Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom.
J Chem Phys. 2011 Jan 14;134(2):024308. doi: 10.1063/1.3519636.
Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2).
飞行时间质谱和二维符合技术已被用于首次确定电子-甲烷碰撞后相对的前体特异性部分电离截面。前体特异性部分电离截面量化了单个、双个和更高水平的电离对形成特定离子（例如 CH(+)）的部分电离截面的贡献，这是在甲烷电子电离后发生的。在 30 至 200 eV 的电离电子能量下，我们呈现了形成 H(+)、H(2)(+)、C(+)、CH(+)、CH(2)(+)和 CH(3)(+)的相对截面，相对于 CH(4)(+)。我们还可以将我们的数据集中的内容减少，以得出甲烷电子电离的相对部分电离截面，以便与早期的测量结果进行比较。这些相对部分电离截面与最近的测定结果非常吻合。然而，我们发现我们的部分电离截面与早期研究中得出的截面之间存在显著差异。检查我们的前体特异性部分电离截面的值表明，这种差异是由于早期研究中能量较高的碎片离子的收集效率较低造成的。我们的符合实验还表明，55 eV 时电子双电离 CH(4)产生的 CH(4)(2+)的低能电子态（基态（3)T(1)、第一激发态（1)E(1)）与 40.8 eV 光致电离产生的相同。（3)T(1)态解离形成 CH(3)(+) + H(+) 和 CH(2)(+) + H(2)(+)，（1)E(1)态解离形成 CH(2)(+) + H(+)和 CH(+) + H(+)。在这个电子能量下，我们还观察到 CH(4)(2+)的第一激发三重态（(3)T(2)）的种群，该态会同时解离成 CH(2)(+) + H(+) + H 和 CH(+) + H(+) + H(2)。
