Let $\Omega$ be a bounded open domain and $v:\Omega\to\mathbb{R}^n$. Consider the following elliptic operator in divergence form, defined on smooth functions $u: \Omega \to \mathbb{R}$

\begin{align} L u=\operatorname{div}[\nabla u-v u] \end{align} with a reflecting boundary condition $$ \boldsymbol{n} \cdot(\nabla u-vu)=0 \text{ on } \partial \Omega. $$ What can we say about the spectrum of $L$? I have been searching for the answer for one week. Any relevant reference is greatly appreciated!

(In fact, I want to verify whether $\sigma(L) \subset \Sigma_{\omega}=\{\lambda \in \mathbb{C} ;|\arg \lambda|<\omega\},$ with some fixed angle $0<\omega<\frac{\pi}{2},$ and the resolvent satisfies the estimate $\left\|(\lambda-L)^{-1}\right\| \leq M /|\lambda|,\lambda \notin \Sigma_{\omega} $ with some constant $M \geq 1$. This is one of the assumptions for generating an evolution operator in nonautonomous parabolic equations.)