Concentration Dependence of Photoemission from Methylene Blue at the Air/Water Interface: Qualitative Insights from Photoemission Yield Spectroscopy in Air and DFT Calculations

Abstract

Photoemission yield spectroscopy in air was used to investigate the concentration-dependent threshold energy of photoemission (E_T) of methylene blue (MB) at the air/water interface under ambient conditions. Aqueous MB solutions of various concentrations were prepared. The results obtained showed that E_T remained nearly constant at low bulk concentrations but decreased at higher concentrations, indicating changes in molecular packing at the interface. To understand the origin of the observed E_T changes, we performed density functional theory calculations that included dispersion corrections and solvent effects. The lowest ionization potentials were found for dimer configurations, thus providing a qualitative explanation for the observed decrease in E_T with increasing MB concentration.