Abstract
The breakdown diagram of propanal cation is reported. Relying on He-Iα photoelectron-photoion coincidence an excitation energy range of 7 eV has been probed and all the major fragment ions have been studied. The specific decay behavior of the title cation is characterized and related to our earlier data on other C3H6O+ isomers. The results are in excellent accord with literature ab initio calculations in terms of the threshold rate and the height of the energy barrier of the least endothermic fragmentation. For low excitation energies, our data imply that propanal radical cation does at most negligibly isomerize to the most stable distonic ion. The lion’s share of the detected C3H5O+ daughter ions has the propanoyl structure irrespective of the internal energy of the parent ion precursor. It is this fact, in conjunction with the extended Franck-Condon gap in the pertinent energy range, that prevents C3H5O+ from being the base peak in the mass spectrum of propanal. In the breakdown diagram of propanal, only the C2H6+ breakdown curve is identical to its counterpart obtained when studying allyl alcohol and cyclopropanal in exactly the same way. Competitive formation of C2H6+ cannot be accounted for by quasi-equilibrium theory calculations. Conceivable models to rationalize the corresponding dissociation involve an electronic predissociation and/or ion-neutral complexes.
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