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Physical chemistry, chemical physics 25(19), 13784 - 13791 (2023). doi:10.1039/D2CP05942H, We present a study on molecular-frame photoelectron angular distributions (MFPADs) of small molecules using circularly polarized synchrotron light. We find that the main forward-scattering peaks of the MFPADs are slightly tilted with respect to the molecular axis. This tilt angle is directly connected to the molecular bond length by a simple, universal formula. We apply the derived formula to several examples of MFPADs of C 1s and O 1s photoelectrons of CO, which have been measured experimentally or obtained by means of ab initio modeling. In addition, we discuss the influence of the back-scattering contribution that is superimposed over the analyzed forward-scattering peak in the case of homo-nuclear diatomic molecules such as N2., Published by RSC Publ., Cambridge

Physical chemistry, chemical physics 24(44), 27121 - 27127 (2022). doi:10.1039/D2CP02408J special issue: "Ions, electrons, coincidences and dynamics: Festschrift for John H.D. Eland", 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., Published by RSC Publ., Cambridge

Physical review letters 129(25), 253201 (2022). doi:10.1103/PhysRevLett.129.253201, We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905 eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space., Published by APS, College Park, Md.

We report on a joint experimental and theoretical study of photoelectron circular dichroism (PECD) in methyloxirane. By detecting O 1s-photoelectrons in coincidence with fragment ions, we deduce the molecule's orientation and photoelectron emission direction in the laboratory frame. Thereby, we retrieve a fourfold differential PECD clearly beyond 50%. This strong chiral asymmetry is reproduced by ab initio electronic structure calculations. Providing such a pronounced contrast makes PECD of fixed-in-space chiral molecules an even more sensitive tool for chiral recognition in the gas phase., Comment: 5 figures

Physical review letters 123(24), 243201 (2019). doi:10.1103/PhysRevLett.123.243201, We investigate angular emission distributions of the 1s photoelectrons of N2 ionized by linearlypolarized synchrotron radiation at hν ¼ 40 keV. As expected, nondipole contributions cause a very strongforward-backward asymmetry in the measured emission distributions. In addition, we observe anunexpected asymmetry with respect to the polarization direction, which depends on the direction ofthe molecular fragmentation. In particular, photoelectrons are predominantly emitted in the direction of theforward nitrogen atom. This observation cannot be explained via asymmetries introduced by the initialbound and final continuum electronic states of the oriented molecule. The present simulations assign thisasymmetry to a novel nontrivial effect of the recoil imposed to the nuclei by the fast photoelectrons andhigh-energy photons, which results in a propensity for the ions to break up along the axis of the recoilmomentum. The results are of particular importance for the interpretation of future experiments at x-rayfree electron lasers operating in the few tens of keV regime, where such nondipole and recoil effects will beessential., Published by APS, College Park, Md.

We investigate the differential ionization probability of chiral molecules in the strong-field regime as a function of the helicity of the incident light. To this end, we analyze the fourfold ionization of bromochlorofluoromethane (CHBrClF) with subsequent fragmentation into four charged fragments and different dissociation channels of the singly ionized methyloxirane. By resolving for the molecular orientation, we show that the photoion circular dichroism signal strength is increased by 2 orders of magnitude.

31st International Conference on Photonic, Electronic, and Atomic Collisions, ICPEAC XXXI, Caen Normandy, France, 23 Jan 2019 - 30 Jan 2019; Journal of physics / Conference Series 1412, 112009 (2020). doi:10.1088/1742-6596/1412/11/112009, (X-ray) free-electron lasers are employed to site specifically interrogate atomic fragments during ultra-fast photolysis of chiral molecules via time-resolved photoelectron circular dichroism., Published by IOP Publ., Bristol

Journal of physics / B 53(19), 1-6 (2020). doi:10.1088/1361-6455/aba3d3 special issue: "Special issue on Frontiers of AMO Science with FELs and Synchrotron Radiation", The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hν = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3–OH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide., Published by IOP Publ., Bristol

Synopsis The spectator resonant interatomic Coulombic decay (SRICD) following the 2s →5p excitation of Ne dimers has been examined experimentally and theoretically. The molecular frame angular distributions (MFADs) of the SRICD electrons depend strongly on the direction of the polarization vector of the exciting photon with respect to the molecular axis. We demonstrate that this effect is caused by the scattering of the low-energy SRICD electron on the density of the spectator electron.

The recent availability of intense and ultrashort extreme ultraviolet sources opens the possibility to investigate ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI@Elettra. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by \emph{ab initio} electronic structure and nuclear dynamics calculations.

We investigate $K$-shell ionization of ${\mathrm{N}}_{2}$ at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the Auger electron, and both ${\mathrm{N}}^{+}$ fragments. These fully differential data show that the dissociation process of the ${\mathrm{N}}_{2}^{2+}$ ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately $30\ensuremath{\mathbin:}70$ between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.

Physical review letters 123(4), 043202 (2019). doi:10.1103/PhysRevLett.123.043202, It is commonly accepted that the magnitude of a photoelectron circular dichroism (PECD) is governed by the ability of an outgoing photoelectron wave packet to probe the chiral asymmetry of a molecule. To be able to accumulate this characteristic asymmetry while escaping the chiral ion, photoelectrons need to have relatively small kinetic energies of up to a few tens of electron volts. Here, we demonstrate a substantial PECD for very fast photoelectrons above 500 eV kinetic energy released from methyloxirane by a participator resonant Auger decay of its lowermost O $1s$ excitation. This effect emerges as a result of the Fano interference between the direct and resonant photoionization pathways, notwithstanding that their individual effects are negligibly small. The resulting dichroic parameter has an anomalous dispersion: It changes its sign across the resonance, which can be considered as an analogue of the Cotton effect in the x-ray regime., Published by APS, College Park, Md.

Physical review letters 121(24), 243002 (2018). doi:10.1103/PhysRevLett.121.243002, We suggest that low-energy electrons, released by resonant decay processes, experience substantial scattering on the electron density of excited electrons, which remain a spectator during the decay. As a result, the angular emission distribution is altered significantly. This effect is expected to be a common feature of low-energy secondary electron emission. In this Letter, we exemplify our idea by examining the spectator resonant interatomic Coulombic decay of Ne dimers. Our theoretical predictions are confirmed by a corresponding coincidence experiment., Published by APS, College Park, Md.

Resonant interatomic Coulombic decay (RICD) has been studied with HeNe dimers, where two decay channels are identified by bound HeNe${}^{+}$ states and low-energetic He+Ne${}^{+}$ fragments, and the state-resolved angular emission distributions of respective RICD electrons are examined in detail.

Resonant Auger (RA) decay spectra of carbon $1s$ excited ${\mathrm{CH}}_{3}\mathrm{Cl}$ molecules are recorded with angular resolution using linearly polarized synchrotron radiation. The selected photon energies corresponding to the C $1s\ensuremath{\rightarrow}8{a}_{1}$ core to lowest unoccupied molecular orbital and C $1s\ensuremath{\rightarrow}4s{a}_{1}$, $4pe$, and $4p{a}_{1}$ core to Rydberg excitations of methyl chloride are used and electrons in the binding energy range of 11--37 eV are detected. The vibrationally unresolved RA electron angular distributions, recorded for participator Auger transitions populating the $X$, $A$, $B$, and $C$ states of the ${\mathrm{CH}}_{3}{\mathrm{Cl}}^{+}$ ion, exhibit strong variations across the selected electronic resonances. These observations are interpreted with the help of ab initio electronic structure and dynamics calculations, which account for electronic-state interference between the direct and different resonant ionization pathways. For spectator transitions, the theory predicts almost isotropic angular distributions with moderate changes of $\ensuremath{\beta}$ parameters around zero, which is in agreement with the experimental observations.

Two-photon ionization of xenon was investigated theoretically for exciting-photon energies from 6.7 to 11.5 eV, which results in the ionization of Xe between $5{p}_{1/2}$ (13.43 eV) and $5s$ (23.40 eV) thresholds. We describe the extension of a previously developed computational technique for the inclusion of relativistic effects to calculate energies of intermediate resonance state and cross sections for two-photon ionization. Reasonable consistency of cross sections calculated in length and velocity form was obtained only after considering many-electron correlations. Agreement between calculated and measured resonance energies is found when core polarization was additionally included in the calculations. The presently computed two-photon photoionization cross sections of Xe are compared with Ar cross sections in our previous work. Photoelectron angular distribution parameters calculated here indicate that intermediated resonances strongly influence photoelectron angular distribution of Xe.

Synopsis The two-photon ionization of the Ne–CH4 and Ar–SH4 isoelectronic systems was studied theoretically for the photon energies exceeding the ionization threshold of the outer shells. The calculations demonstrate that the two-photon ionization of the investigated objects is almost entirely a collective process if the one-photon energy corresponds to the energy of the n ℓ → ε ℓ R ′ giant resonance. The largest contribution to the transition (2) amplitude stems from the Auger-like correlation n ℓ 2 → ( 2 ) ε ℓ R ′ 2 ⇒ n ℓ ε ″ ℓ ″ .

The hitherto unexplored two-photon doubly excited states [Ne∗(2p-13s)]2 were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne2+ ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

Physical review / A 95(5), 053423(2017). doi:10.1103/PhysRevA.95.053423, The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane, C$_3$H$_3$F$_3$O, is studied experimentally and theoretically. Thereby, we investigate the photoelectron circular dichroism (PECD) for nearly symmetric O 1s and F 1s electronic orbitals, which are localized on different molecular sites. The respective dichroic β$_1$ and angular distribution β$_2$ parameters are measured at the photoelectron kinetic energies from 1 to 16 eV by using variably polarized synchrotron radiation and velocity map imaging spectroscopy. The present experimental results are in good agreement with the outcome of ab initio electronic structure calculations. We report a sizable chiral asymmetry β$_1$ of up to about 9% for the K-shell photoionization of oxygen atom. For the individual fluorine atoms, the present calculations predict asymmetries of similar size. However, being averaged over all fluorine atoms, it drops down to about 2%, as also observed in the present experiment. Our study demonstrates a strong emitter and site sensitivity of PECD in the one-photon inner-shell ionization of this chiral molecule., Published by APS, Woodbury, NY

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

In the angle-averaged excitation and decay spectra of molecules, vibronic coupling may induce the usually weak dipole-forbidden transitions by the excitation intensity borrowing mechanism. The present complementary theoretical and experimental study of the resonant Auger decay of core-to-Rydberg excited CH_{4} and Ne demonstrates that vibronic coupling plays a decisive role in the formation of the angle-resolved spectra by additionally involving the decay rate borrowing mechanism. Thereby, we propose that the angle-resolved Auger spectroscopy can in general provide very insightful information on the strength of the vibronic coupling.

We experimentally study $2p$ photoionization of neon dimers (${\mathrm{Ne}}_{2}$) at a photon energy of $h\ensuremath{\nu}=36.56\text{ }\text{ }\mathrm{eV}$. By postselection of ionization events which lead to a dissociation into ${\mathrm{Ne}}^{+}+\mathrm{Ne}$ we obtain the photoelectron angular emission distribution in the molecular frame. This distribution is symmetric with respect to the direction of the charged vs neutral fragment. It shows an inverted Cohen-Fano double slit interference pattern of two spherical waves emitted coherently but with opposite phases from the two atoms of the dimer.

Since their theoretical prediction in 1997, interatomic (intermolecular) Coulombic decay (ICD) and related processes have been in the focus of intensive theoretical and experimental research. The spectacular progress in this direction has been stimulated both by the fundamental importance of the discovered electronic decay phenomena and by the exciting possibility of their practical application, for example in spectroscopy of interfaces. Interatomic decay phenomena take place in inner-shell-ionized clusters due to electronic correlation between two or more cluster constituents. These processes lead to the decay of inner-shell vacancies by electron emission and often also to disintegration of the resulting multiply ionized cluster. Here we review the recent progress in the study of interatomic decay phenomena in singly and multiply ionized clusters.

Photon-induced fluorescence spectroscopy was applied to measure absolute fluorescence emission cross sections from the main 5s- and first satellite levels of XeII upon photoionization from XeI in a broad exciting-photon energy range (23–28 eV) with 20 meV bandwidth of the exciting radiation and in a narrow energy range (23.25–23.50 eV) with a bandwidth of 1.8 meV. The average fluorescence resolution amounted to 0.07 nm. Fluorescence emission cross sections were computed with taking into account many-electron correlations and doubly excited states. Computed cross sections, especially in integrated form, agree well with the results of experiment. Many features observed in the high resolution experiment are identified to originate from nf-Rydberg series.

Total and partial 5p- photoionization cross sections of Xe in the exciting-photon energy range corresponding to the 5s1np Rydberg series were computed including the most important many-electron correlations. Results are compared with available experimental data. The most prominent spectral features observed in the experiments are identified.

In high-intensity laser light, matter can be ionized by direct multiphoton absorption even at photon energies below the ionization threshold. However on tuning the laser to the lowest resonant transition, the system becomes multiply excited, and more efficient, indirect ionization pathways become operative. These mechanisms are known as interatomic Coulombic decay (ICD), where one of the species de-excites to its ground state, transferring its energy to ionize another excited species. Here we show that on tuning to a higher resonant transition, a previously unknown type of interatomic Coulombic decay, intra-Rydberg ICD occurs. In it, de-excitation of an atom to a close-lying Rydberg state leads to electron emission from another neighbouring Rydberg atom. Moreover, systems multiply excited to higher Rydberg states will decay by a cascade of such processes, producing even more ions. The intra-Rydberg ICD and cascades are expected to be ubiquitous in weakly-bound systems exposed to high-intensity resonant radiation., 原子の集団が数珠つなぎに電子を放出する! : 極紫外自由電子レーザーで誘起される新現象解明. 京都大学プレスリリース. 2016-12-12.

Dispersed fluorescence from fragments formed after the de-excitation of the 1s-1π* resonances of N*O and NO* has been measured in the spectral range of 118–142 nm. This range is dominated by lines of atomic nitrogen and oxygen fragments and by the $A ~^{1}{\rm \Pi} \left(v^{\prime}\right) \rightarrow X ~^{1}{\rm \Sigma}^{+}\left(v^{\prime \prime}\right)$ bands in the NO+ ion which result from the participator Auger decay of the 1s-1π* resonances. Ab-initio calculations of the transition probabilities between vibrational levels during the reaction NO $X~^{2}{\rm \Pi}\left(v_{0}=0\right) \rightarrow $ N*O $\left( {\rm NO}^{\ast}\right) $ $1s^{-1}\pi ^{\ast }\left( v_{r}\right) $ ⇒ NO $^{+}~A~^{1}{\rm \Pi} \left(v^{\prime}\right) \rightarrow X~^{1}{\rm \Sigma}^{+}\left(v^{\prime \prime}\right)$ were used to explain the observed intensity dependence for the $A\left(v^{\prime }\right) \rightarrow X\left(v^{\prime \prime}\right)$ fluorescence bands on the exciting-photon energy across the resonances and on both v′ and v′′ vibrational quantum numbers. The multiplet structure of the 1s-1π* resonance and lifetime vibrational interference explain the observed exciting-photon energy dependence of the $A\left( v^{\prime}\right) \rightarrow X\left(v^{\prime \prime}\right)$ fluorescence intensity. A strong spin-orbit coupling between singlet and triplet states of NO+ is proposed to reduce additional cascade population of the $A~^{1}{\rm \Pi}$ state via radiative transitions from the $W~^{1}{\rm \Delta}$ and $A^{\prime}~^{1}{\rm \Sigma}^{-}$ states and to explain remaining differences between measured and calculated integral fluorescence intensities.

The photoionization and photodissociation of an oxygen molecule are investigated theoretically by the method of coupled differential equations. The molecular orbitals of the core are calculated in the MO LCAO approximation. The molecular orbitals of a photoelectron in the discrete and continuous spectra are determined by the single-center method. The wave functions of vibrational motion of the oxygen molecule are obtained within the diabatic approach. The described method is used for calculating the predissociation and autoionization widths of the 2σ u −1 (c 4Σ u − )n/σg, v Rydberg states of the oxygen molecule. The total cross sections of the resonant photoionization and neutral predissociation calculated for the oxygen molecule in the excitation energy range 20.6–24.8 eV are in good agreement with the available experimental data.

A theoretical investigation of N2+ (C2Σu+ → X2Σg+ molecular fluorescence excited through the Auger decay of the 1s−1π* resonance is carried out. The fluorescence cross sections are calculated with due regard for the dependence of the matrix element of the C → X dipole transition on the internuclear distance, the interference between channels of excitation via different vibrational levels vr of the 1s−1π* resonance, the rotational structure of the fluorescence band, and the predissociation of the N2+C2Σu+v′ ≥3) states. The calculated cross sections are in good agreement with the experimental results of recent measurements. The results of the calculations have demonstrated that the observed dependence of the cross section of the (C2Σu+ (v′) → X2Σg+ (v″) fluorescence on the excitation energy and the fluorescence wavelength for a group of bands with equal values of the difference Δv = v′ − v″ is associated with transitions between the vibrational levels of the electronic states involved in the excitation and subsequent cascade decay of the 1s−1π* resonance: N2 (v0 = 0) → N*2(1s−1π*(vr)) ⇒ N2+: (C2Σu+ (v′) → X2Σg+ (v″).

The photoionization cross sections for the 4p shell of ions of the Kr isoelectronic sequence Rb+, Sr2+, and Y3+ are calculated. The configuration interaction theory and the perturbation theory are used to describe the many-electron effects. The relativistic effects are taken into account in the Pauli-Fock approximation. The calculated resonance structure of photoionization cross sections for the 4p shell in the region below the 4s threshold associated with the autoionization of the 4s-np singly excited states and the 4p4p-nln′l′ doubly excited states reproduces the results of recent measurements of total photoabsorption cross sections for the Rb+, Sr2+, and Y3+ ions. It is found that, as the nuclear charge in the isoelectronic sequence increases, the ratio between the direct and correlation parts of amplitudes of the 4s-(n/ɛ)p transition changes and, as the consequence, the minimum of the photoionization cross section of the 4s shell shifts from the continuous spectrum to the region of states of discrete spectrum. This accounts for the strong changes in the shape of the 4s-np resonances in the photoionization cross sections for the 4p shell of Rb+, Sr2+, and Y3+, as well as the distinction between the shapes of the 4s-6p 1/2 mirror resonance in the partial 4p 1/2 and 4p 3/2 photoionization cross sections for the Y3+ ion which do not suppress each other in the total photoionization cross section, as is the case for similar resonances in Rb+ and Sr2+.

The photoionization cross sections of the 4p shell and the 4s main level and 4p 4(3 P) 5s 4 P 5/2, 3/2 satellite subvalence levels of KrII have been calculated in the 4s-near-threshold range of excitation energies from 28.48 to 28.70 eV. The calculation takes into account the core relaxation by the methods of the theory of non-orthogonal orbitals, the interaction between resonant states through autoionization channels by solving the complex secular equation, and the interaction between the channels of the continuous spectrum in all orders of the perturbation theory by the K-matrix method. Good quantitative agreement between the energy-integrated theoretical and experimental photoionization cross sections for the satellite levels has been obtained for the first time. It is shown that only simultaneous consideration of the above-mentioned effects leads to such agreement. The resonant structure of the photoionization cross sections in this excitation energy range is related to the autoionization decay of the 4p 45s(4 P 1/2)np and 4p 45s(2 P 3/2)np Rydberg series. The specificity of this process is that both series manifest themselves not independently but owing to their strong electrostatic interaction with the prominent 4p 4(1 D)5s 2 D 5/2 6p 3/2 resonance, which lies in this excitation energy range.

Participator resonant Auger decay spectra populating the $X, A, B$, and $C$ states of ${\mathrm{CO}}_{2}{}^{+}$ are recorded with angular and vibrational resolution for selected photon energies in the vicinity of the C $1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ resonance of ${\mathrm{CO}}_{2}$ using a narrow photon bandwidth and a high-resolution electron spectrometer. The measured electron spectra and the corresponding angular distribution parameters exhibit significant changes as functions of the photon energy across the resonance and with respect to the vibrational sublevels of a final ionic state. The measured spectra are interpreted by ab initio electronic structure and nuclear dynamics calculations which attribute observed variations to the effects of lifetime vibrational interference and of electronic state interference between the direct ionization amplitude and the resonant amplitudes for the excitation and decay of two overlapping resonant states of different symmetry. The present results provide deeper insight into the femtosecond relaxation dynamics of the core-excited ${\mathrm{CO}}_{2}$, which is not achievable with lower resolution, angle-averaged measurements.

The photoabsorption of Ar is studied experimentally and theoretically for energies between 32.5 eV and 33.0 eV. Resonant excitation and autoionization of doubly excited 3p4(3P)4s2P3/2,1/2 np Rydberg states are observed as dominant mechanism to populate doublet and quartet satellites of the 3p44s, 3d configurations. The perturbation of the Rydberg series by promoter states is reproduced by the calculations. Measured and calculated cross sections for the strong decay channels are in good agreement. A mirror behaviour of the 3s vacancy creation and the satellite production and the influence of the radiative decay of high-lying Rydberg states on the shape of a satellite threshold are manifested.

Predissociation of the N+2 C 2Σ+u(v') vibrational levels with v' ≥ 3 was observed via dispersed C 2Σ+u → X 2Σ+g fluorescence in the spectral range of 165–208 nm after resonant 1s−1π*(vr) excitation of N2 and its subsequent autoionization into the N+2 C state. This range is dominated by lines in atomic nitrogen, by overlapped C 2Σ+u(v') → X 2Σ+g(v'') vibrational band sequences with Δv = const and broad unresolved band systems (D, (2))2Πg(v') → A2Πu(v'') in the N+2 molecular ion. With very high fluorescence resolution of about 0.1 nm FWHM individual C 2Σ+u(v') → X 2Σ+g(v'') vibrational bands have been resolved. Calculation of the observed fluorescence spectra by taking into account predissociation and molecular rotation describes well the shape of both individual vibrational bands C 2Σ+u(v') → X 2Σ+g(v'') and the whole band system.

Fluorescence from fragments formed after the de-excitation of the N*2(1s−1π*) resonance has been measured in the spectral range of 135–190 nm. This range is dominated by lines in atomic nitrogen and lines formed by overlapping C2Σ+u(v') → X2Σ+g(v'') bands with Δv = const in the N+2 molecular ion which result from the spectator Auger decays of the N*2(1s−1π*(vr)) resonances. Ab initio calculations of the corresponding potential curves and transition probabilities showed that the observed irregular intensity dependence of the C2Σ+u(v') → X2Σ+g(v'')(Δv = const) fluorescence lines on the vibrational quantum number vr is due to transitions between vibrational levels during the reaction N2(v0 = 0)→ N*2(1s−1π*(vr)) C2Σ+u(v') → X2Σ+g(v'').

The interaction between resonances through autoionization continua and the interaction between autoionization continua were investigated theoretically and experimentally for the photoionization process of the 4p- and 4s-shells and for the population of 4p4 (3P)5s 4PJ, 4p4 (3P)5s 2PJ and 4p4(3P)4d 4DJ satellites of KrII. Cross sections for the satellite production and the angular distribution parameter of the fluorescence radiation were measured by photon-induced fluorescence spectroscopy after excitationx with linearly polarized monochromatized synchrotron radiation at exciting-photon energies between 28.45 eV and 29.95 eV with an exciting-photon energy resolution of 10 meV (FWHM). Measured cross sections are in good agreement with the computed ones. A refined assignment of resonances in the proximity of the 4s14p65p resonance was performed. It was concluded that there is a strong influence of the core rearrangement, of the interaction between resonances through the autoionization continua and of the interaction between autoionization continua, on the investigated processes on the basis of the observed good overall agreement between the computed and measured quantities.

Photoionization cross-sections for the 4p 4 ( 3 P ) 5s 4 P 5 / 2 , 3 / 2 , 1 / 2 satellites and 4s, 4p main levels of Kr II in the exciting-photon energy range between 28.48 and 28.70 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation were measured utilizing the photon-induced fluorescence spectroscopy. The observed resonances were assigned to the 4p 4 5s( 4 P 1 / 2 )n p and 4p 4 5s( 2 P 3 / 2 )n p Rydberg series on the basis of calculations performed with taking into account core relaxation and interaction between many resonances and many continua. The calculation shows that the resonance structure in the photoionization channels exists due to 4p 4 ( 1 D ) 5s 2 D 5 / 2 6 p 3 / 2 promoter state which also strongly perturbs the above Rydberg series.

Parameters of the resonant 4p 4p –3d e l Auger effect (RA) following the 3d– n p ( n = 5 , 6 ) excitation in Kr were calculated with taking into account the interference between several resonant and direct non-resonant transition amplitudes. For the first time all individual lines of the extended RA spectrum which comprises both the 4 p 4 ( 1 D ) 5p and the 4 p 4 ( 1 D ) 6p groups of final ionic states were considered. It was revealed that each group contains individual lines where the interference contributions have different signs thus providing a weak interference effect on the average over the whole group. Interference effects are found to be more pronounced in the angular distribution of the RA products.

The photoabsorption spectrum of Kr-like Y3+ ions in the very soft x-ray/extreme ultraviolet spectral range where single (4s–np) and double (4p4p–nln'l') electron excitations dominate has been measured using a dual laser plasma photoabsorption technique. The corresponding photoionization cross section has been computed within the framework of the configuration interaction Pauli–Fock approach. The calculations confirm that the 4s–5p resonance drops below the 4p ionization threshold so that the first member of the 4s–np photoionization resonance series becomes 4s–6p. Also, whereas the first such series member in Rb+ and Sr2+ appears as a window resonance, in Y3+ it switches to a normal, slightly asymmetric absorption peak. In addition, although the 4s–5p1/2 resonances in the 4p51/2 and 4p53/2 continua are mirror images in Rb+ and Sr2+ and hence cancel each other, they exhibit quite different shapes in Y3+ and hence we observe the residual signature of the 4s–6p1/2 resonance as a weak partner to the main 4s–6p3/2 resonance.

The competition between autoionization and neutral dissociation (ND) of the vibronic Rydberg states 2σ−1u(c4Σ−u)(ns/nd)σg3Σ−u(v = 0, 1) has been investigated using an experiment of photon-induced fluorescence spectroscopy (PIFS) and the simultaneous measurement of the photo ion yield. The experiments were performed with monochromatized synchrotron radiation from BESSY II with a very narrow bandwidth of ΔE = 1.5 meV at 23 eV where it was possible to observe single rotational branches. The experiments are compared to ab initio calculations for the partial autoionization and dissociation widths and to the corresponding simulations of the spectroscopic shape of the rovibronic branches. For more precise calculations of the dissociation widths, diabatic potential curves of the 2σ−1u(c4Σ−u) states of the O+2 are extracted from the adiabatic ones and improved calculations for molecular orbitals of the Rydberg and Auger electrons are used.

Dispersed Kr VUV-fluorescence excitation spectra and photoion yields were recorded in the exciting photon energy range between 28.4 eV and 28.7 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation used as the exciting photon source. Two series of doubly excited KrI states were observed and identified as the 4s24p4 (3P) 5s 4P1/2 np and the 4s24p4(3P)5s 2P3/2 np series.

Double photoionization of N2 into the N22+D1Σ u+-state as a function of the exciting-photon energy has been investigated experimentally and theoretically. Photon-induced fluorescence spectroscopy using monochromatized synchrotron radiation in the exciting-photon energy range between 50 and 66.5 eV and recording of the intensity of the dispersed VUV fluorescence between 145 and 167 nm has been carried out. Relative cross sections as a function of the exciting-photon energy have been determined. The experimental results are compared with ab initio calculations.

The energy dependence of the alignment parameter A20 for the Kr II 4p45p(E 1J1) states following excitation in the Raman regime with the exciting-photon energy passing through the Kr I 3d9np resonances was investigated theoretically and experimentally. Interference between the resonance and direct photoionization channels explains the dependence of A20 on the excitation energy. Experimentally, A20 was determined after the analysis of 5p–5s fluorescence decay of the 4p45p(E 1J1) states excited with monochromatized synchrotron radiation. The band pass of the synchrotron radiation was set to 10 meV which is smaller than the natural width of the 3d9np resonances (~83 meV). Additionally, a strong energy dependence was predicted for the orientation parameter O10 as well as for the angular distribution parameter of photoelectrons, βe, for several 4p45p(E 1J1) ionic states.

Absolute photoionization cross sections for the population of the Kr II 4p4np states when the exciting-photon energy corresponds to the first four 3d9np resonances were calculated beyond the two-step model for the first time. Good agreement between computed and measured photoionization cross sections proves the importance of taking into account the interference between different resonance channels in understanding the dynamics of the Auger decay of the Kr I 3d9np resonances.

Absolute cross sections for the total fluorescence emission between 97 and 131 nm by atomic oxygen formed after the photoexcitation of 2σ-1u(c4Σ-u)nσg3Σ-u Rydberg states in O2 and their successive predissociation were measured for the first time. The dissociation lifetimes and fluorescence yields were estimated using a stepwise model and the potential curve of the O2+2σ-1u(c4Σ-u) state known from Beebe et al (Beebe N H F, Thulstrup E W and Andersen A 1976 J. Chem. Phys. 64 2080). The calculated lifetimes of the vibrational levels v = 0 and 1 of the 2σ-1u(c4Σ-u) state are 3.4×10-12 and 6.3×10-14 s, respectively. They agree with the measured data of Evans et al (Evans M, Stimson S, Ng C Y and Hsu C-W 1998 J. Chem. Phys. 109 1285) and are consistent with the results of the present experiment. The calculated lifetimes support the model of `fast' dissociation of the O2+2σ-1u(c4Σ-u), v states in contrast to the earlier model of `slow' dissociation of Tanaka and Yoshimine (Tanaka K and Yoshimine M 1979 J. Chem. Phys. 70 1626).

Energy dependences of the total and partial ns-photoionization cross-sections in the vicinity of the ns-subvalence shell threshold were measured for the rare-gas atoms using photon-induced fluorescence spectroscopy. A resolution of 4.5, 3, and 1 meV was attained for Ar, Kr and Xe, respectively. The experimental cross-sections exhibit many well-resolved resonances of different shapes which are connected with the photoionization through the doubly excited atomic states. Calculated photoionization cross-sections of Ar and Kr show that some resonances are determined by doubly excited states including strongly delocalized n′f-excited electrons.