
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given an ellipse with one focus at $F_{1}(0,-2 \sqrt {2})$ and the corresponding directrix equation $y=- \frac {9}{4} \sqrt {2}$. The eccentricity $e$ satisfies that $\frac {2}{3}$, $e$, $\frac {4}{3}$ form a geometric sequence.  
(1) Find the equation of the ellipse;  
(2) Determine whether there exists a line $l$ that intersects the ellipse at two distinct points $M$ and $N$, and the line segment $MN$ is exactly bisected by the line $x=- \frac {1}{2}$. If such a line exists, find the range of the slope angle of $l$; if not, explain why.

Present the answer in LaTex format: \boxed{Your answer}
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