  Let $L$ be a non-negative self-adjoint operator acting on $L^2(X)$, where $X$
is a space of homogeneous type with a dimension $n$. Suppose that the heat
operator $e^{-tL}$ satisfies the generalized Gaussian $(p_0, p'_0)$-estimates
of order $m$ for some $1\leq p_0 < 2$. It is known that the operator $(I+L)^{-s
} e^{itL}$ is bounded on $L^p(X)$ for $s\geq n|{1/ 2}-{1/p}| $ and $ p\in (p_0,
p_0')$ (see for example, \cite{Blunck2, BDN, CCO, CDLY, DN, Mi1}). In this
paper we study the endpoint case $p=p_0$ and show that for $s_0= n\big|{1\over
2}-{1\over p_0}\big|$, the operator $(I+L)^{-{s_0}}e^{itL} $ is of weak type
$(p_{0},p_{0})$, that is, there is a constant $C>0$, independent of $t$ and $f$
so that \begin{eqnarray*} \mu\left(\left\{x: \big|(I+L)^{-s_0}e^{itL}
f(x)\big|>\alpha \right\} \right)\leq C (1+|t|)^{n(1 - {p_0\over 2}) } \left(
{\|f\|_{p_0} \over \alpha} \right)^{p_0} , \ \ \ t\in{\mathbb R}
\end{eqnarray*} for $\alpha>0$ when $\mu(X)=\infty$, and
$\alpha>\big(\|f\|_{p_{0}}/\mu(X) \big)^{p_{0}}$ when $\mu(X)<\infty$. Our
results can be applied to Schr\"odinger operators with rough potentials and
%second order elliptic operators with rough lower order terms, or higher order
elliptic operators with bounded measurable coefficients although in general,
their semigroups fail to satisfy Gaussian upper bounds.
