电荷重排增强X射线电离对分子尺寸敏感性的理论证据
Theoretical evidence for the sensitivity of charge-rearrangement-enhanced x-ray ionization to molecular size
摘要：
最近发现，与孤立原子的电离相比，高X射线强度下的分子电离通过称为CREXIM（分子的电荷重排增强X射线电离）的现象而增强。X射线吸收选择性地电离分子内的重原子，触发电子从相邻原子转移到重原子位点，并使那里的进一步电离成为可能。目前的理论研究表明，CREXIM效应随着分子的大小而增加，作为增加的分子内电子转移的结果，从较大的分子成分连接到重原子。我们比较X射线多光子电离动力学氙，碘甲烷，碘苯后与强烈的X射线脉冲相互作用。虽然它们的光电离截面相似，碘甲烷和碘苯分子比氙原子更电离。此外，我们预测，碘苯的平均总电荷远大于碘甲烷，因为大量的电子在苯环上。从碘位转移到苯环的正电荷被重新分配，使得在远离碘位的末端形成更高的碳电荷。我们的第一性原理计算提供了基本的见解分子与X射线自由电子激光（XFEL）脉冲的相互作用。这些见解需要考虑到解释和设计未来的XFEL实验。
英文摘要：
It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangement-enhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.
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