Based on Hartree-Slater self-consistent potentials, we have studied minima of oscillator strength densities, namely, zeroes of electric dipole matrix elements for excited alkli atoms (here oscillator strength densities proportional to photoabsorption cross sections). Thus we have induced further the systematics about the zeroes of dipole matrix elements between eigenchannels for other excited atoms, namely, the relation between the zeroes and the quantum defect differences for corresponding initial and final channels. Based on such a relation, we predict the photoabsorption windows for excited Ne(3s) and excited Ar(4s), which have been confirmed by a recent experiment.