In a semi-classical hydrodynamical model, giant resonances can be envisioned as collective oscillations of nuclear
matter, much like a liquid drop that can vibrate and oscillate. Isovector giant resonances form a sub-class of such resonances, in which
the proton and neutron matter in the nucleus oscillate out of phase. Two such modes (dipole and monopole) of oscillation are shown in the
following animations ( credit: http://www-linux.gsi.de/~wolle/TELEKOLLEG/KERN/struktur.html).
schematic of the isovector giant dipole resonance
schematic of the isovector giant monopole resonance
In reality, the protons and neutrons cannot seperate as much as is shown in these animations, and the oscillations are actually minor changes
in the density of neutron and proton fluids in the nucleus.
Giant resonances can also be described microscopically. In that framework, they correspond to collective 1-particle 1-hole excitions
that follow certain quantum selection rules.
Isovector giant resonances are most easily studied through a reaction in which the out-of-phase behavior of neutrons and protons is inherent
to the probe used to excite the oscillation, and this is where charge-exchange reactions come into play.
