  Strong-field ionization (SFI) has been shown to prepare wave packets with
few-femtosecond periods. Here, we explore whether this technique can be
extended to the attosecond time scale. We introduce an intuitive model for
predicting the bandwidth of ionic states that can be coherently prepared by
SFI. This bandwidth is given by the Fourier-transformed sub-cycle SFI rate and
decreases considerably with increasing central wavelength of the ionizing
pulse. Many-body calculations based on time-dependent configuration-interaction
singles (TDCIS) quantitatively support this result and reveal an additional
decrease of the bandwidth as a consequence of channel interactions and
non-adiabatic dynamics. Our results further predict that multi-cycle
femtosecond pulses can coherently prepare attosecond wave packets with higher
selectivity and versatility compared to single-cycle pulses.
