The 22 de localized TT-electrons in the 11 conjugated C-C bonds which make up the center...
The 22 de localized TT-electrons in the 11 conjugated C-C bonds which make up the center ß-carotene molecule behave as if they are in a box 2nm long. Given the usual 2 electrons per energy level that you know from chemistry, the highest energy electron (in the HOMO) is in the n = 11 state. What energy photon is needed to bump it to the n = 12 (LUMO) state?