Attosecond Transient Absorption Spectroscopy of doubly-excited states in helium

TL;DR

This paper presents ab-initio calculations that confirm and extend recent experimental observations of ultrafast phenomena in helium's autoionizing states, revealing periodic Fano profile inversion and the appearance of a dark state.

Contribution

The study provides the first accurate theoretical analysis of attosecond transient absorption in helium, predicting new effects like periodic Fano inversion and detection of a dark state.

Findings

Agreement with experimental data on ultrafast resonant phenomena

Prediction of periodic Fano profile inversion with laser intensity

Identification of the dark 2p^2 state in the spectrum

Abstract

Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum of helium dressed by a few-cycle visible pulse [Ott et al., arXiv:1205.0519[physics.atom-ph] ] provide evidence of novel ultrafast resonant phenomena, namely, two-photon Rabi oscillations between doubly-excited states and the inversion of Fano profiles. Here we present the results of accurate \emph{ab-initio} calculations that agree with these observations and in addition predict that (i) inversion of Fano profiles is actually periodic in the coupling laser intensity and (ii) the supposedly dark $2p^2$ {$^1$S} state also appears in the spectrum. Closer inspection of the experimental data confirms the latter prediction.