High-resolution photofragment spectroscopy of the O2+b4Σg− (v′ = 3, 4, 5) ← a4Πu (v′ = 3, 4, 5) First Negative system using coaxial dye-laser and velocity-tuned ion beams

A new photofragment spectrometer employing coaxial tunable single-mode laser and velocity-tuned fast-ion beams has been used to measure transition energies in the O₂⁺b⁴Σ_g^? ← a⁴Π_u First Negative system to an accuracy and precision that are an order of magnitude better than was previously possible in Doppler-limited emission spectroscopy. The technique consists of velocity-tuning a beam of metastable O₂⁺a⁴Π_u ions such that a set of First Negative rotational transitions can be sequentially brought into resonance with the laser wavelength. The subsequent absorption transitions promote the ions to predissociating levels of the b⁴Σ_g^? state and observation of the O⁺ photofragments is the signal that denotes that each absorption transition has occurred. Repetition of the velocity tuning at different dye-laser frequencies provides a scan of the First Negative spectrum for predissociating upper-state vibrational levels, which are inaccessible to emission spectroscopy. The O⁺ photofragment ions have a kinetic energy that depends on the height of the predissociating rotational level above the separatedatom limit. The present apparatus incorporates a photofragment energy analyzer that can often be used to separately record the wavenumbers of transitions to different upper-state rotational levels, but whose wavenumbers could not otherwise be resolved. A set of 359 wavenumbers involving the (4,4), (4,5), (5,5), and (3,3) bands were recorded with an estimated accuracy of ±0.0032 cm^?1 and a precision of 0.0028 cm^?1, the latter being estimated precisely with a statistical technique. These data were fitted to ⁴Σ and ⁴Π Hamiltonians used in recent studies of the First Negative emission spectrum to determine molecular constants for the v′ = 4, 5 and v″ = 4, 5 levels. The former represent an extension of the b⁴Σ_g^? state to new levels and the latter represent substantial improvements over the constants that were available from previous moderate-resolution emission studies. These photofragment molecular constants were merged with those from the previous emission studies to yield a new consistent set of molecular constants and Dunham coefficients for the O₂⁺b⁴Σ_g^? and a⁴Π_u states. In the fit to the photofragment bands, it was found that the Hamiltonians, which were sufficient for the emission data, are inadequate to describe these states within the precision of the present measurements.