Unimolecular reaction rate measurements for molecules with excited overtone vibrations

Unimolecular reaction rate constant measurements on systems with well known total energy contents permit detailed comparison with the predictions of statistical theories. Vibrational overtone excitation is a preparation technique that adds a precise energy increment to the reacting molecules and, when applied to a sample cooled in a free jet expansion, produces an ensemble with a known total energy content. Combining this preparation scheme with time resolved product detection yields the unimolecular reaction rate constant for a large molecule, tetramethyldioxetane. These free jet data serve to fix the parameters in an RRKM calculation, which is then averaged over a thermal distribution of initial vibrational energy for comparison to extensive room temperature measurements on the vibrational overtone initiated reaction. Applying this excitation scheme to a small molecule, hydrogen peroxide, in a free jet expansion permits the spectroscopic inference of its fast unimolecular reaction rate. Observation of the width of individual features in the well resolved vibrational overtone excitation spectrum establishes an upper limit to the unimolecular reaction rate constant for these molecules in their few lowest angular momentum states