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

An infrared-infrared double resonance technique for the study of jet-c ooled polyatomics is reported which offers state-selection, access to one-photon-forbidden vibrations, sub-Doppler resolution, and high sens itivity. It has been applied to the propyne v6, v1 + v6, and 2v1 bands . Several different manifestations of incipient IVR were observed in t hese bands: (i) scattered nonresonant perturbations in the v6 band, (i i) in addition to this, the appearance of individual perturbers as spe ctral features in the v1 + v6 band and (iii) clumps of molecular eigen states 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 e xplicitly observed, and the interactions were treated in the context o f a two-stage coupling mechanism. For the v6 and v1 + v6 bands, the up per states were fit to a distortable rotor Hamiltonian and the followi ng 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.