Knowledge Bank

An infrared-infrared double resonance technique for the study of jet-cooled polyatomics is reported which offers state-selection, access to one-photon-forbidden vibrations, sub-Doppler resolution, and high sensitivity. It has been applied to the propyne $\nu 6.\nu 1+\nu 6$, and $2\nu 1$ bands. Several different manifestations of incipient IVR were observed in these bands: i) scattered non-resonant perturbations in the $\nu 6$ band, ii) in addition to this, isolated, accidental perturbations in the $\nu 1+\nu 6$ band and iii) clumps of molecular eigenstates in the $2\nu 1$ band. The evidence indicates that multiple states are involved in the perturbations in all three cases. For the $2\nu 1$ band, a predicted doorway state was explicitly observed, and the interactions were treated in the context of a two-stage coupling mechanism. For the $\nu 6$ and $\nu 1+\nu 6$ bands, the upper states were fit to a distortable rotor Hamiltonian and constants are reported.