Absolute generalized-oscillator-strength measurement of preionization-edge electronic excitations in the valence and 2p shells of argon

Absolute generalized oscillator strengths (GOS’s) of the 3p⁶→3p⁵(4s,4s^′), 3p⁶→3p⁵(4p,4p^′), and 3p⁶→3p⁵(5s,5s^′,3d,3d^′) valence-shell electronic transitions as well as the 2p_3/2→4s inner-shell transition in Ar have been determined as functions of energy loss and momentum transfer (K) at an impact energy of 2.5 keV. Generally good agreement can be found between the present GOS measurement for the 3p⁶→3p⁵(4s,4s^′) transitions (at 11.8 eV) and the previous experimental profiles for K<1a.u. Calculations based on the first Born approximation appear to be in reasonable agreement with the GOS data only in shape and also for K<1a.u., while the Yukawa frozen-core calculations based on the Glauber approximation appear to converge to the present data even for K>1a.u. The experimental and theoretical results all give consistent values for the K positions of the observed minimum and maximum in the GOS profile of the 3p⁶→3p⁵(4s,4s^′) transitions. The GOS profile for the 3p⁶→3p⁵(4p,4p^′) transitions (at 13.4 eV) in Ar has been determined and found to have a shape characteristic of nondipole excitation with a maximum at K²∼0.7a.u. Like the 3p⁶→3p⁵(4s,4s^′) transitions, the GOS profile for the 3p⁶→3p⁵(5s,5s^′,3d,3d^′) transitions (at 14.2 eV) has the characteristic dipole-dominated shape with a strong maximum at K=0 but a notably larger width and without any discernible extrema. Finally, the GOS profile for the 2p_3/2→4s inner-shell transition (at 244.4 eV) is dominated by a strong maximum at K=0, characteristic of generally dipole-allowed excitations, but with a broad and rather complex shape. The present work illustrates the analytical value of GOS profiles for probing the intricate nature of the valence- and inner-shell electronic excitations.