The evolution of the allowed electronic transitions of the different carbon chains H-(C≡C) n -H (n = 3-9) presented in Fig. (a) clearly shows that the 1 1 Σ u + ← X 1 Σ g + absorption wavelengths of the carbon chains do not increase linearly with the chain length, which can be inferred from the increasingly narrow spacing between the vertical lines representing the peak positions. A plot of the wavelengths of the 1 1 Σ u + ← X 1 Σ g + transition versus the number of C-C units is shown in Fig.