Photofragment excitation (PHOFEX) spectra of NO2 are observed by monitoring the specific quantum states of a fragment NO (2Π1/2;v=0, J=0.5–6.5) in the energy region 0–160 cm−1 above the dissociation limit to NO (2Π1/2) and O (3P2). Preparation of NO2 in a quasibound eigenstate above the dissociation limit is attained by the combination of extremely cooled (∼1 K) parent NO2 in a supersonic jet and a high resolution (∼0.05 cm−1) photolysis laser. The dissociation rate constants are obtained from the peak width of PHOFEX spectra and the smallest rate constant is k=8.5×109 s−1, in the energy region where only J=0.5 of NO (2Π1/2; v=0) is produced. The observation that the rate constant increases stepwise when a new product channel J=1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental proof of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibound state of NO2, though the average value is consistent with the calculation by phase space theory. This state specificity of the rate constant is interpreted in terms of quantum fluctuations associated with individual quasibound eigenstates.
Using a combination of low resolution dispersed A->X fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the f!!st ~ages of vibrational energy flow in the highly vibrationally excited acetylene prepared by AX X emission over the energy range 5 000-18 000 em-I. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the E VIB =7000 cm~l region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling-Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck-Condon bright CC stretch and trans-bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling-Dennison coupling between the cis and trans bends in IVR [J. Chern. Phys. 95, 6336 (1991)]. We prove that the symmetric CH stretch vibration, previously thought to play a crucial role in the redistribution of vibrational energy, is Franck-Condon inactive. We have also shown that vibrational-i-resonance among the states with excitation of both degenerate bending modes, when combined with a Fermi resonance which couples CC stretchltranslcis-bend excited states to the antisymmetric CH stretch, determines the subsequent flow of vibrational energy after the Darling-Dennison bending resonance. These resonances all scale with vibrational excitation in nearly the simple manner expected for the lowest order anharmonic terms in the Hamiltonian, which allows the prediction of the fastest processes at high energy from a detailed study of the high resolution spectrum at lower energy. We find some interesting rules for vibrational energy flow in the short time dynamics: (i) CC stretch excitation is necessary for stretch-bend coupling; (ii) if V~ and V; are the quantum numbers of the initially excited bright state, and vb = v; + Vs is the total bending quantum number of a state coupled to that bright state, then V; ;;;. vb ;;;. (V;-2 V2); (iii) the total stretch quantum number n;' = (vI + v2 + vn is also conserved by the short time dynamics. These are severe ancf well characterized restrictions on the range of quantum numbers accessible to the initial bright state during the first stages of intramolecular vibrational redistribution of energy.
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