QUASISTABLE EXTREME MOTION VIBRATIONAL-STATES OF HFCO ABOVE ITS DISSOCIATION THRESHOLD

The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05-cm-1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero-order vibrational levels observed in the energy range between 13 000 and 23 000 cm-1 are assigned to long Franck-Condon-allowed progressions of extremely high overtones of the C-H out-of-plane bending mode (nu-6) in combination with the C = O stretching mode (nu-2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in nu-6 show the least coupling with other states. More strikingly, as the total energy is increased by adding nu-6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm-1, those that have all of the energy in nu-6 or at most one quantum of excitation in nu-2 are extremely stable against state mixing. These appear to be the regular states of quasiperiodic trajectories predicted in some theoretical studies of two-dimensional systems. Extreme motion of the C-H out-of-plane bending mode seems to localize and prevent coupling with other modes.