Spectral fluctuation properties in the optical-fluorescence excitation spectrum of NO2 are investigated. For this purpose new high-resolution measurements have been performed. We analyze the distribution of intensities and of level spacings and the spectral rigidity. Together with the consideration of effects of missing and spurious lines, the presence of random-matrix fluctuations is demonstrated unambiguously.PACS numbers: 03.65. Ge, 05.40,+j, 05.45.+b, 33.20.Kf The characterization of energy spectra by statistical methods has received much interest in the past years. It has been conjectured that "complex" systems exhibit universal fluctuation patterns of the spectral density. These have first been observed in a class of complicated experimental nuclear 1 " 3 and atomic 4 spectra. Typical statistical level-correlation properties could be modeled by ensembles of random matrices, in particular the Gaussian orthogonal ensemble (GOE). 5 The recent interest was particularly stimulated by numerical studies of simple model systems 6 and semiclassical theories, 7 which established a close connection between quantum energy-level statistics and classical ergodic properties.The presence of GOE-type level statistics in numerically obtained model spectra is by now demonstrated by a large number of investigations. 6 However, the experimental verification of random-matrix (GOE) predictions is still of particular importance because so far there is no general theory which fully explains the success of the GOE and defines the range of its validity. The number of such analyses is very small. From a set of highly resolved nuclear resonance spectra, GOE-type statistical properties have been revealed unambiguously. 3 Similarly, significant results have been reported for selected atomic spectra. 4 Molecules form an important class of systems where the unambiguous verification of GOE predictions has been missing.The basic prerequisite for such a statistical analysis is to have a sufficiently large and complete level sequence which corresponds to quantum states of the same symmetry species. In molecular spectroscopy this is a very difficult task, 8 mainly because of the very high density of states produced by rotational, vibrational, and electronic degrees of freedom.The first signatures of GOE-type spectral fluctuations in molecular spectra have been found in the visible absorption spectrum of NO2, 9 and in the following also in stimulated-emission pumping spectra of acetylene. 10 GOE statistics are also indicated by an investigation of the Fourier transform of a spectrum of methylglyoxal. 1 ] These examples, however, all suffer from the smallness of the data sample and from experimental uncertainties, such as missing lines or spurious lines of wrong symmetry. In particular, the previous statistical analysis of the NO2 spectrum 9 was based on comparatively old data. The possibility of many spurious and mixing levels thus could not be excluded for that example, and the validity of the statistical results has been doubted. 12 The exp...

Measurements of vibronic term values in excited electronic states of NO2 have been performed with cw dye lasers over the whole spectral range from 12117 cm−1 to 24563 cm−1. The low rotational temperatures, achieved by adiabatic cooling of NO2 seeded in a supersonic argon beam, allow the separation of closely lying bands as well as a distinction between transitions to the 2B1 state and the vibronically coupled 2B2 – 2A1 states. The statistical analysis of the distribution P(S), where S is the energy separation between adjacent vibronic levels and of the intensity distribution of all measured bands, suggests that a short range correlation exists between the excited levels. Whether also a long range correlation exists is still under investigation. A definite answer requires an unambiguous determination of all bands even with low intensities and a safe assignment of all hot bands. These points are discussed in detail.

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