  A systematic shift of the photon recoil 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\hbar k, where n is the index of refraction of the gas and k is the
vacuum wavevector of the photon. This systematic effect must be accounted for
in high-precision atom interferometry with light gratings.
