Index of Refraction of Various Gases for Sodium Matter Waves

By inserting a gas cell in one arm of an atom interferometer, we have measured both the attenuation and the phase shift of a sodium matter wave that passes through monatomic (He, Ne, Ar, Kr, and Xe) or molecular gases (${\mathrm{N}}_{2}$, C${\mathrm{O}}_{2}$, N${\mathrm{H}}_{3}$, and ${\mathrm{H}}_{2}$O). This determines the complex index of refraction for Na matter waves and, more accurately, the ratio of the real to the imaginary part of the forward scattering amplitude. These measurements are compared with several semiclassical scattering models.