Eigenvalues for Maxwell's equations with dissipative boundary conditions

Let $V(t) = e^{tG_b},\: t \geq 0,$ be the semigroup generated by Maxwell's equations in an exterior domain $Ω\subset {\mathbb R}^3$ with dissipative boundary condition $E_{tan}- γ(x) (ν\wedge B_{tan}) = 0, γ(x) > 0, \forall x \in Γ= \partial Ω.$ We prove that if $γ(x)$ is nowhere equal to 1, then for every $0 < ε\ll 1$ and every $N \in {\mathbb N}$ the eigenvalues of $G_b$ lie in the region $Λ_ε \cup {\mathcal R}_N,$ where $Λ_ε = \{ z \in {\mathbb C}:\: |\Re z | \leq C_ε (|\Im z|^{\frac{1}{2} + ε} + 1), \: \Re z < 0\},$ ${\mathcal R}_N = \{z \in {\mathbb C}:\: |\Im z| \leq C_N (|\Re z| + 1)^{-N},\: \Re z < 0\}.$

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