where λ and I are the wavelength and the intensity of the driving field, respectively. The electron recombines with its source (see Fig. ) at a time t rec , when z(t rec ) = 0. The energy of the emitted photon created by the recombination event is hν = E KE (t rec ) + IE (5) where "IE" is the molecule's ionization energy. The maximum photon energy (or cutoff energy, E cutoff ) is determined by the maximum kinetic energy in Eq. ( ) under the constraint that z(t rec ) = 0 in Eq. ( ). The result is a cutoff law 2,46,52,53