  Decay via proton emission of isobaric analog states (IAS's) in $^{91}{Nb}$
was studied using the $^{90}{Zr}(\alpha,t)$ reaction at $E_\alpha$=180 MeV.
This study provides information about the damping mechanism of these states.
Decay to the ground state and low-lying phonon states in $^{90}{Zr}$ was
observed. The experimental data are compared with theoretical predictions
wherein the IAS `single-particle' proton escape widths are calculated in a
continuum RPA approach. The branching ratios for decay to the phonon states are
explained using a simple model.
