S. N. Foner

Mass Spectrometry of the HO2 Free Radical

Mass spectrometric studies on the production, identification, and determination of thermochemical energies of HO2 radicals are reported. Reactions found to produce HO2 radicals, and examined in some detail, were: (1) reaction of H with O2, (2) reaction of H with H2O2, (3) reaction of O with H2O2, (4) reaction of OH with H2O2, (5) photolysis of H2O2, and (6) low‐power electrical discharge in H2O2. Of the reactions studied, the low‐power electrical discharge in H2O2 provided the most intense and convenient source of HO2 radicals. Ion—molecule reactions, which are negligible in normal operation of our mass spectrometer, are shown to be a potentially serious source of interference in studies of HO2 with conventional mass spectrometers.The ionization potential of HO2, I(HO)2, and the appearance potential of HO2+ from H2O2, A(HO2+), have been redetermined, and the bond dissociation energies D(H–OOH) and D(H–O2) have been recalculated. The measured values are: I(HO2) = 11.53±0.02 ev, A(HO2+) = 15.36±0.05 ev with...

Multiple Trapping Sites for Hydrogen Atoms in Rare Gas Matrices

Hydrogen atoms have been stabilized in nonequivalent lattice sites in matrices of the rare gases at liquid helium temperature. Electron spin resonance spectra of H atoms in argon, krypton, and xenon show that at least two trapping sites are involved in each case. In a neon matrix, H atoms have been stabilized in only one site. Attainability of the various trapping sites apparently depends on the initial energy of the H atom, a simple doublet spectrum being obtained when the atoms are deposited from the gas phase, while multiple trapping spectra are obtained when the atoms are produced by photolysis in the solid.The hyperfine coupling contants and the electronic g factors for H atoms trapped in the various matrix sites have been determined. The deviation of the hyperfine coupling constant from the free‐state value is positive in some cases and negative in others. The experimental results are in good agreement with theoretical predictions. A complex multicomponent H atom spectrum was obtained by photolysis ...

The Detection of Atoms and Free Radicals in Flames by Mass Spectrometric Techniques

A mass spectrometric method employing a molecular beam gas sampling system has been developed for the detection of atoms and radicals in chemical reactions. Background signals have been virtually eliminated by mechanically modulating the molecular beam and applying phase detection to the ion signal. The application of the method is illustrated by examples of low‐pressure flames. In the hydrogen‐oxygen flame H, O, and OH have been positively identified. The mass spectrum of a simple hydrocarbon flame, such as the methane‐oxygen flame, is complicated by the presence of a large number of stable components generated in the flame. In the methane‐oxygen flame the stable intermediates include C2H2, CO, CH2O or C2H6, CH4O, and C4H2. The methyl radical has been clearly identified in the methane‐oxygen flame. A search for the HO2 radical in the hydrogen‐oxygen flame was made without obtaining positive results. The HO2 detection problem is discussed in detail.

Mass Spectrometric Studies of Metastable Nitrogen Atoms and Molecules in Active Nitrogen

Metastable nitrogen atoms and molecules produced by electrical discharges in N2 and He–N2 mixtures have been studied by mass spectrometry. N(2D) and N(2P) atoms in addition to ground‐state N(4S) atoms were clearly identified in the products observed about 1 msec after leaving the discharge. The concentration of metastables was much higher for a discharge in a He–N2 mixture than for a discharge in pure N2. Assuming that the ionization cross sections for the atoms at corresponding excess energies were equal, the relative concentrations were: N(4S) = 1.00, N(2D) = 0.17, and N(2P) = 0.06 for the He–N2 discharge; and N(4S) = 1.00, N(2D) = 0.0068, and N(2P) = 0.0025 for the pure‐N2 discharge. Metastable N2 molecules with excitation energies of up to several eV were also observed. Ionization processes involving the metastable molecules are discussed. To explain the experimentally obtained N2 ionization curves, it was found necessary to assume that, in addition to vibrationally excited ground‐state molecules, an ...

On the heat of formation of diimide

The heat of formation of trans‐diimide (N2H2) has been determined mass spectrometrically by combining the ionization potential of N2H2 with the appearance potential of N2H+2 in the reaction N2H4+e→N2H+2+H2+2e. The measurements are I.P.(N2H2) =9.65±0.08 eV, in agreement with the photoelectron spectroscopic value, and A.P.(N2H+2) =10.75±0.08 eV leading to ΔH°fo(N2H2) =52.4±2 kcal/mole and ΔH°f298(N2H2) =50.7±2 kcal/mole. Studies of the reaction N2H4+e→N2H+2+2H+2e have provided confirmatory information on this energy assignment and suggest that excess energy in these reactions is very low or zero. The results on the heat of formation of diimide are in good agreement with our earlier work and in remarkable concordance with recent theoretical calculations, but are in marked disagreement with a very recent determination using data from the reaction N2H2+e→N+2+H2+2e. We have been unable to confirm the experimental results reported for this reaction. The measured value of ΔH°fo(N2H2) together with some auxiliary ...

Mass Spectrometric Studies of Atom–Molecule Reactions Using High‐Intensity Crossed Molecular Beams

Elementary gas phase reactions have been studied with high‐intensity crossed molecular beams. A detailed analysis is presented on the sensitivity factors involved in mass spectrometric detection of free radicals formed in ordinary chemical reactions as contrasted to surface ionization detection of the products of alkali atom reactions. Free radicals have been observed in a number of atom–molecule reactions, including: (1) Cl atoms with n‐butane and isobutane, (2) H atoms with NO2 and hydrazine, and (3) O atoms with hydrazine, monomethylhydrazine, and 1,1‐dimethylhydrazine. The O atom reactions with the hydrazines are very interesting because of the marked proclivity for an O atom to abstract two H atoms from opposite ends of the molecule, forming diimide (HNNH) in the case of hydrazine and methyldiazene (HNNCH3) in the case of monomethylhydrazine. Ionization potentials of the free radicals and unstable molecules produced in the reactions have been measured and used in conjunction with appearance potential...

Mass Spectrometry of Free Radicals and Vibronically Excited Molecules Produced by Pulsed Electrical Discharges

Free radicals and metastable molecules produced by short‐duration pulsed electrical discharges have been studied with a mass spectrometer employing a collision‐free molecular‐beam sampling system. Nitrogen atoms generated by extremely short pulses (0.035‐μsec half‐width) have been used to study the dynamics of the gas‐sampling system. NH free radicals produced by electrical discharges in ammonia were detected in the X 3Σ− ground state and in the a 1Δ electronically excited state. It is estimated that approximately 22% of the NH radicals were in the a 1Δ state. Direct measurement of the ionization potential of NH in the ground state gives I (NH) = 13.1 eV. Vibronically excited nitrogen molecules from a discharge in a N2–He mixture have been observed with excitation energies up to about 9 eV, indicating that either or both of the metastable electronic states, a 1Πg and a′ 1Σu−, are substantially populated by the discharge. Evidence for excitation of N2 X 1Σg+ ground‐state molecules to high vibrational quant...

IONIZATION AND DISSOCIATION OF HYDROGEN PEROXIDE BY ELECTRON IMPACT

Mass spectrometric measurements on hydrogen peroxide were carried out with a modulated molecular beam sampling system to eliminate interferences from decomposition products. The mass spectrum of pure hydrogen peroxide differs considerably from previously reported spectra, which did not include an estimate of the H2O+ and O2+ ion intensities because of decomposition problems. Appearance potentials of the ions were measured with the following results: I(H2O2) = 10.92±0.05 ev, A(HO2+) = 15.36±0.05 ev, A(O2+) = 15.8±0.5 ev, A(H2O+) = 14.09±0.10 ev, A(OH+) = 15.35±0.10 ev, and A(O+) = 17.0±1.0 ev. The ionization processes and relevant thermochemical energies are discussed. It is found that the H2O+ and OH+ ions are evolved without excess energy, while the dissociative processes producing O2+ and O+ require excess energy.

Vibrationally excited nitrogen molecules formed in the catalytic decomposition of ammonia on platinum

Catalytic decomposition of ammonia on polycrystalline platinum was studied with a modulated molecular beam mass spectrometer. Threshold ionization measurements on N2 have established the production of significant quantities of vibrationally excited nitrogen molecules (N*2 ). Although the overwhelming majority of molecules have excitation energies less than 1.3 eV, a few of the molecules have energies as high as 2.4 eV, requiring excitation to at least the v=9 vibrational level of N2. It is suggested that the observed vibrational energy is provided by two reactions involving recombination of chemisorbed NH radicals: NHad+NHad→N*2+H2 and NHad+NHad→N*2+2Had. The ionization data on N*2 indicate that during ionization vibrational energy is being efficiently converted into electronic energy via autoionization processes. Measurements on the appearance potential of NH+2 show no evidence for the evolution of NH2 radicals from NH3 decomposition, confirming recently published results. Pulsed heating experiments show...

Ionization potential of the NH free radical by mass spectrometry: Production of ground state and electronically excited NH by F‐atom reactions

Sequential F‐atom reactions with ammonia were used to produce NH radicals in the X 3Σ− ground state and the a 1Δ excited state. Ionization potential measurements on both of these species lead to the determination I(NH) = 13.47±0.05 eV, indicating that the error limits in at least one of the previous studies was seriously underestimated. The F‐atom reactions also produced ground and electronically excited N atoms. Efforts to obtain an independent measurement of the heat of formation of NH using dissociative ionization of NH3 were unsuccessful, apparently because of excess energy involved in the ionization process. Some important N–N and N–H bond dissociation energies have been updated.