  The ionization probability of N$_2$, O$_2$, and CO$_2$ in intense laser
fields is studied theoretically as a function of the alignment angle by solving
the time-dependent Schr\"odinger equation numerically assuming only the
single-active-electron approximation. The results are compared to recent
experimental data [D.~Pavi{\v{c}}i{\'c} et al., Phys.\,Rev.\,Lett.\ {\bf 98},
243001 (2007)] and good agreement is found for N$_2$ and O$_2$. For CO$_2$ a
possible explanation is provided for the failure of simplified
single-active-electron models to reproduce the experimentally observed narrow
ionization distribution. It is based on a field-induced coherent core-trapping
effect.
