A one-electron pseudopotential model is used to calculate the excited states of NanFn-1 clusters in the range 2 less than or equal to n less than or equal to 29. The transition dipole moments are derived to determine the vertical absorption lines from the ground state. Monte Carlo simulations based on the Franck-Condon principle are also achieved to obtain finite temperature absorption spectra. The analysis of ground and excited orbitals in various situations illustrates the rather simple nature of the excitations which are interpreted through an analogy with s-p type transitions in atoms. The influence of electron localization in the initial state is pointed out. Comparison of the simulated spectra with experimental data provides additional keys to understand and discuss the relationship between structure, electron localization, and spectroscopic properties.