Photoelectron Emission by Solutions. II

Experimental conditions were determined for study of photoelectron emission (PEE) by aqueous solutions of sodium and lithium ferrocyanide. PEE currents for emission in pure water vapor were corrected for an appreciable Schottky effect and for electron back scattering. The Thomson equation yielded emission currents of the correct order of magnitude, but average velocities of electrons at emission, deduced from this equation, were incorrect. This result is explained by failure to consider in the Thomson equation the cross section for electron–water-molecule collisions and the energy dependence of this quantity. The error arising from electron attachment to water molecules in the gas phase is evaluated. The PEE spectral response is a sigmoid curve in the 186–214-nm explored range. This shape is accounted for by fluctuations in solvation structure and their effect on the energy required for PEE. The spectral response curve yields the vertical work function of the emitter in solution, i.e., approximately 6.2 eV for ferrocyanide in aqueous solution (0.03–1.3M). This is believed to be the first direct determination of its kind for species in solution. The difference between the vertical work function and the energy for charge transfer to solvent (CTTS) is calculated from the Platzman–Franck polaron model. The energy for PEE is also calculated directly from this model. Agreement with experiment is satisfactory, but the assignment of the CTTS band for ferrocyanide is uncertain. Theory and previous work on CTTS spectra suggest PEE by other systems, e.g., the varified PEE by lithium iodide in acetonitrile.