We present a theoretical interpretation of recent experimental results on multiphoton multiple ionization of xenon by soft-x-ray radiation of photon energy approximately 93 eV and intensity up to 10(16) W/cm2 [A. A Sorokin, Phys. Rev. Lett. 99, 213002 (2007)]. The data are interpreted within multiphoton perturbation theory, taking into account the spatiotemporal distribution of the radiation. Multiphoton cross sections have been obtained through a technique of scaling, with occasional adjustment to the data, provided the two prove to be compatible. Whatever discrepancies between theory and experiment persist can be reasonably attributed to some uncertainty in the experimental conditions and possibly to the value of some cross sections, without, however, any evidence for nonperturbative behavior.

We present measurements of the resonant inelastic x-ray scattering (RIXS) spectra of the CH 3 I molecule in the hard-x-ray region near the iodine L 2 and L 3 absorption edges. We show that dispersive RIXS spectral features that were recognized as a fingerprint of dissociative molecular states can be interpreted in terms of ultrashort natural lifetime of ∼200 attoseconds in the case of the iodine L-shell core-hole. Our results demonstrate the capacity of the RIXS technique to reveal subtle dynamical effects in molecules with sensitivity to nuclear rearrangement on a subfemtosecond time scale.

By means of a high resolution resonant inelastic x-ray scattering spectroscopy, we have for the first time separated spectral features pertaining to different two-electron atomic processes in the vicinity of an innershell threshold. Contributions of shakeoff, shakeup, and resonant 1s3p double excitations were extracted from the Ar KM-M 2;3 M x-ray satellite line intensity measured as a function of photon energy from ½1s3p double excitation threshold to saturation. The isolated ½1s3pnln 0 l 0 excitation spectrum is critically compared to the outcome of the multiconfiguration Dirac-Fock model with relaxation.According to the nature of photon-electron interaction a photon is absorbed only by a single electron. On the other hand, photoionization of an inner-shell electron is sometimes accompanied by removal of another electron (shakeoff) or its excitation into a higher empty atomic level (shakeup), and below the double photoionization threshold, doubly excited atomic states can be created. These multielectron excitations are straightforward manifestations of the breakdown of the independent electron picture as they appear due to the electron-electron interactions. Despite the persistent and intensive research the mechanisms of electron-electron correlations have not been fully elucidated yet. While the well-known shake mechanism (governed by monopole transitions between relaxed states) is commonly taken as a synonym for the double photoionization or excitation process, the presence of dynamical effects in the near-threshold region makes the interpretation of double photoexcitation spectra much more complicated.The main experimental problem hindering the study of near-threshold multielectron excitations is the overlap of the shakeoff, shakeup, and double excitation spectral contributions. Strong dependence of the shakeoff to shakeup probability ratio on the atomic number and atomic shell was employed to bypass this difficulty in some special cases. While the shakeup mechanism fully dominates the electron excitation following 1s photoionization of Be [1], the shakeoff prevails in the 1s2p double photoionization of mid-Z atoms [2]. It was found that a time-dependent perturbation theory [3] describes well the double photoionization of outer-shell electrons in low-Z atoms; for inner shells and higher Z atoms significant deviations were found [4,5], indicating that other mechanisms besides the shakeoff are at work. In the latest work a semiclassical knockout effect was suggested to dominate the near-threshold double 1s photoionization in 3d transition metals [6].One of the most interesting multielectron features is double excitations exhibiting discrete resonances in the preedge region. These processes are extremely weak when both electrons are excited from core levels, but they become more probable if a valence electron takes part in the excitation. In such cases structural solid-state effects obscure spectral details related to multiple atomic excitations, limiting such studies to gaseous atomic targets. This is the mai...

Two-photon excitation of a single-photon forbidden Auger resonance has been observed and investigated using the intense extreme ultraviolet radiation from the free electron laser in Hamburg. At the wavelength 26.9 nm (46 eV) two photons promoted a 3d core electron to the outer 4d shell. The subsequent Auger decay, as well as several nonlinear above threshold ionization processes, were studied by electron spectroscopy. The experimental data are in excellent agreement with theoretical predictions and analysis of the underlying multiphoton processes. DOI: 10.1103/PhysRevLett.104.213001 PACS numbers: 32.80.Rm, 32.80.Fb, 32.80.Hd, 42.50.Hz The chief manifestation of the coupling of electrons bound in atoms or molecules to electromagnetic radiation of high intensity is the onset of nonlinear processes, a feature that could in fact be viewed as the definition of a strong field. Phenomena such as multiphoton multiple ionization, above threshold ionization (ATI), and high order harmonic generation span the broad field of strong field physics that has until recently been restricted to interactions with infrared and optical radiation [1]. Because of the small photon energy, only the outermost electrons are ionized and multiple ionization is obtained via successive stripping of the outer subshell. In contrast, short-wavelength radiation couples predominantly to electrons in lower, more strongly bound shells, producing corehole states, which decay primarily by ultrafast Auger decay. The opportunity to study the underlying multiphoton dynamics arises only now with the availability of free electron lasers (FELs) in the extreme ultraviolet (XUV) to hard-x-ray wavelength regime [2,3].The investigation of nonlinear interactions using FELs started recently and has already highlighted several phenomena, such as, e.g., the formation of very high charged states [4,5] and two-photon double ionization [6,7], that could not be anticipated on the basis of the existing linear experience. A unique access to explore the interaction between matter and strong XUV fields is given by resonant two-photon inner-shell processes. Firstly, two-photon processes enable the study of a class of resonances, which are inaccessible in a single-photon process from the ground state. Secondly, inner-shell resonances are characterized by a dramatic increase of the photoionization cross section and provide, for example, chemical (i.e., atomic) selectivity in the photoionization process of molecules or clusters. The lifetime of a core hole of the order of several femtoseconds is determined by Auger processes, and the excitation in strong XUV fields of similar durations will inevitably result in the competition between sequential ionization and Auger decay. This new phenomenon, which will be present in all processes involving the interaction between intense XUV light and matter, can be addressed only due to the short pulse durations. It is the nonlinear behavior of such interactions that presents a new frontier, with multiphoton excitation and/or ionization o...