  Infrared absorption measurements in the range of 125 to 700 cm^(-1) have been
carried out as a function of temperature upto 5 K in MgB2. The absorption
spectrum is characterised by a broad band centred at 485 cm^{-1}, with
shoulders at 333 and 387 cm^{-1}. Studies on the temperature dependence of
absorption, indicate that these modes initially harden with the lowering of
temperature, and this trend is arrested at ~ 100 K, below which they soften.
Further, in the case of the mode at 333 cm^{-1}, there is a distinct softening
associated with the superconducting transition at 39 K. The implications of
these experimental results in the context of superconductivity in MgB2 are
discussed.
