Fourier-transform analysis of the NO2 spectrum

Abstract

An almost complete knowledge of the NO₂ energy spectrum is now available, from the ground state up to the first limit of dissociation at 25 130 cm^–1. This allows the determination, by Fourier transformation, of the dynamic behaviour of this molecule as a function of energy. We have used the windowed Fourier-transformation technique introduced by Johson and Kinsey (and named ‘vibrogram’ by Gaspard). Four ranges can be distinguished in the NO₂ energy-level ladder: Below 10 000 cm^–1, the 192 vibrational levels of the ground state are assignable and ‘regular’ even if some specific resonances can be observed. Above 10 000 cm^–1, the vibrational levels of the first electronic excited state, whose origin lies at 9732 cm^–1, interact with the highly excited vibrational levels of the ground state. Above about 17 000 cm^–1, these vibronic interactions are so strong, that vibronic quantum chaos exists, while the rotational degrees of freedom are only weakly coupled. In contrast, rovibronic chaos seems to exist just below the limit of dissociation. The low-resolution absorption spectrum of NO₂ has also been analysed; it shows a periodic structure which corresponds to the bending frequency in the òB₂ state. In summary, we discuss the interplay of the frequency and time domains in various energy ranges of the NO₂ molecule.