Observation of a continuum-continuum interference hole in ultrafast dissociating core-excited molecules

The femtosecond dissociation of HCl after core excitation has been studied through the resonant Auger decay. The spectra contain contributions from de cay occurring at both "molecular" and "atomic" internuclear distances. We h ave observed a new interference mechanism in these spectra: An atomic spect ral line develops into a negative spectral contribution, a "hole," when det uning the excitation energy from the maximum of the Cl 2p(-1)sigma* resonan ce. Resonant x-ray scattering theory quantitatively explains this behavior as due to a novel destructive continuum-Continuum interference between mole cular and atomic contributions to the Auger decay.