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A core-excited sulfur state with a lifetime almost one order of magnitude longer than in molecular 2p core-hole states is selectively produced by ultrafast dissociation of S 2p -> sigma* excited OCS. Clear evidence for this is provided by strong atomic peaks (20% of the total intensity) in x-ray fluorescence but very weak ones (2%) in the corresponding resonant Auger spectrum. Corroborating the assignment of the spectra, ab initio calculations explain the enhanced lifetime: the Auger decay of the produced D-3(3) (2p(5)3p(5)) sulfur state is strongly decreased as it contradicts a newly derived propensity rule of the L2,3MM Auger decay.

We have obtained electron decay spectra following core excitation of NO2 below both the nitrogen and the oxygen K-edges. The resonant decay through participator processes is weak below the N K-edge, while is more intense below the O K-edge. The propensity rule stating that two valence electrons tend to participate in the decay with opposite spins was verified in the decay of the O1s → 6a1 core-excited state and the O1s → 2b1 core-excited state(s). Theoretical findings suggest that the relative ordering of the exchange-split core-excited components stems mainly from the valence–valence exchange interaction term being dominant over the core–valence one.

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the Nls --> 4a(1) core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z + l approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH2* and H fragments. (C) 2003 Elsevier Science B.V. All rights reserved.

Resonant Auger spectra of H2O are recorded with unprecedented resolution, for excitation energies tuned across the O 1s→2b2 resonance. Electron emission spectra for decay to the 1b2-1 final state show a partially resolved vibrational envelope whose overall appearance is photon energy dependent. Comparison with ab initio calculations indicates that the two-dimensional nuclear motion of the Auger final state can be controlled by sampling different portions of the potential curve and changing the nuclear motion in the core-excited state.

Resonant excitation to the F1s-σ* dissociative state in hydrogen fluoride gives a photoelectron spectrum where the spectator part contains strong atomic lines but a participator part where such lines are lacking. We demonstrate that this contrasting behaviour between the two parts is due to a strong dynamical suppression of the resonant contribution, making direct main state photoionization the dominating channel even at resonant conditions.

The sulfur L-shell photoelectron spectrum of the carbon disulfide molecule has been studied using monochromated synchrotron radiation with a photon energy of 250 eV. The spectrum is atomic like, showing three major bands that can be associated with the sulfur (2p-1)2P3/2,12 and (2s-1)2S1/2 ionic states. A closer inspection shows that the 2P3/2 state is further split into two components separated by 128 meV due to the molecular field. The resulting ionic states are located at 169.806 eV (0-0 energy), 169.934 eV (0-0 energy), 171.075 eV (0-0 energy) and 237.05±0.2 eV (centroid), respectively. Vibrational progressions in the (2p-1) bands are attributed to the asymmetric ν3 mode, which gives evidence of a localization of the core hole. The following values were obtained for the vibrational constants: ωe = 196.7±1.1 meV; ωexe = 0.2±0.5 meV. A curve fit of the vibrational lines using a Voigt function gave a natural width of 59.6±1.8 meV for the (2p-1) states, corresponding to a lifetime of 11 fs, and a spectrometer broadening of 38.2±1.8 meV. The (2p-1) bands are accompanied by shake-up structures occurring at 6-18 eV higher energies. They are interpreted mainly in terms of excitations to the unoccupied 3πu* orbital in the final ionic state. The (2s-1)2S1/2 band is broad and structureless due to fast Coster-Kronig processes. A fitting of a Voigt function gives a natural line-width of 1.85 eV which corresponds to a lifetime of 0.4 fs.

We employ a pump–probe approach to molecular photoionization to study fast dissociation of Rydberg states in acetylene. By using time-resolved photoelectron spectroscopy to study the electronic state of the resulting ions we are able to monitor the system continuously during dissociation or rearrangement. We find that the predissociative lifetime for the 3R′′′ (v2′=1) Rydberg state is about 150 fs. We demonstrate a powerful new technique using time-correlated femtosecond harmonic generation and laser light pulses to study the time evolution of ultrafast dynamic processes in molecules.

We report the observation of the unusually weak decay of the N 1s --> pi* core-excited N-2 molecule to the (B) over tilde (2)Sigma(u)(+) final state of N-2(+), which is only detectable in an exp ...

By exploiting the core-excitation-induced dissociation of O2, we find that the Auger emission exhibits a Doppler-like energy shift. We show this to be a manifestation of localization of the core hole and propose that the problem of core-hole localization versus delocalization in core-hole spectroscopies may be resolved by considering the nature of the measurement.

The fragmentation of the water molecule upon resonant core excitation and subsequent Auger decay has been studied by electron–ion coincidence spectroscopy. A qualitative comparison between the fragmentation patterns for H 2 O and D 2 O reveals that the anti-bonding nature of the core-excited (*) intermediate state causes appreciable dissociation of the H 2 O* (D 2 O*) into HO*+H (DO*+D) during the lifetime (3.3 fs) of the O1s core hole. A `core–hole clock' model is used to derive approximate quantitative values for the characteristic dissociation time of the core-excited water molecule. The results demonstrate how such a procedure is useful for quantitative experiments with a very high degree of spatial and temporal localization.

Auger decay from the C s core-excited state of CO has been studied by means of angular resolved vibrationally selective electron spectroscopy. It is observed that the so-called strict spectator model provides only a qualitative description of the C Auger decay. We interpret this as evidence for a strong valence character of the 3s orbital. Transitions assigned to the Auger shake-up are revealed. For transitions to the final state with s leading configuration, there are indications of angular asymmetry which depend upon the vibrational level of the final state.

The creation and decay of photon-excited Xe 4d5/2,3/2-1 core-hole states has been studied in the photon energy region 70-110 eV, where photon absorption is dominated by the strong shape resonance. Unprecedented experimental quality has enabled the study of nearly inherent Auger electron lineshapes. Analysis on the basis of a post-collision interaction model allows the first ever experimental verification that the Xe 4d5/2,3/2-1 lifetime widths clearly show dynamic, photon-energy-dependent behaviour.

The Auger decay from the C core-ionized state of CO has been studied with photon energies close to and far above threshold. Close to the threshold post-collision interaction (PCI) redistributes intensity from the low kinetic energy side to the high kinetic energy side of the Auger lines. Taking advantage of this a previously unresolved vibrational level of the B final state has now been observed and we have extracted a vibrational splitting of 0.27(3) eV. This shows how the PCI effect not only broadens the Auger line, but that the associated asymmetric line sharpening can be used to reveal the finer details of the Auger spectrum.

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.

The below-threshold region in core-excited O2 is very complex, consisting of a multitude of exchange-split states with mixed molecular orbital-Rydberg character. We have investigated the nature of these intermediate states by resonant Auger spectroscopy. In particular, we have obtained constant-atomic-final-state yield curves for several atomic peaks in the electron decay spectra which are stemming from ultrafast dissociation. The relative intensity of Auger decay leading to atomic final states is considered a signature of the relative weight of the sigma* character. This method allows one to "filter out" intermediate states with dissociative character. Extensive calculations have been performed by multi-reference configuration interaction at different interatomic distances in order to evaluate the potential curves of the core-excited states and propose a qualitative description of the dissociative molecular dynamics. The calculations show that the core-excited states have a relevant admixture of excitations to orbitals with Rydberg character and excitations to the sigma* orbital with different spin couplings. A diabatization of the adiabatic potential curves shows that the coupling between Rydberg and sigma* diabatic states is very different at the different crossing points and ultrafast dissociation occurs more easily on the lowest sigma* diabatic potential curve. As a consequence, the observation of atomic peaks only in the lower-energy region of the absorption curve is well justified.

We have obtained electron decay spectra following core excitation of NO2 below both the nitrogen and the oxygen K-edges. The resonant decay through participator processes is weak below the N K-edge, while is more intense below the O K-edge. The propensity rule stating that two valence electrons tend to participate in the decay with opposite spins was verified in the decay of the O1s->6a1 core excited state and the O1s->2b1 core -excited state(s). Theoretical findings suggest that the relative ordering of the exchange-split core-excited components stems mainly from the valence-valence exchange interaction term being dominant over the core-valence one.

International audience; High-resolution, angle-resolved resonant Auger electron spectra of the NO molecule in the regions of both N and O 1s→2π core electron excitations are presented. A large number of vibrational final states are resolved due to high energy resolution. Calculations based on lifetime vibrational interference (LVI) theory neglecting interference between different electronic intermediate states and between direct and resonant channels have been performed. A comparison between theoretical and experimental spectra shows that LVI theory describes the major spectroscopic features quite well. The same holds for the evolution of the angular averaged partial cross sections with the change of excitation energy. The angular distribution of particular vibrational final states are, however, not described successfully with LVI calculations at the present level of sophistication. A theoretical analysis supports that one reason for this deviation is electronic state interference.

International audience; We study the dynamics of highly excited states in acetylene initiated by an ultrashort vacuum ultraviolet laser pulse. Electronic states lying in the 4s-3d Rydberg region are excited with one femtosecond pulse, and the dynamic development of the states is monitored by a second short pulse which ionizes the system. We show that even for femtosecond pulses where the bandwidth of the exciting pulse covers several electronic states, it is possible to extract short decay lifetimes through time-resolved photoelectron spectroscopy by using a frequency-modulated (chirped) excitation pulse. We report decay lifetimes for the F 402 and E 4-502 states in acetylene, and for the E 402 and E 502 states in d-acetylene. The time evolution measured in the electron spectra is compared to decay spectra measured using ion yield and the differences in these results are discussed.

International audience; Resonant Auger electron spectra of core excited O3 (1s−12b11) are presented for the first time. The photoionization valence spectrum with sample purity and resolution superior to those published is presented. The first direct experimental evidence for the 2 B1 state of O3+ is found. Ab initio calculations of the resonant Auger electron (RAE) spectrum have been performed supporting the assignment of the -state.

The resonant Auger decay spectra associated with doubly excited core-hole states near the N 1s threshold in the nitrogen molecule was measured. Electron-yield spectrum was obtained by setting a kinetic-energy window at 384 eV and scanning across the threshold region. At ground 410, 414, and 416 eV, photon energy, three dominant structures were observed. The increased signal-to-background ratio, intensity and resolution at third generation facilities were found to cast a different light on the dynamics of core photoionization in molecules.

The photoelectron spectra of argon was demonstrated during resonant photoemissions. The interference effects of the photoelectron spectra of argon was monitored by following the intensity of final states through resonant and direct channels. The need for a unified one-description of the ionization through a direct and core-excitation mediated channel was described.

We present direct evidence for ultra-fast dissociation of molecular water in connection photo-excitation of the O1s→4a1 resonance. The core-excited H2O molecules are shown to dissociate into core-excited O*H and neutral H on a time scale comparable to the core hole lifetime, i.e. a few femtoseconds. This conclusion is based on a resonant Auger study, qualitative arguments and ab initio calculations. ‘Hot bands' connected to Auger transitions from vibrationally excited OH fragments are identified, and these exhibit a dynamical behavior which qualitatively follows the change in excess energy.

Resonant excitation to the F1s-sigma* dissociative state in hydrogen fluoride gives a photoelectron spectrum where the spectator part contains strong atomic lines but a participator part where such lines are lacking. We demonstrate that this contrasting behaviour between the two parts is due to a strong dynamical suppression of the resonant contribution, making direct main state photoionization the dominating channel even at resonant conditions.

International audience; High-resolution electron spectroscopy studies of the NO molecule in the regions of the N 1s→2π core excitations have been performed. By selecting electrons within certain binding energy ranges, either the Auger electron yield - a good approximation for the x-ray absorption spectrum - or the electrons emitted after decay to a particular ionic final state (constant ionic state (CIS)) were detected. By selecting the X 1Σ+ (2π0) final state, the superposition of several intermediate states can be disentangled by exploiting a selection rule which permits only two of the three dipole-allowed intermediate states to decay to this specific final state. This makes it possible to obtain more detailed information on the potential energy curves of the intermediate states than is available from regular absorption measurements. We have also obtained CIS spectra for individual vibrational sublevels within this state. The role of lifetime vibrational interference on the appearance of these spectra is discussed.

The role of avoided crossings for resonant x-ray Raman scattering is explored. The strong dependence of the electronic transition amplitudes on internuclear distance at avoided crossings causes new ...

The femtosecond dissociation of HCl after core excitation has been studied through the resonant Auger decay. The spectra contain contributions from decay occurring at both "molecular" and "atomic" internuclear distances. We have observed a new interference mechanism in these spectra: An atomic spectral line develops into a negative spectral contribution, a "hole," when detuning the excitation energy from the maximum of the Cl2p(-1)sigma(*) resonance. Resonant x-ray scattering theory quantitatively explains this behavior as due to a novel destructive continuum-continuum interference between molecular and atomic contributions to the Auger decay.

When an intense laser field interacts with an atomic gas, odd harmonics of the fundamental frequency are generated.

We present the first photoelectron–photoion–photoion coincidence (PEPIPICO) spectra of ozone, O3, excited at the O1s edge. From symmetry considerations the oxygen atoms at the end of the V-shaped molecule are chemically in-equivalent to the central oxygen atom. For the excitation from one of the terminal O1s orbitals to 7a1( σ ∗ ), experiments combined with simulations yielded that photo-induced breaking of the bond closest to the core-excited terminal atom occurs in at least 70±10% of the cases. We interpret this as due to a `memory' effect of the localized core–hole after the Auger decay. Consequently the sequential two-step model, i.e. core excitation-and-decay resulting in a state with delocalized valence holes followed by dissociation, is not adequate in this case.

International audience; The HCl inner-valence photoelectron band at 26 eV binding energy has been recorded at high resolution. Discrete peaks arising from at least two separate vibrational progressions are superimposed on the broad continuum. Fano profiles are visible in one of the progressions. This indicates interference between superimposed electronic states, where weak avoided crossing allows two adiabatic states to couple. In the isotopic DCl molecular spectrum, the discrete lines are less pronounced, due to slower dissociation and therefore less coupling between the continuum and the bound state.