HIGH RESOLUTION SPECTROSCOPY OF $O_{2}{^{+}} (b^{4}\Sigma_{g}{^{-}}, v' = 4 \leftarrow a^{4}\Pi_{u}, v'' = 4)$ USING COAXIAL LASER AND ION BEAMS

Abstract

A newly developed technique, velocity-tuned predissociation spectroscopy, has been used to obtain energies for the $O_{2}{^{+}}$ transitions $(\underline{b}^{4} \Sigma_{g}{^{-}}, v^{\prime} = 4, N^{\prime} = 9-23) \leftarrow (\underline{a}^{4}\Pi_{u}, v^{\prime\prime} = 4)$ to a substantially higher precision and accuracy than would be possible with normal optical spectroscopy techniques. A single mode dye laser is passed coaxially a Long an ion beam of several kilovolts energy, and photofragments produced by transitions to predissociated levels are monitored as a function of their kinetic energy. The technique takes advantage of the narrowed velocity spread in fast ion beams to obtain linewidths as small as 200 MHz. The laser is tuned discontinuously by rotation of an intracavity etalon, and the absorption wavelength is Doppler-tuned continuously between these ``mode-hops” by scanning the velocity of the ion beam. The absolute wavelength is determined by comparison with a single-mode argon-ion laser tuned to the Iodine absorption at $19 429.815 \pm 0.002\,cm^{-1}$. Over 200 discrete transitions of the (4,4) band have been measured to a precision of $\pm 0.003\,cm^{-1}$ and an absolute accuracy of $\pm 0.01\,cm^{-1}$. Most of the branches which are blended in high resolution optical spectroscopy are resolved in this work. Direct measurement of the energies of the photofragments greatly simplifies the Interpretation of the spectra.