Disentangling the evolution of electrons and holes in photoexcited ZnO nanoparticles.

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy, and ab initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The x-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the x-ray absorption signal is found to rise in ∼1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ∼100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice.

Transient resonant Auger-Meitner spectra of photoexcited thymine.

We present the first investigation of excited state dynamics by resonant Auger-Meitner spectroscopy (also known as resonant Auger spectroscopy) using the nucleobase thymine as an example. Thymine is photoexcited in the UV and probed with X-ray photon energies at and below the oxygen K-edge. After initial photoexcitation to a ππ* excited state, thymine is known to undergo internal conversion to an nπ* excited state with a strong resonance at the oxygen K-edge, red-shifted from the ground state π* resonances of thymine (see our previous study Wolf, et al., Nat. Commun., 2017, 8, 29). We resolve and compare the Auger-Meitner electron spectra associated both with the excited state and ground state resonances, and distinguish participator and spectator decay contributions. Furthermore, we observe simultaneously with the decay of the nπ* state signatures the appearance of additional resonant Auger-Meitner contributions at photon energies between the nπ* state and the ground state resonances. We assign these contributions to population transfer from the nπ* state to a ππ* triplet state via intersystem crossing on the picosecond timescale based on simulations of the X-ray absorption spectra in the vibrationally hot triplet state. Moreover, we identify signatures from the initially excited ππ* singlet state which we have not observed in our previous study.

Photoionization Dynamics of the Tetraoxo Complexes OsO4 and RuO4.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17-90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom.

Proton dynamics in molecular solvent clusters as an indicator for hydrogen bond network strength in confined geometries.

Hydrogen bonding leads to the formation of strong, extended intermolecular networks in molecular liquids such as water. However, it is less well-known how robust the network is to environments in which surface formation or confinement effects become prominent, such as in clusters or droplets. Such systems provide a useful way to probe the robustness of the network, since the degree of confinement can be tuned by altering the cluster size, changing both the surface-to-volume ratio and the radius of curvature. To explore the formation of hydrogen bond networks in confined geometries, here we present O 1s Auger spectra of small and large clusters of water, methanol, and dimethyl ether, as well as their deuterated equivalents. The Auger spectra of the clusters and the corresponding macroscopic liquids are compared and evaluated for an isotope effect, which is due to proton dynamics within the lifetime of the core hole (proton-transfer-mediated charge-separation, PTM-CS), and can be linked to the formation of a hydrogen bond network in the system. An isotope effect is observed in water and methanol but not for dimethyl ether, which cannot donate a hydrogen bond at its oxygen site. The isotope effect, and therefore the strength of the hydrogen bond network, is more pronounced in water than in methanol. Its value depends on the average size of the cluster, indicating that confinement effects change proton dynamics in the core ionised excited state.

Competition between proton transfer and intermolecular Coulombic decay in water.

Intermolecular Coulombic decay (ICD) is a ubiquitous relaxation channel of electronically excited states in weakly bound systems, ranging from dimers to liquids. As it is driven by electron correlation, it was assumed that it will dominate over more established energy loss mechanisms, for example fluorescence. Here, we use electron-electron coincidence spectroscopy to determine the efficiency of the ICD process after 2a1 ionization in water clusters. We show that this efficiency is surprisingly low for small water clusters and that it gradually increases to 40-50% for clusters with hundreds of water units. Ab initio molecular dynamics simulations reveal that proton transfer between neighboring water molecules proceeds on the same timescale as ICD and leads to a configuration in which the ICD channel is closed. This conclusion is further supported by experimental results from deuterated water. Combining experiment and theory, we infer an intrinsic ICD lifetime of 12-52 fs for small water clusters.

Dissociation kinetics of excited ions: PEPICO measurements of Os3(CO)12 - The 7-35 eV single ionization binding energy region.

In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os3(CO)12, in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behavior was observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 ± 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 ± 0.4 eV, corresponding to one electron being ejected from the Os3 center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

Stimulated X-ray Raman scattering - a critical assessment of the building block of nonlinear X-ray spectroscopy.

With the invention of femtosecond X-ray free-electron lasers (XFELs), studies of light-induced chemical reaction dynamics and structural dynamics reach a new era, allowing for time-resolved X-ray diffraction and spectroscopy. To ultimately probe coherent electron and nuclear dynamics on their natural time and length scales, coherent nonlinear X-ray spectroscopy schemes have been proposed. In this contribution, we want to critically assess the experimental realisation of nonlinear X-ray spectroscopy at current-day XFEL sources, by presenting first experimental attempts to demonstrate stimulated resonant X-ray Raman scattering in molecular gas targets.

Mechanisms of site-specific photochemistry following core-shell ionization of chemically inequivalent carbon atoms in acetaldehyde (ethanal).

Site-specific fragmentation upon 1s photoionisation of acetaldehyde has been studied using synchrotron radiation and a multi-electron-ion coincidence technique based on a magnetic bottle. Experimental evidence is presented that bond rupture occurs with highest probability in the vicinity of the initial charge localisation and possible mechanisms are discussed. We find that a significant contribution to site-specific photochemistry is made by different fragmentation patterns of individual quantum states populated at identical ionisation energies.

Ultrafast Molecular Three-Electron Auger Decay.

Three-electron Auger decay is an exotic and elusive process, in which two outer-shell electrons simultaneously refill an inner-shell double vacancy with emission of a single Auger electron. Such transitions are forbidden by the many-electron selection rules, normally making their decay lifetimes orders of magnitude longer than the few-femtosecond lifetimes of normal (two-electron) Auger decay. Here we present theoretical predictions and direct experimental evidence for a few-femtosecond three-electron Auger decay of a double inner-valence-hole state in CH_{3}F. Our analysis shows that in contrast to double core holes, double inner-valence vacancies in molecules can decay exclusively by this ultrafast three-electron Auger process, and we predict that this phenomenon occurs widely.

Ionization and photofragmentation of Ru3(CO)12 and Os3(CO)12.

In this paper, we use a combination of photoelectron spectroscopy, mass spectrometry, and density functional theory calculations to get a detailed understanding of valence single and double ionization and the subsequent dissociation processes. This is exemplified on benchmark systems, trimetallo-dodecacarbonyls M3(CO)12 with M = Ru, Os, where the energy remaining in the molecule after photoionization can be retrieved by measuring the degree of fragmentation of the molecular ion. The intensity of different mass peaks can thus be directly related to ionization cross sections obtained by photoelectron spectroscopy. We find that the M-CO dissociation energy rises as the number of CO ligands decreases due to dissociation. Moreover, ionization of the CO ligands has a higher cross section than that of the metal center for both single and double ionization. After advanced fragmentation, a CO bond can break and the carbon atom remains bonded to the metal core. In addition, we found that the valence ionization cross sections of M3(CO)12 are maximal at about 40 eV photon energy thus showing a more pronounced shape resonance than Ru and Os-complexes with a single metal atom center. Finally, an np → nd giant resonance absorption causes a significant increase of the ionization cross section above 50 eV for Ru3(CO)12.

Internal conversion versus intersystem crossing: what drives the gas phase dynamics of cyclic α,ß-enones?

We investigate the competition between intersystem crossing (ISC) and internal conversion (IC) as nonradiative relaxation pathways in cyclic α,ß-unsaturated enones following excitation to their lowest lying (1)ππ* state, by means of time-resolved photoelectron spectroscopy and ab initio computation. Upon excitation, the (1)ππ* state of 2-cyclopentenone decays to the lowest lying (1)nπ* state within 120 ± 20 fs. Within 1.2 ± 0.2 ps, the molecule subsequently decays to the triplet manifold and the singlet ground state, with quantum yields of 0.35 and 0.65, respectively. The corresponding dynamics in modified derivatives, obtained by selective methylation, show a decrease in both IC and ISC rates, with the quantum yields of ISC varying between 0.35 and 0.08. The rapid rates of ISC are explained by a large spin orbit coupling of 45-60 cm(-1) over an extended region of near degeneracy between the singlet and triplet state. Furthermore, the rate of IC is depressed by the existence of a well-defined minimum on the (1)nπ* potential energy surface. The nonadiabatic pathways evinced by the present results highlight the fact that these molecular systems conceptually represent "intermediate cases" between ultrafast dynamics mediated by vibrational motions at conical intersections versus those by statistical decay mechanisms.

The photoelectron angular distribution of water clusters.

The angular distribution of photoelectrons emitted from water clusters has been measured by linearly polarized synchrotron radiation of 40 and 60 eV photon energy. Results are given for the three outermost valence orbitals. The emission patterns are found more isotropic than for isolated molecules. While a simple scattering model is able to explain most of the deviation from molecular behavior, some of our data also suggest an intrinsic change of the angular distribution parameter. The angular distribution function was mapped by rotating the axis of linear polarization of the synchrotron radiation.

Autoionization mediated by electron transfer.

Electron-electron coincidence spectra of Ar-Kr clusters after photoionization have been measured. An electron with the kinetic energy range from 0 to approximately 1 eV is found in coincidence with the Ar 3s cluster photoelectron. The low kinetic energy electron can be attributed to an Ar + Kr+ + Kr+ final state which forms after electron transfer mediated decay. This autoionization mechanism results from a concerted transition involving three different atoms in a van der Waals cluster; it was predicted theoretically, but hitherto not observed.

Interatomic Coulombic decay in mixed NeKr clusters.

We report the occurrence of interatomic Coulombic decay (ICD) in mixed NeKr clusters. A well-defined feature ranging from 9 to 12 eV in kinetic energy is observed in coincidence with the Ne 2s photoelectrons. It derives from an ICD process, in which an initial Ne 2s vacancy is filled by a Ne 2p electron and an electron is emitted from a 4p level on a neighboring Kr atom. We have studied the dependence of the effect on photon energy, cluster composition, and cluster size. Interestingly, the ICD electron energy increases slightly and grows a shoulder on going from 2% to 5% Kr in the coexpansion process, which we interpret in terms of surface versus bulk effects.

Valence ionization of water clusters: from isolated molecules to bulk.

The valence photoelectron spectra of water clusters are studied experimentally and by ab initio calculations. The size dependence of the vertical ionization energy of the outermost orbitals is explicitly shown. A shift toward lower values is observed. For small cluster sizes, it can be rationalized as an effect of charge delocalization as the system is becoming more extended. Ionization energies of larger clusters decrease linearly with inverse cluster radius and asymptotically approach the value of liquid water. In the calculations, we apply a reflection principle approach based on sampling a quantum mechanical distribution of different initial-state geometries to clusters. An excellent agreement of peak shapes calculated thus with measured ones is shown. Using additional polarization fields, the extension of this approach to the photoionization of liquid water is demonstrated. Upon deuteration of the water clusters, we experimentally and theoretically find slightly larger absolute values of the vertical ionization energies. We suggest that the measurement of electron ionization energies can be used as an alternative means to characterize water cluster sizes, which can complement the use of scaling laws.