Vibrational mode mixing in terminal acetylenes: High-resolution infrared laser study of isolated J states

Abstract

Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1-butyne and 1-pentyne is studied via high-resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1-butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1-butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J′ shows that anharmonic terms in the potential, rather than Coriolis effects, contribute most strongly to the coupling. The acetylinic CH stretch spectrum of 1-pentyne (2400 states/cm−1) reveals only broad rovibrational transitions with ∼0.01 cm−1 Lorentzian width, even at our 10−4 cm−1 resolution. J′ independent, anharmonic coupling with a minimum of 1/3 of all states must be invoked to reproduce the observed broadening. In contrast, the 1-pentype methyl CH stretch spectrum shows broadening greater than five times larger than that observed at the acetylinic end. Via Fourier transform methods, the spectra for both 1-butyne and 1-pentyne indicate vibrational energy localization in the CH stretch for ∼500 ps. However, for the methyl CH stretch, energy redistribution takes place in <40 ps, clearly indicating the presence of mode specific, nonRRKM vibrational relaxation pathways.