  We report on angle resolved photoemission spectroscopic studies on a parent
topological insulator (TI), Bi$_2$Se$_3$. The line width of the spectral
function (inverse of the quasi-particle lifetime) of the topological metallic
(TM) states shows an anomalous behavior. This behavior can be reasonably
accounted for by assuming decay of the quasi-particles predominantly into bulk
electronic states through electron-electron interaction and defect scattering.
Studies on aged surfaces reveal that topological metallic states are very much
unaffected by the potentials created by adsorbed atoms or molecules on the
surface, indicating that topological states could be indeed protected against
weak perturbations.
