Resonance-enhanced two-photon ionization mass spectroscopy of ephedrine: Indication for a state-selective fragmentation in a flexible molecule

The vibronic structure of the S1 ← S0 spectrum of ephedrine was measured by resonance-enhanced two-photon ionization spectroscopy with mass resolution under cold molecular beam conditions. The spectra recorded at four different mass channels, m/z = 165 (parent), 58, and the hitherto unknown 71, 85 fragment ions show dissimilar vibronic fine structure and the observed mass pattern strongly depends on the selected intermediate vibrational state. This points to an intermediate state-selected process resulting in a different fragmentation mass pattern. Ab initio calculations at the MP2/6-311++G∗∗ level demonstrate that the AG (a) conformer is more stable by 238 cm−1 than the next stable GG (a) conformer.