Quasi-free Electron Energy in Near Critical Point He

We present the quasi-free electron energy $V_0(\rho)$, where $\rho$ is the density, in He from low density to the density of the gas/He-I/He-II triple point, at various noncritical temperatures and on an isotherm near the critical isotherm. These data, which were obtained using field enhanced photoemission, represent the first extended density overview of $V_0(\rho)$ for He, especially near the critical point. A novel critical point effect is observed and is accurately fitted to the local Wigner-Seitz model, thereby showing that this model, which was developed for attractive systems, can also be applied to repulsive systems.\\[4pt] The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund (45728-B6) and from the National Science Foundation (NSF CHE-0956719).