Investigating charge-up and fragmentation dynamics of oxygen molecules after interaction with strong X-ray free-electron laser pulses.

During the last decade, X-ray free-electron lasers (XFELs) have enabled the study of light-matter interaction under extreme conditions. Atoms which are subject to XFEL radiation are charged by a complex interplay of (several subsequent) photoionization events and electronic decay processes within a few femtoseconds. The interaction with molecules is even more intriguing, since intricate nuclear dynamics occur as the molecules start to dissociate during the charge-up process. Here, we demonstrate that by analyzing photoelectron angular emission distributions and kinetic energy release of charge states of ionic molecular fragments, we can obtain a detailed understanding of the charge-up and fragmentation dynamics. Our novel approach allows for gathering such information without the need of complex ab initio modeling. As an example, we provide a detailed view on the processes happening on a femtosecond time scale in oxygen molecules exposed to intense XFEL pulses.

Photon-recoil imaging: Expanding the view of nonlinear x-ray physics.

Addressing the ultrafast coherent evolution of electronic wave functions has long been a goal of nonlinear x-ray physics. A first step toward this goal is the investigation of stimulated x-ray Raman scattering (SXRS) using intense pulses from an x-ray free-electron laser. Earlier SXRS experiments relied on signal amplification during pulse propagation through dense resonant media. By contrast, our method reveals the fundamental process in which photons from the primary radiation source directly interact with a single atom. We introduce an experimental protocol in which scattered neutral atoms rather than scattered photons are detected. We present SXRS measurements at the neon K edge and a quantitative theoretical analysis. The method should become a powerful tool in the exploration of nonlinear x-ray physics.

Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon.

Short wavelength free-electron lasers (FELs), providing pulses of ultrahigh photon intensity, have revolutionized spectroscopy on ionic targets. Their exceptional photon flux enables multiple photon absorptions within a single femtosecond pulse, which in turn allows for deep insights into the photoionization process itself as well as into evolving ionic states of a target. Here we employ ultraintense pulses from the FEL FERMI to spectroscopically investigate the sequential emission of electrons from gaseous, atomic argon in the neutral as well as the ionic ground state. A pronounced forward-backward symmetry breaking of the angularly resolved emission patterns with respect to the light propagation direction is experimentally observed and theoretically explained for the region of the Cooper minimum, where the asymmetry of electron emission is strongly enhanced. These findings aim to originate a better understanding of the fundamentals of photon momentum transfer in ionic matter.

Note: Soft X-ray transmission polarizer based on ferromagnetic thin films.

A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L3-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element.

Rotational analysis for the Doppler-free photoelectron spectrum of water using the spectator model.

A photoelectron spectrum of H(2)O has been recorded at a resolution of 2 meV under Doppler-free conditions. Complex rotational structures appear in the individual vibrational states of the electronic X̃(+ 2)B(1) and Ã(+ 2)A(2) states in H(2)O(+). The rotational structures are analyzed and well reproduced using a spectator orbital model developed for rotationally resolved photoelectron spectroscopy.

Internal inelastic scattering satellite probed by molecular-frame photoelectron angular distributions from CO2.

The molecular-frame photoelectron angular distribution (MFPAD) of the satellite accompanying the C 1s photoline of the CO2 molecule has been measured at the C 1s(2sigmag)-->4sigmau* shape resonance, using electron-ion multicoincidence momentum spectroscopy. The observed MFPAD indicates that the conjugate satellite is excited by internal inelastic scattering. In this scenario, a photoelectron is ejected from the C 1s(2sigmag) orbital along the molecular axis and collides with an O lone-pair electron in the highest occupied molecular orbital 1pig. Then one of the colliding electrons is trapped to the lowest unoccupied molecular orbital 2piu*, while the other is emitted as a satellite photoelectron of sigmag symmetry, losing the information of the original photoelectron emission direction and parity.

Breakdown of the two-step model in K-shell photoemission and subsequent decay probed by the molecular-frame photoelectron angular distributions of CO2.

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s-->sigma* resonance: experiment and theory.

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.

Probing the valence character of O 1s-->Rydberg excited O2 by participator Auger decay measurements and partial ion yield spectroscopy following x-ray absorption.

The valence character of O 1s-->Rydberg excited O2 is investigated by means of participator Auger decay spectroscopy, performed at selected photon energies across the K-shell resonance region, and by means of partial ion yield x-ray absorption spectroscopy. For several of the excitation energies studied, the authors find substantial sigma*(4Sigmau-, 2Sigmau-) valence character being mixed with nssigma and npsigma (4Sigmau-, 2Sigmau-) Rydberg states. An experimental indication of a coupling between the channels associated with quartet and doublet ion cores is considered and discussed. New spectroscopic constants are derived for the singly ionized X 2Pig state of O2 based on the observation of at least 20 vibrational sublevels.

C1s and O1s photoelectron satellite spectra of CO with symmetry-dependent vibrational excitations.

The photoelectron shake-up satellite spectra that accompany the C1s and O1s main lines of carbon monoxide have been studied by a combination of high-resolution x-ray photoelectron spectroscopy and accurate ab initio calculations. The symmetry-adapted cluster-expansion configuration-interaction general-R method satisfactorily reproduces the satellite spectra over a wide energy region, and the quantitative assignments are proposed for the 16 and 12 satellite bands for C1s and O1s spectra, respectively. Satellite peaks above the pi(-1)pi(*) transitions are mainly assigned to the Rydberg excitations accompanying the inner-shell ionization. Many shake-up states, which interact strongly with three-electron processes such as pi(-2)pi(*2) and n(-2)pi(*2), are calculated in the low-energy region, while the continuous Rydberg excitations are obtained with small intensities in the higher-energy region. The vibrational structures of low-lying shake-up states have been examined for both C1s and O1s ionizations. The vibrational structures appear in the low-lying C1s satellite states, and the symmetry-dependent angular distributions for the satellite emission have enabled the Sigma and Pi symmetries to be resolved. On the other hand, the potential curves of the low-lying O1s shake-up states are predicted to be weakly bound or repulsive.

Angular distribution of different vibrational components of the X and B states reached after resonant Auger decay of core-excited H2O: experiment and theory.

Vibrationally resolved spectra have been obtained for the lowest-lying cationic states X (2)B(1), A (2)A(1), and B (2)B(2) of the water molecule reached after participator resonant Auger decay of core-excited states. The angular distribution has been measured of the first four vibrational components of the X state in the photon energy regions including the O 1s-->4a(1) and the O 1s-->2b(2) core excitations, and for different portions of the vibrational envelope of the B state in the photon energy region including the O 1s-->2b(2) core excitation. For the X state, a large relative spread in beta values of the different vibrational components is observed across both resonances. For the B state, a very different trend is observed for the high binding energy side and the low binding energy side of the related spectral feature as a function of photon energy. A theoretical method based on the scattering K matrix has been used to calculate both the photoabsorption spectrum and the beta values, by taking both interference between direct and resonant photoemission and vibrational/lifetime interference into account. The numerical results show qualitative agreement with the trends detected in the experimental values and explain the conspicuous variations of the beta values primarily in terms of coupling between direct and resonant photoemission by interaction terms of different sign for different final vibrational states.

Doppler effect in resonant photoemission from SF6: correlation between Doppler profile and Auger emission anisotropy.

Fragmentation of the SF6 molecule upon F 1s excitation has been studied by resonant photoemission. The F atomiclike Auger line exhibits the characteristic Doppler profile that depends on the direction of the photoelectron momentum relative to the polarization vector of the radiation as well as on the photon energy. The measured Doppler profiles are analyzed by the model simulation that takes account of the anisotropy of the Auger emission in the molecular frame. The Auger anisotropy extracted from the data decreases with an increase in the F-SF5 internuclear distance.

Anisotropic ultrafast dissociation probed by the Doppler effect in resonant photoemission from CF4.

The resonant Auger spectrum from the decay of F 1s-excited CF4 is measured. Several lines exhibit a nondispersive kinetic energy as the exciting photon energy is tuned through the resonance region. The F 1s(-1) atomiclike Auger line is split into two components due to the emission of Auger electrons by a fragment in motion, when electron emission is observed along the polarization vector of the light. This Doppler splitting is direct evidence that the core excitation leads to T(d)-->C(3v) symmetry lowering, by elongation of a specific C-F bond preferentially aligned along the polarization vector of the incident photon.

Auger electron emission from fixed-in-space CO.

We have measured the angular distribution of carbon K-Auger electrons from fixed in space, core-ionized, CO molecules in coincidence with the kinetic energy release of the C+ and O+ fragments. We find a very narrow ejection of Auger electrons in the direction of the oxygen and an oscillatory diffraction pattern. Even for similar electron energies, the angular distribution strongly depends on the symmetry of the final state.

Doppler-free resonant Raman auger spectroscopy of Ne+ 2s2p(5)3p excited states.

Using very high resolution achieved by the Doppler-free resonant Raman Auger technique, we have resolved the lowest terms of the series of inner-valence excitations 2s2p(5)((1,3)P)np 2S, 2P, and 2D in Ne+. The measured Auger anisotropic parameters and branching ratios help to establish the assignments of these levels. The measured lifetime widths are in reasonable agreements with ab initio calculations available in the literature.

Interference effects in the resonant photoemission channels to the Ne+ 2p(4)(1D2)3p 2P, 2D, and 2F states in the Ne 1s excitation region.

We present measurements of total and partial photoionization cross sections of Ne in the 1s excitation region. The total cross section exhibits resonances with symmetric profiles, whereas the branching ratios to the Ne+ 2p(4)(1D2)3p 2P, 2D, and 2F states present strong oscillations in the interresonance regions. We prove that not only the interference between the direct and resonant ionization processes but also the interference among the ionization processes via different nonisolated resonances are important for this effect.

Symmetry-dependent multielectron excitations near the C 1s ionization threshold and distortion of the shape resonance in CO(2).

Satellite bands accompanying the C 1s photoline for the CO2 molecule parallel to the electric vector of the incident radiation E are found to be more intense than those for CO2 perpendicular to E in the shape resonance region. This indicates that multielectron excitations are caused in part by the interaction of the outgoing C 1s photoelectron with the valence electrons. The photoelectron-impact valence excitations couple with the C 1s single-hole ionization and distort the shape resonance significantly. We assign the broad resonance at approximately 312 eV to a distorted Sigma(u) shape resonance.

Symmetry-resolved vibrational spectroscopy for the C 1s(-1)2 pi(u) Renner-Teller pair states in CO(2).

Symmetry-resolved excitation spectra have been measured for the Renner-Teller pair states A(1) and B(1) split from the core-excited C 1s(-1)2 pi(u) state in CO(2). A vibrational progression with the spacings of approximately 145 meV is found in both the A(1) and B(1) spectra at different energies and assigned to the symmetric stretching mode caused in the B(1) linear state, with the help of ab initio calculations. Appearance of the vibrations in the A(1) spectrum is interpreted as due to non-adiabatic coupling between the A(1) and B(1) states via the bending motion.

The Molecular Constants of 12CH3I in the Ground and v6 = 1 Excited Vibrational State.

A set of 26 new measurements have been recorded by a Doppler-free double-resonance technique, with a relative accuracy of about 10(-8). These data are combined to previous FTIR measurements to refine the molecular constants of 12CH3I. All the available literature data have been revisited, a few assignments updated, and some frequencies changed according to new laser standards. A simultaneous fit to all the measurements of infrared, microwave, and radiofrequency transitions produced an improved set of parameters, independent of previously determined constants. Copyright 1998 Academic Press.