High resolution Raman spectroscopy of gases with laser sources. XIV —The pure rotational spectra of dimethylacetylene and dimethyldiacetylene

The pure rotational Raman spectra of dimethylacetylene (DMA), dimethylacetylene‐ d 6 (DMA‐ d 6 ) and dimethyldiacetylene (DMDA) vapor excited by the λ 0 = 4579 Å,4765 Å,4880 Åand 5145 Å lines of a single mode argon ion laser have been photographed under high resolution. The spectra were recorded on Kodak IIIa‐J spectroscopic plates hypersensitized by baking in nitrogen. The spectra of DMA and DMA‐ d 6 are those of symmetric top rotors showing the R‐ and S‐branches, whereas in the spectrum of DMDA only the S‐branch is observed. The spectra of DMA and DMA‐ d 6 were analyzed by including an H j ( J + 3/2) 5 term in the frequency formula for the S‐branch, while for DMDA the conventional expression was employed. The results for the molecular constants are, in cm −1 ,These constants are average over the ground and excited vibrational states. The quoted errors are five standard deviations. The r CC and r CC bond lengths in dimethylancetylene are related by the expression.