The singlet π*←n spectrum of jet-cooled acetaldehyde

The supersonic jet fluorescence excitation spectrum of the singlet π*←n transition of CH₃CHO has been recorded between 29 700 and 30 800 cm^-1. The lower frequency vibrational peaks have been analyzed in terms of two upper electronic state (S₁) vibrations; the out-of-plane bend (ν′₁₄) and the CH₃ torsion (ν′₁₅). A threefold potential with a barrier height of Vprime;₃=653 cm^-1 gave an accurate fit of the observed ν′₁₅ levels of the S₁ state. The allowed transition to the electronic origin has been observed. The value of T ₀=29 771 cm^-1 is considerably higher than recent estimates.