Jost R, Michalski G, Thiemens M
Laboratoire de Spectrométrie Physique, Centre National de la Recherche Scientifique-Unité Mixte de Recherche 5588, Université Joseph Fourier-Grenoble 1, BP 87, 38402 Saint Martin d'Hères Cedex, France.
J Chem Phys. 2005 Aug 1;123(5):054320. doi: 10.1063/1.1978873.
We have measured the rovibronic densities of four symmetric (C2v) and two asymmetric (Cs) 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(2pi(1/2)) + O(3P2) is much faster than the radiative decay. As a consequence, in a narrow energy range below D0, 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, 16O14N16O, 18O14N18O, 16O15N16O, and 18O15N18O, 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, 18O14N16O and 18O15N16O, 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 C2v to those of Cs) 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.
我们测量了二氧化氮的四种对称（C2v）和两种不对称（Cs）同位素异构体在其光解离阈值以下的振转电子密度。在解离阈值和射流条件下，激光诱导荧光会突然消失，因为解离成NO(2π(1/2)) + O(3P2)的速度比辐射衰变快得多。因此，在低于D0的狭窄能量范围内，可以观察并分类J = 1/2和J = 3/2的最高束缚振转电子能级。对相应的振转电子密度、能量间距和振转电子跃迁强度进行了统计分析。观察到的强度分布与波特-托马斯分布一致。这种分布使人们能够估计缺失能级的数量，从而确定并比较振转电子密度和振动电子密度。四种对称的NO2同位素异构体，16O14N16O、18O14N18O、16O15N16O和18O15N18O，J = 1/2和J = 3/2的振转电子密度总和分别为18±0.8、18.3±1.4、18.4±2.7和19.8±3.5 cm(-1)，而对于两种不对称同位素异构体，18O14N16O和18O15N16O，相应的密度分别为20.9±4.5和23.6±5.6 cm(-1)。只有当我们同时考虑对称物种的合并（从C2v到Cs）以及势能面沿解离坐标的长程尾部的贡献时，相应的振动电子密度才一致。讨论了同位素取代对解离速率的影响以及与质量无关的同位素分馏的可能关系。
