  The relations between photoionization cross sections and photon radius are
obtained on basis of quantum mechanics and the particle-like properties of a
photon. The photoionization cross sections of H atom and H-like ions, He atom
and He like ions, alkali metal atoms, and Rydberg atoms are calculated using
the relations. The calculation results are found to be good agreement with the
known experimental data. The results show that the photoionization cross
section is always smaller than the cross section of the photon to ionize the
atom or ion and can be expressed as the product of the cross section of the
photon and the probability that electron meets with the photon. These provide
the intuitive understanding for the photoionization phenomena and open a new
avenue of research on interaction between a photon and an atom or ion.
