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Abstract

The far-infrared spectrum of HC₃ N has been recorded at high resolution between 70 and 500 cm −1 using synchrotron radiation. Four prominent features, i.e., ν₇ , ν₆ −ν₇ , ν₄ −ν₆ , and 2 ν₇ have been identified in the spectrum together with many associated hot bands. In addition, rotational transitions for the in- teracting v ₄ = v ₇ = 1 , (v ₆ = 2 , v ₇ = 1) , (v ₅ = 1 , v ₇ = 2) , and v 7 = 5 vibrationally excited states have been recorded in the millimeter/submillimeter region. The newly assigned transitions, together with those re- ported previously, form a comprehensive data set including about 17 0 0 0 transitions, which involves al- most all the vibrational states of HC ₃ N lying below 1300 cm ⁻¹ plus some excited states with energies between 2075 and 3550 cm ⁻¹ . These data have been fitted to an effective Hamiltonian which takes into account rotational and vibrational l-type resonance effects, together with a number of anharmonic inter- action terms. On average, all the analysed data are reproduced within the experimental accuracy. About 90 0 0 0 rotational and ro-vibrational transition frequencies have been computed on the basis of the spec- troscopic constants obtained from the global fit in order to support data interpretation and astronomical searches in the interstellar medium and planetary atmospheres. Part of these data is included in the 2020 release of the HITRAN database.