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

Western Michigan University, Kalamazoo MI

Investigators

Abstract

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).