  Non-collinear four-wave mixing (FWM) techniques at near-infrared (NIR),
visible, and ultraviolet frequencies have been widely used to map vibrational
and electronic couplings, typically in complex molecules. However, correlations
between spatially localized inner-valence transitions among different sites of
a molecule in the extreme ultraviolet (XUV) spectral range have not been
observed yet. As an experimental step towards this goal we perform
time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses.
The first two pulses (XUV-NIR) coincide in time and act as coherent excitation
fields, while the third pulse (NIR) acts as a probe. As a first application we
show how coupling dynamics between odd- and even-parity inner-valence excited
states of neon can be revealed using a two-dimensional spectral representation.
Experimentally obtained results are found to be in good agreement with ab
initio time-dependent R-matrix calculations providing the full description of
multi-electron interactions, as well as few-level model simulations. Future
applications of this method also include site-specific probing of electronic
processes in molecules.
