Evaporation of Cold Metallic Hydrogen

Solid metallic hydrogen is only metastable at low pressures, but the transition to the molecular form may require surface phenomena. The "evaporation" of pairs of adjacent surface atoms (forming free $\mathrm{H}_2^{}{}_{}{}^{-}$ molecules) is discussed. A monolayer of surface atoms is lost in $e^G$ times a lattice vibration period, where $G\lesssim 20$ at zero pressure and temperature. $G$ increases with pressure and decreases with temperature above $T_c\sim 150$ °K.