Infrared Analysis of the Anharmonic Resonance between nu8 + nu9 and the Dark State nu6 + nu7 of HNO3.

The high-resolution FTIR spectrum of the nu8 + nu9 band of HNO3 around 1205 cm-1 has been measured and analyzed. The bright state was found to be strongly perturbed by its neighboring dark state nu6 + nu7 at the coincident levels. Taking account of the DeltaK = ±2 anharmonic resonance, a simultaneous fit of nu8 + nu9 and nu6 + nu7 was performed leading to a good reproduction of the nu8 + nu9 band with a rms uncertainty of 0.00046 cm-1 . A set of rovibrational constants for nu8 + nu9 were accurately determined, while for nu6 + nu7 the rotational constant B and the band origin were obtained.