Appearance Potentials Of 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.

Thermodynamic Properties of Lanthanum Chlorides

The enthalpies of atomization of LaCl, LaCl2, and LaCl3 are determined by measuring the vapor pressure over liquid and solid LaCl3 and assessing the equilibrium constants of the gas-phase reactions BaCl + La = Ba + LaCl and 2BaCl + La = 2Ba + LaCl2, using the appearance potentials of ions and the ionization potentials of the lanthanum chlorides: Δat H 0(LaCl, 298 K) = 502.4 ± 0.5 kJ/mol, Δat H 0(LaCl2, 298 K) = 999.2 ± 0.8 kJ/mol, and Δat H 0(LaCl3, 298 K) = 1524.6 ± 2.0 kJ/mol. In addition, the enthalpies of formation of gaseous lanthanum chlorides are determined using new data on the thermodynamic functions of condensed LaCl3 and gaseous LaCl, LaCl2, and LaCl3: Δf H 0(LaCl, g, 298 K) = 48.9 ± 1.0 kJ/mol, Δf H 0(LaCl2, g, 298 K) = –326.6 ± 1.2 kJ/mol, and Δf H 0(LaCl3, g, 298 K) = –730.7 ± 2.0 kJ/mol. The ionization and appearance potentials of ions resulting from electron impact ionization of LaCl3 are determined.

The electron-impact dissociative ionization ofCCl2F2

An investigation of the formation channels and properties of ionfragments following electron-impact dissociative ionization of theCCl2F2 molecule using electron kinetic energies in the 0–100 eV range isreported. Measurements of ion appearance potentials (APs) and nascenttranslational energy distributions were made on a supersonic expansion ofCCl2F2 in a time-of-flight mass spectrometer. A discussion of the correlation betweenthe channel APs, the precursor bond characters as calculated from thepopulation analysis, and the low-resolution photoelectron spectrum of theCCl2F2 molecule is presented.

Homologous micro-clusters of carbon disulphide

Mass spectrometric investigations of homologous micro-clusters ( n⩽6) of carbon disulphide are studied by using an electron impact ion source and a double focusing sector field mass spectrometer. Clusters are produced by adiabatic gas expansion of mixture of CS 2 vapour and Ar. The appearance potentials for ions (CS 2) n S +, (CS 2) n CS +, (CS 2) n S + 2 ( n=1 and 2), CS 2C + and (CS 2) 3S + are determined. The temperature dependences of selected clusters are also presented.

The adsorption of methanol on large water clusters

We have studied the adsorption of methanol monomers onto large water clusters by using the pick-up technique to deposit methanol onto water clusters formed in a supersonic expansion of pure water vapour. The clusters are ionised by electron impact and mass analysed. The major low-mass methanol-containing ion peaks (< 100u) correspond to unclustered and unprotonated methanol and have similar cracking patterns to free methanol monomers, although there are some heterocluster ions observed containing no more than one water and two methanol molecules. The measured ion appearance potentials indicate that the methanol is directly ionised. In contrast to previous results for methanol adsorbed onto large argon clusters [F. Huisken and M. Stemmler, J. Chem. Phys. 98 (1993) 7680], we do not find any evidence for the formation of dimers or higher clusters of methanol on the water clusters.

Absolute oscillator strengths for the photoabsorption, photoionization and ionic photofragmentation of phosphine. I. The valence shell

Dipole (e, e) spectroscopy has been used to obtain the first reported measurements of the absolute photoabsorption oscillator strengths (cross sections) for the valence shell of phosphine in the equivalent photon energy range 5–130 eV. Dipole (e, e+ion) coincidence spectroscopy has been used to collect photoionization time-of-flight mass spectra from the first ionization potential up to 50 eV to yield ionic photofragmentation branching ratios. Absolute partial photoionization oscillator strengths for production of the molecular and fragment ions have been obtained from the measured absolute photoabsorption oscillator strengths, the photoionization efficiency and the ionic photofragmentation branching ratios. Radiation-induced breakdown pathways of phosphine have been deduced using observed ion appearance potentials and the presently reported partial photoion oscillator strengths, together with the known electronic state ionization potentials of PH 3. Absolute electronic state partial photoionization oscillator strengths have been derived by summing the appropriate portions of the ionic photofragmentation partial oscillator strengths according to the proposed radiation-induced breakdown scheme.

A thermokinetic method for the determination of heats of reaction of gas phase processes

A novel method is presented for the determination of heats of reaction for gas phase processes by simultaneous measurement of the reaction rate and the internal temperature rise at the centre of a spherical vessel. The reaction stoichiometry must be known, and it is necessary to choose reaction conditions giving temperature rises of between 0.5°C and 5°C. The method makes use of well established relationships from self heating theory. The only additional essential datum is the thermal conductivity of the mixture; our experiments use mixtures with a high proportion of argon, typically 90 mol%, to ensure that this can be estimated to good accuracy. The method is demonstrated by measurements on the decomposition reaction of di- t-butyl peroxide and the isomerisation reaction of methyl isocyanide. It is applied to the determination of heats of reaction for the decomposition reactions of 1,1,2,2-tetrafluoroethyltrifluorosilane and 1,1,2,2-tetrafluoroethyltrimethylsilane and for the reaction of trichloromethyltrichlorosilane with trans-but-2-ene. From this last reaction system, the heat of formation of dichlorocarbene is obtained as Δ H xxx f.298(CCl 2) = 230 kJ mol −1 with an uncertainty of the order of 12 kJ mol −1. The most recent previous values for this quantity are 163 ± 12 kJ mol −1, derived from measurements of gas phase proton transfer processes, and 230 ± 35 kJ mol −1 from the appearance potentials of ions from the dissociative photodissociation of CF 2CCl 2.

Absolute oscillator strengths for photoabsorption, photoionization and ionic photofragmentation of silane. I. The valence shell

Absolute photoabsorption oscillator strengths (cross sections) for the valence shell of silane have been measured using dipole (e, e) spectroscopy in the equivalent photon energy range 8–100 eV. Photoionization time-of-flight (TOF) mass spectra have been collected using dipole (e, e+ion) coincidence spectroscopy from the first ionization potential up to 52 eV. Combination of the photoabsorption oscillator strengths with the photoionization efficiency and the ionic photofragmentation branching ratios determined from the mass spectra yielded oscillator strengths for all dissociative photoionization channels. No stable molecular ions were detected. Using the measured ion appearance potentials and previously reported ionization potentials of silane, the various contributions to each of the dissociative photoionization channels arising from decomposition of the valence T 2 and A 1 ionic states (vertical ionization energies ≈ 12.4 (T 2) and ≈ 18.2 eV (A 1)) are estimated. Based on this information electronic state partial photoionization cross sections are inferred for the T 2 and A 1 states.

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.

Formation enthalpies of hetaryl ions in the gas phase

Photoionization was used to determine the appearance potentials of ions from 2-acetylfuran, 2-acetylthiophene, 2-acetylpyrrole, 2,5-dimethylfuran, 2,5-dimethylthiophene, and 2,5-dimethylpyrrole. The enthalpies of formation were determined for RC≡0+ ions (R=2-furyl, 2-thienyl, and 2-pyrroloyl), pyrilium, and thiopyrilium ions.

Thermochemical features of some ?-diketones and their fragments in the gas phase

This paper is a continuation of work on the determination bythe action of the enthalpy of formation of cations and radicals using data on the ioinization potentials of molecules and the appearance potentials of ions obtained by photoionization. Diketones were treated, the thermochemical characteristics of the fragments of which had not been studied.

Mass spectrometric investigation of NH 3 and CO 2 cluster ions: appearance potentials and stability

Electron impact ionization of (CO 2) x and (NH 3) y (x up to 5, y up to 10) was investigated in a molecular beam experiment coupled with a double focusing mass spectrometer. A special ion optical system and operating mode (deflection method) was used to discriminate between beam species and background species. For both neat and seeded (He, Ar, Kr) beams we have studied neutral cluster beam properties. The following cluster ion appearance potentials were obtained: (CO 2) 2+e→(CO 2) + 2 AP=13.1±0.2 eV (NH 3) 2+e→NH 4 + AP=9.95±0.3 eV (NH 3) 3+e→(NH 3) 2H + AP=9.50±0.3 eV The exothermicity of the reaction of CO + 2 with CO 2 to form (CO 2) + 2 was found to be 17.1±5 kcal/mole in good agreement with previous high pressure mass spectrometry determination. The dissociation energy for the proton bound dimer to NH 4 ++NH 3 was found to be 0.60±0.2 eV in good agreement with a previous photoionization result of 0.60 eV, in contrast to a number of recent ion molecule studies supporting a value of about 1.1 eV. In addition, metastable transitions occurring in the mass spectrum of C 3H 8, N 2O and (CO 2) x, (NH 3) y were investigated by means of decoupling the acceleration and deflection fields of the mass spectrometer, yielding the first results on the stability of cluster ions.

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.

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.

Mass spectrometry of uronic acid derivatives. XI--Structure elucidation by mass spectrometry of aldobiouronic and pseudoaldobiouronic acids and their beta-elimination products as per-o-methyl derivatives.

Electron impact mass spectra of a series of aldobiouronic and pseudoaldobiouronic acid per-O-methyl derivatives and of the corresponding 4,5-unsaturated analogues, found normally among the products of methylation of uronic acid containing disaccharides as a result of methylation accompanying beta-elimination, have been studied. Using labelling experiments, metastable transition measurements and high resolution mass spectrometry, the fragmentation mechanisms of substances of this class have been deduced. Application of the information to the structure elucidation of this type of compound is discussed. It is concluded that from the mass spectra alone it is possible to determine the molecular weight, the cycle masses as well as the mode of linkage between the monomeric units. The appearance potentials of ions formed by cleavage of the glycosidic linkages have also been determined and the energetic differences encountered in the fission of the glycosidic linkages of various types of uronic acid containing oligosaccharides are discussed.

Identification of methyl (methyl O-acetyl- O-methylhexopyranosid)uronates by mass spectrometry

Fragmentation of methyl (methyl O-acetyl- O-methyl-α- d-gluco- and -galactopyranosid, uronates has been studied at 70 and 12 eV. At 12 eV, the production of ions resulting from secondary and further processes is greatly diminished and the spectra are simpler and easier to interpret. The energy required for the elimination of acetic acid and ketene has been calculated from the ion-appearance potentials. The number and location of methyl groups in methyl (methyl O-acetyl- O-methyl-hexopyranosid)uronates can be determined. The procedure is particularly suitable for g.l.c.-m.s. of the uronic acid portion of methanolysates of methylated biopolymers and other substances containing uronic acids. From the presence or absence of peaks for molecular ions in the 12-eV spectra, gluco and galacto isomers which do not contain a methoxyl group at C-4 can be distinguished. The synthesis of methyl (methyl 2,3-di- O-methyl-α- d-galactopyranosid)uronate is also described.

Positive ion appearance potentials measured in CeI3

The positive ions produced by electron impact on CeI3 have been studied with a mass spectrometer having a heatable collision chamber with an attached, separately heatable sample reservoir. The individual ion appearance curves have been measured with a resolution of 0.15 eV, using the retarding potential difference technique. The ion appearance potentials have been determined from the appearance curves by a linear extrapolation technique which accounts for the curvature in the immediate vicinity of the threshold. The appearance potentials give estimates of the heats of formation of the various ions and of the ionization potentials of the corresponding fragments of CeI3. The resulting recommended values are ΔHf(CeI+3) =128 kcal/mole (5.53 eV), I.P.(CeI3) =9.05 eV; ΔHf(CeI+2) =113 kcal/mole (4.92 eV), I.P.(CeI2) =6.75 eV; ΔHf(CeI+) =171 kcal/mole (7.41 eV); I.P.(CeI) =5.9 eV; ΔHf(Ce+) =228 kcal/mole (9.90 eV), I.P.(Ce) =5.65 eV.

A simple method of obtaining excitation functions in digital form: application to electron impact dissociative ionization of CO 2

A method for obtaining excitation functions in digital form is described which is applicable to measurements of ion appearance potentials by electron impact in mass spectrometry. The method employs a sampling time-to-pulse height converter and synchronously pulsed operational amplifier. Excitation functions for the ions O + and C + produced by electron impact on a CO 2 gas beam are presented. An appearance potential of 19.4±0.3 eV was obtained for O +, probably corresponding to the threshold for excitation of the predissociation C 2Σ g + state of CO 2 +. The observed C + onset of 29.8±0.3 eV corresponds to total dissociation of the CO 2 molecule by the process e+CO 2 → C + + O+O+2 e.