Polarization Control in Two-Color Above-Threshold Ionization of Atomic Helium

Abstract

Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field. DOI: 10.1103/PhysRevLett.101.193002 PACS numbers: 32.80.Fb, 32.80.Rm Multiphoton single-color ionization in intense optical or infrared laser fields has been the subject of multiple experimental and theoretical studies for more than two decades and is by now a very well understood process (e.g., [1]). The extension of these studies to multiphoton absorption in the photoionization continuum was followed by the discovery that high order harmonics of the fundamental laser frequency are emitted in the extreme ultraviolet (XUV) when a strong femtosecond optical laser pulse interacts with a gas jet (e.g., [2,3]). The combination of different wavelengths, one in the XUV and the other in the visible or near infrared, opens new opportunities. It has recently permitted the investigation of above-threshold ionization (ATI) as the result of the combined interaction of both fields [4][5][6]. In this case the dominant contribution comes from processes in the course of which the emitted electron exchanges photons with the dressing laser field via stimulated emission (or absorption) resulting in a comb of sidebands disposed on both sides of the main photoelectron line.Theoretical studies have established that the sideband intensity depends on the electron kinetic energy as well as on the strength and polarization state of the optical laser field [7]. Fitting theoretical profiles to the measured sideband signals should yield the main parameters which govern the photon-atom interaction in this regime. For example, changing the polarization of either of the radiation beams gives rise to ''dichroic effects'' in the photoelectron spectrum. It therefore opens the possibility to control the relative contributions of photoionization channels with different angular momenta.This approach has been extensively used in studies of atomic ionization by weak monochromatic radiation from synchrotrons and continuous lasers, where at least one resonant intermediate state is involved, and the basic photon-electron interaction is completely dominated by this resonant excitation [8]. The use of high harmonic XUV sources to generate similar processes in the nonresonant continuum is complicated by very difficult analysis, since contributions from several harmonics and their mutual interferenc...

Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs)

“…The well-defined characteristics of both sources, and in particular their high degree of linear polarization is further used to get even greater insight into the dynamics of the photoionization process. In figure 14 the polarization dependence of the two-colour process is shown [73]. The spectra were recorded in the high field regime of the optical laser, i.e.…”

“…For the case of low dressing fields, i.e. for IR laser intensities below 10 11 W/cm 2 , when only one sideband is seen on each side, this variation can theoretically be described within second order perturbation theory by the rather elegantly simple expression [73]:…”

Probing the energy flow in Bessel light beams using atomic photoionization

“…Simple core-Hartree wave functions in a screened potential have been applied to calculate all the necessary (one-electron) atomic matrix elements [56], cf. (20).…”

“…This non-perturbative interaction of the electrons with both, the XUV and NIR fields then leads to the well-known 'sidebands' that typically occur as satellites to the normal photolines [14,15]. Such sidebands were first observed in the two-color ATI of noble gases as well as in laser-assisted Auger processes [16][17][18][19][20][21]. More recently, these sidebands have been applied in pump-probe photoelectron spectroscopy [22] or in imaging the molecular orbitals of H 2 O, O 2 and N 2 [23].…”

“…This orientation dependence has been explored especially by Meyer and coworkers [20] for the angular distribution of the sidebands in the photoionization of helium. Later, it was shown theoretically [26] that the two-color ATI sideband spectra are rather sensitive also with regard to the circular polarization of both, the XUV and NIR light, and an asymmetry in the photoelectron spectra was found, if the circular polarization of one of the field is changed from same to the opposite direction, a phenomenon that is termed today as circular dichroism in two-color ATI.…”