RYDBERG-RYDBERG EMISSION SPECTRUM OF CARBON MONOXIDE IN THE NEAR-INFRARED REGION: AN {AB INITIO} STUDY OF NEWLY OBSERVED $^{3}\Pi - ^{3}\Sigma^{+}$ TRANSITION

Recently we have observed a new emission spectrum of carbon monoxide (CO) in the near-infrared region, $8240 cm^{-1}$, which can he ascribed to the transition between $^{3} \\Pi$ and $^{3} \\Sigma$ states. In order to identify the states, potential energy and transition dipole moment curves for the lowest four valence $(a ^{3} \\Pi, a^{\\prime}{^{3}\\Sigma}^{+}, d^{3} \\Delta, \\ and \\ e {^{3}\\Sigma}^{-})$ and the lowest three Rydberg $(b {^{3}\\Sigma}^{+}, j {^{3}\\Sigma}^{+},\\ and \\ c ^{3}\\Pi)$ triplet suites of CO were calculated theoretically by the highly correlated {ab initio} Multi-Configuration Self-Consistent Field and internally contracted Multi-Reference Configuration Interaction (MCSCF-MRCI) method with basis sets of double-zeta plus polarisation quality. Observed $^{3}\\Pi-^{3}\\Sigma$ spectra can be assigned to none of the transitions between valence $^{3}\\Pi$ and $^{3}\\Sigma$ styles nor to those- between valence and Rydberg states. Only possibility remained is the transition between the low-lying c $^{3}\\Pi$ and \\ $b ^{3}\\Sigma$ Rydberg states. The potential curves for these Rydberg states are isolated clearly enough to give simple emission spectrum, and the transition dipole moment between these states is very large, 8 Debye. These indicate that the newly observed spectrum should be attributed to the transition between c $^{3}\\Pi$ and $b^{3}\\Sigma^{+} $states.