Laser fluorescence excitation spectrum of jet-cooled tropolone: The à 1B2–X̃ 1A1 system

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Abstract
Five of the lowest frequency species a1, b1, and b2 vibrational modes of tropolone in the à 1B2 electronic state are studied using high resolution laser fluorescence excitation spectroscopy of the jet‐cooled sample. The ν’26(b1) mode at 38 cm−1, most probably a ‘‘folding’’ vibration of the seven‐membered and O⋅⋅⋅HO chelated ring systems of tropolone, is observed in the progression 26v0 with primarily even valued quantum numbers to v26=14. Tunneling doublets in the progression are observed to decrease from 18.90 cm−1 in the vibrationless state to unresolvable values when v’26 is larger than 8, thereby demonstrating highly mode‐specific quenching of the ‘‘H atom’’ tunneling process by these low frequency, out‐of‐plane, ring deformation vibrations. The progression 251026v0 is observed to v26=9 and no tunneling doubling is resolved for any of the transitions. These progressions show that the tunneling energy level splitting of tropolone in the vibrationless level of the ground electronic state can be at most about 0.3 cm−1. Additional highly mode‐specific interactions with the tunneling process are observed for the in‐plane ring deformation modes ν’13(a1) and ν39(b2), but the ν’14(a1) mode at 378.24 cm−1, the lowest frequency a1 mode, is found to interact very little with the tunneling coordinate.