Fig. 4: Absorption spectrum of the R branch of the v₁₁ band of benzene.

Experimentally measured transmission at the output of a 0.67 m long cell at a pressure of 13.3 Pa and room temperature. The spectral features have a typical shape, with the K-structure degrading regularly towards higher wavenumbers and an apparent intensity alternation of subsequent lines. The insets show zoomed-in views of R_K(6) and R_K(33) features (left and right inset, respectively): for the latter, the K-splitting is much more manifest together with intensity alternation of K components, owing to spin statistics, that varies as 10:11:9:14, for K = 6p, 6p ± 1, 6p ± 2, 6p ± 3, respectively, (p = 0, 1, 2, 3…)⁴¹.