Critical behavior of hydrogen

Abstract

We have studied the critical behavior of equilibrium ${\mathrm{H}}_{2}$ using optical techniques. From measurements of the liquid-vapor coexistence curve covering the reduced temperature range 3.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$7.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ we find the critical exponent \ensuremath{\beta}=0.326\ifmmode\pm\else\textpm\fi{}0.003 and the corrections-to-scaling exponent \ensuremath{\Delta}=0.46\ifmmode\pm\else\textpm\fi{}0.02. From compressibility measurements we find the critical exponent \ensuremath{\gamma}=1.19\ifmmode\pm\else\textpm\fi{}0.05 and the universal amplitude ratio ${\ensuremath{\Gamma}}_{0}^{+}$/${\ensuremath{\Gamma}}_{0}^{\mathrm{\ensuremath{-}}}$=5.2\ifmmode\pm\else\textpm\fi{}0.4. These results are in accord with theoretical predictions. Measurements of the coexistence curve diameter show deviations from the classical law of rectilinear diameter in the opposite direction from those observed in other fluids, possibly indicating the presence of attractive many-body interactions in fluid hydrogen.