Thickness of the Surface of Liquid Argon near the Triple Point

The coefficient of ellipticity of the liquid-vapor interface of liquid argon has been measured to be ρ×10⁴=5.7±0.4 at 90^∘{}K, rising to 8.8±{}0.6 at 120^∘{}K. Assuming a Fermi density profile and isotropy of the dielectric constant in the interface region, this yields a 10%-90% interface thickness t(90^∘K)=7.9±0.5 {} and t(120^∘K)=15.2±1.0 {}, averaged over a surface dimension of the wavelength of light. The measurement is compared with predictions, and theoretical uncertainties are emphasised.