Continuing our research interests in material molecules with special topological structure , this contribution reports in detail the optical excitation properties of carbon chains H-(C≡C) n -H (n = 3-9,15) in the gas phase and reveals the nature of their size dependence at the electronic structure level. The suggested fitting formulas for electronic and optical characteristics can be extrapolated to longer carbon chains and are expected to be used to predict the properties of sp-hybridized carbon allotrope carbyne.