  The influence of ambient air on the flowing afterglow of an atmospheric
pressure Ar/O2 radiofrequency plasma has been investigated experimentally.
Spatially resolved mass spectrometry and laser induced fluorescence on OH
radicals were used to estimate the intrusion of air in between the plasma torch
and the substrate as a function of the torch-to-substrate separation distance.
No air is detected, within the limits of measurement uncertainties, for
separation distances smaller than 5 mm. For larger distances, the effect of
ambient air can no longer be neglected, and radial gradients in the
concentrations of species appear. The Ar 4p population, determined through
absolute optical emission spectroscopy, is seen to decrease with separation
distance, whereas a rise in emission from the N2(C--B) system is measured. The
observed decay in Ar 4p and N2(C) populations for separation distances greater
than 9mm is partly assigned to the increasing collisional quenching rate by N2
and O2 molecules from the entrained air. Absorption measurements also point to
the formation of ozone at concentrations from 10^14 to 10^15 cm-3, depending
both on the injected O2 flow rate and the torch-to-substrate separation
distance.
