  In this paper, we investigate dispersive estimates for the time evolution of
Hamiltonians $$ H=-\Delta+\sum_{j=1}^N\langle\cdot\,, \varphi_j\rangle
\varphi_j\quad\,\,\,\text{in}\,\,\,\mathbb{R}^d,\,\, d\ge 1, $$ where each
$\varphi_j$ satisfies certain smoothness and decay conditions. We show that,
under a spectral assumption, there exists a constant $C=C(N, d,
\varphi_1,\ldots, \varphi_N)>0$ such that $$ \|e^{-itH}\|_{L^1-L^{\infty}}\leq
C t^{-\frac{d}{2}}, \,\,\,\text{for}\,\,\, t>0. $$
  As far as we are aware, this seems to provide the first study of
$L^1-L^{\infty}$ estimates for finite rank perturbations of the Laplacian in
any dimension.
  We first deal with rank one perturbations ($N=1$). Then we turn to the
general case. The new idea in our approach is to establish the Aronszajn-Krein
type formula for finite rank perturbations. This allows us to reduce the
analysis to the rank one case and solve the problem in a unified manner.
Moreover, we show that in some specific situations, the constant $C(N, d,
\varphi_1,\ldots, \varphi_N)$ grows polynomially in $N$. Finally, as an
application, we are able to extend the results to $N=\infty$ and deal with some
trace class perturbations.
