Watanabe Noboru, Takahashi Masahiko
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.
J Chem Phys. 2020 Apr 30;152(16):164301. doi: 10.1063/5.0006256.
We experimentally investigate the molecular-orientation dependence of high-energy electron-impact ionization of CO. The direction of the molecular-axis with respect to the momentum transfer vector K is deduced from the angular correlation between the fragment ion and the scattered electron. The experimental results on the 3 Π ionization reveal that at small momentum transfer, the ionization probability near the threshold is higher when K points toward the C atom along the molecular axis than when it is in the opposite direction. Such a forward-backward asymmetry does not appear in single-photon ionization and requires non-dipole contributions. It is also shown that the {4 Σ + 5 Σ + 6 Σ} ionization preferentially takes place in the vicinity of the molecular orientation parallel to K at small momentum transfer, while non-dipole contributions cause the decrease in the relative intensity of the parallel direction.
我们通过实验研究了一氧化碳（CO）的高能电子碰撞电离对分子取向的依赖性。根据碎片离子与散射电子之间的角关联，推导出分子轴相对于动量转移矢量K的方向。关于3Π电离的实验结果表明，在小动量转移情况下，当K沿分子轴指向C原子时，阈值附近的电离概率高于其相反方向时的电离概率。这种前后不对称性在单光子电离中不会出现，并且需要非偶极贡献。研究还表明，在小动量转移时，{4Σ + 5Σ + 6Σ}电离优先发生在与K平行的分子取向附近，而非偶极贡献导致平行方向相对强度降低。
