Photon Recoil Momentum in Dispersive Media

A systematic shift of the photon recoil momentum due to the index of refraction of a dilute gas of atoms has been observed. The recoil frequency was determined with a two-pulse light grating interferometer using near-resonant laser light. The results show that the recoil momentum of atoms caused by the absorption of a photon is $n\hslash k$, where $n$ is the index of refraction of the gas and $k$ is the vacuum wave vector of the photon. This systematic effect must be accounted for in high-precision atom interferometry with light gratings.