MEASUREMENT AND ANALYSIS OF THE FERMI RESONANCE BETWEEN NU-5 AND 2-NU-9 OF NITRIC-ACID

New tunable diode laser measurements have been made on the ν5 and 2ν9 infrared absorption bands of HNO3 between 853 and 919 cm-1. These two bands are observed to be about 17 cm-1 apart and are coupled by Fermi resonance which causes a considerable displacement of all the energy levels and transition frequencies. An effective resonance interaction of the type 〈ν5, J, k + 2|2ν9, J, k〉 is particularly important for understanding the appearance of the spectrum. The Fermi resonance has been taken into account in a global fit of both bands that has allowed us to assign all of the strongest transitions. The Fermi interaction constant found is Wf = 8.13 ± 0.14 cm-1 and the unperturbed band separation is 6.0 ± 0.4 cm-1. Other higher-order interactions were also considered in the analysis, including a ΔK = ±2 Coriolis interaction. Over 1400 diode laser transitions were fitted with a rms deviation of 0.00099 cm-1. Transition wavenumbers, assignments, and lower state energies are made available for the strong transitions of HNO3 between 853 and 920 cm-1. © 1992.