Photons and electrons are two common relaxation products upon X-ray absorption, enabling fluorescence yield and electron yield detections for X-ray absorption spectroscopy (XAS).The ions that are created during the electron yield process are relaxation products too, which are exploited in this study to produce ion yield for XA detection. The ionic currents measured in a liquid cell filled with water or iron(III) nitrate aqueous solutions exhibit characteristic O K-edge and Fe L-edge absorption profiles as a function of excitation energy. Application of two electrodes installed in the cell is crucial for obtaining the XA spectra of the liquids behind membranes. Using a single electrode can only probe the species adsorbed on the membrane surface. The ionic-current detection, termed as total ion yield (TIY) in this study, also produces an undistorted Fe L-edge XA spectrum, indicating its promising role as a novel detection method for XAS studies in liquid cells.Key words: total ion yield (TIY), ionic current, liquid flow-cell, total electron yield (TEY), X-ray absorption spectroscopy (XAS) TOC GraphicPage 2 of 17 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 Studying the electronic structure of liquid water and aqueous solutions by soft X-rays has attracted much attention in recent years, 1-6 and continues to be a vital research field. Resonant excitation by X-rays is highly element-specific, which makes X-ray absorption spectroscopy (XAS) a widely used tool in many scientific disciplines. Due to the vacuum requirement for soft X-ray propagation, detection of XA spectra for liquid (volatile) samples in vacuum is very challenging. One of the most applied techniques to introduce liquid samples into a vacuum chamber is a liquid flow-cell with an ultra-thin membrane separating the liquid from the vacuum. 7,8 When equipped with multiple electrodes, such a liquid cell can act as a standard electrochemical cell. It is therefore of great interest to combine XAS and the liquid cell technique for in situ/operando investigations on liquid-based materials. [9][10][11][12] When a sample's thickness exceeds the penetration depth of soft X-rays, which is the case for the liquid cell adopted in this study, detection of XA spectra in the transmission mode is not applicable. Therefore, fluorescence yield (FY) or electron yield (EY) must be employed to acquire XA spectra. Due to the significant thickness of typical membranes, e.g. Si 3 N 4 and SiC membranes (~ 100 nm), the electrons created within liquid solutions cannot penetrate the membrane and escape into vacuum. The FY was thus considered the only feasible way to probe the liquid-phase species behind membranes. However, EY has been recently realized in liquid cell studies, thanks to the newly developed graphene membra...

Detection of ionic current with two electrodes installed in a liquid cell has been established previously as an effective method, termed as total ion yield (TIY), to acquire X-ray absorption (XA) spectra of liquid solutions behind a membrane. In this study, the exact locations where TIY signals are generated are further investigated and unequivocally identified. The detected ionic current stems dominantly from the bulk solution species while only marginally from the species located at the membrane-solution interface. Such a two-electrode TIY detection in a liquid cell combines the advantages of bulk sensitivity of fluorescence yield and high signal strength (for light elements) of electron yield, exhibiting its novel and promising role in the XA spectroscopy measurements of liquid cells.

Soft X-ray emission, absorption, and resonant inelastic scattering experiments have been conducted at the nitrogen K-edge of urea and its derivatives in aqueous solutions and compared with density functional theory and time-dependent density functional theory calculations. This comprehensive study provides detailed information on the occupied and unoccupied molecular orbitals of urea, thiourea, acetamide, dimethylurea and biuret at valence levels. By identifying the electronic transitions that contribute to the experimental spectral features the energy gap between the highest occupied and the lowest unoccupied molecular orbital of each molecule is determined. Moreover, a theoretical approach is introduced to simulate resonant inelastic X-ray scattering spectra by adding an extra electron to the lowest unoccupied molecular orbital mimicking the real initial state of the core electron absorption before the subsequent relaxation process.