Static-Electric-Field-Induced, High-Energy Plateau for Scattered X-Ray Photons in Laser-Assisted, X-Ray–Atom Scattering

Control of intense laser-atom processes with strong static fields

“…They found plateaulike structures in the differential cross section (DCS) as a function of the number of photons n exchanged with the laser field primarily for n < 0 (i.e., emitted photons), indicating scattered x-rays having lower energies. MiloSeviC and Starace [4] showed that the addition of a static electric field gives rise to an extended plateau for n > 0 (i.e., absorbed photons), indicating scattered x-rays having substantially higher energies close to the "water window" energy region. Fig.…”

“…Thus, in Ref. [4] we demonstrated how a strong static electric field may induce a high-energy plateau for scattered xray photons in laser-assisted, x-ray-atom scattering in which the incident x-rays were assumed t o have an energy of 50 eV. The scattered x-rays were shown to have energies up to well over 200 eV, making such a process an attractive one for realizing coherent x-rays in the "water window" [between the K shell absorption edges of C (284 eV) and 0 (532 eV)], which would have important applications to imaging living biological structures by means of x-ray holography [6].…”

“…Key goals of research in this area are to increase the intensities and frequencies of coherent light produced in these processes. In two recent works [4,5] we have demonstrated theoretically the possibility of controlling intense laser-atom interaction processes by employing strong, but experimentally feasible, static electric or magnetic fields. Thus, in Ref.…”

“…In this case, the third step can explain the appearance of the plateau in AT1 with its cutoff at IOUp [13]. The classical three-step model is consistent with results of quantum-mechanical calculations [14-201. Although we use the 3-step physical model to interpret our results, all of our results are obtained from quantum-mechanical calculations, as described in detail elsewhere [4,5,. In brief, we solve the three-dimensional, time-dependent Schrodinger equation for an electron moving in the laser plus static field(s).…”

“…The initial atomic or ionic state is assumed to be unaffected by the fields. A key approximation in both our own work [4,5, and that of others [14-201 is the so-called strong-field approximation (SFA), in which the Green's propagator of the total system is replaced in intermediate states by the Volkov Green's propagator, i.e., the influence of the atomic potential on the electron is neglected in comparison with that of the laser field (and the static external fields if present). The SFA fails [21] if the number of photons exchanged with the laser field is small: for AT1 this corresponds to photoelectrons with energies close to the threshold, while for HHG it corresponds to low-order harmonics.…”