Steering chemical branching in photodissociation of molecules in a high vibrational overtone by combining laser and synchrotron radiation

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Initial vibrational excitation in molecules may influence the chemical branching in photodissociation, if two or more different dissociation pathways are accessible from an electronically excited state. Much attention has been focused on the work of Crim, who could accomplish the selective bond-breaking of heavy water, HOD. We are planning to use synchrotron radiation (SR) to promote vibrationally excited molecules to electronically excited states in the vacuum UV region. The main objective is to elucidate the properties of dissociative states by sampling a wide range of their potential energy surfaces, such as dynamics determining the final-state distributions of the products, nonadiabatic transitions on dissociation, and assignments and characterization of unknown multiply-excited states produced by Auger decay. We have developed an experimental system for laser-SR two photon ionization the crux of which are an ultrahigh resolution continuous titanium-sapphire laser and beam alignment devices for strict overlapping of the two photon beams. The apparatus was connected to a beam line of the Photon Factory in Tsukuba. The wavenumber of the laser was determined precisely by observing photoacoustic signal from laser excited H2O measured with a lock-in modulation technique. The photon beam alignment devices are situated upstream and downstream of a central photoionization chamber. Each device is comprised of two SUS plates fitted on two-dimensional motion feedthroughs. These plates can be inserted across the photon beam axis without breaking the system vacuum, which permits us to easily attain good spatial overlap of the two beams. The OH+ ions produced via dissociation of H2O*(4νO-H)+hν→OH+(Χ3Σ-)+H(n=1)+e- were detected by time-of-flight mass spectrometry at SR photon energies near the dissociation threshold of 18.05 eV with respect to the neutral ground state of water.