* Astronomy

How do free electrons originate?

Scientists at Max Planck Institute of Plasma Physics (IPP) in Garching and Greifswald and Fritz Haber Institute in Berlin have discovered a new way in which high-energy radiation in water can release slow electrons. Their results have now been published in the renowned journal, "Nature Physics". Free electrons play a major role in chemical processes. In particular, they might be responsible for causing radiation damage in organic tissue.
When ionising radiation impinges on matter, large quantities of slow electrons are released. It was previously assumed that these electrons are ejected by the high-energy radiation from the electron sheath of the particle hit - say, a water molecule. In their experiment the Berlin scientists bombarded water clusters in the form of tiny ice pellets with soft X-radiation from the BESSY storage ring for synchrotron radiation. As expected, they detected the slow electrons already known. In addition, however, they discovered a new process: Two adjacent water molecules work together and thus enhance the yield of slow electrons.
First the energy of the X-radiation is absorbed in the material: A water molecule is then ionised and releases an electron. But this electron does not absorb all of the energy of the impinging X-ray photon. A residue remains stored in the ion left behind and causes another electron to be released just very few femtoseconds later. (A femtosecond is a millionth of a billionth of a second. For example, the electrons in a chemical process take a few femtoseconds to get re-arranged.) This process is known as autoionisation, i. e. the molecule ionises itself.
The Max Planck scientists have now discovered that two adjacent water molecules can work together in such an autoionisation process. Working in conjunction, they achieve a state that is more favourable energy-wise when each of them releases an electron. What happens is that the molecular ion produced first transfers its excess energy to a second molecule, which then releases an electron of its own. This energy transfer even functions through empty space, no chemical bonding of the two molecules being necessary.

Source Max-Planck-Institut fuer Plasmaphysik