Observation and quantification of OH radicals in the far downstream part of an atmospheric microwave plasma jet using cavity ringdown spectroscopy

An atmospheric argon microwave plasma jet with a plasma column 8 mm long and 1-2 mm wide is studied. Existence of hydroxyl (OH) radicals down to 28 mm away from the plasma orifice is evidenced by UV cavity ringdown spectroscopy. The measured OH (v(')=0, J(')=3.5) number density at 28 mm is 7.2x10(12) molecule/cm(3). The plasma gas flow velocity, along with the known OH lifetimes, suggests that OH radicals observed in the downstream are not the OH radicals formed in the plasma column but were formed in the downstream.