Momentum-transfer dependence of Fano parameters for the(1s2)1S → (2s2p)1P doubly excited transition in helium

Abstract

We report the first detailed characterization of absolute generalizedoscillator strength (GOS) of the (1s2)1S→(2s2p)1P doublyexcited transition in helium as a function of momentum transfer (K) byangle-resolved electron energy loss spectroscopy at 2.5 keV impactenergy. The GOS profile is found to be dominated by a strong maximum atK = 0, characteristic of dipole-dominated interactions. Extrapolating ourGOS profile to K = 0 using a semi-empirical power series (Lassettreseries) gives an absolute OS of 0.0052, in good accord with theoreticaland experimental optical values reported in the literature. Changes in theFano parameters for the doubly excited transition are determined for thefirst time as a function of momentum transfer. The Fano profile index(q) increases rapidly from -2.75 in the K = 0 limit (observed earlieroptically) to -1.5 at K2 ~ 1.5 au and levels off to -1.0 at K2 ~ 4 au, while the Fano correlation coefficient (ρ2) decreasesslowly from 0.04 at K ~ 0 to 0.03 at K2 ~ 4 au. Thesevariations reflect the intricate nature of the transition matrix elementsinvolved in the interference between direct ionization and autoionization.Finally, we also obtain clear evidence for the presence of a non-dipoleautoionizing resonance near 58.1 eV in He in the higher-K region,confirming an earlier theoretical prediction.