Comparison of rovibronic density of asymmetric versus symmetric NO2 isotopologues at dissociation threshold:: Broken symmetry effects -: art. no. 054320

We have measured the rovibronic densities of four symmetric (C-2v) and two asymmetric (C-s) isotopologues of nitrogen dioxide just below their photodissociation threshold. At dissociation threshold and under jet conditions the laser-induced fluorescence abruptly disappears because the dissociation into NO((2)Pi(1/2))+O(P-3(2)) is much faster than the radiative decay. As a consequence, in a narrow energy range below D-0, the highest bound rovibronic energy levels of J=1/2 and J=3/2 can be observed and sorted. A statistical analysis of the corresponding rovibronic density, energy spacing, and rovibronic transition intensities has been made. The observed intensity distributions are in agreement with the Porter-Thomas distribution. This distribution allows one to estimate the number of missing levels, and therefore to determine and compare the rovibronic and the vibronic densities. The four symmetric NO2 isotopologues, (ONO)-O-16-N-14-O-16, (ONO)-O-18-N-14-O-18, (ONO)-O-16-N-15-O-16, and (ONO)-O-18-N-15-O-18, have, respectively, a sum of J=1/2 and J=3/2 rovibronic densities of 18 +/- 0.8, 18.3 +/- 1.4, 18.4 +/- 2.7, and 19.8 +/- 3.5/cm(-1), while for the two asymmetric isotopologues, (ONO)-O-18-N-14-O-16 and (ONO)-O-18-N-15-O-16, the corresponding densities are 20.9 +/- 4.5 and 23.6 +/- 5.6/cm(-1). The corresponding vibronic densities are in agreement only if we include both the merging of symmetry species (from those of C-2v to those of C-s) and the contribution of the long-range tail(s) of the potential-energy surface along the dissociation coordinate. The effects of isotopic substitution on dissociation rates and the possible relation to mass-independent isotopic fractionation are discussed. (C) 2005 American Institute of Physics.