  Magnesium atoms embedded in superfluid helium nanodroplets have been
identified to arrange themselves in a metastable network, refered to as foam.
In order to investigate the ionization dynamics of this unique structure with
respect to a possible light-induced collapse the femtosecond dual-pulse
spectroscopy technique is applied. Around zero optical delay a strong feature
is obtained which represents a direct probe of the foam response. We found that
upon collapse, ionization is reduced. A particlar intensity ratio of the pulses
allows to address either direct ionization or photoactivation of the neutral
complexes, thus affecting reaction pathways. A simplified excitation scheme
visualizes possible scenarios in accordance with the experimental observations.
