On the validity of the sum rule for multiphoton free-free transitions
Rosalba Daniele, Francesca Morales, Fabio Trombetta · Journal of Physics B Atomic and Molecular Physics · 1986
The authors report numerical calculations showing that the well known sum rule for multiphoton-free-free transitions does not hold, contrary to expectations, at small scattering angles. Compared with the field-free results, significant enhancement occurs in both the differential and total cross sections when scattering at small angles is important, especially in the case of strong particle-field coupling and relatively high incident energy (a few hundred eV). A derivation is also reported showing explicitly the new restrictions to the range of validity of the sum rule.