PHOTODISSOCIATION SPECTRUM OF $CO^{-}_{3}$

Abstract

Photodissociation of thermal $CO^{-}_{3}$ to produce $O^{-} +CO_{2}$ has been observed throughout the wavelength range of 6390 -- 5640 {\AA} using a tunable dye laser with a resolution of 0.5 {\AA} and a drift tube mass spectrometer. The photodissociation cross section has significant structure over this photon energy range. Between 6390 and 6050 A there are six well-defined peaks, plus an additional three peaks which are found by a simple deconvolution. These peaks have a FWHM of about 77 cm$^{-1}$ and a mean separation of about 100 cm$^{-1}$. At least nine other peaks are spaced irregularly between 6050 and 5640 \AA. The photodissociation cross section varies from 0.4 to 3.3 $\times$ 10$^{-18}$ cm$^{2}$ over the wavelength range observed. The absolute magnitude and the wavelength dependence of the cross section are independent of photon intensity and gas pressure, indicating a single photon process which involves no collisional dissociation. We do not yet have a full explanation for the detailed structure of the cross section. However, it is almost certainly related to a bound metastable excited state of $CO_{3}^{-}$ which dissociates rapidly either through a curve crossing to a dissociating state, or by a change in geometry brought about by vibration which leads to an unstable configuration. The envelope centered at 6200 {\AA} is probably related to the $\nu$$_{3}$ or $\nu$$_{4}$ bending modes of the excited $CO_{3}^{-}$. We expect to extend these measurements to longer and shorter wavelengths in the near future, and to investigate other ions.