Charge Transfer Reactions
Konrad Dennerl
Charge transfer, or charge exchange, describes a process in which an ion takes one or more electrons from another atom. Investigations of this fundamental process have accompanied atomic physics from its very beginning, and have been extended to astrophysical scenarios already many decades ago. Yet one important aspect of this process, i.e. its high efficiency in generating X-rays, was only revealed in 1996, when comets were discovered as a new class of X-ray sources. This finding has opened up an entirely new field of X-ray studies, with great impact due to the richness of the underlying atomic physics, as the X-rays are not generated by hot electrons, but by ions picking up electrons from cold gas. While comets still represent the best astrophysical laboratory for investigating the physics of charge transfer, various studies have already spotted a variety of other astrophysical locations, within and beyond our solar system, where X-rays may be generated by this process. They range from planetary atmospheres, the heliosphere, the interstellar medium and stars to galaxies and clusters of galaxies, where charge transfer may even be observationally linked to dark matter. This review attempts to put the various aspects of the study of charge transfer reactions into a broader historical context, with special emphasis on X-ray astrophysics, where the discovery of cometary X-ray emission may have stimulated a novel look at our universe.
1 Introduction
Charge transfer is a fundamental process in atomic physics which has been studied in various
contexts for a long time. During the last decade, it has received renewed attention, when
the discovery of cometary X-ray emission revealed that this process can be an important
source of X-ray emission, which was overlooked before. Since this discovery (announced on
April 4, 1996; Lisse et al. 1996a) and its correct interpretation as the direct result of charge
transfer between highly charged heavy ions in the solar wind and cometary neutrals (Cravens
1997), investigations on charge transfer reactions have expanded considerably, comprising
experimental studies in the laboratory and theoretical calculations of the underlying atomic
physics as well as astrophysical studies, which are not restricted to comets anymore but
cover a wide range, from planetary atmospheres to clusters of galaxies. The fact that X-rays
are not the result of excitation by hot electrons, but by ions picking up electrons from cold
gas has opened up an entirely new field of X-ray studies, which is likely to continue to grow.
In view of this exciting development it may be now, 14 years after the discovery of
cometary X-ray emission, a good time for a broad review of the history and current
status of charge transfer studies and their various implications. Specific aspects of this topic
have already been addressed in earlier reviews: Krasnopolsky et al. (2004) and Lisse et al.
(2004) reviewed charge transfer reactions in the context of the X-ray and EUV emission
from comets. Krasnopolsky et al. (2004) discussed also alternative excitation processes and
presented detailed discussions of theoretical modeling methods, spectra, and an extensive
list of relevant laboratory measurements of charge transfer reactions. Bhardwaj et al. (2007)
reviewed the various processes which give rise to X-rays from solar system objects,
including charge transfer reactions. Wargelin et al. (2008) presented a brief review of the
astrophysical relevance of charge transfer reactions in the solar system and beyond, and focused
on charge exchange spectra, experimental approaches, and theoretical modeling efforts.
This review attempts to provide a concise overview on the various aspects of charge
transfer reactions in a historical context, with specific emphasis on X-ray astrophysics. It
is outlined in the following way: In Sect. 2, a short overview about the physics of charge
transfer and its investigations by theoretical studies, laboratory experiments, and
astrophysical observations, will be given. Section 3 will cover the history of charge transfer studies,
from its very beginnings to the present, and the next two sections will summarize our current
knowledge about the astrophysical environments where charge transfer reactions are known
or expected to take place, both in the solar system (Sect. 4) and beyond (Sect. 5). The paper
will close with conclusions and a short outlook in Sect. 6.
An alternative name for charge transfer is charge exchange. Both notations will be
adopted in this paper, with some preference for the designation which appears to be more
common in the corresponding context. For brevity, the abbreviation CXE will be used for
Charge Exchange (or Charge Transfer) induced X-ray Emission.
2 Physics of Charge Transfer
In its basic principle, charge transfer is a very simple process: charge (in the form of one
or more electrons) is transferred from an atom or molecule to an ion that gets into an
e (...truncated)