Fermi Coupling and l-Type Resonance Effects in the Hot Bands of Acetylene: The 2650-cm−1 Region

The hot bands originating from the low-lying ν4 (Πg) state to the ν3(Σ+u) fundamental and (ν2 + ν4 + ν5)2,0± (Σ+u, Σ−u, Δe,fu) combination states of acetylene have been analyzed in the high-resolution FTIR spectrum recorded in the 2600-cm−1 region. Fermi resonance between the ν3(Σ+u) and (ν2 + ν4 + ν5 )0 (Σ+u) levels, as well as the l-type resonances within the combination state, have been explicitly taken into consideration in the analysis of the data. Transitions to all the sublevels within the combination state as well as to the ν3 state have been identified, and the data have been fitted with an overall standard deviation of 1.8 × 10−4 cm−1. Values for the perturbation parameters as well as deperturbed values for the band origins and rotational and centrifugal distortion constants are reported.