  We introduce a semi-classical wavefunction (SCWF) model for strong-field
physics and attosecond science. When applied to high harmonic generation (HHG),
this formalism allows one to show that the natural time-domain separation of
the contribution of ionization, propagation and recollisions to the HHG process
leads to a frequency-domain factorization of the harmonic yield into these same
contributions, for any choice of atomic or molecular potential. We first derive
the factorization from the natural expression of the dipole signal in the
temporal domain by using a reference system, as in the quantitative
rescattering (QRS) formalism [J. Phys. B. 43, 122001 (2010)]. Alternatively, we
show how the trajectory component of the SCWF can be used to express the
factorization, which also allows one to attribute individual contributions to
the spectrum to the underlying trajectories.
