K. E. McCulloh

Photoionization of carbon dioxide

The yields of photoions from CO2 cooled to 150 °K have been measured at a resolution of 0.22 A for CO2+ and 0.4 A for the O+ and CO+ fragments, in the photon energy region extending from onset for each species to approximately 20 eV. Most of the observed structure of the molecular ion yield curve recapitulates the well known features of the absorption spectrum, but a number of anomalies are reported. Autoionization peaks at photon energies just above the first molecular ionization limit of 13.773 ± 0.002 eV do not fit into a pattern characteristic of Rydberg series converging to excited vibrational levels of the ion. Although the Tanaka‐Ogawa series and Hennings sharp series can be assigned as ns Rydberg series on the basis of quantum defects, the expected 3s members could not be observed in the present study. The most prominent feature of the O+ yield curve is a step at 19.39 eV, suggesting that the principal mechanism for production of this fragment is predissociation of CO2+ (C 2 Σg+) in its ground vi...

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...

On the mechanisms of C6H6 ionization and fragmentation

Abstract Photoionization and electron monochromator studies have been carried out on benzene, 1,5-hexadiyne, 2,4-hexadiyne, pyridine and some C4H4 isomers to develop new information concerning the details of benzene ion fragmentation. Present results, together with those of other workers, indicate that a number of C6H6+ ion isomers have energies comparable to that of the first excited state of the benzene ion. These could act as intermediate states in skeletal fragmentation of the benzene ion and account for the observed small kinetic shift. Results on vinyl acetylene and butatriene indicate that these ion structures are not formed at the fragmentation threshold in benzene. Results on 1,5-hexadiyne and pyridine indicate the formation of a new C4H4+ ion structure, more stable than the two linear isomers. The evidence for lack of competition between hydrogen loss and skeletal fragmentation in the benzene ion is reviewed. Photoionization of benzene reveals autoionization contributions to parent ion and fragment ion production; assignments of the autoionizing levels are given. The photoionization behavior of 1,5-hexadiyne suggests that this also fragments via two pairs of non-competing reactions, as postulated for benzene.

Energetics and mechanisms of fragment ion formation in the photoionization of normal and deuterated water and ammonia

Abstract Fragment photoion yield curves have been obtained for OH + from H 2 O at 215 and 298 K and for NH 2 + from NH 3 at 160 and 298 K. The results indicate that for both molecules the initial rotational energy is completely available for fragmentation. Appearance potentials, corrected to 0 K, are 18.115±0.008 eV for OH + from H 2 O and 15.768±0.004 eV for NH 2 + from NH 3 . Assuming zero excess energy at threshold, the values 309.0±0.2 and 302.7±0.1 kcal mol −1 are obtained for the standard enthalpies of formation of OH + and NH 2 + , respectively, leading to the derived ionization potentials I (OH) = 12.995±0.009 eV and I (NH 2 ) = 11.17±0.05 eV. The observed yield of OH + from H 2 O deviates markedly from values calculated on the assumption that fragmentation occurs exclusively by predissociation from excited vibrational levels of H 2 O + (B 2 B 2 ), suggesting that Rydberg series converging to these levels play an important role in OH + production. Appearance potentials for OD + , NHD + and ND 2 + are reported.

Observations on hot bands in the molecular and dissociative photoionization of acetylene and the heat of formation of the ethynyl ion

Photoion yield curves in the vicinity of threshold are obtained for the molecular and the ethynyl ions of acetylene and acetylene‐d2 at ion source temperatures of 360, 298, and 130 °K. Weak ionization below the adiabatic threshold for C2H2+ and C2D2+ is ascribed to the ionization of molecules excited by one quantum of the bending vibrations, ν4 and ν5. Consideration of selection rules suggests a change in symmetry from the linear ground state molecule to a bent gound state ion. The 0 °K curves for C2H+ and C2D+ are estimated from the observed 130 °K data. Satisfactory agreement is obtained when the 298 °K data are compared with a curve calculated from the 0 °K curve by convolution with vibrational and rotational distributions. The 0 °K thresholds corrected for kinetic energy are used to calculate Δ H f0°(C2H+)=17.47 ± 0.01 eV (402.8 ± 0.2 kcal mol−1) and ΔH f °0(C2D+) = 17.43 ± 0.02 eV (402.0 ± 0.5 kcal mol−1). The ionization energy of the ethynyl radical is estimated to be 11.96 ± 0.05 eV.

Direct Observation of the Decomposition of Multiply Charged Ions into Singly Charged Fragments

A new technique has been developed for the direct observation of the decomposition of multiply charged polyatomic ions. Positive ions or ion pairs and ejected electrons, formed in a uniform electrostatic field by a 1‐keV electron beam, are accelerated in opposite directions to multiplier detectors. The masses of the positive ions are determined by measuring the time interval between electron and ion pulses by delayed coincidence. The formation of positive ion pairs is demonstrated and their masses determined by electron—ion—ion delayed coincidence techniques. The effects of initial kinetic energy on mass‐spectrum peak shapes and travel time correlations between members of an ion pair are discussed. Both of these effects are experimentally observed. The technique has been applied to the study of ionization and fragmentation of CO2, CF4, CH4, and C3H6. Numerous decompositions leading to the formation of positive ion pairs are observed. Significant fractions of particular high‐kinetic‐energy fragment ions ar...

Photoionization Study of the Dissociation Energy of Fluorine and the Heat of Formation of Hydrogen Fluoride

Mass spectra and photoionization yield curves are obtained for the molecular and atomic ions of fluorine and hydrogen fluoride. The atomic ions of both molecules are formed by ion‐pair and by dissociative ionization processes. The F2+ curve has a weak onset at 15.58 eV, ascribed to a hot band, followed by an abrupt, intense onset at 15.69 eV, ascribed to the 0–0 transition. Several broad bands of autoionization are observed. The onset of the dissociative ionization process occurs at 18.76 eV, from which we calculate D°0(F2) = 1.34 ± 0.03 eV (30.9 kcal mole−1). The only observed ion‐pair process is spin forbidden, producing F+(3P) + F−(1S), for which the thermodynamic threshold is not reached. The HF+ curve has a slightly tailing onset at 15.92 eV. No F+ ion is observed. The dissociative ionization process yielding H+ occurs at 19.34 eV, permitting the calculation of D°0(HF) = 5.74 eV, and ΔHf°0(HF) = −2.83 ± 0.03 eV (−65.3 kcal mole−1). Identical ion‐pair curves have a threshold at 15.87 eV resulting in E...

Effect of hot bands on the ionization threshold of some diatomic halogen molecules

Abstract Photoion-yield curves are obtained for the molecular ions of the homonuclear diatomic halogens and for iodine monochloride and iodine monobromide. Excepting iodine, ionization thresholds of ground-state and vibrationally-excited molecules are separately identified by measurements made at several ion-source temperatures. The adiabatic ionization energies (in eV) are: I(F2 = 15.69, I(Cl2) = 11.48, I(Br2) = 10.52, I(ICl) = 10.07, and I(IBr) = 9.79. A preferred value of I(I2) = 9.37 eV is proposed.

Experimental Test of the Franck‐Condon Principle: Double Ionization of Molecular Hydrogen

The double ionization of molecular hydrogen by electron impact has been observed by coincidence detection of the resulting proton pairs in a system especially designed for absolute kinetic energy measurements. Monte Carlo calculations provided an accurate knowledge of the resolution with which the kinetic energy measurements were made. Experimental results agree closely with predictions based on accurate Franck‐Condon overlap integrals for a model involving Morse oscillator wavefunctions for the molecule and Coulomb wavefunctions for the ion. In contrast, the reflection approximation and the harmonic oscillator approximation for molecular vibration, two approximations frequently made in such Franck‐Condon calculations, do not agree well either with experiment or accurate calculations. Observations at electron energies of 0.5, 0.75, and 1 keV show that the kinetic energy distribution is independent of electron energy and suggest that the double ionization cross section is inversely proportional to electron...

Observation of the Products of Ionic Collision Processes and Ion Decomposition in a Linear, Pulsed Time‐of‐Flight Mass Spectrometer

A retarding potential technique has been investigated which permits study of secondary processes occurring after ion acceleration in a time‐of‐flight mass spectrometer. By application of a retarding field near the end of the flight path, peaks due to neutral species and fragment ions formed after acceleration are separated from their parent ion peaks and displayed in the same panoramic mass spectrum. Charge exchange, collision‐induced dissociation, and spontaneous (metastable) ion decomposition are easily observed. The identities of initial ions and fragment ions involved in secondary processes can be determined unambiguously without complications of peak overlap.