Search

The direct observation of triple photoionization involving one inner shell and two valence electrons is reported. The energy distribution of the three photoelectrons emitted from Ne is obtained using a very efficient multielectron coincidence method using the magnetic bottle electron spectroscopic technique. A predominance of the direct path to triple photoionization for the formation of Ne3+ in the 1s 2s2 2p4 configuration is observed. It is demonstrated that the energy distribution evolves with photon energy and indicates a significant difference with triple photoionization involving only valence electrons.

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields ($\ensuremath{\sim}24.1\text{ }\text{ }\mathrm{eV}$, $\ensuremath{\sim}5\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$) is presented. Resonances to the doubly excited states converging to the ${\mathrm{He}}^{+}$ $N=3$ level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr\"odinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.

We present an experimental and theoretical study of triply charged hydrogen bromide ions formed by photoionization of the inner 3d shell of Br. The experimental results, obtained by detecting the 3d photoelectron in coincidence with the two subsequent Auger electrons, are analyzed using calculated potential energy curves of HBr{sup 3+}. The competition between the short-range chemical binding potential and the Coulomb repulsion in the dissociative process is shown. Two different mechanisms are observed for double Auger decay: one, a direct process with simultaneous ejection of two Auger electrons to final HBr{sup 3+} ionic states and the other, a cascade process involving double Auger decay characterized by the autoionization of Br*{sup +} ion subsequent to the HBr{sup 2+} fragmentation.

The decay processes of core-valence doubly excited states near the N K edge of NO have been studied using electron spectroscopy. Electron yields measured as a function of photon energy and kinetic energy enable the clear identification of atomic Auger lines associated with the dissociation of doubly excited states. The atomic Auger lines exhibit Doppler profiles, allowing the entire reaction scheme of such dissociation processes to be determined.

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21 eV, ∼ 5 TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).

The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.

A simple asynchronous mechanical light chopper, based on modification of a turbo-molecular pump, has been developed to extend the interval between light pulses in single bunch operation at the Photon Factory storage ring. A pulse repetition rate of 80 kHz was achieved using a cylinder rotating at 48000 rpm, with 100 slits of 80 microm width. This allows absolute timing of particles up to 12.48 micros instead of the single-bunch period of 624 ns. We have applied the chopper together with a light pulse monitor to measure multielectron coincidence spectra using a magnetic bottle time-of-flight electron spectrometer. With such a system, the electron energies are determined without any ambiguity, the folding of coincidence spectra disappears and the effect of false coincidences is drastically reduced.

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

Simultaneous two-electron emission upon $2p$ core-electron excitation in Ar has been studied using a state-of-the-art multielectron coincidence method. Simultaneous two-electron emission effectively populates Rydberg-excited ${\text{Ar}}^{2+}$ states, in which the excited electron behaves as a spectator of the direct double Auger decay of the core hole. This observation constitutes experimental evidence that the shake-off mechanism is not sufficient to model direct double Auger decay of a $2p$ hole in Ar and suggests an important contribution from the knock-out mechanism.

We report both experimental and theoretical studies on x-ray absorption measured using partial Auger yields of gas phase nitrogen, carbon monoxide, and oxygen molecules near the N 1s ->pi(*), O 1s ->pi(*), and O 1s ->pi(*) regions, respectively. The main tool of our study is a two-dimensional map in which resonant Auger yields are plotted as a function of photon and kinetic energy. The partial yields of the three molecules are analyzed in detail by extracting profiles along various directions in the map. Narrowing of the absorption resonances is observed along the direction of constant kinetic energy. It is shown that such profiles are similar to the conventional x-ray-absorption spectrum for a broad class of molecules whose potential energy surfaces of the final and core-excited states are almost parallel. However, substantial differences with the conventional x-ray-absorption profiles are observed in the general case of nonparallel surfaces due to the lifetime vibrational interference. Here, we suggest a systematic way to eliminate the lifetime vibrational interference.

We report on an experimental investigation of energy correlation among three electrons emitted in valence triple photoionization (TPI) of Ar. The energy correlations reveal a predominant contribution from sequential TPI processes involving intermediate Ar2+ formation, which suggests that such indirect contributions should be included in the formulation of the threshold law for TPI cross sections. The differential cross section for direct TPI at about 150 eV above threshold producing one slow electron with a few eV kinetic energy is found to have a deeply hollow U-shaped profile in energy sharing between the other two electrons.

We report both experimental and theoretical studies on x-ray absorption measured in the resonant Auger scattering mode of gas phase carbon monoxide near the O1s{yields}2{pi} region. Both experiment and theory display a crucial difference between the x-ray absorption profiles obtained in the conventional and resonant scattering modes. Lifetime vibrational interference is the main source of the difference. It is demonstrated that such interference, which arises from a coherent excitation to overlapping intermediate levels, ruins the idea for obtaining x-ray absorption spectra in a lifetime broadening free regime.

We demonstrate that the observation of slow electrons emitted in the decay of molecular core-excited states can be a sensitive probe of the double Auger processes, and that in combination with electron-electron coincidence spectroscopy, it can provide clear insight into the mechanisms involved. The present study identifies all cascade Auger paths from the C1s-to-Rydberg states in CO to final states of CO{sup 2+}. One pathway includes the first directly identified case of molecular level-to-level autoionization of a cation and shows remarkable selectivity for a specific final state.

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.

Autoionization dynamics of core-valence doubly excited states in ${\mathrm{N}}_{2}$ is studied by measuring angular dependence and photon energy evolution of inner-shell photoelectron spectra. The anisotropic electron emissions in the autoionization are interpreted in terms of symmetries of the doubly excited states. The autoionization features in the photoelectron spectra demonstrate clearly the electronic decay closely related to nuclear dynamics in the femtosecond regime. A nonadiabatic picture is required to describe the autoionization of a Rydberg-type doubly excited state.

We demonstrate ultrafast multiphoton coherent excitation in near-infrared intense laser fields (≤10 TW/cm2) using excited helium in the 1s2p (1P) state as the target. Strong two-photon coupling between the 2p state and the 5f and 6f Rydberg states drives ultrafast Rabi oscillations in tens of femtosecond. Theoretical calculations solving the time-dependent Schrodinger equation were carried out to elucidate the strong field effects.

We have studied the site-specific formation of metastable OCS2+ in the C1s and the O1s Auger decays of OCS molecules by using an Auger-electron–ion coincidence technique. The coincidence measurement reveals the metastable character of the Auger final states with the (3π)−2 configuration. The formation of metastable OCS2+ is observed negligibly in the C1s Auger decay and only weakly in the O1s Auger decay, in contrast to the favorable metastable formation previously reported for S2p Auger decay. It is found that the origin of the site-specificity in the formation of metastable OCS2+ arises from the localization effect of the valence orbital involved in the Auger transition. The dissociative dication states related to individual fragmentation pathways are also derived from the coincidence measurement.

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).

Experimental investigation of the threshold electron / fast electron coincidences allows one to select lines which are associated with two Auger electron emission. Such an investigation carried out for near threshold photoionization of Xe 4d shell reveals a considerable distortion of the lineshapes due to Post Collision Interaction (PCI). Analysis of the PCI influence on the Auger lineshapes allows us to clarify dynamics of the two Auger electron ejection. Our study shows that both double Auger decay (DA) and cascade Auger decay (CA) could contribute to the dynamics of the decay process.

The emission of low-energy electrons from H2O has been investigated at photon excitation energies in the vicinity of the O 1s ionization threshold. Neutral oxygen Rydberg atoms (O*) were found to form, and the correlation between the initial inner-shell excited state of H2O and the Rydberg state of O* was determined. The initially excited electron in a Rydberg orbital is shown to remain associated with O* even after the cleavage of two O-H bonds. We also show that the energy discrepancy between two Rydberg states of H2O and O* can be explained by the influence of the post-collision interaction, which becomes stronger as the excitation energy approaches the 1s ionization threshold.

We have studied double photoionization from molecular inner-shell orbitals and investigated the properties of the resultant double core-hole states in molecules, by multi-electron coincidence spectroscopy with a magnetic bottle electron spectrometer. A brief summary of our previous studies is presented.

Direct measurements of the photoelectrons or Auger electrons associated with inner shell ionization of positively charged ions are extremely difficult and rarely realized. We use here an alternative method to simulate such measurements, based on core-valence double photoionization of the neutral species. As an example, we obtain the spectroscopy, lifetimes and Auger decays of the states arising from 2p inner shell ionization of an Ar+ ion. Observations compare well with theoretical predictions obtained with multiconfigurational Dirac-Fock formalism.

We have observed single photon one-site (K?2) and two-sites (K?1K?1) double K-shell ionization in C2H2n (n=1-3) sequence and in isoelectronic C2H2, N2 and CO molecules. The spectroscopy of K?1K?1 states is much more sensitive to the bond length than conventional ESCA spectroscopy based on single K-shell ionization. The mechanism of single photon two-sites is due to a knock-out process in which a primary photoionized 1s electrons eject another 1s electron on the neighbor atom.

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O $K$ shell of oriented CO molecules near the ionization threshold in the region of a ${\ensuremath{\sigma}}^{*}$ shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six $l\ensuremath{\sigma}$ partial waves with $0\ensuremath{\le}l\ensuremath{\le}5$ to the ${\ensuremath{\sigma}}^{*}$ shape resonance is demonstrated. A comparison with our calculations in the relaxed core Hartree-Fock approximation reveals only a qualitative agreement, therefore a more advanced theory is needed.

The concept, design and performance of Japan's compact free-electron laser (FEL) facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications in mainly atomic, molecular and optical science are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multi-photon processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump?probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multi-photon ionization by intense x-ray FEL pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.

Electron reemission following photoelectron recapture due to post-collision interaction has been studied at 0.7 eV the inner-shell photoionization threshold of water molecules, using a multi-electron coincidence method. Electron reemissions after single Auger decay occur from O and OH fragments which are produced by the dissociations of high-n Rydberg H2O(+) states populated through photoelectron recapture. In addition, electron reemissions after double Auger decay are identified in triple coincidence events, where autoionization lines from O and O(+) fragments are observed.

Three-photon double excitation of He in intense EUV FEL fields is studied by the shot-by-shot photoelectron spectroscopy, revealing the enhancement by resonances to the doubly excited states converging to the He+ N =3 level.

Auger decay of the C(2)H(2) double core-hole (DCH) states, including the single-site DCH (C1s(-2)), two-site DCH (C1s(-1)C1s(-1)), and satellite (C1s(-2)π(-1)π∗(+1)) states, has been investigated experimentally using synchrotron radiation combined with multi-electron coincidence method, and theoretically with the assumption of the two-step sequential model for Auger decay of the DCH states. The theoretical calculations can reproduce the experimental two-dimensional Auger spectra of the C(2)H(2) single-site DCH and satellite decays, and allow to assign the peaks appearing in the spectra in terms of sequential two-electron vacancy creations in the occupied valence orbitals. In case of the one-dimensional Auger spectrum of the C(2)H(2) two-site DCH decay, the experimental and calculated results agree well, but assignment of peaks is difficult because the first and second Auger components overlap each other. The theoretical calculations on the Auger decay of the N(2) single-site DCH state, approximately considering the effect of nuclear motion, suggest that the nuclear motion, together with the highly repulsive potential energy curves of the final states, makes an important effect on the energy distribution of the Auger electrons emitted in the second Auger decay.

Single photon double K-shell ionization of small molecules (N2, O2, CO, CO2) has been observed by coincidence detection of two photoelectrons with two Auger electron and reveals properties of hollow molecules.

We performed multielectron coincidence measurements for inner-shell photoionizations of ethyl trifluoroacelate molecules (C4H5F3O2) using a magnetic bottle electron spectrometer. From a two dimensional coincidence map between a photoelectron and Auger electron for C 1s ionizations, we extracted site-specific Auger electron spectra for each carbon site and corresponding binding energy of doubly charged states.

We observed an interference originating from coincidence detection of two indistinguishable electron pairs emitted upon photoionization of 4d electron in Xe. At 89.9 eV photon impact the energy of photoelectron ejected from 4d5/2 orbital equals energy of Auger electron emitted in decay of 4d3/2 hole into the [5s5p1P] state, and for the same final state the energy of the 4d3/2 photoelectron equals the Auger electron energy in decay of 4d5/2 hole. An angle-integrated coincidence yield as a function of photon energy is measured with the magnetic bottle time-of-flight spectrometer and shows a peak at the expected energy position.

Cascade Auger electron emission following Xe 3d photoionization has been investigated using a multi-electron coincidence technique, which utilizes an electron spectrometer of magnetic bottle type. It has been found that the Xe2+ states of the 4p−14d−1 configuration, formed by the Auger decay of the Xe+ 3d3/2, 5/2−1 states, dominantly turn into triply charged states of the 4d−25p−1/4d−25s−1 configurations. The Xe2+ 4s−14d−1 states, formed by the 3d Auger decay, yield the 4p−14d−15p−1 states as well as the 4d−3 states. From the coincidence spectrum among three Auger electrons, it is suggested that the Xe2+ 4p−14d−1 states give rise to the following cascade processes: 4p−14d−1 → 4d−25p−1 → 4d−15p−3.

Auger decay of an inner shell hole is an efficient way to create multiply charged ions in the gas phase. We illustrate this with the example of the argon 2s decay, and show that multi-electron coincidence spectroscopy between the 2s photoelectron and all released Auger electrons leads to a complete reconstruction of the Ar 2s decay cascade. Spectra of the intermediate and final Ar(n+) states are obtained and are compared with a theoretical model.

International audience; Electron correlations in three electron emission associated with innershell photoionization are investigated for the first time through a combined theoretical and experimental approach. Namely, different kinds of Post Collisional Interactions (PCI) occurring in the decay of Argon 2p holes by emission of two Auger electrons are isolated experimentally with a powerful coincidence technique and are modeled with an eikonal approach. It is shown that PCI distortion of the photoelectron line shape is stronger than in the single Auger paths and depends critically on the process: in direct Double Auger decays where two Auger electrons are emitted simultaneously, direct interaction of the 2p photoelectron with the two Auger electrons is demonstrated ; in cascade Double Auger processes, the PCI distortion depends only slightly on the last electron emitted in the cascade. This perturbation depends on and reveals the lifetime of the intermediate Ar 2+ state.

We have studied Auger decays of the 1s shake-up and shake-off states in N2 molecules with a state-of-the-art multi-electron coincidence method. The multi-electron coincidences reveal overall features of the Auger transitions from the individual core-hole states. The decay features of the shake-up states are interpreted by spectator and participant roles of the exited electrons. It is observed that the valence holes formed at the initial shake-off process remain unchanged during the Auger decay from the shake-off states.

Core–valence doubly ionized states of OCS, which are produced by the Coster–Kronig transitions following the S 2s ionization, have been investigated by using threshold photoelectron-Auger electron coincidence spectroscopy. The sub-natural linewidth regime achieved with this coincidence method enables us to locate the core–valence OCS2+ states, while the corresponding structures are almost smeared out on the S 2s Auger spectrum measured with conventional Auger electron spectroscopy.

Double photoionization into states which have holes in one core and one valence orbitals has been observed in CO using a state-of-the-art multielectron coincidence method. The core-valence CO2+ structures exhibited on the electron coincidence spectra are assigned by comparison with the available calculation [H. Schulte et al., J. Chem. Phys. 105, 11108 (1996)]. Features of the spectrum confirm that the properties of the CO2+ states are characterized by the interaction between the localized valence holes and the core holes.

Single-shot photoelectron spectroscopy was performed for nonresonant EUV-UV two-color two-photon ionization of He. From data analysis on the shot-by-shot basis, the absolute cross-section was determined to be σ(2)(59.7 nm, 268 nm) = 4.1(6) × 10−52 cm4 s.