Intermolecular Radiative Decay: A non-local decay mechanism providing an insider's view of the solvation shell

TL;DR

Intermolecular Radiative Decay (IRD) is demonstrated as a non-local X-ray emission process in which a water-solvent electron fills a core hole on a solvated ion, producing a photon and enabling chemically selective probing of the first solvation shell. Combined experimental X-ray emission spectroscopy on Na^{+} and Mg^{2+} in water with HF/RS-SCF and MD-based calculations shows IRD features arise from water–ion hybridized orbitals, dominated by metal np character, while the final hole localizes on surrounding water molecules. The study validates the one-center approximation for IRD, reveals strong distance- and orientation-dependence of IRD signals, and demonstrates IRD’s potential to reveal solvation-shell properties, including ion pairing and shell disorder, from within the solvent environment. These findings establish IRD as a powerful tool for chemically selective analysis of solvation shells with implications for environmental, biological, and materials chemistry.

Abstract

Aqueous solutions are crucial in chemistry, biology, environmental science, and technology. The chemistry of solutes is influenced by the surrounding solvation shell of water molecules, which have different chemical properties than bulk water due to their different electronic and geometric structure. It is an experimental challenge to selectively investigate this property-determining electronic and geometric structure. Here, we report experimental results on a novel non-local X-ray emission process, Intermolecular Radiative Decay (IRD), for the prototypical ions Na$^{+}$ and Mg$^{2+}$ in water. We show that, in IRD, an electron from the solvation shell fills a core hole in the solute, and the released energy is emitted as an X-ray photon. We analyze the underlying mechanism using theoretical calculations, and show how IRD will allow us to meet the challenge of chemically selective probing of solvation shells from within.

Figures (15)

• Figure 1: Schematic illustration of the Intermolecular Radiative Decay (IRD) process for a solvated M$^{q}$ ion. From the left, we show the ground state with a M$^{q}$ ion surrounded by six water molecules, followed by core photoionization, which results in a core-ionized M$^{q+1}$ ion. In the IRD process, an electron from a solvation-shell water molecule fills the M core hole, and the released energy is emitted as an X-ray photon. In the final state, the M$^{q}$ ion is back to its ground state, and the hole is in the valence levels of the solvation-shell water molecules.
• Figure 2: Top: Schematic illustration of the electronic transitions in the local K$_{\alpha}$ decay (green box) and non-local IRD decay (red box). Middle: Experimentally measured X-ray emission spectra for Na$^{+}$ and Mg$^{2+}$ after $1s$ ionization of the metal ion M in the energy range of K$_{\alpha}$ and IRD. For both ions, the spectra exhibit two main features, of which the most intense one is due to the local K$_{\alpha}$ emission, M$^{q+1}$1s$^{-1}$$\rightarrow$ M$^{q+1}$2p$^{-1}$ + h${\nu}$. The weaker features at higher photon energies agree well with the energies estimated for the non-local intermolecular radiative decay IRD, M$^{q+1}$1s$^{-1}$ + H$_{2}$O $\rightarrow$ M$^{q}$ + H$_{2}$O$^{+}$val$^{-1}$ + h${\nu}$. Bottom: Theoretical X-ray emission spectra for Na$^{+}$ and Mg$^{2+}$ calculated for M$^{q}[\text{H}_2\text{O}]_6$ clusters. The absolute energy scales of the theoretical spectra is shifted so that the K$_{\alpha}$ emission aligns with the experimental value. The feature around 1277 eV in the Mg spectrum is due to IRD involving the inner valence orbital 2a$_{1}$ of water, which mainly consists of O 2s. This will not be further discussed, as the corresponding spectral region was not measured.
• Figure 3: Mulliken charges in the ground, intermediate, and final states for Na$^{+}$[H$_{2}$O]$_{6}$ (lower panel) and Mg$^{2+}$[H$_{2}$O]$_{6}$ (upper panel). The total charge (black) is decomposed into charge on the metal ion (red), and on the water molecules (blue). For both metal ion and water, the final-state charge after both local K$_{\alpha}$ decay and non-local IRD is shown. The connecting lines are only guides for the eye.
• Figure 4: Comparison of one-hole spectra. From the top, liquid-water PES spectrum recorded using a photon energy of 532 eV, i.e. under relatively bulk-sensitive conditions Thurmer_PRL_2013 (blue), Na IRD spectrum (red, experimental solid and theory dashed), Mg IRD spectrum (green, experimental solid and theory dashed). Experimental and theoretical IRD spectra are enlargements of the data shown in Fig. \ref{['img:spectra']}. Solid black curves indicate a fitting of the experimental data using a minimal number of Gaussian curves. Original theoretical data for Na and Mg are represented by black bars, and the dashed curves for their convolution with a Gaussian function (FWHM = 0.8 eV) to facilitate the comparison with the experimental spectra. Vertical solid lines indicate calculated binding energies from Ref. ICDmanuscript.
• Figure 5: The experimental X-ray spectrum after Mg$^{2+}$$1s$ ionization in the energy range of K$_{\alpha}$ and IRD measured at P04. The spectrum shown is a sum of several spectra recorded with different photon energies above and just below the Mg$^{2+}$$1s$ ionization threshold. Within the experimental resolution these spectra exhibit no significant differences. To improve the statistics, the spectra were therefore summed. The spectrum contains two peaks, of which the most intense one is the K$_{\alpha}$, Mg$^{3+}$1s$^{-1}$$\rightarrow$ Mg$^{3+}$1s$^{2}$2p$^{-1}$ + h$\nu$. The weaker peak at 1290.4 eV agrees well with the energy estimated for the non-local radiative decay, Mg$^{3+}$1s$^{-1}$ + H$_{2}$O $\rightarrow$ Mg$^{2+}$ + H$_{2}$O$^{+}$ val$^{-1}$ + h$\nu'$. The vertical dashed lines are guides for the eye for the K$_{\alpha}$ and IRD features.