  Octupole deformations and related collective excitations are analyzed using
the framework of nuclear density functional theory. Axially-symmetric
quadrupole-octupole constrained self-consistent mean-field (SCMF) calculations
with a choice of universal energy density functional and a pairing interaction
are performed for Xe, Ba, and Ce isotopes from proton-rich to neutron-rich
regions, and neutron-rich Se, Kr, and Sr isotopes, in which enhanced octupole
correlations are expected to occur. Low-energy positive- and negative-parity
spectra and transition strengths are computed by solving the
quadrupole-octupole collective Hamiltonian, with the inertia parameters and
collective potential determined by the constrained SCMF calculations.
Octupole-deformed equilibrium states are found in the potential energy surfaces
of the Ba and Ce isotopes with $N\approx 56$ and 88. The evolution of
spectroscopic properties indicates enhanced octupole correlations in the
regions corresponding to $N\approx Z\approx 56$, $Z\approx 88$ and $Z\approx
56$, and $N\approx 56$ and $Z\approx 34$. The average $\beta_{30}$ deformation
parameter and its fluctuation exhibit signatures of octupole shape phase
transition around $N=56$ and 88.
