An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

We have measured the nondipolar contribution to the Ar ls photoelectron angular distribution over the 30 -2000 eV electron-energy range. The nondipolar interaction results in a forward or backward asymmetry with respect to the photon beam. The asymmetry is directed backward near threshold, is symmetric near 230 eV, and becomes increasingly forward directed at higher energies. The measured asymmetries are in excellent agreement with theoretical calculations, which include interference between the electric-dipole and electric-quadrupole photoionization amplitudes.PACS numbers: 32.80.Fb Current understanding of atomic photoionization phenomena is largely based on the dipole approximation [1][2][3][4]. Within this approximation, the standard transition matrix element used to describe photoionization between initial and final states, M;f = (f~e xp(ik r)c p~i), is simplified. In this expression, exp(ik . r)E describes the photon field (k is the photon propagation vector, r is the electron position vector, and c the photon polarization vector), and p is the electron momentum operator. In the dipole approximation, only the first term of the expan-0031-9007/95/75(26)/4736(4) $06.00

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

The Xe 5s nondipole photoelectron parameter is obtained experimentally and theoretically from threshold to 200 eV photon energy. Significant nondipole effects are seen even in the threshold region of this valence shell photoionization. In addition, contrary to previous understanding, clear evidence of interchannel coupling among quadrupole photoionization channels is found. DOI: 10.1103/PhysRevLett.91.053002 PACS numbers: 31.25.Eb, 32.80.Fb Until recently, conventional wisdom had assumed nondipole effects in photoionization were negligible at relatively low photon energies, perhaps for energies up to a few keV, but certainly for photon energies below a few hundred eV [1][2][3]. Indeed, despite indications to the contrary [4 -7], the usual practice in the field of photoionization, particularly for experiment, had been to ignore effects beyond the dipole approximation for photon energies as high as several keV. While this may be a reasonable assumption for integrated cross sections, recent work has shown it is certainly wrong for differential cross sections (i.e., photoelectron angular distributions). Experiments have shown the importance of nondipole effects in the 1-3 keV photon-energy region [8,9], in the hundreds-of-eV range [10], and, in one case, at 13 eV [11]. Concurrently, theory has predicted significant nondipole contributions to electron angular distributions for atomic valence shells down to threshold at a few tens of eV photon energy [12 -14].In addition, for dipole photoionization, interchannel coupling, which is simply configuration interaction in the continuum, has been shown to be important for most subshells of most atoms at most energies [15,16]; this work was contrary to the previous conventional wisdom that the independent-particle approximation (IPA) was generally valid away from thresholds. It had been suggested, however, that such interchannel coupling was not important in quadrupole photoionization channels [17], but recent theory has suggested that interchannel coupling can indeed be significant in quadrupole photoionization channels as well [14].To test these two ideas, significant nondipole contributions to the photoionization of a valence shell in the threshold region, and the existence of interchannel coupling effects in quadrupole photoionization channels, we have performed a benchmark experiment on the differential photoionization cross section of Xe 5s from 26 eV (close to threshold) to 200 eV to obtain the nondipole contribution to the photoelectron angular distribution which arises from interference between dipole (E1) and quadrupole (E2) channels. The differential cross section is given by [6,[18][19][20] where is the angle-integrated cross section, is the dipole anisotropy parameter, P 2 cos 3cos 2 ÿ 1=2, and and are nondipole asymmetry parameters. The coordinate axes have the positive x axis along the direction of the photon propagation vector, the z axis along the photon polarization vector, and and are the polar and azimuthal angles of the photoelectron momentum vector. ...

The partial cross-section and asymmetry parameters of the 2 s 1 , 2 A 1 and 2 B 2 ionic states of H 2 o+ have been measured at photon energies of 18-31 eV, by time-of-flight photoelectron spectroscopy using synchrotron radiation. The atomic character of the molecular orbitals is discussed in the interpretation of similarities and differences between the measurements for these states. Other synchrotron radiation data, resonance line measurements, and (e,2e) dipole results are ineluded. A multiple-scattering Xa (MSXa) calculation and a ground-state inversion method calculation are in good agreement with our crosssection measurements. There is excellent agreement between experimental asymmetry parameters and those computed from the MSXa method.Stieltjes-Tchebycheff imaging model calculations suggest the presence of resonant channels. This may be the origin of weak resonance structure present in the measurements.