Interpretation and analysis of [lambda]-resonance effects upon linear molecules with application to acetylene in the 14[mu]m region

31 absorption bands of acetylene in the 14µm bending region observed with a resolution of 0.0025cm^-1 using FT-spectrometric (FTS) data obtained at the National Solar Observatory on Kitt Peak, Arizona, have been analyzed to obtain an improved set of molecular constants for the ν₄ and ν₅ bending modes of acetylene. The fundamental ν₅ and the seven hot bands ν₄ + ν₅ ← ν₄ and 2ν₄ ← ν₅ of the lesser isotope ¹³C¹²CH₂ were observed in the natural sample up to two excited vibrational quanta. Measured transitions of the major isotope up to three excited vibrational quanta, i.e. 2ν₄ + ν₅, ν₄ + 2ν₅, and 3ν₅, and ¹³C¹²CH₂ have been fitted to an appropriate Hamiltonian of a linear molecule containing vibrational and rotational ℓ-resonances as off-diagonal matrix elements.

From the unitary transformation which diagonalizes the Hamiltonian, mixing coefficients of the perturbed levels have been used to calculate effective dipole transition moments. It is evident that ℓ-type doubling effects have a non-negligible contribution to the infrared intensities of linear molecules. Combining high-resolution Doppler limited tunable diode laser (TDL) measurements of intensities with those obtained from the FTS spectra, band intensities of the ν₅ fundamental of both isotopes and the seven strongest hotbands of the major isotope in the 14µm region have been derived. By calculating ℓ-resonance intensity factors, W_J,ΔJ from the effective dipole transition moments, linear Herman-Wallis factors have been experimentally determined apart from the ℓ-type resonances for all seven hot bands. While band intensities retrieved from the two type of measurements (FTS and TDL) agree well, Herman-Wallis factors derived from the FTS data tend to be larger by a factor of 2-5. Possible explanations for these observed discrepancies are discussed. Nevertheless, for the first time ℓ-resonance effects on hotband intensities of the linear acetylene molecule have been quantitatively treated to obtain absolute band intensities for the strongest bands observed in the bending mode region of acetylene.