermochemical values deduced from the study are: D(CH3–H) = 4.41 eV, D(CD3−D) = 4.55 eV, D(CH2–H) = 4.87 eV, and ΔHf (CH2) = 4.07 eV. Zero‐point differences for methyl and methylene ions and neutrals are also estimated.

Electron Impact Studies of Aromatic Hydrocarbons. I. Benzene, Naphthalene, Anthracene, and Phenanthrene

A systematic survey of ionization‐dissociation processes of fused‐ring aromatic compounds is initiated. Mass spectra and appearance potentials of the singly and doubly charged molecule ions are reported.The observed ionization potentials of benzene (9.38 ev), naphthalene (8.26 ev), anthracene (7.55 ev), and phenanthrene (8.03 ev) are compared with available spectroscopic data, with values obtained from molecular orbital calculations, and with other electron impact data. The empirical method of group equivalents is extended to the calculation of the ionization potentials of the fused‐ring aromatic compounds.

Enthalpy of formation of methyl and methylene radicals of photoionization studies of methane and ketene

Photoion yield curves for CH3+ and CH2+ from methane have been measured near threshold at 295 and 115 °K, and the curves for CH2+ from ketene have been obtained at 295 and 130 °K. Although the detection efficiences for positive and negative ions were nearly equal, a search for the ion‐pair process yielding CH3++H− gave negative results. The methane data are successfully fitted on the assumption that the full rotational energy is available for formation of CH3+, but that only two rotational degrees of freedom contribute to the available energy for the process yielding CH2+. Neglecting excess energy at threshold, the values ΔHf°0(CH3) =149.4±0.5 kJ/mole (35.70±0.12 kcal/mole and ΔHf°0(CH2) =392.5±2.1 kJ/mole(93.8±0.5 kcal/mole) from methane. Correction of the threshold for CH2+ from ketene for rotational energy results in the concordant value ΔHf°0(CH2) =390.8±1.7 kJ/mole (93.4±0.4 kcal/mole) on the assumption that excess energy can be neglected at threshold. The mean of the two determinations is selected a...

Mass‐Spectrometric Study of Photoionization. VIII. Dicyanogen and the Cyanogen Halides

Photoionization‐yield curves are obtained for the molecule and selected radical ions of C2N2, FCN, ClCN, BrCN, and ICN from threshold to 600 A. Vibrationally excited states of ions and autoionization of Rydberg levels in the molecules are observed and discussed briefly. The X+ ion thresholds in the heavier cyanogen halides are used to obtain: ΔHf0o (CN) = 101.5 kcal mole−1, ΔHf0o (CN+) = 430.0 kcal mole−1, and I (CN) = 14.2 eV. These are applied to compute ΔHf0o (FCN) = 5.6 kcal mole−1, D (F–CN) = 5.0 eV, I (C2) = 12.15 eV, and other thermodynamic properties. The formation of CN+ from dicyanogen apparently includes about 0.6 eV excess energy near the threshold.

Electron Impact Studies of Sulfur Dioxide and Sulfuryl Fluoride

Mass spectra and appearance potentials have been obtained for the principle positive and negative ions of SO2 and SO2F2. Ionic heats of formation and probable ionization‐dissociation procesess are tabulated. The electron‐impact data for SO2 support the values of 3.3 ev for D(S2) and 5.15 ev for D(SO). The molecular ionization potential is in good agreement with recent photoionization measurements. The ΔHf(SO2F2) = — 205 kcal/M is calculated from the appearance potential of the SO2+ ion. Other ionic heats of formation are based on this value. The ΔHf(SO+) is in good agreement for both molecules. A minimum value of 3.0 ev is calculated for the electron affinity of F2 from the appearance potential of the F2‐ ion.

Kinetics of Nitrogen Atom Recombination

The rate of recombination of nitrogen atoms in the nitrogen afterglow has been measured in a flow system using NO as a titrant and determining the NO content continuously by mass spectrometer. Above about 3 mm of mercury the reaction is predominantly homogeneous and third order. The rate constant, 5.7×1015 cc2 mole—2 sec—1, is independent of temperature in the interval studied (195–450°K). The rate is reduced by substituting helium or argon for N2 as the third body, the ratios of the third‐order rate constants with these third bodies being about the same as those for recombination of iodine and bromine atoms. Below about 3 mm of mercury a pseudo‐first‐order wall recombination becomes important. The recombination coefficient was found to be 1.6×10—5 on a glass wall that undoubtedly was partly poisoned by water.