  For ultrashort VUV pulses with a pulse length comparable to the orbital time
of the bound electrons they couple to we propose a simplified envelope
Hamiltonian. It is based on the Kramers-Henneberger representation in
connection with a Floquet expansion of the strong-field dynamics but keeps the
time dependence of the pulse envelope explicit. Thereby, the envelope
Hamiltonian captures the essence of the physics, -- light-induced shifts of
bound states, single-photon absorption, and non-adiabatic electronic
transitions. It delivers quantitatively accurate ionization dynamics and allows
for physical insight into the processes occurring. Its minimal requirements for
construction in terms of laser parameters make it ideally suited for a large
class of atomic and molecular problems.
