Electron Emission from Quasimolecules

Up to now we have mainly considered the autoionization of atoms which can be regarded as more or less isolated particles. Only for the case of a charged collision partner at low velocities was the influence of the charge on the final state potential curve taken into account in the description of postcollision interaction. A first step in describing the autoionization process as starting from quasimolecular levels is the treatment by Stolterfoht 

If an ion collides with its parent atom A in such a way that an autoionizing atom A is formed, there are two possible ways for a transition to the final state A + A + c, namely, (i) excitation of the target atom and (ii) charge exchange into an excited state of the projectile atom. For He -He collisions this can be written as 

Boskamp et have analyzed such interferences experimentally 

This demonstrates that the collision system has to be described as a quasimolecule even for very large intemuclear distances. Although there is no longer any overlap between the electron wave functions of target and projectile, the coherence of the two emission amplitudes is conserved and is reflected in the interference structures of the angular distribution. 

Electron emission from collision complexes at small intemuclear distances often yields complicated spectra for the following reasons (i) The energy difference between excited and ionized state varies with the inter-nuclear distance and therefore no sharp lines can be expected, (ii) It is not clear how large a Doppler effect will be suffered by the electrons since it 

---

Two other groups of processes in which electron emission from quasimolecules takes place are Penning ionization and transfer ionization processes. However, they are different from those discussed here in that the electronic energy of the collision system in the initial state is high enough for an ionization process to occur. This leads to ionization even at large impact parameters and very low kinetic collision energies. For a more detailed discussion the reader is referred to recent review articles. ... 

Table I shows the possible processes (1) to (6) which can result from conversion of translational into electronic energy. The excited states A or B can either be metastable or can decay by electron or photon emission. A or B can also denote molecules. Common to processes (1) to (3) is the transition from the ground state to some excited states of the quasimolecule AB formed during the collision. When the particles separate again, there are several possibilities ...

---