Discrete spectrum of laplacian $ \sigma_{disc}(\Delta)$ on $ H^2(\mathbb{R}^n)$?

Question

Let $\Delta: H^2(\mathbb{R}^n)\subseteq L^2(\mathbb{R}^n)\rightarrow L^2(\mathbb{R}^n)$ be the Laplace operator in the weak sense.

What is the discrete spectrum of $\Delta$?

the discrete spectrum consists of isolated eigenvalues $ \lambda$ such that $\mathrm{ker}(\lambda-\Delta)$ is finite dimensional