Striking anharmonic resonances in N2O4:: supersonic jet Fourier transform spectra at 13.3, 7.9, 5.7 and 3.2 μm

The spectrum of N2O4 has been obtained in the 3.2, 5.7, 7.9 and 13.3 mu m regions, using a supersonic slit jet system coupled to a Fourier transform spectrometer with a spectral resolution of 0.005 cm(-1). Analysis of the spectra obtained indicates that the temperature of the molecular beam is close to 30 K. In the three lowest frequency spectral regions, not only were the nu(9), nu(11) and nu(12) fundamental bands observed, but each fundamental was accompanied by an intense combination band which appears because of a strong anharmonic resonance leading to a transfer of intensity from the fundamental band to the combination band. In the 13,3 mu m region, two a-type bands with nearly equal intensities, are seen. Their band centers are nu(0)(nu(12))= 747.85495(20) cm(-1) and nu(0)(nu(6) +/- nu(10)) = 755.37492(20) cm(-1). The 7.9 mu m region also contains two a-type bands whose band centers are nu(0)(nu(11)) = 1261.08011(20) cm(-1) and nu(0)(DB) = 1265.03219(20) cm(-1) where DB stands for "dark band". This label is chosen because the assignment of the perturbing state can not be well established. In this region the combination band has about one-third the intensity of the fundamental. In the 5.7 mu m region, two b-type bands are observed with band centers nu(0)(nu(9))= 1756.76217(20) cm(-1) and nu(0)(nu(6) +/- nu(11))= 1733.15573(20) cm(-1) with the combination band having about a quarter of the intensity of the fundamental band. In addition, a much weaker combination band, nu(1) + nu(9), was observed in the 3.2 mu m region: nu(0)(nu(1) divided by nu(9)) = 3125.25987(30) cm(-1). All the bands were successfully analyzed leading to ground state combi- nation differences which were combined with those obtained in a previous diode-laser study and fit to obtain ground state rotational constants. The upper state energy levels were fit using a Hamiltonian that explicitly rakes into account the anharmonic resonances. The experimental data could be fitted to within their experimental uncertainties. (C) 2000 Elsevier Science B.V. All rights reserved.