On the low-frequency approximation for scattering of an electron in a laser field: An improved approximation
Potential and electron-atom scattering in the presence of an intense low-frequency laser field is considered. Forms of the T matrices convenient for low-frequency treatment are developed. Unambiguous answers are obtained on convergence and on the upper bounds of the common, on-shell low-frequency results in terms of the expansion parameters, which are dependent either on the laser field intensity or on the frequency. The compact form of the T matrix, which includes corrections in the laser electric field amplitude to the low-frequency approximation, is derived. It has the same form for both potential and electron-atom scattering and is applicable up to laser intensities of 1014W cm-2 in the low-frequency range.