Knowledge Bank

Two-photon and one-photon resonance multiphoton ionization spectra in the methyl torsional region were obtained for jet cooled acetaldehyde-$h4$ and acetaldehyde-$d4 3s\leftarrow n$ Rydberg and $\pi \ast \leftarrow n$ transitions, respectively. Up to three quanta of the ground state torsion and two quanta in the excited state were observed, e.g $\nu 15\prime \prime =109.6cm-1,2\nu 15\prime \prime =211.0$ and $212.5cm-1,3\nu 15-298.2$ and $309.7cm-1,\nu 15=153.4cm-1$, and $2\nu 15=306.8cm-1$, all for $acetaldehyde-d4$. There are significant modifications from published values. The accurately measured torsional frequencies allow a ground state potential function (barrier $423cm-1$ which is higher than recent estimates) to be obtained which simulates known frequencies within $1cm-1$. The barrier is substantially higher than the barrier obtained from recent ab initio skeletal flexing model calculations for torsional vibrations. The 3s Rydberg state potential barrier $(822cm-1)$ calculated from the excited state frequencies is substantially lowered from previous estimates.