Absolute Photoionization Cross-Section of the Methyl Radical

Abstract

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.

An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

“…In Kiefer et al's work [5], the rate constants of R1 were measured in experiments and calculated by RRKM theory at various pressures (33,67,133,200 and 266 mbar) in the temperature range of 1300-2000 K. They suggested that the uncertainty of rate constants is within ±30% throughout the range of pressure and temperature. In this work, their calculated rate constant at 33 mbar is used and divided by a factor of 1.5 in the simulation to get a better agreement with the experimental measurement.…”

Combustion chemistry of the propanol isomers — investigated by electron ionization and VUV-photoionization molecular-beam mass spectrometry

“…For these species, the quantitative profiles typically agree to within a factor of 2-4 between the EI and PI measurements, but discrepancies up to an order of magnitude are observed. For example, the photoionization cross section of the methyl radical had not been measured until recently [38] and no experimental value is available for the electron ionization process. The calibration factor in the electron ionization measurements was based on the convolution of the value calculated with the Binary Encounter Bethe (BEB) theory [39] with the experimentally determined electron-energy distribution function.…”

Photodissociation dynamics of acetone studied by time-resolved ion imaging and photofragment excitation spectroscopy

“…The ionization crosssections of CH3 and HCO are similar in magnitude near 118 nm and relatively small (~5×10 -18 cm 2 ). 42,53,54 However, ionization is universal. That is, the ionization efficiency is independent of the rovibrational state of the products, which readily compensates for the apparent limitation on the detection sensitivity.…”