The ion-thin-film interaction during plasma chemical vapor deposition of diamond like carbon (DLC) films from a methane electron cyclotron resonance (ECR) plasma was investigated by means of in-situ ellipsometry. The ion bombardment during growth leads to a disturbed surface layer, which becomes apparent in the in-situ ellipsometric results by optical constants, which are higher than those for the bulk material. The optical constants of the modified surface layer increase with ion energy during deposition, but decrease with the addition of hydrogen to the source gas. After switching off the plasma, the increased optical constants of the surface layer relax to their equilibrium state. The sources of this modified film surface and the mechanisms for its relaxation are discussed in this paper.