Perturbations in the b${\mathrm{}}^4$${\Sigma }_g^{\mathrm{-}}$ state of ${\mathrm{O}}_2$${\mathrm{}}^{+}$ immediately below the predissociation limit, studied by high-resolution optical-optical double-resonance photofragment spectroscopy

Optical-optical double-resonance photofragment spectroscopy of molecular ions is, for the first time, performed with a Doppler-limited resolution of 0.003 ${\mathrm{cm}}^{\mathrm{-}1}$. The high resolution allows the observation of perturbations in the b${\mathrm{}}^4$ ${\Sigma }_g^{\mathrm{-}}$ state arising from a vibrationally excited level in the d${\mathrm{}}^4$ ${\Sigma }_g^{+}$ state lying only 29 ${\mathrm{cm}}^{\mathrm{-}1}$ below the predissociation limit. The matrix element for the interaction between the b and d states is obtained from a perturbation analysis and is used to estimate the contribution of the d state to the predissociation of the b state. Precise effective Hamiltonians are reported for the a${\mathrm{}}^4$ ${\Pi }_u$(v’ ’=4,5) and b${\mathrm{}}^4$ ${\Sigma }_g^{\mathrm{-}}$(v’=4) states.