Knowledge Bank

We have investigated the (4, 1) local-local OH stretch combination band of hydrogen peroxide using infrared-optical double resonance. A pulse from a Nd:YAG pumped optical parametric oscillator excites an OH stretch fundamental of HOOH and selects a single or small subset of rotational states. A dye laser pulse then promotes the molecules above the threshold for O-O bond dissociation by a $4\leftarrow 0$ vibrational overtone excitation of the other OH stretch. The overtone absorption is detected by monitoring the OH dissociation fragment via laser induced fluorescence. The rotationally resolved vibrational overtone spectra generated by this technique indicate that the band origin of the $4\leftarrow 0$ OH stretch transition in an HOOH molecule containing one quantum of OH stretch in the other oscillator is shifted $by-13.2 cm^{-1} $from the direct $4\leftarrow 0$ transition of the ground state $molecule.1$ These spectra also provide the opportunity to prepare reactant molecules in single rovibrational states at energies close to the dissociation threshold. The OH product state distributions resulting from the decomposition of these selectively excited molecules reveals the presence of a small barrier in the exit channel of the potential energy surface. The overtone excitation spectra indicate that the top of the barrier lies at $17,054cm-1\pm 5cm-1$, and the OH product state distributions set a lower limit of $190cm-1$ for the barrier height.