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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.

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.

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.

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.

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.

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.

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

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.

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.

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

The coincident detection of particles is a powerful method in experimental physics, enabling the investigation of a variety of projectile-target interactions. The vast majority of coincidence experiments is performed with charged particles, as they can be guided by electric or magnetic fields to yield large detection probabilities. When a neutral species or a photon is one of the particles recorded in coincidence, its detection probability typically suffers from small solid angles. Here, we present two optical assemblies considerably enhancing the solid angle for photon detection in the extreme ultraviolet to visible spectral range. The efficiency and versatility of these assemblies are demonstrated for electron-photon coincidence detection, where electrons and photons emerge from fundamental processes after photoexcitation of gaseous samples by synchrotron radiation.

EUV-Spectra of electron-impact induced fluorescence from rare gases were recorded for an impact energy of 3.5 keV intending their application in the first step for spectrometer wavelength scale calibrations. The spectra are in the investigated range of 165–314 nm in good agreement with previously measured and calculated spectra. Nearly all observed lines could be identified including 196 new lines. 183 lines mainly for krypton and xenon could not be identified. The spectra are presented and discussed. A compendium of 1670 lines and their identification is added as supplementary tables . The spectra show a number of strong lines which are well suited for spectrometer wavelength scale calibrations and once their intensity relation is deduced can be used for a sensitivity determination of a spectrometer–detector combination as described in the introductory and experimental sections of the paper.

We report Lyman-series emission cross sections of neutral hydrogen dissociation fragments after valence (15--34 eV) and inner-shell (533--542 eV) excitation of water vapor with monochromatic synchrotron radiation as functions of the exciting-photon energy. In the valence excitation energy region, the thermodynamical limits of the production of the differently excited hydrogen fragments are directly observed and absolute emission cross sections are determined. For resonant inner-shell excitations, the fluorescing excited hydrogen state is found to be strongly dependent on the molecular or Rydberg-like character of the excitation.

The investigation of environmental effects in weakly bound clusters has evolved as a comprehensive field in atomic physics during the last two decades. Recently it was shown that fluorescence spectroscopy can be utilized as complementary tool to track such processes. Here, we report on progress in fluorescence spectroscopy experiments on prototypical noble gas clusters.

Tremendous progress has been made in the research on the structure and dynamics of liquids due to the development of advanced experimental techniques such as liquid microjets, enabling investigations on volatile samples in ultrahigh vacuum environments. The spectroscopy of charged particles, e.g. photoelectron or Auger electron spectroscopy on liquids, is an established field by now. Here, we report on the successful application of a fluorescence spectrometer to measure optical emission spectra from liquids irradiated with soft X-ray synchrotron radiation.

Angular distributions of the ${\mathrm{N}}_{2}$${\mathrm{O}}^{+}(X)$ and ${\mathrm{N}}_{2}$${\mathrm{O}}^{+}(A)$ photoelectrons were measured as functions of the exciting-photon energy with small bandwidth in the vicinity of the resonant ${\mathrm{N}}_{t}(1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*})$, ${\mathrm{N}}_{t}(1s\ensuremath{\rightarrow}{\ensuremath{\sigma}}^{*})$, and ${\mathrm{N}}_{c}(1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*})$ core excitations of ${\mathrm{N}}_{2}\mathrm{O}$. For selected exciting-photon energies in this range, angular distribution parameters of the ${\mathrm{N}}_{2}$${\mathrm{O}}^{+}(A\ensuremath{\rightarrow}X)$ fluorescence were also determined. These data are interpreted by ab initio calculations. Long-range exciting-photon energy dependencies of the photoelectron angular distribution parameters ${\ensuremath{\beta}}_{X}^{e}(\ensuremath{\omega})$ and ${\ensuremath{\beta}}_{A}^{e}(\ensuremath{\omega})$, recorded vibrationally unresolved, are attributed to an electronic state interference between the direct and the different resonant amplitudes for the population of the final ionic states. In addition to that, strong influence of lifetime vibrational interference is identified in the observed dispersion of the vibrationally resolved fluorescence angular distribution parameter $\ensuremath{\beta}{2}_{A}^{X}(\ensuremath{\omega})$ across the electronic resonances.

Fluorescence spectrometry is used to determine absolute cross sections of cluster-specific decay processes following photon excitation. Absolute values for selected processes in partially condensed supersonic jets are determined using atomic benchmarks. This method is applicable to a wide variety of processes in clusters in which a photon is emitted. In a proof-of-principle experiment, absolute cross sections of processes in two prototype systems were measured: (I) inner-valence excitation and ionization of Ar clusters and (II) resonant interatomic Coulombic decay (rICD) in Ne clusters. Example II is the first measurement of absolute cross sections of ICD whatsoever.

For the electronic- and charge-state specific determination of VUV?VIS fluorescence emission cross sections after collisions between heavy ions and neutral gases or electrons a fluorescence spectrometer for the VUV?VIS spectral range is planned. Tentative experiments showed that signal rates after collisions between Xe atoms and ions are high enough to allow efficient experiments.

We present emission cross section functions for Lyman series fluorescence emission after photoexcitation of water molecules with monochromatized synchrotron radiation as function of the exciting-photon energy. Both the valence and inner-shell excitation energy range of the water molecule are covered. The results are compared to literature and contribute to complete the model of dissociative relaxation dynamics of excited small molecules.

We have measured the emission spectrum of gaseous hydrogen molecules in the far ultraviolet spectral range over the large interval of excitation energies between 11 eV and 17 eV. This regime is governed by singly excited electronic states below the ionization threshold. While these features have been already investigated separately in the past, the completeness of the present measurements allows a very intuitive representation of the participating molecular effects and their respective correlations. It also contains the first complete investigation of the Condon diffraction bands visible for hydrogen through transitions into the ground state continuum.

Dispersed fluorescence excitation functions and the relative total ion yield as a function of the exciting-photon energy in the range between 26.150 eV and 26.400 eV have been measured for Xe after excitation with monochromatized synchrotron radiation with the very narrow bandwidth of 2.5 meV. Fluorescence excitation functions have been simultaneously measured in the VUV and in the visible spectral range to track and identify radiative decay cascades after photoionization into excited XeII states of 5p46p electron configuration. Additionally the angular fluorescence distribution parameter βfl-parameter was experimentally determined for some of the observed fluorescence lines.