Measurements of absolute cross sections from excited states via double resonance ion-imaging mass spectrometry: the m p 1 / 2 5 n ( p ′ / f ′ ) autoionizing Rydberg states of argon

Abstract We demonstrate measurements of absolute cross sections from excited states via a double resonance scheme in an ion-imaging mass spectrometer. The method is applied to determine photoexcitation (PE) cross sections of the transition from metastable argon (Ar*) in the 3 p 3 / 2 5 4 s [ 3 / 2 ] 2 state to the 3 p 1 / 2 5 9 f ′ or 11 p ′ Autoionizing Rydberg States (ARS) of Ar, whose lineshapes and cross sections are sensitive probes for high-level electronic structure theory. The Ar* atoms are generated via electron impact excitation in a pulsed supersonic molecular beam, and a subsequent in situ one-photon transition excites the Ar* atoms to the 9 f ′ and 11 p ′ ARS resonances that rapidly evolve into Ar + ions. The lineshapes of the PE spectra of the two ARS of Ar are in excellent agreement with published experimental spectra and high-level ab initio calculations. However, the measured absolute cross sections are systematically larger than the theoretical predictions by factors of 2–3, providing new benchmarks to constrain many-electron correlations, core polarization potential, and s-d mixing interactions in electronic structure theory.