Giovanni Meloni

Absolute Photoionization Cross-Section of the Methyl Radical†

The absolute photoionization cross-section of the methyl radical has been measured using two completely independent methods. The CH3 photoionization cross-section was determined relative to that of acetone and methyl vinyl ketone at photon energies of 10.2 and 11.0 eV by using a pulsed laser-photolysis/time-resolved synchrotron photoionization mass spectrometry method. The time-resolved depletion of the acetone or methyl vinyl ketone precursor and the production of methyl radicals following 193 nm photolysis are monitored simultaneously by using time-resolved synchrotron photoionization mass spectrometry. Comparison of the initial methyl signal with the decrease in precursor signal, in combination with previously measured absolute photoionization cross-sections of the precursors, yields the absolute photoionization cross-section of the methyl radical; sigma(CH3)(10.2 eV) = (5.7 +/- 0.9) x 10(-18) cm(2) and sigma(CH3)(11.0 eV) = (6.0 +/- 2.0) x 10(-18) cm(2). The photoionization cross-section for vinyl radical determined by photolysis of methyl vinyl ketone is in good agreement with previous measurements. The methyl radical photoionization cross-section was also independently measured relative to that of the iodine atom by comparison of ionization signals from CH3 and I fragments following 266 nm photolysis of methyl iodide in a molecular-beam ion-imaging apparatus. These measurements gave a cross-section of (5.4 +/- 2.0) x 10(-18) cm(2) at 10.460 eV, (5.5 +/- 2.0) x 10(-18) cm(2) at 10.466 eV, and (4.9 +/- 2.0) x 10(-18) cm(2) at 10.471 eV. The measurements allow relative photoionization efficiency spectra of methyl radical to be placed on an absolute scale and will facilitate quantitative measurements of methyl concentrations by photoionization mass spectrometry.

Products of the benzene + O(3P) reaction.

The gas-phase reaction of benzene with O((3)P) is of considerable interest for modeling of aromatic oxidation, and also because there exist fundamental questions concerning the prominence of intersystem crossing in the reaction. While its overall rate constant has been studied extensively, there are still significant uncertainties in the product distribution. The reaction proceeds mainly through the addition of the O atom to benzene, forming an initial triplet diradical adduct, which can either dissociate to form the phenoxy radical and H atom or undergo intersystem crossing onto a singlet surface, followed by a multiplicity of internal isomerizations, leading to several possible reaction products. In this work, we examined the product branching ratios of the reaction between benzene and O((3)P) over the temperature range 300-1000 K and pressure range 1-10 Torr. The reactions were initiated by pulsed-laser photolysis of NO(2) in the presence of benzene and helium buffer in a slow-flow reactor, and reaction products were identified by using the multiplexed chemical kinetics photoionization mass spectrometer operating at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory. Phenol and phenoxy radical were detected and quantified. Cyclopentadiene and cyclopentadienyl radical were directly identified for the first time. Finally, ab initio calculations and master equation/RRKM modeling were used to reproduce the experimental branching ratios, yielding pressure-dependent rate expressions for the reaction channels, including phenoxy + H, phenol, cyclopentadiene + CO, which are proposed for kinetic modeling of benzene oxidation.

Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr(3) at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomer-specific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C(2)H(4) reaction the C(3)H(5) intermediate decays by H atom loss to yield 70(+/-8)% allene, 30(+/-8)% methylacetylene, and less than 10% cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C(3)H(2) isomer is contrary to a previous RRKM calculations that predicted linear triplet propargylene to be 90% of the total H-atom coproducts. High-level CBS-APNO quantum calculations and RRKM calculations for the CH + C(2)H(2) reaction presented in this manuscript predict a higher contribution of the cyclic C(3)H(2) (27.0%) versus triplet propargylene (63.5%) than earlier predictions. Extensive calculations on the C(3)H(3) and C(3)H(2)D system combined with experimental isotope ratios for the CD + C(2)H(2) reaction indicate that H-atom-assisted isomerization in the present experiments is responsible for the remaining discrepancy between the new RRKM calculations and the experimental results. Cyclic isomers are also found to represent 30(+/-6)% of the detected products in the case of CH + methylacetylene, together with 33(+/-6)% 1,2,3-butatriene and 37(+/-6)% vinylacetylene. The CH + allene reaction gives 23(+/-5)% 1,2,3-butatriene and 77(+/-5)% vinylacetylene, whereas cyclic isomers are produced below the detection limit in this reaction. The reaction exit channels deduced by comparing the product distributions for the aforementioned reactions are discussed in detail.

Direct detection of polyynes formation from the reaction of ethynyl radical (C2H) with propyne (CH3–CCH) and allene (CH2CCH2)

The reactions of the ethynyl radical (C2H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C2H radical is prepared by 193-nm photolysis of CF3CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron–photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C5H4 produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50–70% diacetylene (H–CC–CC–H) and 50–30% C5H4, with a C5H4-isomer distribution of 15–20% ethynylallene (CH2CCH–CCH) and 85–80% methyldiacetylene (CH3–CC–CCH). The reaction of allene with ethynyl gives 35–45% ethynylallene, 20–25% methyldiacetylene and 45–30% 1,4-pentadiyne (HCC–CH2–CCH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan’s atmospheric chemistry are discussed.

Synchrotron Photoionization Mass Spectrometry Measurements of Kinetics and Product Formation in the Allyl Radical (H2CCHCH2) Self-Reaction†

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

Photoelectron spectroscopy of ClH2− and ClD2−: A probe of the Cl+H2 van der Waals well and spin–orbit excited states

Photoelectron (PE) spectra of ClH2− and ClD2− were measured at 299 nm (4.154 eV). Photodetachment of these anions accesses the prereactive van der Waals well on the ground state Cl+H2 potential energy surface, as well as the low-lying spin–orbit excited states resulting from the interaction of Cl and Cl* with H2. The PE spectra are dominated by two relatively narrow peaks corresponding to transitions to the neutral Cl⋅H2 and Cl*⋅H2 complexes. The energetics and widths of these features are interpreted in terms of the properties of the anion and neutral potential energy surfaces.

Spectroscopic characterization of the ground and low-lying electronic states of Ga2N via anion photoelectron spectroscopy

Anion photoelectron spectra of Ga(2)N(-) were measured at photodetachment wavelengths of 416 nm(2.978 eV), 355 nm(3.493 eV), and 266 nm(4.661 eV). Both field-free time-of-flight and velocity-map imaging methods were used to collect the data. The field-free time-of-flight data provided better resolution of the features, while the velocity-map-imaging data provided more accurate anisotropy parameters for the peaks. Transitions from the ground electronic state of the anion to two electronic states of the neutral were observed and analyzed with the aid of electronic structure calculations and Franck-Condon simulations. The ground-state band was assigned to a transition between linear ground states of Ga(2)N(-)(X (1)Sigma(g) (+)) and Ga(2)N(X (2)Sigma(u) (+)), yielding the electron affinity of Ga(2)N, 2.506+/-0.008 eV. Vibrationally resolved features in the ground-state band were assigned to symmetric and antisymmetric stretch modes of Ga(2)N, with the latter allowed by vibronic coupling to an excited electronic state. The energy of the observed excited neutral state agrees with that calculated for the A (2)Pi(u) state, but the congested nature of this band in the photoelectron spectrum is more consistent with a transition to a bent neutral state.

Negative ion photodetachment spectroscopy of the Al3O2, Al3O3, Al4Ox, Al5Ox(x=3–5), Al6O5, and Al7O5 clusters

The Al3O2, Al3O3, Al4Ox, Al5Ox (x = 3–5), Al6O5, and Al7O5 clusters are studied using negative ion photoelectron spectroscopy. At 266 nm (4.661 eV) laser photodetachment wavelength the spectra of Al3O2 and Al3O3 present vibrationally resolved features. They show three electronic transitions, due to two different isomers. From Franck–Condon simulations of the Al3O2 and Al3O3 photoelectron spectra, several vibrational frequencies together with the normal coordinate changes were derived. We obtained approximate electron affinities for Al3O2 and Al3O3 using the Gaussian 2 model, and calculated isomerization energies for both the anionic and neutral geometries and the experimental adiabatic detachment energies (ADE) of bands X′ and X. The larger aluminum oxide clusters present several structureless bands which likely also result from multiple isomers. The ADEs for the larger clusters increase with size within a cluster series, with the exception of Al5O5.

Structure and thermodynamic stability of the OsC and OsC2 molecules by theoretical calculations and by Knudsen cell mass spectrometry

Knudsen cell mass spectrometric equilibrium measurements together with theoretical computations have been employed to gain structural and thermodynamic information of the OsC and OsC2 molecules. Several levels of theory have been applied to determine the structures, molecular parameters, and physico-chemical properties of OsC(g) and OsC2(g), and their singly charged ions. Complete active space self-consistent field (CASSCF) calculations were performed on the apparent 3Σ− ground state and first 3Δ excited state of OsC. From the analyzed gaseous equilibria and the thermal functions calculated from the computed molecular parameters, the following atomization enthalpies, ΔaH0o(OsC,g) and ΔaH0o(OsC2,g), and enthalpies of formation, ΔfH298.15o(OsC,g) and ΔfH298.15o(OsC2,g), in kJ mol−1, have been obtained: OsC, 605.6±14.0 and 895.4±14.0; OsC2, 1154.6±18.0 and 1059.5±18.0. The results have been compared with those for the other platinum metal carbides and oxides.

Atomization enthalpies and enthalpies of formation of the germanium clusters, Ge5, Ge6, Ge7, and Ge8 by Knudsen effusion mass spectrometry

The high-temperature mass spectrometric method was employed to measure the equilibrium partial pressures of small germanium clusters above liquid germanium contained in a graphite Knudsen cell. These data were combined with new thermal functions, calculated from recent theoretical and spectroscopic molecular parameters, to evaluate the atomization enthalpies and enthalpies of formation of Ge5–Ge8. Mass spectrometric equilibrium data available in literature were also reevaluated. The following atomization enthalpies, ΔaH0o(Gen,g) and enthalpies of formation ΔfH298.15o(Gen,g), in kJ mol−1, have been obtained: Ge5, 1313±27 and 548±27, Ge6, 1649±33 and 583±33, Ge7, 2008±42 and 598±42, Ge8, 2359±60 and 618±60. The atomization energies are compared with available theoretical values.