Radiative Double Electron Capture (RDEC) of Ions with Quasi-free Electrons

一幅流行的原子图像显示电子绕着原子核旋转，就像行星绕着太阳旋转一样。为了改进这个类比，我们必须了解原子中的电子是如何因负电荷而相互排斥的，以及电子是如何遵守量子力学定律的。这个项目研究电子之间的相互作用如何使它们的运动变得相关。这很重要，因为这种相关性对原子、分子和材料的物理和化学性质有很大的影响。为了研究电子的相关性，这个项目将观察两个电子从一个原子转移到一个离子并同时发射一个x射线光子的情况。当这种情况发生时，整个过程可以被认为是单光子双电离（去除两个电子）的时间逆。这与光电效应有关，最早是由阿尔伯特·爱因斯坦在1905年解释的。然而，由于电子相互作用的方式，双光电离的速率很难预测。当两个电子被转移到一个移动的抛射体上时，类似的物理现象也很重要。科学家们有兴趣了解这一过程，因为它发生在抛射离子和目标原子之间的碰撞中，以及单个光子对原子的双光子电离的时间逆转过程中。这两个过程都可能发生在天体物理等离子体中。双电子过程的概率计算得到了截然不同的结果，这给实验解决这些差异留下了空间。由于观察事件需要很长的测量时间，测量过程的尝试非常困难。博士生参与这项研究项目，使他们在最先进的物理实验方面得到了宝贵的培训，并在准备合作手稿和摘要方面获得了经验，并使他们能够参加会议报告结果，并帮助他们发展成为富有成效的年轻科学家。两个电子向离子的转移伴随着一个光子的发射被称为辐射双电子捕获，简称RDEC。这一建议的智力价值在于它在离子与准自由电子碰撞领域的内在重要性，以及它与高电荷离子的光子相互作用的密切关系。这种关系的产生是因为RDEC是双光离的时间逆。当入射全剥离离子发生RDEC和双电子系统发生双光离时，这些过程本质上是相同的，在这种情况下，两个电子之间的相互作用，即相关，起着基本的作用。RDEC类似于通常观察到的辐射电子捕获（REC）的单电子过程，与光电效应相反，但不需要电子相关。RDEC这个项目对氦、氮和氖气体靶的新研究将解决先前发表的涉及氧和氟离子入射在薄c箔靶上的离子-固体相互作用的数据的复杂性。气体目标不像固体目标那样存在多重碰撞效应。RDEC可以与几个已发表的理论计算进行比较，然而，它们相差几个数量级。西密歇根大学的6 mv串联范德格拉夫加速器正在对完全剥离的氟离子进行测量（已经获得了氮和氖目标的一些初步数据）。发射的x射线与单荷和双荷射弹之间的一致性被记录下来，后者代表RDEC的特征。由于RDEC截面相对较小（小于1个仓），测量非常耗时，并且需要对每个目标连续计数一个月或更长时间。这些研究是由PI、联合PI和两名研究生进行的，还包括来自圣母大学和波兰克拉科夫雅盖隆大学的合作者。

A popular image of atoms shows electrons circling the nucleus like planets orbiting the sun. To improve upon this analogy, one must understand how electrons in atoms repel each other due to their negative charges, and how electrons obey the laws of quantum mechanics. This project studies how interactions between electrons causes their motion to become correlated. This is important because such correlations have a big impact on the physical and chemical properties of atoms, molecules, and materials. To investigate electron correlations, this project will observe situations in which two electrons are transferred from one atom to an ion and an x-ray photon is simultaneously emitted. When this happens, the entire process can be thought of as the time inverse of double ionization (removal of two electrons) by a single photon. This is related to the photoelectric effect, which was originally explained by Albert Einstein in 1905. However, the rate for double photoionization is difficult to predict because of the way electrons interact with one another. Similar physics is important when two electrons are transferred to a moving projectile. Scientists are interested in understanding this process as it occurs in collisions between projectile ions and target atoms and from the time-reversed process of double photoionization of an atom by a single photon. Both processes are likely to occur in astrophysical plasmas. Probabilities for the two-electron process have been calculated with widely differing results, leaving the field open for experiments to resolve the differences. Attempts to measure the process have been very difficult due to the long measuring times required to observe the events. The involvement of Ph.D. students in this research project gives them valuable training in a state-of-the-art physics experiment and experience in preparing collaborative manuscripts and abstracts and allows them to attend conferences to report the results and assists them as they develop into productive young scientists.The transfer of two electrons to an ion accompanied by the emission of a single photon is called radiative-double-electron capture, or RDEC for short. The intellectual merit of this proposal lies in its intrinsic importance in the field of ion collisions with quasi-free electrons, and its close relationship to photon interactions with highly-charged ions. This relationship comes about because RDEC is the time inverse of double photoionization. These processes are essentially identical when RDEC occurs for incident fully-stripped ions and double photoionization for two-electron systems, in which case the interaction between the two electrons, i.e., correlation, plays a fundamental role. RDEC is similar to the commonly observed one-electron process of radiative-electron capture (REC), the inverse of the photoelectric effect, which, however, does not require electron correlation. New studies in this project of RDEC for gas targets of helium, nitrogen and neon will address complications of previously published data involving ion-solid interactions for oxygen and fluorine ions incident on a thin C-foil target. Multiple collision effects are not present for gas targets as they are for solid targets. RDEC can be compared with several published theoretical calculations, which, however, differ by several orders of magnitude. The measurements are being done (Some preliminary data have already been obtained for nitrogen and neon targets.) for fully-stripped fluorine ions using the 6-MV tandem Van de Graaff accelerator at Western Michigan University. Coincidences between x rays emitted and singly- and doubly-charged projectiles are recorded, with the latter representing a signature for RDEC. The measurements are very time-consuming due to the relatively small RDEC cross sections (less than ~1 barn) and require continuous counting times of a month or more for each target. The studies are carried out by the PI, the co-PI, and two graduate students and also include collaborators from the University of Notre Dame and Jagiellonian University in Krakow (Poland).