Interface sensitivity in electron/ion yield x-ray absorption spectroscopy: the TiO₂–H₂O interface

The liquid phase has been posing a challenge to soft X-ray electron spectroscopies. While the broad availability of near-ambient pressure techniques improved the situation, accessing true dynamic in-flow processes remains the domain of closed-cell systems. These lend themselves particularly well to X-ray absorption spectroscopy (XAS), although the interpretation is complex. In our recent contribution, we elaborate on the detection mechanisms for XAS in an electrochemical flow cell. Specifically, we solve the controversy over the depth sensitivity of the so-called total-electron- (TEY) and total-ion-yield (TIY) detection modes. We show that the two detection modes carry identical signal, conforming to the charge neutrality principle. The information depth, on the order of a few nanometers, is determined by the penetration of the secondary electron cascade from the cell membrane. We then use the sensitivity to the solid–liquid interface to evidence the spectroscopic similarity of water near the surface of anatase to ice. The paper is featured on the cover of the current issue of the Journal of Physical Chemistry Letters.