For the ellipse $\frac { x ^ { 2 } } { 36 } + \frac { y ^ { 2 } } { 20 } = 1$, let F and $\mathrm { F } ^ { \prime }$ be the two foci, and let A be the vertex closest to focus F. For a point P on this ellipse such that $\angle \mathrm { PFF } ^ { \prime } = \frac { \pi } { 3 }$, find the value of $\overline { \mathrm { PA } } ^ { 2 }$. [4 points]