Extracting generalized multiphoton ionization cross-sections from nonperturbative time-dependent calculations: An application in positroniumL. B. Madsen, L. A.A. Nikolopoulos and P. LambropoulosEur. Phys. J. D, 10 1 (2000) 67-79DOI: https://doi.org/10.1007/s100530050527