On quadrupole resonances in atomic photoionization

It is shown that quadrupole matrix elements corresponding to the np --> eps ilonf transitions in Xe have strong maxima near ionization thresholds very similar to the well known maxima in the dipole nd --> epsilonf transitions. The latter manifest themselves as giant resonances in the total photoioniz ation cross section and appear due to the double-well shape of the effectiv e potential for the epsilonf partial waves. The quadrupole resonances have the same origin because, as we show, the short range phase shift of the eps ilonf partial waves as a function of photoelectron energy epsilon is increa sing by pi in both dipole and quadrupole resonances. The quadrupole resonan ces reveal themselves in strong deviations of angular distributions and spi n polarization of photoelectrons near ionization thresholds from the predic tions given by the electric dipole approximation.