  A neutron number dependence of the E1 0+ --> 1- reduced transition
probability in spherical even--even nuclei is analysed within the Q--phonon
approach in the fermionic space to describe the structure of collective states.
Microscopic calculations of the E1 0+ --> 1- transition matrix elements are
carried out for the Xe isotopes based on the RPA for the ground state wave
function. A satisfactory description of the experimental data is obtained.
