Abstract
The high-resolution infrared spectrum of the ν3+ν8 combination band of propyne (CH3CCH) is presented for the first time. Continuous-wave cavity ring-down spectroscopy is used to measure this weak infrared band in the 3175cm−1 region using a supersonic free jet. The rotational analysis of the experimental spectrum results in accurate spectroscopic parameters for the ν3+ν8 combination vibrational state. Severe perturbations are found for K=3 and 4 rotational levels that are likely due to near-resonant or non-resonant interactions between the ν3+ν8 and other vibrational states. Moreover three parallel-transition type subbands are observed and their analysis is presented as well.
Highlights
Propyne, known as methylacetylene (H3CAC„CH), is a small unsaturated hydrocarbon of astrophysical importance
It is believed to play a role in the chemistry of a number of hydrocarbon-rich astronomical objects, including the atmosphere of Titan [1], the dark cloud TMC-1 [2], the circumstellar shell of the AGB star IRC+10216 [3], and two protoplanetary nebulae CRL 618 [4] and SMP LMC 11 [5], where it has been observed in the infrared (IR) through the m9 (HAC„C bending) mode, and by radio astronomy through pure rotational transitions
Studies of spectra that are perturbed through weak near-resonant couplings to background vibrational states, as seen in other transitions of propyne, make it of interest for studying intramolecular vibrational relaxation (IVR) [11,12,13,10,14,15,16,17]
Summary
Known as methylacetylene (H3CAC„CH), is a small unsaturated hydrocarbon of astrophysical importance. It is believed to play a role in the chemistry of a number of hydrocarbon-rich astronomical objects, including the atmosphere of Titan [1], the dark cloud TMC-1 [2], the circumstellar shell of the AGB star IRC+10216 [3], and two protoplanetary nebulae CRL 618 [4] and SMP LMC 11 [5], where it has been observed in the infrared (IR) through the m9 (HAC„C bending) mode, and by radio astronomy through pure rotational transitions. The close spacing of the rotational transitions of different K0 subbands, and the relatively low dipole moment (l = 0.78 D) [6] make propyne an ideal probe of the interstellar medium’s kinetic temperature; since the excitation temperature increases as K0 increases [7,8,9]. Comparison between high-resolution measurements as presented here for propyne and ab initio methods offers a good test of the accuracy of the Hamiltonians used to describe the involved molecular energy levels
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