β -Ga 2 O 3 has the widest energy gap of the transparent conducting oxides. The interest in its electronic properties has recently increased because of its applications in various optoelectronic devices, semiconductor lasers, and ultrasensitive gas detecting systems. In contrast, information on the electronic structure of β-Ga2O3 is very scarce. Here, we present the experimental valence-band structure of β-Ga2O3 single crystals determined by high-resolution angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. We find good matching of the experimental band structure with the advanced density functional theory calculations employing hybrid functionals and projector augmented wave potentials.

L-edge soft X-ray spectroscopy has been proven to be a powerful tool to unravel the peculiarities of electronic structure of transition metal compounds in solution. However, the X-ray absorption spectrum is often probed in the total or partial fluorescence yield modes, what leads to inherent distortions with respect to the true transmission spectrum. In the present work, we combine photon- and electron-yield experimental techniques with multi-reference first principles calculations. Exemplified for the prototypical FeCl2 aqueous solution we demonstrate that the partial yield arising from the Fe3s → 2p relaxation is a more reliable probe of the absorption spectrum than the Fe3d → 2p one. For the bonding-relevant 3d → 2p channel we further provide the basis for the joint analysis of resonant photoelectron and inelastic X-ray scattering spectra. Establishing the common energy reference allows to assign both spectra using the complementary information provided through electron-out and photon-out events.

Revealing the delicate balance between protonation/deprotonation and surface/bulk partitioning of organic acids and bases in aqueous solutions.

Photoionization is at the heart of X-ray photoelectron spectroscopy (XPS), which gives access to important information on a sample's local chemical environment. Local and non-local electronic decay after photoionization-in which the refilling of core holes results in electron emission from either the initially ionized species or a neighbour, respectively-have been well studied. However, electron-transfer-mediated decay (ETMD), which involves the refilling of a core hole by an electron from a neighbouring species, has not yet been observed in condensed phase. Here we report the experimental observation of ETMD in an aqueous LiCl solution by detecting characteristic secondary low-energy electrons using liquid-microjet soft XPS. Experimental results are interpreted using molecular dynamics and high-level ab initio calculations. We show that both solvent molecules and counterions participate in the ETMD processes, and different ion associations have distinctive spectral fingerprints. Furthermore, ETMD spectra are sensitive to coordination numbers, ionsolvent distances and solvent arrangement.Site-selectivity and sensitivity to the local chemical environment have made X-ray photoelectron spectroscopy (XPS) a powerful tool for probing both gas phase and condensed matter-its development has also led to a deeper understanding of the complex and competitive processes that occur as a result of X-ray-substrate interactions. The creation of deep inner-shell electron holes through X-ray photoionization is followed by relaxation processes that provide additional important insight into electronic structure and correlation in the valence-electron region. One such process is Augerelectron decay, in which, within the same initially ionized species, a valence electron relaxes to fill a core vacancy, causing the emission of an electron from a higher state. Auger processes have found widespread applications in many areas of research, especially in materials science and surface-composition analysis. Element-selectivity of these X-rayinduced de-excitations opens a way for targeted energy deposition, which can be used in medicine, in particular for cancer treatment1 or for selective transformations of molecules and materials2. However, Auger decay is not the only relaxation process that can occur after initial photoelectron emission. Several experimental and theoretical works have demonstrated electronic relaxation processes that can efficiently compete with local Auger decay and that are 'non-local' in nature, that is, they involve species other than the initially ionized monomer. The best studied process is intermolecular Coulombic decay (ICD)3, which occurs in weakly interacting systems such as rare gases and hydrogen-bonded complexes4,5. In an ICD process, the energy gained after refilling the initial hole created by ionization or excitation is used to eject an electron from a neighbouring species, resulting in the formation of two singly charged units that subsequently separate by Coulomb repulsion. The competition of non-local an...

Recently, a new family of autoionization processes has been identified in aqueous phases. The processes are initiated by core-electron ionization of a solute molecule and involve proton transfer along the solute-solvent hydrogen bond. As a result, short-lived singly charged cations form with structures sharing a proton between solute and solvent molecules. These molecular transients decay by autoionization, which creates reactive dicationic species with the positive charges delocalized over the entire molecular entity. Here, we investigate the ultrafast electron and nuclear dynamics following the core ionization of hydrated ammonia and glycine. Both molecules serve as models for exploring the possible role of the nonlocal relaxation processes in the chemical reactivity at the interface between, for instance, a protein surface and aqueous solution. The nature of the postionization dynamical processes is revealed by high-accuracy Auger-electron spectroscopy measurements on liquid microjets in vacuum. The proton-transfer-mediated processes are identified by electron signals in the high-energy tail of the Auger spectra with no analogue in the Auger spectra of the corresponding gas-phase molecule. This high-energy tail is suppressed for deuterated molecules. Such an isotope effect is found to be smaller for aqueous ammonia as compared to the hydrated H2O molecule, wherein hydrogen bonds are strong. An even weaker hydrogen bonding for the hydrated amino groups in glycine results in a negligibly small proton transfer. The dynamical processes and species formed upon the nitrogen-1s core-level ionization are interpreted using methods of quantum chemistry and molecular dynamics. With the assistance of such calculations, we discuss the conditions for the proton-transfer-mediated relaxation processes to occur. We also consider the solvent librational dynamics as an alternative intermolecular ultrafast relaxation pathway. In addition, we provide experimental evidence for the umbrella-type motion in aqueous ammonia upon core ionization. This intramolecular channel proceeds in parallel with intermolecular relaxation processes in the solution.

Detection of secondary emissions, fluorescence yield (FY), or electron yield (EY), originating from the relaxation processes upon X-ray resonant absorption has been widely adopted for X-ray absorption spectroscopy (XAS) measurements when the primary absorption process cannot be probed directly in transmission mode. Various spectral distortion effects inherent in the relaxation processes and in the subsequent transportation of emitted particles (electron or photon) through the sample, however, undermine the proportionality of the emission signals to the X-ray absorption coefficient. In the present study, multiple radiative (FY) and nonradiative (EY) decay channels have been experimentally investigated on a model system, FeCl3 aqueous solution, at the excitation energy of the Fe L-edge. The systematic comparisons between the experimental spectra taken from various decay channels, as well as the comparison with the theoretically simulated Fe L-edge XA spectrum that involves only the absorption process, indicate that the detection of the Fe 3s → 2p partial fluorescence yield (PFY) gives rise to the true Fe L-edge XA spectrum. The two key characteristics generalized from this particular decay channel-zero orbital angular momentum (i.e., s orbital) and core-level emission-set a guideline for obtaining undistorted X-ray absorption spectra in the future.

Recent studies on sea spray aerosol indicate an enrichment of Ca 2+ in small particles, which are often thought to originate from the very surface of a water body when bubbles burst. One model to explain this observation is the formation of ion pairs between Ca 2+ (aq) and surface-active organic species. In this study, we have used Xray photoelectron spectroscopy to probe aqueous salt solutions and artificial sea spray aerosol to study whether ion pairing in the liquid environment also affects the surface composition of dry aerosol. Carboxylic acids were added to the sample solutions to mimic some of the organic compounds present in natural seawater. Our results show that the formation of a core-shell structure governs the surface composition of the aerosol. The core-shell structure contrasts previous observations of the dry sea spray aerosol on substrates. As such, this may indicate that substrates can impact the morphology of the dried aerosol.

Non-local analogues of Auger decay, such as Intermolecular Coulombic Decay, are increasingly recognized as important relaxation processes in aqueous electrolytes.