
 Dipole Approximation 
  Within the dipole approximation, the differential cross section (cross section per unit solid angle) for angleresolved photoemission with linearly polarized x rays is described by three quantities: the partial cross section, s(hn), the angulardistribution parameter, b(hn), and the angle (q) of the photoelectron trajectory relative to the polarization vector. 
  In the dipole approximation, a single term describes electron angular distributions as a function of the angle q relative to the polarization, E, of the radiation. 
  The complex angular dependence of the differential cross section means that which corrections to the dipole approximation are needed depends on the experimental geometry, but the new results demonstrate that researchers need to be ready to include nondipole effects through at least the second order in analyzing their results. 
 wwwals.lbl.gov /als/science/sci_archive/dipole_approx.html (976 words) 
