We discuss how a recent pump-probe study [Kelkensberg et al., Phys. Rev. Lett.
103, 123005 (2009)] of the dissociative ionization of H2, under the combined
effect of a single extreme ultraviolet attosecond pulse and an intense near-
infrared pulse, actually represents a transition-state spectroscopy of the
strong-field dissociation step, i.e., of the (probe-pulse-)dressed H2+
molecular ion. The way the dissociation dynamics is influenced by the duration
of the near-infrared probe pulse, and by the time delay between the two
pulses, is discussed in terms of adiabatic versus nonadiabatic preparation and
transport of time-parametrized Floquet resonances associated with the
dissociating molecular ion. Under a long probe pulse, the field-free
vibrational states of the initial wave packet are transported, in a one-to-one
manner, onto the Floquet resonances defined by the field intensity of the
probe pulse and propagated adiabatically under the pulse. As the probe pulse
duration shortens, nonadiabatic transitions between the Floquet resonances
become important and manifest themselves in two respects: first, as a
vibrational shake-up effect occurring near the peak of the short pulse, and
second, through strong interference patterns in the fragment's kinetic energy
spectrum, viewed as a function of the time delay between the pump and the
probe pulses
