Helium and neon photoelectron satellites at threshold

Photoionization of helium and neon to excited satellite states, ${\mathrm{He}}^{+}$ nl and ${\mathrm{Ne}}^{+}$ 1$s^2$2$s^2$2$p^4$nl, was studied with synchrotron radiation and threshold electron analysis. Photoelectron satellites have been directly measured at threshold for the first time to our knowledge. The relative satellite cross sections were determined over the kinetic energy range from 0 to 1 eV. The angular distributions were also evaluated close to threshold. Strong correlation effects were observed in two cases. For He near threshold, the angular-distribution asymmetry parameter β is near zero for the n=2 satellite and is increasingly negative for the higher-n satellites, in agreement with the theoretical prediction of Greene. In the threshold photoelectron spectrum of Ne, many final states are present, some with quartet spin multiplicity and others with high-L values.