The infrared absorption spectrum of the ngr;4fundamental band of (NO)2has been recorded at 0.3 cmminus;1resolution in a cell cooled down to 110 K. From the band contour analysis and comparison with a simulated spectrum accurate values of the rotational constants in the ngr;4state are derived, showing a reduction of the van der Waals bond on vibrational excitation. Absorption measurements performed using the various lines emitted by an isotopic CO laser suggest an unresolved rotational fine structure. A lower limit of the time for energy redistribution of 50 to 100 ps is proposed. Also, infrared photopredissociation of (NO)2has been directly observed in a crossed laserhyphen;molecular beam experiment using bolometric detection of the beam attenuation. An upper limit of the dissociation lifetime can be set to 10minus;3s.
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