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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 …