We examine the role of electronic continua in time-resolved photoelectron spectroscopy studies of polyatomic nonadiabatic dynamics. We have investigated the two limiting cases for such studies. We consider here the limiting case of complementary ionization correlations where the two nonadiabatically coupled excited electronic states (S-2 and S-1) correlate (in the Koopmans' picture) to different cation electronic states. We show, using an example of ultrafast internal conversion a linear polyene, that this favorable case allows for disentangling of the electronic population dynamics from the coupled vibrational dynamics. In the following paper, we investigate the unfavorable case of corresponding ionization correlations.