  We studied the far-infrared reflectance of superconducting MgB2 in and out of
equilibrium using pulsed synchrotron radiation. The equilibrium data can be
described by a single energy gap at 2 Delta / hc = 43 cm-1. The fit improves
slightly at lower frequencies when two gaps (at 30 cm-1 and 56 cm-1$) are
assumed to exist. The non equilibrium data is obtained when excess
quasiparticles are generated in the MgB2 film by a laser pulse, decreasing the
energy gap(s). The temperature dependence of the number of excess
quasiparticles indicates that only a single gap near 30 cm-1 controls the
non-equilibrium process. Fits to the photo-induced reflectance show the
presence of the two gaps but weakening of an energy gap at 30 cm-1 only. Our
data can be understood if one considers the picture where excited
quasiparticles created by the laser impulse relax to the lowest gap band before
recombining into Cooper pairs.
