8. Let $a \in \mathbf{R}$, and the complex numbers
$$
z_{1}=a+\mathrm{i}, z_{2}=2 a+2 \mathrm{i}, z_{3}=3 a+4 \mathrm{i} \text {. }
$$

If $\left|z_{1}\right|, \left|z_{2}\right|, \left|z_{3}\right|$ form a geometric sequence, then the value of the real number $a$ is . $\qquad$