Electron mobility maximum in dense argon gas at low temperature

We report measurements of excess electron mobility in dense Argon gas at the two temperatures $T=152.15$ and 162.30 K, fairly close to the critical one ($T_c =150.7$ K), as a function of the gas density $N$ up to 14 atoms$\cdot$nm$^{-3}$ ($N_{c}=8.08$ atoms$\cdot$nm$^{-3}$). For the first time a maximum of the zero-field density-normalized mobility $\mu_{0}N$ has been observed at the same density where it was detected in liquid Argon under saturated vapor pressure conditions. The existence of the $\mu_{0}N$ maximum in the liquid is commonly attributed to electrons scattering off long-wavelength collective modes of the fluid, while for the low-density gas a density-modified kinetic model is valid. The presence of the $\mu_{0}N$ maximum also in the gas phase raises therefore the question whether the single scattering picture valid in the gas is valid even at liquid densities.