A FREE-JET INFRARED DOUBLE-RESONANCE STUDY OF THE THRESHOLD REGION OF IVR - THE NU(6),NU(1)+NU(6), AND 2-NU(1)BANDS OF PROPYNE

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 v6, v1 + v6, and 2v1 bands. Several different manifestations of incipient IVR were observed in these bands: (i) scattered nonresonant perturbations in the v6 band, (ii) in addition to this, the appearance of individual perturbers as spectral features in the v1 + v6 band and (iii) clumps of molecular eigenstates in the 2v1 band. The qualitative behavior, multiple perturbing states and indications of z-axis Coriolis interactions, is consistent for the three bands. The extent of mixing increases monotonically with vibrational energy. For the 2v1 band, a predicted doorway state was explicitly observed, and the interactions were treated in the context of a two-stage coupling mechanism. For the v6 and v1 + v6 bands, the upper states were fit to a distortable rotor Hamiltonian and the following constants are reported: v0=2980.8522(9) cm-1, A=5.2969(8) cm-1, B=0.285039(12) cm-1, zeta=0.07250(16), for the v6 band, and v0=6315.7998(9)cm-1, A=5.2915(8)cm-1, B=0.284379(10)cm-1, zeta=0.07158(16), for the v1 + v6 band.