Are Laplace Transforms a Special Case of Fourier Transforms?
Solution 1:
Your formula for the Laplace transform is wrong. It should be $F(\xi) = \int_0^\infty f(x) e^{-\xi x}\, dx$. But yes, when $\xi$ is imaginary you have (up to normalization) the Fourier transform of $f$ (considered as a function on $(-\infty, \infty)$ which is 0 on $(-\infty, 0)$).