  In the context of polymer surface treatment, the afterglow of an atmospheric
pressure Ar/O$_2$ radiofrequency plasma is characterized by optical emission
spectroscopy, laser induced fluorescence and mass spectrometry. The influence
of the O$_2$ gas flow rate and the source power on the plasma properties (gas
temperature, Ar excitation temperature, relative concentrations of O atoms and
OH radicals) are evaluated. We show that for plasma torch-to-substrate
distances lower than 6 mm, the afterglow creates a protective atmosphere, thus
the plasma gas composition interacting with the substrate is well controlled.
For higher distances, the influence of ambient air can no longer be neglected
and gradients in Ar, O$_2$ and N$_2$ concentrations are measured as a function
of axial and vertical position.
