H. K. Tseng

Limitations on the validity of the non-relativistic dipole approximation for photoelectron angular distributions☆

Abstract We present results which exhibit hazards in the use of the non-relativistic dipole approximation in the interpretation of experimental photoelectron angular distributions. Significant deviations from the non-relativistic dipole approximation occur for both low and high Z atoms. In light elements these effects are found in the keV range and even below. In heavy elements they are found even for energies very close to threshold. For total cross sections, in contrast to this angular distribution situation, the surviving integrated relativistic and multipole corrections tend to cancel, so that the non-relativistic dipole approximation holds to surprisingly high energies.

Electron Bremsstrahlung Energy Spectra, 1 keV–1000 MeV

With a partial-wave expansion method we found that the partial cross sections of bremsstrahlung differential in photon energy, from neutral atoms, are a smoothly varying function of the orbital angular momentum quantum number l of the electron. A modified partial-wave method is therefore possible, which directly calculates a finite set of l-values on a grid in l whose spacing increases with l, and then interpolates the intermediate terms. Electron bremsstrahlung energy spectra from neutral atoms with atomic number Z=13 and 92 are presented for incident electrons of kinetic energy T1 from 1 keV to 1000 MeV. This uses numerical data obtained with the modified partial wave method for T1=5 and 10 MeV, our previous tabulated data for T1≤2 MeV, results obtained from Born approximation modified by form-factor screening and the Elwert-factor and results obtained from high energy theory.

Electron Bre msstrahlung Angular Distributions in the keV Energy Range

Simpler theories for the shapes of bremsstrahlung cross sections differential in photon energy and angle are examined using numerical data obtained with the partialwave method for incident electrons of kinetic energy from 1 keV to 500 keV, under the assumption the process is described as a single electron transition in an atomic central potential. For low-Z elements the Born approximation or the Elwert-Haug approach, modified with form-factor screening, give good predictions for the shape of the angular distributions except at forward and backward angles, i.e., the results are good in the main region which contributed to the energy spectrum. While for high-Z elements, such predictions are accurate within about 15%. Analytic properties of the shape of the angular distribution from simpler theories suggest a simple way to paramaterize the results.