  Electron--impact excitation of Xe atoms in pure Xe gas and in a Xe(10
%)--Ar(90 %) mixture has led to the discovery of infrared (IR) luminescence of
Xe$_{2}$ excimers. The investigation of the emission spectrum at low gas
density has allowed the identification of the molecular states involved in the
transition. When the gas density is increased to values up to 40 times larger
than the density of the ideal gas at standard temperature and pressure, the
interaction of the excimer with the dense environment produces a strong
red--shift of the spectrum that is interpreted in terms of many--body effects.
