  For a Lie algebra ${\mathcal L}$ with basis $\{x_1,x_2,\cdots,x_n\}$, its
associated characteristic polynomial $Q_{{\mathcal L}}(z)$ is the determinant
of the linear pencil $z_0I+z_1\text{ad} x_1+\cdots +z_n\text{ad} x_n.$ This
paper shows that $Q_{\mathcal L}$ is invariant under the automorphism group
$\text{Aut}({\mathcal L}).$ The zero variety and factorization of $Q_{\mathcal
L}$ reflect the structure of ${\mathcal L}$. In the case ${\mathcal L}$ is
solvable $Q_{\mathcal L}$ is known to be a product of linear factors. This fact
gives rise to the definition of spectral matrix and the Poincar\'{e} polynomial
for solvable Lie algebras. Application is given to $1$-dimensional extensions
of nilpotent Lie algebras.
