Determination of the Absolute Photoionization Cross Sections of CH3 and I Produced from a Pyrolysis Source, by Combined Synchrotron and Vacuum Ultraviolet Laser Studies

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• 作者：Brenger Gans ; Luiz A. Vieira Mendes ; Sverine Boy-Pronne ; Stphane Douin ; Gustavo Garcia ; Hloï ; se Soldi-Lose ; Barbara K. Cunha de Miranda ; Christian Alcaraz ; Nathalie Carrasco ; Pascal Pernot ; Dolores
• 刊名：Journal of Physical Chemistry A
• 出版年：2010
• 出版时间：March 11, 2010
• 年：2010
• 卷：114
• 期：9
• 页码：3237-3246
• 全文大小：338K
• 年卷期：v.114,no.9(March 11, 2010)
• ISSN：1520-5215

文摘

A pyrolysis source coupled to a supersonic expansion has been used to produce the CH₃ radical from two precursors, iodomethane CH₃I and nitromethane CH₃NO₂. The relative ionization yield of CH₃ has been recorded at the SOLEIL Synchrotron Radiation source in the range 9.0−11.6 eV, and its ionization threshold has been modeled by taking into account the vibrational and rotational temperature of the radical in the molecular beam. The relative photoionization yield has been normalized to an absolute cross section scale at a fixed wavelength (118.2 nm, σ_i^CH₃ = 6.7_−1.8^+2.4 Mb, 95% confidence interval) in an independent laboratory experiment using the same pyrolysis source, a vacuum ultraviolet (VUV) laser, and a carefully calibrated detection chain. The resulting absolute cross section curve is in good agreement with the recently published measurements by Taatjes et al.,(1) although with an improved signal-to-noise ratio. The absolute photoionization cross section of CH₃I at 118.2 nm has also been measured to be σ_i^CH₃I = (48.2 ± 7.9) Mb, in good agreement with previous electron impact measurements. Finally, the photoionization yield of the iodine atom in its ground state ²P_3/2 has been recorded using the synchrotron source and calibrated for the first time on an absolute cross section scale from our fixed 118.2 nm laser measurement, σ_i^I2P_3/2 = 74₋₂₃⁺³³ Mb (95% confidence interval). The ionization curve of atomic iodine is in good agreement, although with slight variations, with the earlier relative ionization yield measured by Berkowitz et al.(2) and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene.(3) It is demonstrated that, in the range of pyrolysis temperature used in this work, all the ionization cross sections are temperature-independent. Systematic care has been taken to include all uncertainty sources contributing to the final confidence intervals for the reported results.