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The forbidden c3Π–X1Σ+ (0–0) absorption band of carbon monoxide has been reinvestigated under different pressure conditions using the 10.7-m VUV spectrograph at Meudon. Overlap with the allowed C1Π–X1Σ+ (0–0) band at lower transition energy has been taken into account. We have identified a new rotational branch corresponding to an S-type branch and extended the analysis to both higher and lower J. An analysis of the band structure and the low J transition lines suggests that the band gains its intensity predominantly as a result of an interaction of the c3Π state with a 1Σ+ state, most likely the C1Σ+ (v = 0) state. Molecular constants have been obtained for the c3Π state that are in reasonable agreement with those previously published. The apparently anomalous small value for the centrifugal distortion constant is explained by a homogeneous perturbation with the k3Π valence state.

The vacuum ultraviolet band spectrum system B1Σ+-X1Σ+ of 12C16O, 13C16O, 12C18O, and 13C18O has been extensively investigated both in absorption and in emission with the Meudon Observatory national 10-m vacuum spectrograph. A band lying at 109.9 nm in 12C16O is observed for all isotopes but only in absorption. It is shown to be the B-X(2-0) band. Molecular parameters of the B(v = 0, 1, 2) levels have been derived from simultaneous least-squares fits of the absorption and emission data pertaining to each of these vibrational levels. In addition, a simultaneous fit of the data for the different isotopes provided mass independent Dunham coefficients and equilibrium constants for the B state. Our values obtained for ωe and ωexe confirm the recent determinations which are in disagreement with the values tabulated in standard references. The emission bands exhibit sudden weakenings typical of predissociation. This predissociation further manifests itself in the v = 2 level by a conspicuous broadening of the absorption lines. It may also be the cause of the displacement and anomalous B values observed for this level. From the Dunham coefficients, the following molecular constants (in cm−1 except for re) for the B state have been derived: T e =86926.9 r 3 =1.1197 A . ω e ω e x e B e α e 10 6 D e 12C16O 2161.75 39.84 1.9613 0.0262 6.5 13C16O 2113.56 38.08 1.8749 0.0245 6.0 12C18O 2109.53 37.93 1.8677 0.0244 5.9 13C18O 2060.12 36.18 1.7812 0.0227 5.4 14C16O 2071.52 36.58 1.8010 0.0231 5.5

Cross sections for photoabsorption and photodissociation of CO between 88.5 and 115 nm have been measured at a spectral resolution of 0.015 nm using a synchrotron radiation source. Most of the photodissociation is found to occur in rotationally resolved bands and not in continua. The region below 100 nm predominates in the photodissociation rate. The band absorption cross sections of the first Rydberg states are considerably increased with respect to previous studies following a better appreciation of pressure saturation effects. The integrated photodissociation cross section above 91.2 nm, which is of interest in many astrophysical situations, is measured to be 300 ± 40 × 10 −18 cm 2 nm, sharply increased with respect to usually accepted values.