  Let $A$ and $B$ be non-negative self-adjoint operators in a separable Hilbert
space such that its form sum $C$ is densely defined. It is shown that the
Trotter product formula holds for imaginary times in the $L^2$-norm, that is,
one has % % \begin{displaymath}
  \lim_{n\to+\infty}\int^T_0 \|(e^{-itA/n}e^{-itB/n})^nh - e^{-itC}h\|^2dt = 0
\end{displaymath} % % for any element $h$ of the Hilbert space and any $T > 0$.
The result remains true for the Trotter-Kato product formula % %
\begin{displaymath} \lim_{n\to+\infty}\int^T_0 \|(f(itA/n)g(itB/n))^nh -
e^{-itC}h\|^2dt = 0 \end{displaymath} % % where $f(\cdot)$ and $g(\cdot)$ are
so-called holomorphic Kato functions; we also derive a canonical representation
for any function of this class.
