Unimolecular reactions near threshold: the overtone vibration initiated decomposition of HOOH(5νOH)

Laser induced fluorescence detection of the fragments from the unimolecular dissociation of hydrogen peroxide initiated by direct excitation of the fourth overtone of the OH stretching transition (5νOH) is a means of measuring both vibrational overtone predissociation spectra and populations of individual quantum states of the OH products. Because the excitation adds insufficient energy to break the OO bond, the measurements detect reactions of molecules that initially possess substantial thermal vibrational–rotational energy (≥1100 cm−1). The spectroscopic data, in which hot band and high energy transitions are particularly prominent, are fit by a vibrational–torsional model that adiabatically separates the low frequency torsional vibration from the higher frequency OH and OO stretching vibrations. The spectroscopic analysis shows that the effective trans barrier in the torsional potential increases by 75 cm−1 for each quantum of OH stretching excitation but decreases by 50 cm−1 upon excitation of the OO stretching vibration. The product state population distributions demonstrate that the lowest rotational level is preferentially populated in the unimolecular decomposition. This result is inconsistent with phase space theory, which predicts substantially more rotational excitation, but agrees with the statistical adiabatic channel model. The predictions of the latter model agree with the data for HOOH (5νOH) as well as with previous results for HOOH (6νOH). The statistical adiabatic channel model contains one parameter in addition to the ones required in phase space theory, and the value of this parameter that provides the best agreement with the measurements is consistent with an independent analysis of thermal recombination data. The experiment serves to test statistical calculations critically because it explores reactions near the threshold energy, where the predicted product state distributions are most sensitive to the details of the theory.