Volume 68. number I CHEMICAL PHYSICS LETTERS 1 December 1979 MULTIPHOTON VIBRATIONAL PUMPING OF OPTICALLY PREPARED NO2 MOLECULES*

Reported is the collisionless infrared multiphoton excitition of NO2 mokcules initially prepared in sn eIectronicaUy excited state. From the shape of the infrared induced blue-shifted fluorescence spectrum the probnbditp distrrbution P(Jz) for the net absorption of tr photons has been deduced. The shape of the probability distribution function P(n) for Iz-photon infrared excitation of molectdes has been the subject of lively discussion in the liter-ature [l-3] _ In this work, we report the observation of infrared multiphoton vibrational excitation of.r molecule initially prepared in an electronically ex-cited state _ By monitoring the transient vistble fluo-rescence spectrum, we determined the energy dis-tribution function produced by collisionIess infrared multiphoton pumping. Vibrational excitation of an electronically excited molecule has been reported earlier [4] for the jA, state of biacetyl. That excitation involved a collision-al ener7 exchange between vibrationally excited ground state moIecdes and optically prepared tri-plet molecules. The present experiment involves single photon optical excitation of NO, molecuIes from the?A1 ground state to high levels of the strongly mixed zB2-zA1 vibronic manifold, followed by direct in-frared multiphoton pumping with a very short CO, laser pulse _ This work was prompted by an earlier report [5], in which weak double resonance signals were observed in NO? molecules exposed simultane-ously to cw A? and CO2 laser beams. The experimental arrangement is shown in fig. la