15min:

SYLVAIN HEILLIETTE, ANTOINE DELON, RÉMY JOST AND PATRICK DUPRÉ, Grenoble High Magnetic Field Laboratory, CNRS-MPI, BP 166, 25 Rue des Martyrs 38042 GRENOBLE, Cedex 9, France.

The \underbarCavity \underbarRing \underbarDown \underbarSpectroscopy (CRDS) technique is a unique tool for probing non fluorescing absorbing molecular species. High resolution in achievable in the blue-UV energy region by intracavity frequency doubling a CW laser source (here a Ti:Sa) and by using a seeded supersonic jet (slit nozzle) expansion.

The \( \mathrmNO₂ \) radical is known to strongly absorb in the blue energy range. However, if a LIF signal can be easily detected up to the dissociation threshold \( D₀ \) (\( \mathrmNO₂ \mathrmNO(² _1/2)+\mathrmO(³P₂) \)), above, a lack of fluorescence is observed which is typical of a photodissociation process. The usual techniques for level detection above the dissociation threshold (PHOFEX or Fluorescence Depletion Pumping, for example) are based on pulsed sources (laser bandwidth limited). At the opposite, the CW CRDS technique (residual Doppler width: \( 400 \mathrmMHz \)) allows to probe resonances above the threshold without laser bandwidth limitation. Resonances, whose the width spreads from \( 0.055 \mathrmcm^-1 \) (corresponding to a dissociation time of \( 200 \mathrmps \)) just above \( D₀ \), to larger shapes \( ( 1 \mathrmcm^-1) \) without clear structure \( 10 \mathrmcm^-1 \) higher, are identified.