Cold Jet Plasma Studies by Cavity Ring Down Absorption Spectroscopy (CRDS)

Reactive intermediates are of crucial importance both for combustion and atmospheric chemistry. We have set up a cold radical plasma source based on a pulsed slit jet supersonic expansion (5 cm long) and a transverse DC discharge. This source is inserted inside a CRDS cavity running in the near and mid infra-red ranges (the high resolution laser source is described in the second paper). We will present results obtained in the rotational temperature range of 15 to 30~K on various species including the metastable nitrogen (transition: $B\,^{3}\Pi_{g}(v'=2)\leftarrow A\,^{3}\Sigma_{u}^{+}(v''=4)$), hydroxyl radical (first overtone), methyl radical (fundamental transition of the antisymmetric CH stretch mode) and preliminary results on the weak $\widetilde{A}-\widetilde{X}$ transition of methyl peroxy (${\rm CH}_{3}{\rm O}_{2})$. Density number of certain of these unstable species can reach $\sim4\times10^{14}/{\rm cm}^{3}$ inside the throat.% \footnote{S.~Wu, P. Dupr\'e and T.A.~Miller, Phys. Chem. Chem. Phys. 2006, 8, 1682% } Sensitivities better than $40\,{\rm ppb}/{\rm pass}/\sqrt{{\rm Hz}}$ have been reached.