Parametric perspective on highly excited states: case studies of CHBrClF and C2H2

Considerable insights can be obtained regarding the nature of highly excited states by computing the eigenbasis expectation values of the resonance operators associated with an effective spectroscopic Hamiltonian. The expectation values are related to the parametric derivative of the eigenvalues with respect to specific resonance strengths, i.e., level velocities. Sensitivity of the level velocities to the existence of closed orbits in the underlying classical phase space provides for a dimensionality independent route to a dynamical assignment of the states. In this Letter, taking CHBrClF (polyad P = 5) and the [16,0]g+ bend polyad of C2H2 as examples, we show that the level velocities can signal the birth of new modes and highlight sequences of localized eigenstates.