On invariant measures for the group of diffeomorphisms of the circle

In a previous paper the author constructed biinvariant measures (possibly having values in a line bundle) for a loop group LK (with compact simply connected K) acting on the formal completion of its complexification LG. One motivation for this was to find a geometric construction for the unitary structure for the positive energy representations of LK. In this paper we pursue an analogous construction for the group of diffeomorphisms of the circle.