As the chain length increases, the absorption band is clearly shifted to longer wavelengths. The oscillator strengths of the 1 1 Σ u + ← X 1 Σ g + transition in carbon chains also increase with the chain length, just like Vuitlon and Scemama et al. found in their theoretical works . Coincidentally, the value of oscillator strength that we calculated for H-(C≡C) n -H (n = 3-9) is very close to the n value in the chain, and generally slightly smaller than the corresponding semi-empirical result . It is worth mentioning that the oscillator strength of cyclo carbon at the same calculation level is f = 3.02 , which is much smaller than that of the carbon chain of similar size. Green and blue regions denote the positive and negative orbital phases, respectively.