Momentum-transfer-dependence behavior of the 3s autoionization resonances of argon studied by high-resolution fast electron scattering

Abstract

The asymmetric Fano profile, which is caused by the two-channel interference between discrete states and relevant continuum states, presents a great challenge to atomic and molecular physics. This study investigates the inner-valence excitations of argon $3s$ electrons in the energy range 24.5--29.5 eV using an electron energy-loss spectrometer. The absolute generalized oscillator strength densities of the $3s$ excitations and the Fano profile parameters (i.e., $q$, ${f}_{a}$, $\ensuremath{\rho}{}^{2}$, $f$, $S$) in $0<{K}^{2}<2.2$ a.u. for the $3s3{p}^{6}ns$ ($n=4$ and 5), $3s3{p}^{6}np$ ($n=4$ and 5), and $3s3{p}^{6}3d$ autoionization states are determined. For the same Rydberg series, similar momentum-transfer-dependence behaviors are observed for the Fano profile parameters $q$, ${f}_{a}$, $\ensuremath{\rho}{}^{2}$, $f$, and $S$. The magnitudes of $q$ and ${f}_{a}$ are found to be nearly equal for the same Rydberg series, while the corresponding $f$ and $S$ parameters decrease as the principal quantum number increases, confirming the theoretical predictions of Fano and Cooper [U. Fano and J. W. Cooper, Phys. Rev. 137, A1364 (1965)]. Moreover, the typical dipole-allowed or dipole-forbidden ${K}^{2}$-dependence behaviors of the Fano profile parameters are used to elucidate the properties of electric transition multipolarities. The parameters of the Fano profile derived in a large ${K}^{2}$ region offer an opportunity to understand the wave functions far away from the nucleus.