Ion distributions at charged aqueous surfaces by near-resonance X-ray spectroscopy

Near-resonance X-ray spectroscopy is used to determine ion distributions and their local environment at charged aqueous interfaces. Energy scans at fixed momentum-transfers under specular reflectivity conditions near the L_III Cs⁺ resonance reveal the formation of a diffuse Gouy-Chapman layer at a charged surface formed by a Langmuir monolayer (dihexadecyl phosphate) spread on CsI solution. The energy scans exhibit a periodic dependence on photon momentum-transfer (Q_z) with a line-shape that consists of a Q_z-dependent linear combination of the dispersive f'(E) and absorptive f''(E) fine-structure corrections. The results in the Born approximation are discussed and more quantitatively by using the dynamical method numerically (i.e. recursive or matrix methods to calculate the reflection of electromagnetic waves from stratified media). The ion distributions obtained from the analysis of the spectroscopy are in excellent agreement with those obtained from anomalous reflectivity measurements, providing further confirmation to the validity of the renormalized surface-charge Poisson-Boltzmann theory for monovalent ions. The fine structures of f'(E) and f''(E) obtained in the process differ significantly from the multi-electron photoexcitation spectra of the isolated ion, revealing the local environment of a Cs⁺ ion in the solution at the interface. The comparison with similar X-ray absorption fine structures suggests that the Cs⁺ ion is surrounded by a shell of eight O atoms.