We have indeterminateness of type
oo/oo,
i.e. limit for the numerator is
$$\lim_{x \to \infty} \sqrt{x} = \infty$$
and limit for the denominator is
$$\lim_{x \to \infty} e^{x} = \infty$$
Let's take derivatives of the numerator and denominator until we eliminate indeterninateness.
$$\lim_{x \to \infty}\left(\sqrt{x} e^{- x}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{\frac{d}{d x} \sqrt{x}}{\frac{d}{d x} e^{x}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{- x}}{2 \sqrt{x}}\right)$$
=
$$\lim_{x \to \infty}\left(\frac{e^{- x}}{2 \sqrt{x}}\right)$$
=
$$0$$
It can be seen that we have applied Lopital's rule (we have taken derivatives with respect to the numerator and denominator) 1 time(s)