  Attosecond ionization time-delays at photoelectron energies above typically
10 eV are usually interpreted using the so called asymptotic approximation as a
sum of the atomic or molecular delays with a universal laser-induced
contribution. Here, we employ a two-harmonic RABITT (Reconstruction of
Attosecond Beating by Interference of Two-photon Transitions) configuration to
isolate the multiphoton pathways and measure the ionization time delays as a
function of the dressing field intensity. We show that the validity of the
asymptotic theory can be extended to the threshold or to higher-order
contributions by rigorously treating the angular-momentum dependence of the
continuum-continuum transitions into universal and easily computable
partial-wave-specific correction factors. Our asymptotic treatment is also
valid for higher-order interfering amplitudes while significantly simplifying
their evaluation and providing a transparent physical interpretation. The
validity of the method for atomic and molecular targets in the vicinity of
resonances, ionization thresholds, and for both the emission-integrated and
angularly resolved signal is confirmed by comparison to ab initio calculations
over a wide energy range.
