 Such effects are generally negligible in the bulk, but may become important at the surface.The quadratic term in formula_13 can be instead safely neglected, since its contribution in a typical photoemission experiment is about one order of magnitude smaller than that of the first term .In first-order perturbation approach, the one-electron Hamiltonian can be split into two terms, an unperturbed Hamiltonian formula_20, plus an interaction Hamiltonian formula_21, which describes the effects of the electromagnetic field