Asymptotic behaviour of the integral

Question

Suppose I have the integral $$ \tag 1 I\left[p\equiv -\frac{1}{2}\pm ia, z\right] \equiv \frac{1}{\Gamma(-p)}\int \limits_{0}^{\infty}e^{-xz -\frac{x^{2}}{2}}x^{-p-1}dx $$ I'm interested in asymptotic behaviour of this integral for the case $a\to \infty$, $a \to 0$. I know that for limit $|z| \to \infty$, $|z| >>|p|$ the asymptotic behaviour of $(1)$ is $$ I\sim e^{-\pi a} $$ Could someone clarify to me how to extract behaviour of $(1)$ for $a\to \infty$, $z$ is arbitrary?