When an atom with ionisation potential E0 is irradiated by a strong electromagnetic field of frequency w, it ionises. Energy balance requires that at least (n+1) photons have been absorbed (n is defined by nhw < E0 < (n+l)hw). The energy spectrum of emitted electrons consists of several peaks, evenly spaced. Peak number k corresponds to electrons of energy $${{\text{E}}_{\text{k}}}\left( {{\text{n + k}}} \right){\text{hw - }}{{\text{E}}_{\text{0}}}$$ (1) ejected when the atom has absorbed n+k photons. The latter description is consistent with a perturbative interpretation of multiphoton absorption. In such a frame, the height of k-th peak, NK, is predicted to vary as the (n+k)-th power of light intensity I. A perturbative description is justified as long as (n+k)-photon process is more efficient than (n+k+1)-photon process,and thus the heights of peaks are expected to decrease when k increases. These features have been observed for moderate in-tensities such as the ones required to ionise alkali atoms [,2,33].