Strong salt depletion at the surface of highly concentrated aqueous solutions of CsF, studied by HeI-UPS

We measure He(I) photoelectron spectra from the surface of concentrated aqueous solutions of CsF. The composition ranges from CsF · 4.2H 2O to CsF · 2.6H 2O. We find the three bands characteristic for the water orbitals 1b 1, 3a 1 and 1b 2. We evaluate the respective phase transition energy shifts to 1.68, 1.29 and 1.7 eV with an experimental uncertainty of ± 0.1 eV. Further, we identify signals originating from the ions Cs + and F −. With the aid of absolute photoionization cross sections it is possible to relate the signal strengths of the ions to those of the water bands. We conclude that the salt concentration is strongly depleted in the surface range accessible by UPS. Assuming a salt-free overlayer we determine its thickness to d ≈ 7.5 A ̊ (± 50%) which would be equivalent to about 2 monolayers of water on top of the highly concentrated bulk phase.