Appearance Potentials Of Fragment Ions Research Articles

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene (POPOP) molecule by electron impact in gaseous phase is studied. Potentials of appearance of some fragments of the molecule under study are determined from the experimentally measured dependences of ionization cross-section on the ionizing electron energy. A new ion with m/ z = 144 [C 9H 6ON] + is detected in the mass spectrum of the POPOP molecule, being complementary to the fragment with m/ z = 220 [C 15H 10ON] +. The threshold of appearance of this ion is determined ( E ap = 9.51 eV) as well as the first ionization potential of the POPOP molecule and fragment ion appearance potentials.

Synchrotron Radiation Vacuum Ultraviolet Photoionization and Photodissociation of Diethylzinc

Photoionization and photodissociation of diethylzinc (ZnC4H10) were performed using vacuum ultraviolet (VUV) synchrotron radiation and a time-of-flight mass spectrometer (TOF-MS). The photoionization mass spectra of all the observed ions from diethylzinc and their photoionization efficiency (PIE) curves with the photon energy from 8 to 22 eV were measured. From their PIE curves, the ionization potential (IP) of ZnC4H10 (IP=8.20±0.05 eV) and appearance potentials of fragment ions (ZnC2H5+, ZnH+, Zn+, C2H5+, C2H3+, etc.) were obtained. According to the gas phase energetic data from references, the formation enthalpies of chief ions were evaluated and their possible channels of dissociative photoionization as well as their branch rates were discussed. The results indicated that the chief channels were the simple cleavage of Zn—C bond in the parent ions to form fragmental ions C2H5+ and ZnC2H5+ which further dissociated to Zn+ ion. More than 75% of the total production ions belonged to Zn-containing ions.

Photoionisationsmassenspektren von MethyI(trifluormethyl)sulfan CH3SCF3 [1]/Photoionization Mass Spectra of Methyl(trifluoromethyl)sulfan CH3SCF3 [1

Abstract The ionization potential of CH3SCF3 and the appearance potentials of fragment ions have been measured in the photon energy range 9 -19 eV using synchrotron, radiation. From these results thermochemical data of CH3SCF3, CH3SCF3 + and fragment ions have been evaluated.

Photoionenspektroskopie an Schwefelchloridpentafluorid SF5Cl, das Ionisationspotential von Schwefelpentafluorid SF5/Photoion Spectroscopy of Sulfurchloridepentafluoride SF5Cl, the Ionization Potential of Sulfurpentafluoride SF5

Abstract The appearance potentials of fragment ions from SF5Cl have been measured in the energy range 12 - 20 eV by means of photoionization mass spectrometry. From these data, the ionization potential of SF5 comes to 9.65 eV.

Thermochemistry of the plutonium fluorides from electron impact studies

The ionization potentials of the molecules PuF5, PuF4, and PuF3 have been measured using electron impact ionization. The values are, respectively, 14.5±0.1, 11.8±0.4, and 8.6±0.5 eV. The parent ion PuF+6 was not detected. From the ionization potentials and the fragment ion appearance potentials the bond dissociation energies of the molecules F5Pu–F, F4Pu–F, F3Pu–F, and F2Pu–F have been calculated to be 53±2, 108±12, 113±5, and 141±16 kcal/mol, respectively. Upon comparison with data available on the plutonium fluorides the bond energies of F3Pu–F and F2Pu–F are accurate to within the stated uncertainty. The values of the bond energies of F5Pu–F and F4Pu–F disagree with the reported enthalpy of formation of PuF6(s). Analysis of available data indicates that the enthalpy of formation of PuF6(s) is most likely −484±10 kcal/mol.

Photoionisation mass spectrometric study of acetylene in the VUV region

Photoionisation mass measurements on acetylene have been made between 1100 and 400 AA by using synchrotron radiation as a light source. Photoionisation efficiency curves are obtained for the molecular ion C2H2+ and also for fragment ions C2H+, C2+, CH2+ and CH+. To assign the structures of C2H2+, ab initio calculations have been performed for the excitation from the second (3 sigma g) and third (2 sigma u) occupied molecular orbitals. Two broad maxima at 930 and 810 AA between the first and second ionisation thresholds are assigned to the autoionisation from valence-type excited states. New Rydberg series (np pi u) converging to the second ionisation threshold were found. The appearance potentials of fragment ions by photoexcitation are given and discussed in terms of the heats of formation of molecular ions. For each of C2+ and CH+, two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C2+ is estimated from the appearance potential to be 1980 kJ mole-1.

A photoionization mass spectrometric investigation of CH3CN and CD3CN

A photoionization mass spectrometric investigation of CH3CN and CD3CN from the onset of ionization to ∼20 eV is reported. Photoionization efficiency (PIE) curves of the acetonitrile parent ions and of the fragments C2H2N+, C2HN+, CH2N+, CH+3, CH+2 and their deuterated analogs have been measured. The parent ion curves display isotope-dependent direct ionization as well as autoionization structure. Excitation of one quantum of the doubly degenerate CCN bending mode (ν8) in the ground state of CH3CN+ is observed, indicating the presence of Jahn–Teller coupling. The measured ionization potentials are 12.194±0.005 eV (CH3CN) and 12.235±0.005 eV (CD3CN). Autoionizing Rydberg series converging to the first and second excited electronic states have been observed and assigned. At their thresholds, C2H2N+ and C2D2N+ are produced exclusively from parent ion states which are populated by autoionization. Appearance potentials of the fragment ions for which PIE curves were obtained are reported, and their heats of formation have been calculated where more reliable literature values are not available. The observed fragment ion appearance potentials and relative ion yields indicate that H-atom migrations in the molecular ion are important in the fragmentation mechanisms.

Laser induced multiphoton ionization mass spectrum of benzene

Gas phase benzene is multiphoton ionized under low pressure, collision-free conditions in the source region of a time of flight mass spectrometer using UV excimer laser radiation. All of the ions generated are identifiable and derive from C6H6. The differences between KrF and ArF laser induced mass spectra and total ionization yields are interpreted on the basis of known fragment ion appearance potentials, and a rate equations model which describes the ionization process. The observed mass spectra exhibit certain characteristics not found in conventional electron impact spectra. These idiosyncracies may enhance its potential as a mass spectrometric ionization source.

à → X̃ band systems of 2X + ( 2Π → 2Π, and of CH 3???CC??? nX + ( 2E → 2E) n = 1,2 with X = Cl, Br

Emission spectra of the cations given in the title have been detected in the gas phase following low energy electron impact excitation. The band systems are assigned to the indicated electronic transitions by reference to photoelectron spectroscopic data. An assignment of the stronger bands in the emission spectra of the substituted diacetylene cations yields the frequencies of several of the totally symmetric fundamentals for the X̃ states and of one such fundamental for the à states. These frequencies are supplemented by values obtained from photoelectron spectra which have been recorded under high resolution, and include those of the corresponding iodo- derivatives. The lifetimes of the title cations in the lowest vibrational levels of the à states have been measured, and their relaxation behaviour is discussed. Although in the case of the halo-methylacetylene and the halo-methyldiacetylene cations the à states lie above the fragment-ion appearance potentials, radiative decay is nevertheless detected for the chloro- and bromo- derivatives.

Study of the radical cation of all trans-1,3,5,7-octatetraene by its emission, Ã2Au → X̃2Bg, and by photoelect

The gas phase emission spectrum of the radical cation of all trans-1,3,5,7-octatetraene has been detected following excitation by a low-energy electron beam. The electronic transition was identified and assigned as à 2 A u → X̃ 2 B g on the basis of the He(Iα) photoelectron spectrum, which is also presented. The fine structure in the emission and photoelectron spectra are analysed to yield the vibrational frequencies of some of the totally symmetric fundamentals in the X̃ 2 B g and à 2 A u states of the octatetraene radical cation. This cation belongs to the select group of organic cations for which the à state lies above the fragment ion appearance potentials and yet radiative decay is also detected.

Emission spectra of two C 6H 6+ cations in the gas phase: competition of the radiative, Ã(π −1) → X̃(π −1), and fragmentation decay of the Ã(π −1) excited state of 2,4- and 1,3-hexadiyne radical cations

The radiative relaxation of the first excited states, Ã(π −1), of two C 6H 6 +radical cations in the gaseous phase has been detected following excitation by low energy electron impact. The à 2E u → X̃ 2E g emission band system of 2,4-hexadiyne cation, and the O 0 0 band of the corresponding emission of 1,3-hexadiyne cation, which are identified by comparison with their photoelectron spectra are presented and discussed. The decay of the Ã(π −1) excited states of these C 6H 6 ∓ions is exceptional as the fragmentation and radiative pathways are competitive and are both detected experimentally. The lifetime of the à 2E u state of 2,4-hexadiyne radical cation was measured to be 24 ± 2 ns, which considered together with the upper limit of 0.6 for the quantum yield of emission inferred from the cation breakdown diagram of the à 2E u state, indicates that the radiative and non-radiative pathways deplete the à 2E u state at approximately the same rate of 2 × 10 7 s −1. The implications of the observations of the radiative relaxation of these two electronically excited C 6H 6 ∓ radical cations lying above the fragment ion appearance potentials are considered with respect to the isomerization and fragmentation of C 6H 6 ∓isomers.

Energy transfer and primary product ion excitation in dissociative charge-transfer and penning ionization reactions

The near-adiabatic theory has been applied to experimental data obtained for some dissociative charge-transfer reactions of propane. The results support the proposition that the electronic energy levels of the C 3H 8 + primary product ion from dissociative charge-transfer correspond to published values of the appearance potentials of fragment ions from propane. The internal energy distribution of the C 3H 8 + primary product ion is obtained by plotting the relative abundances of the fragment ions against their appearance potentials.

Total and Dissociative Charge Transfer of Positive Ions with Perfluoroalkanes Studied by Mass Spectrometry

Total and dissociative charge-transfer cross sections of rare gas and molecular positive ions with CF4, C2F6, C3F8, n-C4F10, and cyclo-C4F8 are reported. The positive ions had an average energy of 50 eV so an electron jump mechanism occurs. When the total cross sections are plotted against the projectile ion recombination energy pseudoresonance effects are seen. It is suggested that the total cross section depends on the Franck—Condon factors for the transition of the neutral target molecule to a vibrationally excited ion. Thus higher vibrational states are less probable since they are generally associated with lower Franck—Condon factors. The highest cross sections occur with N2+. For a given projectile ion the total cross sections increase with the molecular weight of the fluorocarbons. The dissociative cross-section data indicates fragment ion appearance potentials in remarkably good agreement with recent photoionization measurements except for CF3+ from CF4.

Appearance Potentials of Metastable Molecular Ions

Differences between the appearance potentials of fragment ions formed a few μ secs after the ionization (so-called metastables) and normal fragment ions were measured. It is shown, however, that no immediate meaning regarding the primary fragmentation mechanism can be attached to these quantities. Instead, the experimental ionization curves of metastable and normal fragments were explained by a theoretical model which assumes 1) a linear threshold law, 2) a linear relation between the logarithm of the rate constant and the internal energy in the molecular ion, and 3) a constant density of populated states on the energy scale. In the case of HCN loss from benzonitrile ions, an excellent fit between the experimental and theoretical curves can then be achieved if it is assumed that an increase of 0.65 eV internal energy increases the rate constant by one order of magnitude. This is a surprisingly slow rise of k (E), compared with the few existing calculations in other cases.

Appearance Potentials of Positive and Negative Ions by Mass Spectrometry

Appearance potentials (AP) of ions were obtained by the retarding potential difference (RPD) technique using a time-of-flight mass spectrometer. Confirmatory measurements were made using a 180° magnetic instrument. Information about processes such as ion-pair formation, electronic excitation of ions, rearrangement of ions, autoionization and metastable transitions is inferred from the structure of the ionization efficiency (IE) curves of Xe, H2, CH4, CD4, C2H4, C2H6, C3H8, and C4H10 over about a 4 eV range. The AP's in Xe at 12.02, 12.35, 12.75, and 14.0 correspond in a qualitative way to autoionization levels. The structure observed in parent and fragment IE curves for hydrocarbons can, in some systems, be correlated with known processes. Structure due to vibrational excitation of ions was not resolved. The IE curves for polyatomic ions indicate that many ions formed by electron impact are formed in electronically excited states, and that appearance potentials of fragment ions often correlate with upward breaks in the parent IE curves. Parent ion intensities did not decrease with increasing electron energy even at large degradation ion intensities. An AP at about 0.7 eV above onset was observed in all the IE curves of R+ from RH and shown to arise from R++H— formation. The ion-pair process is as probable as the ground-state ion—neutral reaction in the systems investigated. The structure in the fragment IE curves can explain some of the difficulties encountered in measuring appearance potentials. An example of interference by ion—molecule reactions is also reported. Appearance potential data obtained by RPD and by conventional techniques are compared.

Étude du Comportement D'Isomères Géométriques Sous L'Impact Électronique V. Comparaison des données obtenues, par différentes méthodes, pour les 1,2‐diméthylcyclohexanes cis et trans

AbstractIonization potentials of cis and trans 1,2‐dimethylcyclohexanes have been determined by electron impact as well as appearance potentials of fragment ions first produced by breakdown of these molecules. Ionization efficiency curves treated by Warren's extrapolated difference method yield values in good agreement with those obtained by Lossing's semilogarithmic method.Identical ionization potentials and different appearance potentials of main fragment ions measured for both isomers can be related to the extra energy stored in the cis compound because of its steric conformation.Comparison is attempted between present electron impact data and photoionization data for the compounds investigated and qualitative agreement is found.