Measuring concentration depth profiles at liquid surfaces: Comparing angle resolved X-ray photoelectron spectroscopy and neutral impact collision scattering spectroscopy

Measuring concentration depth profiles is important for analyzing surfaces. The surface excess, the change in concentration from the surface to the bulk and separation of the constituents are some of the features that can be derived from concentration depth profiles and are most important in particular for analyzing liquid surfaces. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and neutral impact collision ion scattering spectroscopy (NICISS) are methods used for determining concentration depth profiles at liquid surfaces. Here we compare concentration depth profiles determined with both methods from the constituents of a solution of the ionic surfactant tetrabutylphosphonium bromide in the polar solvent formamide. ARXPS is performed with a laboratory X-ray source and NICISS with 4.5 keV helium ions. Agreement is found in the surface excess and in the shape of the cation concentration depth profiles. Disagreement was found in the shape of the anion concentration depth profiles. The separation of charges as found with NICISS for projectiles with low kinetic energies could not be reproduced. The advantages of each of the methods and the criteria for selecting the projectile energy in NICISS are discussed.