Absolute Photoionization Cross-Section of the Methyl Radical † Craig A. Taatjes,* David L. Osborn, Talitha M. Selby, and Giovanni Meloni Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, LiVermore, California 94551-0969 Haiyan Fan ‡ and Stephen T. Pratt* Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: March 16, 2008; ReVised Manuscript ReceiVed: April 29, 2008 The absolute photoionization cross-section of the methyl radical has been measured using two completely independent methods. The CH 3 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; σ CH 3 (10.2 eV) ) (5.7 ( 0.9) × 10 -18 cm 2 and σ CH 3 (11.0 eV) ) (6.0 ( 2.0) × 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 CH 3 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) × 10 -18 cm 2 at 10.460 eV, (5.5 ( 2.0) × 10 -18 cm 2 at 10.466 eV, and (4.9 ( 2.0) × 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. Introduction The continuing implementation of synchrotron photoioniza- tion mass spectrometric measurements for investigations of chemically reacting systems such as low-pressure flames, 1–17 laser-initiated chemical kinetics 18–20 and aerosol chemistry 21 has led to a renewed interest in measurements of absolute photo- ionization cross-sections. Absolute cross-sections for photoion- ization are necessary to convert observed photoionization signals to mole fractions in molecular beam mass spectrometry inves- tigations of low pressure flames, and to quantify branching fractions in elementary chemical kinetics measurements. Mea- surement of cross-sections for stable species has been carried out by a number of methods. Typically, the primary measure- ments of photoionization cross-sections are decomposed into a measurement of the absolute absorption cross-section and a determination of the ionization yield (i.e., the probability that absorption of a photon results in ionization). Dyke and cowork- ers 22 have measured absolute photoionization cross-sections of reactants and intermediates in the Cl 2 + dimethyl sulfide reaction by comparing photoelectron spectra of the species to reference photoelectron spectra of Ar under the same conditions. Person and Nicole 23–25 measured absolute cross-sections for absorption of isotopomers of several hydrocarbons and determined the ionization yield relative to that of NO, using the NO photoion- ization yield measurements of Watanabe et al. 26 to place the ionization cross-sections on an absolute scale. Recently, Cool and coworkers 1,10,27 have measured photoionization cross- sections of many stable combustion intermediates relative to hydrocarbon molecules whose absolute photoionization cross- sections are known from those earlier experiments. However, understanding the detailed chemistry of flames or of any reacting system requires knowledge of the radical species concentrations, and photoionization cross-sections of radicals are far less well- known. One difficulty with many techniques to determine absolute photoionization cross-sections is the determination of the absolute concentration of radicals. One way to circumvent this issue is to use photodissociation of a suitable precursor to produce the desired radical in conjunction with a species of known cross-section. For example, Flesch et al. 28,29 have determined the absolute photoionization cross-section of ClO by photodissociating ClO 2 and Cl 2 O, and making use of the known absolute photoionization cross-sections of O and Cl. More recently, Neumark and coworkers 30–32 have developed a related, but more general, approach that is applicable to larger radicals. This approach is based on translational spectroscopy and the photoionization of momentum-matched fragments produced by photodissociation. By using radical-chloride pre- cursors, the known absolute photoionization cross-section of Cl could be used to extract the absolute cross-section of the radical. This novel approach has been successfully applied to allyl, 31 † Part of the “Stephen R. Leone Festschrift”. * Authors to whom correspondence should be addressed. E-mail: cataatj@sandia.gov (C.A.T.); stpratt@anl.gov (S.T.P.). ‡ Current address: Department of Chemistry, Concordia College, 901 8th Street South, Moorhead, MN 56562. J. Phys. Chem. A 2008, 112, 9336–9343 9336 10.1021/jp8022937 CCC: $40.75  2008 American Chemical Society Published on Web 06/21/2008