vibrations. This interpretation is supported by quantum dynamics calculations, displayed in Figure , that show that the wavefunctions of the 6 2 and 5 1 H2CC vibrational levels have substantial amplitude at the HCCH geometry (upper right corner). The overall interpretation of these results is that there is coupling between H2CC and HCCH vibrational energy levels, but this coupling is vibrationally statespecific and only activated upon excitation of the 5 and 6 modes of H2CC.