The absorption spectra of the first electronic exited state of alkali metal atoms on helium nanodroplets formed of both He-4 and He-3 isotopes were studied experimentally as well as theoretically. In the experimental part new data on the 2p <- 2s transition of lithium on 3 He nanodroplets are presented. The absorption spectrum changes drastically when compared to 4 He droplets, in contrast to sodium where only marginal differences were observed in former studies. To explain these large differences and to answer some still open questions concerning the interaction of alkali metal atoms with helium nanodroplets, a model calculation was performed. New helium density profiles as well as a refined model allowed us to achieve good agreement with the experimental findings. For the first time the red-shifted intensities in the lithium and sodium spectra are explained in terms of enhanced binding configurations in the excited state displaced spatially from the ground state configurations.