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Research Activities

Collisions Involving Charged Particles

Dr B Peart
 
We are presently concerned with a variety of collision processes involving charged particles, i.e. electron-ion and ion-ion interactions. Bearing in mind that more than 99% of the universe consists of charged particles, it can be appreciated that these measurements find wide application. They are most relevant to astrophysics, thermonuclear fusion, plasma chemistry and fundamental aspects of atomic and molecular structure. Many new techniques and results have originated in this laboratory.

Electron impact ionization processes control the charge state of thermonuclear plasmas and stellar atmospheres and hence they determine principal properties of the outer layers of the sun and stars. We have established that autoionization (which involves inner-shell electrons) can sometimes dominate outer shell ionization to a very surprising degree. For example, one of the inner shell (3p) electrons in Ca+ can be excited (3p --> nl) so that it decays, not by emitting radiation, but by ejecting a more weakly bound outer shell (4s) electron.

Recent research has shown that there are several other indirect ionization processes and a new method has been developed to study autoionization of ions. This is producing vastly more detailed results than previously obtained. Recent work on mesuring the electron impact ionization of Ca(+) has been highlighted by SERC as being of "outstanding merit".

Ion-ion reactions of the type, 


              A(+)  +  B(-)  ---->  A  +  B
are important in many environments, but particularly in the ionised layers of the upper atmosphere. These reactions remove free charges from the atmosphere and radically alter its properties. A "merged-beam" apparatus is being developed to study a variety of such mutual neutralization processes at very low interation energies.

We have also measured the cross section for photoionisation of atomic ions. These measurements are unique; no other absolute cross sections for positive atomic ions exist. The greatest interest, however, derives from the wealth of detail that the measurements revealed about the inner shell structure of ions and the way that inner shells can profoundly influence ionisation. These were elaborate experiments that required intense VUV radiation from the Daresbury synchrotron.

Two major reviews summarise most of our recent work: