COMMENT ON X-RAY-ABSORPTION BY ATOMS UNDER INTENSE LASER FIELDS

In connection with a paper by Kalman [Phys. Rev. A 39, 2428 (1989)], w e present an independent analysis of the behavior of the matrix elemen t of r and p, relevant for the ground-state photoelectric cross sectio n in hydrogen, evaluated in the Born approximation. This analysis, per formed in momentum space, proves once again that, in the plane-wave ap proximation, the result obtained in momentum gauge is the correct one in the lowest order in alpha Z (alpha being the fine-structure constan t, Z the atomic number).