Decoding the dynamical information embedded in highly mixed quantum states

The standard description of the vibrational and rotational motion of polyat omic molecules, as expressed by the distortable rotor/harmonic oscillator a pproximation, provides an adequate description of the molecular quantum sta tes only in regions of low total state density. When the total state densit y is large, exceeding 100 states/cm(-1), the vibrational dynamics are "diss ipative" and the fundamental process of intramolecular vibrational energy r edistribution is operative. The presence of intramolecular vibrational ener gy redistribution leads to molecular quantum states of a qualitatively diff erent nature. With respect to a normal-mode vibrational basis, these quantu m states are "highly mixed" in their vibrational character and represent nu clear motion that is a combination of all the normal-mode motions. This rev iew describes frequency domain spectroscopy techniques designed to probe th e vibrational, rotational, and structural composition of these eigenstates. Recent work that investigates spectroscopy between highly mixed states is also reviewed.