Threshold fragmentation of atomic and molecular systems by charged particle impact

The Wannier theory –originally derived for the ionisation of atoms by electrons —is extended to general fragmentation reactions which produce three charged particles of arbitrary masses and charges. The center of mass fragmentation cross section and the angular correlation are shown to be given in terms of two energy-independent, characteristic indices as in Wannier's case. These indices vary drastically as a function of masses and charges of the fragments. It is also shown that these indices may have singularities at critical however physical values of charges and masses. In such cases a generalized Wannier threshold law with a suitable, fractional exponent does not exist. The theory developed in this article applies to energies up to several eV above the fragmentation threshold.