High‐resolution Raman spectroscopy of ν 2 and associated hot bands of 13 CH 12 CH: global analysis of the anharmonic resonances

The Raman spectrum of the 13 CH 12 CH molecule in the region of its ν 2 vibrational band was recorded at a resolution of 2.6 × 10 −3 cm −1 and analyzed. Several Q branches were observed and identified, corresponding to the ν 2 fundamental and the associated hot bands ν 2  + ν 4 – ν 4 , ν 2  + ν 5 – ν 5 , ν 2  + 2ν 4 – 2ν 4 and 2ν 2 – ν 2 . The usual normal modes of vibration in acetylene, with the symmetric (ν 1 ) and antisymmetric (ν 3 ) CH stretchings, the CC stretching (ν 2 ), the degenerate trans (ν 4 ) and cis (ν 5 ) bendings were adopted. All the spectra were obtained using the experimental technique of quasi‐continuous stimulated Raman spectroscopy. The fundamental ν 2 band and the ν 2  + ν 4 – ν 4 and ν 2  + ν 5 – ν 5 hot bands were recorded at room temperature, while ν 2  + 2ν 4 – 2ν 4 was recorded at a temperature of 355 K. Finally, the spectrum of the 2ν 2 – ν 2 band was recorded at 170 K using a Raman‐Raman double resonance technique in which a stimulated Raman pumping process is used to populate a single rotational level of the v 2  = 1 state, prior to the spectroscopy stage. A 15 ns delay between pump and spectroscopy is introduced to allow partial collisional relaxation of the rotational population of v 2  = 1 among rotational levels. The assigned Raman transitions and several previously observed infrared bands have been simultaneously fitted in order to derive a set of deperturbed molecular parameters, including the quartic anharmonic coupling coefficients K 1,255 and K 3,245 . The ro‐vibrational pattern of the stretching and stretching‐bending combination states and an accurate description of the anharmonic resonances in this molecule have been obtained. Copyright © 2013 John Wiley & Sons, Ltd.