  Techniques of gauge theory are used to define and compute an invariant of
certain diffeomorphisms of 4-manifolds. The invariant vanishes for any
diffeomorphism which is smoothly isotopic to the identity. As an application,
we give the first example of a diffeomorphism of a simply-connected 4-manifold
which is homotopic to the identity map, but not smoothly isotopic to the
identity.
