The transport mechanisms of radicals (H and D) and neutrals (Ar) in plasma expansions have been studied by means of laser-based diagnostic techniques: TALIF, LIF and UV Rayleigh scattering. The flow of radicals does not fulfill the well-established free-jet shock wave picture. H(D) atoms escape the supersonic domain of the plasma jet. This outward diffusion process is driven by the huge density gradients induced in the background by wall-recombination. For the first time it is clearly shown that processes at surfaces can strongly influence the radical flow pattern.