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Scientific reports 11(1), 505 (2021). doi:10.1038/s41598-020-79818-6, Molecules can sequentially absorb multiple photons when irradiated by an intense X-ray pulse from a free-electron laser. If the time delay between two photoabsorption events can be determined, this enables pump-probe experiments with a single X-ray pulse, where the absorption of the first photon induces electronic and nuclear dynamics that are probed by the absorption of the second photon. Here we show a realization of such a single-pulse X-ray pump-probe scheme on N2 molecules, using the X-ray induced dissociation process as an internal clock that is read out via coincident detection of photoelectrons and fragment ions. By coincidence analysis of the kinetic energies of the ionic fragments and photoelectrons, the transition from a bound molecular dication to two isolated atomic ions is observed through the energy shift of the inner-shell electrons. Via ab-initio simulations, we are able to map characteristic features in the kinetic energy release and photoelectron spectrum to specific delay times between photoabsorptions. In contrast to previous studies where nuclear motions were typically revealed by measuring ion kinetics, our work shows that inner-shell photoelectron energies can also be sensitive probes of nuclear dynamics, which adds one more dimension to the study of light-matter interactions with X-ray pulses., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

We present chemical kinetics measurements of the luminol oxydation chemiluminescence reaction at the interface between two aqueous solutions, using liquid jet technology. Free-flowing iquid microjets are a relatively recent development that has found its way into a growing number of applications in spectroscopy and dynamics. A variant thereof, called flat-jet, is obtained when two cylindrical jets of a liquid are crossed, leading to a chain of planar leaf-shaped structures of the flowing liquid. We here show that in the first leaf of this chain the fluids do not exhibit turbulent mixing, providing a clean interface between the liquids from the impinging jets. We also show, using the example of the luminol chemiluminescence reaction, how this setup can be used to obtain kinetics information from friction-less flow and by circumventing the requirement for rapid mixing but by intentionally suppressing all turbulent mixing and instead relying on diffusion.

The toolbox for time-resolved direct measurements of electron dynamics covers a variety of methods. Since the experimental effort is increasing rapidly with achievable time resolution, there is an urge for simple and robust measurement techniques. Within this paper prove of concept experiments and numerical simulations are utilized to investigate the applicability of a new setup for the generation of ultrashort electron pulses in the energy range of 300 eV up to 1.6 keV. The experimental approach combines an in-vacuum liquid microjet and a few-cycle femtosecond laser system, while the threshold for electron impact ionization serves as a gate for the effective electron pulse duration. The experiments prove that electrons in the keV regime are accessible and that the electron spectrum can be easily tuned by laser intensity and focal position alignment with respect to the water jet. Numerical simulations show that a sub-picosecond temporal resolution is achievable., 13 Pages, 6 Fugures

Charge transfer processes between weakly bound entities play an important role in various chemical and biological environments. In this combined experimental and theoretical work, we investigate the nature of charge-transfer processes in homogeneous atomic and heterogeneous atomic-molecular clusters. Our results reveal fundamentally different processes to be at play in pure argon clusters compared to mixed argon-nitrogen systems: We demonstrate that the former species decay via photon-mediated charge transfer while a nonradiative direct process is found dominant in the atomic-molecular cases. Our results are of general interest for studies on charge redistribution in more complex and biologically relevant samples where molecules are involved.

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

The photoelectron circular dichroism (PECD) of the O 1s-photoelectrons of trifluoromethyloxirane(TFMOx) is studied experimentally and theoretically for different photoelectron kinetic energies. The experiments were performed employing circularly polarized synchrotron radiation and coincidentelectron and fragment ion detection using Cold Target Recoil Ion Momentum Spectroscopy. The corresponding calculations were performed by means of the Single Center method within the relaxed-core Hartree-Fock approximation. We concentrate on the energy dependence of the differential PECD of uniaxially oriented TFMOx molecules, which is accessible through the employed coincident detection. We also compare results for differential PECD of TFMOx to those obtained for the equivalent fragmentation channel and similar photoelectron kinetic energy of methyloxirane (MOx), studied in our previous work. Thereby, we investigate the influence of the substitution of the methyl-group by the trifluoromethyl-group at the chiral center on the molecular chiral response. Finally, the presently obtained angular distribution parameters are compared to those available in literature., 6 figs

A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent excitation of the satellite channel leads to an additional 10% increase in the photoabsorption cross section in the double-ionization threshold region., 6 pages, 3 figures, 22 bibliography

Dispersed fluorescence of wavelengths between 75 and 180 nm has been recorded after excitation of room temperature H 2 X 1 Σ g + v = 0 by narrow bandwidth monochromatized synchrotron radiation. Spectra have been taken in the exciting-photon energy range between 11 and 18 eV for discrete energy steps of 2.5 meV at a bandwidth of about 10 meV. The exciting-photon energy bandwidth has been narrow enough to induce transitions between individual rovibronic states. The resolution in the dispersed fluorescence detection was partly sufficient to resolve fluorescent transitions between individual rovibronic states. The recorded intensities are presented as two-dimensional photon-excitation photon-emission (PhexPhem) maps. They give a complete overview of experimental dispersed VUV/ FUV fluorescence spectra of H2 for these excitation energies, covering excitation from H 2 X 1 Σ g + v = 0 to all known singly excited states. The presented proof of principle experiments show a way to generate spectroscopic reference data for theory benchmarking and the interpretation of astrophysical observations.

X-ray as well as electron diffraction are powerful tools for structure determination of molecules. Studies on randomly oriented molecules in the gas-phase address cases in which molecular crystals cannot be generated or the interaction-free molecular structure is to be addressed. Such studies usually yield partial geometrical information, such as interatomic distances. Here, we present a complementary approach, which allows obtaining insight to the structure, handedness and even detailed geometrical features of molecules in the gas phase. Our approach combines Coulomb explosion imaging, the information that is encoded in the molecular frame diffraction pattern of core-shell photoelectrons and ab initio computations. Using a loop-like analysis scheme we are able to deduce specific molecular coordinates with sensitivity even to the handedness of chiral molecules and the positions of individual atoms, as, e.g., protons., Comment: 13 pages, 3 figures

The journal of physical chemistry letters 12(30), 7146 - 7150 (2021). doi:10.1021/acs.jpclett.1c01879, The predominant reason for the damaging power of high-energy radiation is multiple ionization of a molecule, either direct or via the decay of highly excited intermediates, as, e.g., in the case of X-ray irradiation. Consequently, the molecule is irreparably damaged by the subsequent fragmentation in a Coulomb explosion. In an aqueous environment, however, it has been observed that irradiated molecules may be saved from fragmentation presumably by charge and energy dissipation mechanisms. Here, we show that the protective effect of the environment sets in even earlier than hitherto expected, namely immediately after single inner-shell ionization. By combining coincidence measurements of the fragmentation of X-ray-irradiated microsolvated pyrimidine molecules with theoretical calculations, we identify direct intermolecular electronic decay as the protective mechanism, outrunning the usually dominant Auger decay. Our results demonstrate that such processes play a key role in charge delocalization and have to be considered in investigations and models on high-energy radiation damage in realistic environments., Published by ACS, Washington, DC

Electronic excitations in the valence shell of Ne clusters were studied by fluorescence spectroscopy. The measured fluorescence excitation functions contain information about the nature and number of excitonic states and the mean cluster size of the produced size distribution. Mean cluster sizes were determined by comparing surface and bulk contributions using a multidimensional fitting algorithm, with good agreement to commonly used scaling laws. The influence of different size distributions, which were not considered in previous investigations, on homogeneous noble gas cluster jets is implemented in the proposed model. The present work is the first approach using fluorescence spectroscopy for the determination of the mean size of Ne cluster jets created by supersonic expansion.

We report on the experimental observation of the direct decay of a core vacancy in van der Waals clusters by emission of a fast electron from a neighboring atom. The process can be regarded as an interatomic Coulombic decay of core holes (core-level ICD). We identify it unambiguously by electron-electron and electron-electron-photon coincidence spectroscopy of the decay of 2p vacancies in Ar clusters. While several earlier works reported the absence of this channel, we find core-level ICD to be of considerable significance and quantify the branching ratio of this nonlocal electron emission to conventional local Auger decay as (0.8±0.2)%. Our results are supported by calculations on smaller clusters and show a reasonable agreement. This report on a successfully performed electron-electron-photon coincidence experiment provides a perspective for explorations of matter exposed to ionizing radiation. The observed core-level ICD is proposed to be of general importance for studies on charge redistribution after core-level photoionization where van der Waals clusters are often used as prototype systems.

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

The interatomic resonant Auger effect in N2O is investigated experimentally and theoretically. We observe variations of the ratio between the yields of 1s-photoionization of the central and terminal nitrogen atom in the photon energy range across the O 1s → π* excitation. The present ab initio calculations of electronic structure and dynamics attribute these variations to the Fano interference between the direct N 1s-photoionizations and the resonant O 1s → π* excitation followed by Auger decays into the respective core–shell continua. The theory reveals that this interatomic core–hole-transfer effect is governed entirely by an energy transfer mechanism, and not by charge transfer.

Over a thousand spectral lines in the photoexcitation spectrum of molecular deuterium (D2) to np 1 Σ u + and 1 Π u + Rydberg levels ( n ⩾ 4 ) were measured for rotational levels N ′ = 1 –6 in the 117 000–137 000 cm−1 spectral range by two different types of experiments at two synchrotron radiation sources: a vacuum ultraviolet (VUV) Fourier-transform (FT) spectrometer at SOLEIL, Paris and a 10 m-normal-incidence monochromator (NIM) at BESSY II, Berlin. The experimental energies, the absorption cross sections, Einstein A-coefficients, and line widths are compared with ab initio multi-channel quantum defect (MQDT) calculations for these levels. More than 350 R(0) or P(2) lines were assigned, some 280 R(1) or P(3) lines, some 270 R(2) or P(4) lines, over 100 R(3) or P(5) lines, over 90 R(4) lines, and 24 R(5) lines to extract information on the N ′ = 1 –6 excited levels. Transition energies were determined up to excitation energies of 137 000 cm−1 above the ground state, thereby extending earlier work by various authors and considerably improving the spectral accuracy ( 0.1 cm−1), leading to several reassignments. The absorption and the dissociation, ionization and fluorescence excitation cross sections from the NIM experiment are measured on absolute scale and are used to calibrate intensities in the VUV-FT spectra. The overall agreement between experiment and first principles calculations, without adjustable parameters, is excellent in view of the multi-state interferences treated within the MQDT-framework: For the low N ′ values the averaged deviations between those observed in the FT-SOLEIL spectra and those calculated with MQDT are ∼ 0.1 cm−1 with a spread of ∼ 0.5 cm−1. The line intensities in terms of Einstein coefficients are well represented in the MQDT-framework, as are the level widths representing the lifetimes associated with the sum of the three decay channels. These line intensities follow, in general, the 1 / n 3 scaling behavior as characteristic in Rydberg series, but deviations occur and those are explained by MQDT. The decay dynamics of the excited N Rydberg levels is analyzed on the basis of the measured quantum yields for ionization, dissociation and fluorescence observed in the NIM experiment in terms of absolute cross sections for the distinctive channels. In particular in the n = 4 manifolds dissociation is found to play a major role, where in the n = 5 manifolds the behavior is most erratic due to strong competition between decay channels. At n = 6 , ionization takes over as the dominant channel. Despite the excellent agreement between observations and the outcome of the MQDT calculations for both level energies and dynamics, some pronounced deviations are found as in the splitting of the 5 p π , v = 4 –6, N ′ = 1 levels. The shortcomings of the MQDT calculations are ascribed to the treatment of the excited states in terms of a 1 snp single electron configuration, therewith neglecting possible interferences with 1 snf or 2 s core excited states. Some 27 lines remained unassigned; in view of their observation in fluorescence it is stipulated that these lines probe levels in the nf manifold.

Resonant interatomic Coulombic decay (RICD) in inner-valence excited neon clusters is observed by a combination of vacuum-ultraviolet (VUV) and UV/visible fluorescence spectroscopy. These ultrafast interatomic electronic processes efficiently quench radiation emission from inner-valence excited clusters. After RICD took place, outer-valence excited clusters relax further by emission of fluorescence. The direct correspondence of the structures observed in the VUV and UV/visible fluorescence signals implies that the final states of the spectator RICD decay by a cascade of radiative decays: First, by the Rydberg-to-Rydberg transitions in the UV/visible spectral range, and then, by the Rydberg-to-valence transition in the VUV range. Our study demonstrates a possibility of detecting interatomic electronic processes by UV/visible fluorescence spectroscopy.

International audience; We report the observation of the radiative decay of singly charged noble gas ground-state ions embedded in heterogeneous van der Waals clusters. Electron-photon coincidence spectroscopy and dispersed photon spectroscopy are applied to identify the radiative charge transfer from Kr atoms to a Ne þ 2 dimer, which forms after single valence photoionization of Ne atoms at the surface of a NeKr cluster. This mechanism might be a fundamental decay process of ionized systems in an environment.

The angular correlation functions (ACFs) of a pair of Lyman-$\ensuremath{\alpha}$ photons in photodissociation of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ are measured with linearly polarized incident light at a 33.66-eV incident photon energy in a wider angular range at a narrower angular step with smaller distortion than before [Y. Nakanishi et al., Phys. Rev. A 90, 043405 (2014)] so that we identify the atom-pair state emitting the pair of Lyman-$\ensuremath{\alpha}$ photons and find out whether the atom pair is entangled or not. Searching for reasonable $2p$ atom-pair states that reproduce the experimental ACFs to solve the issue, we show that hydrogen molecules are photoexcited to the ${Q}_{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1)$ state in the Franck-Condon region and then the ${Q}_{2}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1)$ state comes to superpose with the ${Q}_{2}{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}(2)$ state as the internuclear distance increases to infinity. The superposition is brought about by the spin-orbit coupling, which is effective around infinite internuclear distance because the potential-energy curves of those states are close to each other, but is negligibly small around the Franck-Condon region because they are apart from each other. The $2p$ atom pairs turn out to be in the ${1}_{u}$ superposition state, which is entangled. We therefore conclude that an entangled pair of hydrogen atoms is spontaneously produced through the photodissociation of a hydrogen molecule, and the entanglement originates from the ${1}_{u}$ symmetry properties, which are invariant during the dissociation from the Franck-Condon region towards infinite internuclear distance.

The two-photon above-threshold 3p-ionization of argon for the photon energies exceeding the 3p-ionization threshold is studied using the correlation function technique. Generalized two-photon ionization cross sections were calculated taking into account many-electron correlations. The calculations demonstrate that the two-photon ionization of Ar at the 3p4 threshold is almost entirely a collective process. The decisive contribution in the above-threshold two-photon 3p-ionization at the exciting-photon energies corresponding to the 3p→ed giant resonance comes from the Auger-like many-electron correlations of e′ de″d − 3pef type.

A set of cross sections for the formation of a pair of $2p$ atoms on an absolute scale is determined against the incident photon energy in the double photoexcitation of the isotopomers ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$, incorporating the same cross sections of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ obtained in our recent experiments [K. Hosaka et al., Phys. Rev. A 93, 063423 (2016)], and the oscillator strengths for the formation of a pair of $2p$ atoms from the precursor ${Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1)$ state, ${f}_{2p2p}^{{\text{H}}_{2}/\text{HD}/{\text{D}}_{2}}({Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1))$, are determined from the cross sections. The oscillator strength of HD, ${f}_{2p2p}^{\text{HD}}({Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1))$, is found to be larger than the value expected from ${f}_{2p2p}^{{\text{H}}_{2}/{\text{D}}_{2}}({Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1))$, considering the same decay mechanism for ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$ molecules photoexcited to the ${Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1)$ state, a mechanism which was revealed in our recent experiments for ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ mentioned above. The origin of the enhancement in the oscillator strength for HD is discussed and we show that the enhancement is attributed to nonadiabatic transitions between a gerade electronic state and an ungerade one through a term neglected in the Born-Oppenheimer approximation that vanishes in the homonuclear isotopomers (${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$) but does not vanish in the heteronuclear isotopomer (HD). It turns out that approximately 10--20 % of ${f}_{2p2p}^{\text{HD}}({Q}_{2}\phantom{\rule{0.16em}{0ex}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}(1))$ originates from such nonadiabatic transitions due to the breaking of the space-inversion symmetry for electrons.

Energy and charge transfer processes play an important role in many fundamental reactions in chemistry, biochemistry, and even technology. If an entity that is part of a larger system is photoexcited, its energy will dissipate, for example, by rearrangement of electron density in a large molecule or by photon emission (fluorescence). Here, we report the experimental observation of free electrons from a heterogeneous van der Waals cluster, in which some sites act as electron emitters receiving their energy efficiently from other "antenna" sites that are resonantly excited in the UV range. By complementing electron spectroscopy with fluorescence detection, we can directly observe that electron emission via this mechanism completely quenches fluorescence once the channel opens. We suggest this mechanism to be important for both quenching of fluorescence as well as resonantly enhancing free electron production in a variety of systems.

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.

The detection of a single photon is the most sensitive method for sensing of photon emission. A common technique for single photon detection uses microchannel plate arrays combined with photocathodes and position sensitive anodes. Here, we report on the combination of such detectors with grating diffraction spectrometers, constituting a low-noise wavelength resolving photon spectroscopy apparatus with versatile applicability. We recapitulate the operation principle of such detectors and present the details of the experimental set-up, which we use to investigate fundamental mechanisms in atomic and molecular systems after excitation with tuneable synchrotron radiation. Extensions for time and polarization resolved measurements are described and examples of recent applications in current research are given.

Due to its rising importance in science and technology in recent years, particle tracking in videos presents itself as a tool for successfully acquiring new knowledge in the field of life sciences and physics. Accordingly, different particle tracking methods for various scenarios have been developed. In this article, we present a particle tracking application implemented in Python for, in particular, spherical magnetic particles, including superparamagnetic beads and Janus particles. In the following, we distinguish between two sub-steps in particle tracking, namely the localization of particles in single images and the linking of the extracted particle positions of the subsequent frames into trajectories. We provide an intensity-based localization technique to detect particles and two linking algorithms, which apply either frame-by-frame linking or linear assignment problem solving. Beyond that, we offer helpful tools to preprocess images automatically as well as estimate parameters required for the localization algorithm by utilizing machine learning. As an extra, we have implemented a technique to estimate the current spatial orientation of Janus particles within the x-y-plane. Our framework is readily extendable and easy-to-use as we offer a graphical user interface and a command-line tool. Various output options, such as data frames and videos, ensure further analysis that can be automated., Comment: Accepted preprint submitted to Computer Physics Communications

We report the adaptation of an electron–photon coincidence detection scheme to the multibunch hybrid mode of the synchrotron radiation source BESSY II (Helmholtz-Zentrum Berlin). Single-event-based data acquisition and evaluation, combined with the use of relative detection times between the coincident particles, enable the acquisition of proper coincidence signals from a quasi-continuous excitation pattern. The background signal produced by accidental coincidences in the time difference representation is modeled using the non-coincident electron and photon spectra. We validate the method by reproducing previously published results, which were obtained in the single bunch mode, and illustrate its usability for the multibunch hybrid mode by investigating the photoionization of CO2 into CO2+B satellite states, followed by subsequent photon emission. The radiative lifetime obtained and the electron binding energy are in good agreement with earlier publications. We expect this method to be a useful tool to extend the versatility of coincident particle detection to arbitrary operation modes of synchrotron radiation facilities and other excitation sources without the need for additional experimental adjustments.

Relative ionization cross sections and fluorescence intensities as functions of the exciting-photon energy were recorded for the chiral molecules carvone, α-pinene, limonene and fenchone after excitation by monochromatized synchrotron radiation with energies of the exciting-photons between 284 eV and 289 eV. At selected exciting-photon energies dispersed fragment fluorescence spectra in the wavelength range between 365 nm and 505 nm were obtained. Time dependent density functional theory (TD-DFT) computations were performed to analyze the experimentally observed resonance-structures. Comparison of the computed and recorded spectra demonstrates the possibility of a predominant or even specific excitation of one particular stereocenter site in a molecule with more than one stereocenter.

The absolute absorption cross section and the branching ratios for the competing decay channels fluorescence, dissociation, and ionization of photoexcited long-lived 1 Π u - superexcited molecular levels in D2 have been measured with a spectral resolution of 0.001 nm from the ionization threshold of D2 up to the D( n = 1 ) + D( n = 3 ) dissociation limit. The experimental energies and absorption line intensities are compared with fully ab initio multichannel quantum defect theory (MQDT) calculations Glass-Maujean et al. (2011) which are based on quantum-chemical potential energy curves and transition moments of Wolniewicz and collaborators Wolniewicz and Staszewska (2003) and http://fizyka.umk.pl/ftp/pub/publications/ifiz/luwo . The overall agreement between experiment and theory is good. The branching ratios for the competing decay channels are also reproduced by the calculations including their substantial variations from level to level. A comparison of the vibronic interactions in the isotopomers H2 and D2 is made.

Tandem mass spectroscopy ($\textrm{MS}^2$) combined with single photon excitation in the VUV range (photon energy 4.5-9 eV) was performed on protonated uracil ($\textrm{UraH}^{+}$) and uridine ($\textrm{UrdH}^{+}$). The precursor ions with $m/z\;113$ and $m/z\;245$ respectively were produced by an Electrospray Ionization source (ESI) and accumulated inside a quadrupole ion trap mass spectrometer. After irradiation with tunable synchrotron radiation, product ion mass spectra were obtained. Fragment yields as a function of exciting energy show several maxima that can be attributed to the photo-excitation into different electronic states. For uracil, vertically excited states were calculated using the equation-of-motion coupled cluster approach (EOM-CCSD) and compared to the observed maxima. This allows to establish correlations between electronic states and resulting fragment masses and can thus help to disentangle the complex deexcitation and fragmentation pathways of nucleic acid building blocks above the first electronically excited state. Photofragmentation of the nucleoside uridine shows a significantly lower variety of fragments, indicating stabilization of the nucleobase by the attached sugar., author version

Superexcited states of ${\mathrm{H}}_{2}\mathrm{S}$ have been investigated with determining the cross sections for emission of dispersed and nondispersed atomic fluorescence against the incident photon energy in the range 11--40 eV to address the ionization and excitation of the valence electrons. This method enables us to extract the discrete electronic state from the superposition with continuous electronic states. The cross sections for H($2p$) formation have been put on an absolute scale. Ten superexcited states have been found, two in the range 13--15 eV are singly excited $2{b}_{2}^{\ensuremath{-}1}$(mo) states with a single configuration and the other eight states in the range 16--25 eV are doubly excited states with multiple configurations. State-resolved dipole oscillator strengths for H($2p$) formation in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{S}$ have been determined. Similar experiments have been performed for ${\mathrm{H}}_{2}\mathrm{O}$. The state-resolved dipole oscillator strengths for H($2p$) formation in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{S}$ are $\ensuremath{\sim}{10}^{\ensuremath{-}3}$, whereas those in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{O}$ range from $\ensuremath{\sim}{10}^{\ensuremath{-}3}$ to $\ensuremath{\sim}{10}^{\ensuremath{-}2}$. It is found that major fragment atoms are H($2p$) atoms in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{S}}^{*}$ in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{S}$. The superexcited states of ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{H}}_{2}\mathrm{S}$ are compared based on the similarity and difference of the electronic structures. It turns out from the comparison that (i) the smaller values of the dipole oscillator strengths for H($2p$) formation in the photoexcitation of ${\mathrm{H}}_{2}\mathrm{S}$ are related to the change of the major fragment atoms and (ii) the energy splitting of the doubly excited ``$4{a}_{1}^{\ensuremath{-}1}({\text{mo}}^{\ensuremath{'}})$'' states of ${\mathrm{H}}_{2}\mathrm{S}$ is enhanced in comparison with that of the doubly excited ``$2{a}_{1}^{\ensuremath{-}1}({\text{mo}}^{\ensuremath{'}})$'' states of ${\mathrm{H}}_{2}\mathrm{O}$. This enhancement is caused by the stronger electron correlation in ${\mathrm{H}}_{2}\mathrm{S}$ than in ${\mathrm{H}}_{2}\mathrm{O}$. The similarity in shape is indicated between the inner valence band and inner shell band in the fluorescence cross sections against the incident photon energy for ${\mathrm{H}}_{2}\mathrm{S}$ and ${\mathrm{H}}_{2}\mathrm{O}$.

To identify the precise molecular processes to induce DNA lesions, we attempt a novel spectroscopy of X-ray induced luminescence (XIL) using soft X-ray synchrotron radiation, which is a non-destructive analysis of the reaction intermediates in the elementary reaction pathway of damage induction and self-organized restoration. Using a liquid micro-jet technique to introduce aqueous samples in a vacuum chamber, we measure UV-visible luminescence from nucleotide solution as a function of the soft X-ray energy from the nitrogen to oxygen K-edge region. The XIL intensities for the nucleotide solutions are significantly enhanced in the soft X-ray region (410-530 eV) which is ascribed to the K-shell excitation/ionization of nitrogen atoms in the nucleobases. Furthermore, the XIL spectra do not show any signature of X-ray absorption near-edge structure (XANES) of the nucleobases. This is because the luminescence intensities collected from the integral area of the micro-jet only reflect the quantum yield of luminescence of the absorbed X-ray into UV-visible light irrespective of the absorption cross sections, i.e. of XANES. Thus the present result is the first evidence of luminescence as a result of X-ray absorption of aqueous nucleotides.

Nature Communications 9(1), 4659 (2018). doi:10.1038/s41467-018-07152-7, 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., Published by Nature Publishing Group UK, [London]

An apparatus for the coincident detection of two photons in the visible and VUV spectral range is presented equipped with two position- and time resolving detectors. The equipment enables angular resolution for the detected photons and thus allows an angular correlation between the two detected photons without changing the target cell geometry. Two different configurations of this apparatus are presented and compared in terms of solid angle coverage, imaging properties and suitability for their use in gas phase experiments at synchrotron radiation facilities.

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.

The journal of physical chemistry / B 121(10), 2326 - 2330 (2017). doi:10.1021/acs.jpcb.7b00096, Despite its importance, the structure anddynamics of liquid water are still poorly understood in manyapsects. Here, we report on the observation of opticalfluorescence upon soft X-ray irradiation of liquid water.Detection of spectrally resolved fluorescence was achieved bya combination of the liquid microjet technique and fluorescencespectroscopy. We observe a genuine liquid-phase fluorescencemanifested by a broad emission band in the 170−340 nm (4−7 eV) photon wavelength range. In addition, another narroweremission near 300 nm can be assigned to the fluorescence ofOH (A state) in the gas phase, the emitting species beingformed by Auger electrons escaping from liquid water. We arguethat the newly observed broad-band emission of liquid water isrelevant in search of extraterrestrial life, and we also envision the observed electron-ejection mechanism to find application forexploring solutes at liquid−vapor interfaces., Published by ACS, Washington, DC

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