  Using synchrotron radiation we simultaneously ionize and excite one helium
atom of a helium dimer (He_2) in a shakeup process. The populated states of the
dimer ion (i.e. He^[*+](n = 2; 3)-He) are found to deexcite via interatomic
coulombic decay. This leads to the emission of a second electron from the
neutral site and a subsequent coulomb explosion. In this letter we present a
measurement of the momenta of fragments that are created during this reaction.
The electron energy distribution and the kinetic energy release of the two He^+
ions show pronounced oscillations which we attribute to the structure of the
vibrational wave function of the dimer ion.
