Resonant Dissociation Research Articles

Electron impact studies of tin tetrahalides SnCl 4, SnBr 4 and SnI 4

Abstract Negative ions formed by dissociative resonance capture from SnCl4, SnBr4 and SnI4 have been studied. Each compound gave the ions SnX3−, SnX2−, X2− and X−. Ionization-efficiency curves were determiend for each ion and in all but one case the translational energies of the ions were measured. Extrapolation of the translational energy to thermal value yields the energy of the reaction. The electron affinities of the SnX3 and SnX2 radicals have thus been determined. The X2− and X− ions appear in most cases to be formed along with the SnX2 and SnX3 radicals in electronically excited states and the transition energies have been determined. The translational energies give some insights into the structure of the decomposition products.

Electron impact induced dissociative electron resonance capture with nitrotoluenes

Electron impact induced dissociative electron resonance capture with nitrotoluenes

Negative ion electron impact studies of arsenic trihalides: AsF3, AsCl3, and AsBr3

Low energy electron impact of AsF3, AsCl3, and AsBr3 gases gave many ions by dissociative resonance capture. Appearance potentials have been determined by deconvolution of the capture curves, and translational energies of the majority of the ions have been measured through the resonance processes. The X− ions are formed along with electronically excited neutral AsX2, and the results allow the electronic transition energies to be determined: 7.3, 4.3, and 2.6 eV for AsF2, AsCl2, and AsBr2, respectively. From the processes giving AsX−2+X, the experimental data allow the heats of formation of the negative ions ΔHf(AsX−2) to be derived: −129.7, −65.3, and −72.5 kcal mole−1 for AsF−2, AsCl−2, and AsBr−2, respectively. The ions AsCl− and AsBr− appear to be formed along with electronically excited X2 neutral: ΔHf(AsCl−) =−2.2 kcal mole−1 and ΔHf(AsBr−) =1.7 kcal mole−1 are derived. The ions X−2 appear to be formed with electronically excited AsX neutral, allowing the electronic transition energies to be derived: E*el(AsBr) ≳0.6 eV and E*el(AsCl) =2.5 eV.

Low energy electron attachment to gaseous chromium oxides

Abstract Chromium oxide (CrO3) was vaporized from a Knudsen cell into the ionization chamber of a time-of-flight mass spectrometer. The ions CrO3−, (CrO3)2− and (CrO3)3− were observed. The dimer and trimer showed resonance capture processes at approximately O eV electron energy. In addition, the dimer ion was formed by dissociative resonance capture at three resonances having onsets at 2·8, 3·8 and 4·8 eV. These are attributed to formation of (CrO3)2− from the neutral pentamer, tetramer and trimer respectively. When the appearance potentials are corrected for excess energy the electron affinity of (CrO3)2 is found to be 1·6 ± 0·3 eV. The trimer ion exhibited two dissociative resonance capture peaks at 2·7 and 4·1 eV attributable to the formation of (CrO3)3− from the pentomer and tetramer respectively. Excess energy could not be measured, but the electron affinity was estimated to be approximately 1·8 eV.

Negative ion electron impact studies of inorganic halides: Niobium and tantalum pentachlorides

Abstract Appearance potentials of the negative ions Cl − , NbCl 4 − and Cl − , TaCl 4 − formed by dissociative resonance capture of NbCl 5 and TaCl 5 have been measured, along with translational energy measurements on the Cl − ions. The processes giving rise to the Cl − are accompanied by the formation of electronically excited neutrals, the electronic excitation energy being 4·3 and 4·4 eV for NbCl 4 and TaCl 4 , respectively. Lower limits of the electron affinities of NbCl 4 and TaCl 4 of 1·4 eV are established.

Negative ion electron impact studies of titanium tetrahalides

The introduction of the vapors of TiCl4, TiBr4 and TiI4 into the ion source of a time-of-flight mass spectrometer, followed by low-energy electron impact, gave the ions X− and TiX3−. Appearance potentials have been determined by deconvolution of the dissociative resonance capture curves, and translational energies of the ions have been measured through the resonance processes. The results for TiX3− allow ΔHf(TiX3− and EA(TiX3) to be established: ΔHf(TiCl3−) = −142.8±3.2, ΔHf(TiBr3− = −108.1±3.5, ΔHf(TiI3−) < −57.3±4.3 kcal mole−1; and EA(TiCl3) = 0.6±0.2, EA(TiBr3) = 0.8±0.3 and EA(TiI3) > 0.9 eV. The ion X− is formed along with electronically excited neutral TiX3, and the results allow the electronic transition energy to be determined: 4.8±0.2, 3.3±0.3 and 2.2±0.5 eV, for TiCl3, TiBr3 and TiI3, respectively. Only the parent negative ion was observed over TiF4.

High temperature negative ions. Electron impact study of tetratomic phosphorus vapor

Appearance potentials and ion translational energies have been measured for the negative ions P, P2−, and P3− formed by dissociative resonance capture of P4. The following thermochemical values (all in kcal mole−1) have been obtained: Δ H °f,298 (P2−) = 37.2 ± 4.8; E.A. (P2) = 5.5 ± 5.3; and Δ H°f,298 (P3−) = 38.2 ± 4.5. The P− ion appears to be formed in its first excited state on low-energy electron impact of P4, and the derived heat of formation of P3, ΔH°f,298 (P3,g) =59.4±4.0, is proposed. The measurement of the increase in translational energy across resonance gave α (where α serves as the fraction of the oscillators in the transition complex which are effective for the distribution of the excess energy) = 0.43 for the three processes studied.

Partitioning of excess energy in dissociative resonance capture processes

The translational energies of selected negative ions formed by dissociative resonance capture processes from the polyatomic systems NF3, BF3, CF4, C2F6, C3F8, and c-C4F8 have been measured as functions of excess energy over the resonances. The excess energy in the molecular negative ion intermediate prior to dissociation has been calculated and partitioned into translational, vibrational, and, in some cases, electronic excitation of the dissociation products. The degree of vibrational activation in the intermediate state before dissociation is found to depend on the particular molecule under investigation and to vary from one dissociation channel to another. These observations are discussed in relation to theoretical concepts of dissociative resonance capture and given a qualitative explanation. The measurement of translational energy has led to a more complete interpretation of the states involved in the various processes and in computing ground state thermochemical properties of the decomposition products.

Mass spectra of certain initiators of radical reactions and resonance capture of slow electrons by 2-cyanoisopropyl and tert-butoxyl radicals

1. There is an intense peak of the molecular ion in the spectrum of dissociative resonance capture of electrons of azodiisobutyronitrile (I), in contrast to the mass spectrum of positive ion. 2. In the mass spectra of positive ions of tert-butylhydroperoxide (II) and tert-butylperoxide (III), the maximum peaks correspond to hydrocarbon fragments, and the peaks corresponding toα-cleavage are absent. 3. Some data were obtained on the resonance capture of slow electrons by 2-cyanoisopropyl and tert-butoxyl radicals, formed in the thermolysis of (I)–(III) under the conditions of ionization.

High temperature negative ions: Electron impact of As4 vapor

Appearance potentials and ion translational energies have been measured for the negative ions As−, As2−, and As3− formed by dissociative resonance capture of As4. The following thermochemical values (all in kilocalorie/mole) have been obtained: Δ Hf,298o(As2−)=43.1± 4.5; Δ Hf,298o(As3−)=38.3± 4.3; E.A.(As2)=2.4± 4.1; E.A.(As3)=19.3± 8.2 and Δ Hf,298o(As3)=57.6± 3.9. The measurement of the increase in translational energy through resonance gave α = 0.43 for the three processes studied.

Use of translational energy measurements in the evaluation of the energetics for dissociative attachment processes

The translational energies of negative ions formed by dissociative resonance capture processes from CO, NO, CO2, and SO2 have been measured as functions of excess energy. The sums of the translational energies of O− and the corresponding neutral from CO and NO were found to be equal to the electron energy above onset over a range of about 1 eV. At higher energies, the translational energies dropped down from the expected values because of loss of the more energetic ions to the walls. With CO2 and SO2 the total translational energy was always E*/N, where E* is excess energy and N the number of vibrational modes, 3 in each case. The measurement of translational energy has also helped in interpreting the states involved in the various processes and in computing the ground state thermochemical properties of the decomposition products.

Study of Negative Ion Formation by Electron Impact II. C- and O- Ion Formations from Carbon Monoxide

The negative ions C- and O- produced in the dissociative resonance electron capture(DREC)and ionpair formation(IPF)from carbon monoxide have been measured by means of the RPD method using a single focusing mass spectrometer. In the DREC, the cross section of the C- ion formation is2×10-22 cm2 at the peak maximum, which is about 10-3 times for that of theO- ion formation. Within the resolution of this method, the electron energy dependence of the cross section of the C- ion formation near the threshold, as well as that of theO- ion, shows probably vertical onset, and the threshold energy of the C- ion is0.34±0.05eV higher than that of theO- ion. The shapes of the energy dependence suggest that the potential energy curves of the compound states cross to the respective dissociation limit in the Franck-Condon region, therefore, these ions have no kinetic energy at their thresholds. Consquently, the difference of the threshold energies means the difference of the electron affinities. From this point of view, the electron affinity of the C atom is estimated to be 1.22±0.05eV, taking1.465±0.005eV for the electron affinity of the oxygen atom. The potential energy curves of the compound molecular-negative ion states of CO are also discussed. The energy dependence of the cross sections of the C- and O- ion formations in the IPF processes has also been measured. The step functional curves involved one and two excited levels were observed for C- and O- ions, respectively. The results were also confirmed by using CO containing 56% of 13C and the same results were obtained, and any isotope effect neither for cross section nor for threshold energy has been observed within the experimental accuracy.

Ionisation and dissociation of hexafluoroethane, and of 1,1,1-trifluoroethane and fluoroform, by electron impact

Abstract Negative ion formation as a result of the electron bombardment of hexa-fluoroethane has been investigated. Several dissociative resonance and ion-pair processes have been suggested to account for ion formation. The C2F5− ion was observed and values of ≦ 2.4 eV and ≦ 2.6 eV estimated for the electron affinities of the pentafluoroethyl and trifluoromethyl radicals, respectively. The onset of fluoride ion formation at 12.5 eV has been shown to be the result of secondary electron capture, an excited state of hexafluoroethane being involved in the production of the slow electron. Fluoride ion formation has also been observed following electron bombardment of 1,1,1-trifluoroethane and fluoroform but the low intensity of ion formation has enabled only a semi-quantitative evaluation of ion formation processes to be made.

Ion–Molecule Reactions of Negative Ions. I. Negative Ions of Sulfur

Negative ions from carbonyl sulfide and carbon disulfide have been studied in a time-of-flight mass spectrometer. The principal ions formed by dissociative resonance electron capture in carbonyl sulfide are S− and O−. Those from carbon disulfide are S− and CS−. The onset for the formation of S− by a resonance capture process has been found to occur at 1.2 eV from carbonyl sulfide, and the translational energy of the resulting ion has been found to be approximately 0.04 eV. ΔHf(S−) is thus found to be 20 kcal/mole in agreement with previous values. At pressures of several microns in the source, secondary ions are formed in both carbonyl sulfide and carbon disulfide. In the electron capture region the principal secondary ion is S2−, formed by reaction of S− with the corresponding parent molecule. At higher electron energies where greater ion intensities are observed, ions of the general formula Sn− are formed with n varying from 2 to 6. These must result from sequential reactions of Sn−1− with the parent molecule. An important ion in the high-pressure spectrum of carbon disulfide is CS2−, presumably formed by charge transfer. The rate constant from the reaction of S− with COS is about 3.0 × 10−10 cc/molecule·sec.

Temporary negative ion resonances in benzene and benzene derivatives

Abstract New, intense, energy loss resonances below the first excitation potential were discovered in the electron impact excitation spectra of benzene and six of its derivatives. The observed resonances for o-C6H4CH3Cl, C6H5Cl, C6H5Br and o-6H4Cl2 peak at exactly the same energy as the dissociative electron capture resonances previously reported for these compounds, suggesting that both resonances proceed via a common temporary negative ion state of the parent molecules.

A survey of negative ions in mass spectra of polyatomic molecules

A s urvey was made of negative-ion production in 27 different molecules covering a wide variety of chemical compounds. In nine compounds no ions were found. For the other cOlr>pounds rough measurements of the appearance potentials and relative abundance of the ions are reported . About 60 different ionization processes are observed in 18 compounds. Formation of ion s by dissociative resonance attachment is t he most common process, but there are several cases of ion pair formation and of attachment without dissociation. Perchlorylfluoride gives by far t he greatest abundance and variety of ions. with lanthanum boride was substituted for the pure tungsten filament that is used routinely as an electron source. This resulted in an ion source temperature of about 150 0 C instead of 250 0 C. Negative ions formed by attachment processes were first detected by repeated sweeping of portions of the spectrum while varying t h e electron energy from 0 to 20 v, and negative ions formed by ion-pair processes were detected at 100 v. Data for ioniza­ tion-efficiency curves were then obtained by varyin g t he electron energy in 0.2-v step s from 0 to 20 v and in larger increments up to 100 v. Curves were plotted on a lineal' scale. The r elative efficiency for collecting negative ions produced by resonance capture as compared with positive ions depends criticall y on the operating conditions. The collecting efficiency of the 1800 mass spectrometer has been estimated by a method used by Marriott and Craggs (2). Lozier (6) made measurements of the relative abundance of positive and negative ions from oxygen in a nearly field-free space and found the ratio 0 - 10 +=0.04 at 35 v. This instrument gave a ratio of about 0.0125 and the collecting efficiency is taken to be 31 percel~t. This paper reports observed appearance potentials without any correction for effects of contact poten­ tials and the electric field across the ionization chamber. A comparison with published resul ts in­ dicates t.hat the correction would be less than 1 v. 3. Results

268. Dissociation constants, activation energies, and resonance

268. Dissociation constants, activation energies, and resonance