Laboratoire de Photophysique Moléculaire, CNRS UPR 3361, Univ. Paris-Sud 11, F-91405 Orsay Cédex, France.
J Phys Chem A. 2010 Mar 11;114(9):3237-46. doi: 10.1021/jp909414d.
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, sigma(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., 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 sigma(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, sigma(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. and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene. 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.
一种热解源与超音速膨胀相结合，已被用于从两种前体，碘甲烷 CH3I 和硝基甲烷 CH3NO2 中产生 CH3 自由基。在 SOLEIL 同步辐射源中，在 9.0-11.6 eV 的范围内记录了 CH3 的相对电离产率，并通过考虑自由基在分子束中的振动和旋转温度来模拟其电离阈值。相对光致电离产率已在独立实验室实验中归一化为固定波长（118.2nm，sigma(i)(CH3)=6.7(-1.8)(+2.4)Mb，95%置信区间）的绝对截面标度，该实验使用相同的热解源、真空紫外（VUV）激光和经过精心校准的检测链。得到的绝对截面曲线与 Taatjes 等人最近发表的测量结果非常吻合，尽管信噪比有所提高。在 118.2nm 处 CH3I 的绝对光致电离截面也已测量到为 sigma(i)(CH3I)=(48.2 +/- 7.9)Mb，与以前的电子碰撞测量结果吻合良好。最后，使用同步辐射源记录了基态 2P(3/2)的碘原子的光致电离产率，并首次在我们固定的 118.2nm 激光测量的绝对截面标度上进行了校准，sigma(i)(I2P(3/2))=74(-23)(+33)Mb（95%置信区间）。原子碘的电离曲线与 Berkowitz 等人早期测量的相对电离产率非常吻合，尽管略有变化，并且与 Robicheaux 和 Greene 早期计算的碘截面也进行了比较。结果表明，在所使用的热解温度范围内，所有的电离截面都是与温度无关的。在报告结果的最终置信区间中，已经系统地考虑了所有对不确定度来源的贡献。
