Knowledge Bank

The laser induced $T1(n\pi \ast )\leftarrow S0$ phosphorescence excitation spectrum of jet-cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion-wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 614.30 and $864.45cm-1$, respectively. Minima in the potential hypersurface at $\theta =60.30\circ$ and $\alpha =42.16\circ$ defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding $S1$-state parameters showed that the torsion barrier (in $cm-1$) does not greatly change, $S1$/$T1$ = 710.8/614.3, whereas the barrier height for the wagging-inversion barrier increases dramatically, 574.4/864.5.