Near-Threshold Photodissociation Of Cool Oh+ To O + H+ And O+ + H

We have measured the spectrum of laser photodissociation of OH+ molecular ions to O + H+ and O+ + H fragments for photon energies of 38 100-40 900 cm(-1). The OH+ ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N '' = 0-11 of the vibrational ground state X-3 Sigma(-)(v '' = 0), close to room temperature (approximate to 300 K). The many resonances in the spectra reveal the energies, widths, and O/O+ branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v ' = 9 and 10 of the A(3)Pi electronic state. For the A(3)Pi(0,1) substates, isolated levels with v ' = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v ' = 0 and 1 states of the shallow 1(5)Sigma(-) electronic state, borrowing oscillator strength from nearby A(3)Pi levels. Together, the near-threshold levels deliver a new value D-0 = 40 253.8(1.1) cm(-1) for the dissociation energy of OH+. Through a two-step photodissociation process, 72 levels from the lower bound states A(3)Pi(v ' = 7-8) appear as well and are rotationally analyzed. The level energies are used to construct improved A(3)Pi and 1(5)Sigma(-) Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH+ photodissociation or of the related charge-exchange reaction O + H+ -> O+ + H.