Electron and ion spectroscopy of azobenzene in the valence and core shells.

Azobenzene is a prototype and a building block of a class of molecules of extreme technological interest as molecular photo-switches. We present a joint experimental and theoretical study of its response to irradiation with light across the UV to x-ray spectrum. The study of valence and inner shell photo-ionization and excitation processes combined with measurement of valence photoelectron-photoion coincidence and mass spectra across the core thresholds provides a detailed insight into the site- and state-selected photo-induced processes. Photo-ionization and excitation measurements are interpreted via the multi-configurational restricted active space self-consistent field method corrected by second order perturbation theory. Using static modeling, we demonstrate that the carbon and nitrogen K edges of azobenzene are suitable candidates for exploring its photoinduced dynamics thanks to the transient signals appearing in background-free regions of the NEXAFS and XPS.

Photoelectron-photoion(s) coincidence studies of molecules of biological interest.

Photoelectron-photoion(s) coincidence, PEPICO, experiments with synchrotron radiation have become one of the most powerful tools to investigate dissociative photoionization thanks to their selectivity. In this paper their application to the study of molecular species of biological interest in the gas phase is reviewed. Some applications of PEPICO to the study of potential radiosensitizers, amino acids and small peptides and opportunities offered by the advent of novel methods for the production of beams of these molecules are discussed.

Graphene-Induced Improvements of Perovskite Solar Cell Stability: Effects on Hot-Carriers.

Hot-carriers, that is, charge carriers with an effective temperature higher than that of the lattice, may contribute to the high power conversion efficiency (PCE) shown by perovskite-based solar cells (PSCs), which are now competitive with silicon solar cells. Hot-carriers lose their excess energy in very short times, typically in a few picoseconds after excitation. For this reason, the carrier dynamics occurring on this time scale are extremely important in determining the participation of hot-carriers in the photovoltaic process. However, the stability of PSCs over time still remains an issue that calls for a solution. In this work, we demonstrate that the insertion of graphene flakes into the mesoscopic TiO2 scaffold leads to stable values of carrier temperature. In PSCs aged over 1 week, we indeed observe that in the graphene-free perovskite cells the carrier temperature decreases by about 500 K from 1800 to 1300 K, while the graphene-containing cell shows a reduction of less than 200 K after the same aging time delay. The stability of the carrier temperature reflects the stability of the perovskite nanocrystals embedded in the mesoporous graphene-TiO2 layer. Our results, based on femtosecond transient absorption measurements, show that the insertion of graphene can be beneficial for the design of stable PSCs with the aim of exploiting the hot-carrier contribution to the PCE of the PSCs.

An experimental and theoretical investigation of XPS and NEXAFS of 5-halouracils.

The C, N and O 1s excitation and ionization processes of 5X-uracil (X = F, Cl, Br, and I) were investigated using near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies. This study aims at the fine assessment of the effects of the functionalization of uracil molecules by halogen atoms having different electronegativity and bound to the same molecular site. Two DFT-based approaches, which rely on different paradigms, have been used to simulate the experimental spectra and to assign the corresponding features. The analysis of the screening of the core holes of the different atoms via electronic charge density plots has turned out to be a useful tool to illustrate the competition between the partially aromatic and partially conjugate properties of this class of molecules.

Insights into 2- and 4(5)-Nitroimidazole Decomposition into Relevant Ions and Molecules Induced by VUV Ionization.

Nitromidazoles are relevant compounds of multidisciplinary interest, and knowledge of their physical-chemical parameters as well as their decomposition under photon irradiation is needed. Here we report an experimental and theoretical study of the mechanisms of VUV photofragmentation of 2- and 4(5)-nitromidazoles, compounds used as radiosensitizers in conjunction with radiotherapy as well as high-energy density materials. Photoelectron-photoion coincidence experiments, measurements of the appearance energies of the most important ionic fragments, density functional theory, and single-point coupled cluster calculations have been used to provide an overall insight into the energetics and structure of the different ionic/neutral products of the fragmentation processes. The results show that these compounds can be an efficient source of relevant CO, HCN, NO, and NO2 molecules and produce ions of particular astrophysical interest, like the isomers of azirinyl cation ( m/ z 40), predicted to exist in the interstellar medium, and protonated hydrogen cyanide ( m/ z 28).

Circular Dichroism in Multiphoton Ionization of Resonantly Excited He

Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He^{+}(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed.

Fragmentation of pure and hydrated clusters of 5Br-uracil by low energy carbon ions: observation of hydrated fragments.

The fragmentation of the isolated 5-bromouracil (5BrU) molecule and pure and nano-hydrated 5BrU clusters induced by low energy 12C4+ ions has been studied. A comparison indicates that the environment, on the one hand, protects the system against the complete break-up into small fragments, but, on the other hand, triggers 'new' pathways for fragmentation, for example the loss of the OH group. The most striking result is the observation of several series of hydrated fragments in the hydrated cluster case, with water molecules bound to hydrophilic sites of 5BrU. This highlights the strong interaction between 5BrU and water molecules and the blocking of specific fragmentation pathways, such as the loss of the BrC2H group for example.

Determination of Energy-Transfer Distributions in Ionizing Ion-Molecule Collisions.

The ionization and fragmentation of the nucleoside thymidine in the gas phase has been investigated by combining ion collision with state-selected photoionization experiments and quantum chemistry calculations. The comparison between the mass spectra measured in both types of experiments allows us to accurately determine the distribution of the energy deposited in the ionized molecule as a result of the collision. The relation of two experimental techniques and theory shows a strong correlation between the excited states of the ionized molecule with the computed dissociation pathways, as well as with charge localization or delocalization.

Communication: "Position" does matter: The photofragmentation of the nitroimidazole isomers.

A combined experimental and theoretical approach has been used to disentangle the fundamental mechanisms of the fragmentation of the three isomers of nitroimidazole induced by vacuum ultra-violet (VUV) radiation, namely, 4-, 5-, and 2-nitroimidazole. The results of mass spectrometry as well as photoelectron-photoion coincidence spectroscopy display striking differences in the radiation-induced decomposition of the different nitroimidazole radical cations. Based on density functional theory (DFT) calculations, a model is proposed which fully explains such differences, and reveals the subtle fragmentation mechanisms leading to the release of neutral species like NO, CO, and HCN. Such species have a profound impact in biological media and may play a fundamental role in radiosensitising mechanisms during radiotherapy.

Site- and state-selected photofragmentation of 2Br-pyrimidine.

The fragmentation of the 2Br-pyrimidine molecule following direct valence photoionization or inner shell excitation has been studied by electron-ion coincidence experiments. 2Br-pyrimidine has been chosen as a model for the class of pyrimidinic building blocks of three nucleic acids and several radiosensitizers. It is known that the site- and state-localization of energy deposition, typical of inner shell excitation, results in the enhancement of the total ion yield as well as in changes in the relative intensity of the different fragmentation channels. Here we address the question of the origin of this selective fragmentation by using electron-ion coincidence techniques. The results show that the fragmentation is strongly selective in the final singly charged ion state, independently of the process that leads to the population of that state, and the dominant fragmentation patterns correlate with the nearest appearance potential.

Signature of two-electron interference in angular resolved double photoionization of Mg.

The double photoionization of Mg has been studied experimentally and theoretically in a kinematic where the two photoelectrons equally share the excess energy. The observation of a symmetrized gerade amplitude, which strongly deviates from the Gaussian ansatz, is explained by a two-electron interference predicted theoretically, but never before observed experimentally. Similar to the Cooper minima in the single photoionization cross section, the effect finds its origin in the radial extent and oscillation of the target wave function.

Resonant Auger spectroscopy at the carbon and nitrogen K-edges of pyrimidine.

The resonant Auger electron spectra obtained after photoexcitation below the C and N 1s ionization thresholds in the pyrimidine molecule have been measured at several photon energies. The results show the relevance of the localization of the inner hole and of the matching between the symmetries of the intermediate and final states in the decay spectra via participator transitions. The comparison with the Auger electron spectra suggests some assignment for the two-hole-one-particle states reached via spectator transitions. The analysis of the participator decay is supported by state-of-the art density functional theory calculations.

Vibrationally resolved photoionization of N2 near threshold.

A recently developed velocity map imaging spectrometer has been used to study the photoionization of molecular nitrogen near threshold. The potentialities of the spectrometer have been exploited to measure simultaneously the energy and angular distribution of the photoelectrons corresponding to the residual N(2)(+) X(2)Σ(g) v = 0-3 ion states. In a single experiment all the experimental observables, i.e., the total and partial cross sections, their branching ratios and the asymmetry parameter of the angular distributions have been determined.

A study of selected fragmentation paths of the ethyne dication: theory and experiment.

The fragmentation of the C(2)H(2)(2+) dication, formed upon inner shell ionization and the subsequent Auger decay, has been studied by means of Auger electron-ion and Auger electron-ion-ion coincidence spectroscopy at four different kinetic energies of the Auger electron. The experimental investigation of three fragmentation paths leading to the C(2)H(+)/H(+), C(2)(+)/H(+) and C(+)/H(+) pairs has been complemented by theoretical calculations of the Potential Energy Surfaces (PES). It is found that when the amount of internal energy of the dication increases this is mainly transferred into the kinetic energy of the fragments of the second step of the dissociation.

Radiationless decay in the region of the 2t2g and 4eg resonances in SF6.

The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.

Photoabsorption and S 2p photoionization of the SF6 molecule: resonances in the excitation energy range of 200-280 eV.

Photoabsorption and S 2p photoionization of the SF(6) molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t(2g) and 4e(g) shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s(-1)6t(1u) excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.

Pyrimidine and halogenated pyrimidines near edge x-ray absorption fine structure spectra at C and N K-edges: experiment and theory.

The inner shell excitation of pyrimidine and some halogenated pyrimidines near the C and N K-edges has been investigated experimentally by near edge x-ray absorption fine structure spectroscopy and theoretically by density functional theory calculations. The selected targets, 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine, allow the effects of the functionalization of the pyrimidine ring to be studied either as a function of different halogen atoms bound to the same molecular site or as a function of the same halogen atom bound to different molecular sites. The results show that the individual characteristics of the different spectra of the substituted pyrimidines can be rationalized in terms of variations in electronic and geometrical structures of the molecule depending on the localization and the electronegativity of the substituent.

Ion optics simulation of an ion beam setup coupled to an electrospray ionization source, strengths, and limitations.

A unified approach to achieve a start-to-end ion optics simulation of an ion beam apparatus coupled to an electrospray ionization source is presented. We demonstrate that simulations enable reliable information on the behavior and operation of the apparatus to be obtained, but due to the collisions with the buffer gas in the initial stages of the setup, the results concerning the kinetic energy of the ion beam must be treated with care.

The OCS S L3MM auger spectrum and angular distributions studied by photoelectron-auger electron coincidence experiments.

The selectivity of the photoelectron-Auger electron coincidence technique has been used to isolate the S L(3)MM contribution to the L(2,3)MM Auger spectrum in OCS. In this way, a direct comparison of the energies, widths, and relative intensities of the final OCS(2+) states with theoretical calculations has been achieved. Moreover, the angular distributions of some selected Auger electrons have been measured in coincidence with the photoelectron at two different photon energies. In contrast with the results of noncoincidence measurements, the coincidence angular distributions show a significant asymmetry and a dependence on the photon energy and OCS(2+) final state.

Investigation of halogenated pyrimidines by X-ray photoemission spectroscopy and theoretical DFT methods.

The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets-5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine-allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.