Recent state-resolved investigations of unimolecular dissociation and collisional relaxation of NO, at chemically significant internal energies are outlined. Two powerful double-resonance techniques are described which permit the investigation of these processes on a quantum-state-resolved level of detail. A sequential optical double-resonance technique with sensitive laser-induced fluorescence detection has been employed for assignments of the molecular eigenstates of NO, in the energy range at 17 700 cm-'. Subsequently, we were able to measure state-to-state rotational and vibrational energy transfer in NO,-NO, self-collisions using a time-resolved doubleresonance technique. From these data, direct information about propensity rules and intermolecular interactions for rotational and vibrational energy transfer in NO, self-collisions at high vibrational excitation could be obtained. In addition, we have used a folded high-resolution V-type doubleresonance technique in a free jet to access and to assign rovibronic states of NO, above and below the dissociation threshold, E , . From the doubleresonance spectra, linewidths at around 25 130 cm-' as a function of internal energy, E, and total angular momentum, J , could be extracted. Specific rate constants, k(E, J), calculated from the homogeneous linewidths, have been compared with results from SACM calculations, predictions from a statistical random matrix model, and ps time-domain measurements. Highly ExcitedStates of NO2 2 * B ~ states J, N ', K' strongly perturbed predissociating A \ \ / J " , N " = 2, K" = 0 J " , N " = 0, K" = 0 x 2A, K l a = K ", bright state dark state LL X 2A, N::+ 1 N "'-1 4-1 N " + 1 N '' N " -1

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The features and mechanisms of intramolecular energy redistribution (IVR) and state mixing of HOCl at chemically significant energies have been investigated using intra-cavity laser absorption spectroscopy (ICLAS). These studies revealed that for the observed vibrational states up to about 75% of the dissociation energy E, both strong and weak interactions (Fermi-type, Coriolis) exist but producing only localized perturbations in the spectra. The 5 v, band, however, at about 85% of the dissociation threshold E,, (0-Cl bond) has been found to be globally and strongly mixed with the 4v, + 2 v 2 + vj zero-order vibrational state, and both states appeared in turn coupled to other background states by additional (weaker) interactions. The transfer from frequency to time domain led to the conclusion that this spectral signature is characteristic for efficient but nevertheless restricted intramolecular vibrational energy redistribution in the overtone spectrum of HOCI. The intramolecular dynamics of HOCl is compared to the dynamics of HCN and discussed in terms of low-order interactions between zeroorder states (ZOS) grouped in different tiers.

Cross sections and rate constants for OH + H2 reaction on three different potential energy surfaces for rovibrationally excited reagents High resolution spectra of the 4 1 and 5 1 OH-stretch overtone and the weak 4 1 ϩ 2 combination band of transient HO 35͑37͒ Cl in the energy range 12 500-16 500 cm Ϫ1 have been recorded using ultrasensitive intracavity laser absorption spectroscopy ͑ICLAS͒. For this investigation, two different spectrometers, a titanium:sapphire and a dye-jet ICLA spectrometer have been employed. We report line assignments for both isotopomers, refined Dunham parameters, and the spectroscopic constants for the excited rovibrational states. Strong and weak Fermi-type resonances as well as Coriolis interactions at Ϸ65%-85% of the dissociation energy E 0 have been found and the role of dark perturbers is discussed. The dark 2 1 ϩ3 2 ϩ3 3 state has been analyzed and identified to be the perturber of 3 1 ϩ2 2 . From the detailed analysis of the experimental data the anharmonicity parameters of the asymmetric stretch 3 are refined. The 5 1 band at about 85% of E 0 has been found to be strongly mixed with the 4 1 ϩ2 2 ϩ 3 zero order vibrational state, and both states are coupled to other background states by additional ͑weaker͒ interactions. The extent and the magnitude of perturbations in all K a bands of the 5 1 /4 1 ϩ2 2 ϩ 3 system and additional interactions with other background states may be regarded as the signature of the onset of efficient but restricted intramolecular vibrational energy redistribution ͑IVR͒ in the overtone spectrum of HOCl. The intramolecular dynamics of this small molecule after coherent short pulse excitation of the 5 1 zero order state is discussed in terms of a simple tiers model.