Knowledge Bank

Both parallel-plate and mass spectroscopic multiphoton ionization studies of $CD3CDO,CD3CHO1,2$ and $CH3CDO$ fragmentation have been carried out, extending previous work on CH,CHO. A $N2$-pumped dye laser with bandwidth, capable of providing a 7ns, $0.63$ mJ pulse at 363 nm was used as the ionization source. From the variation of total ion production with wavelength, the vibrational structure of the 2-Photon resonant $n\to 3s$ Rydberg st e is assigned. Variation of splitting patterns due to the $\nu 15$ methyl torsional barrier are discussed as a function of deuteration. Mass spectroscopic analysis of the fragments indicates that for the partially deuterated species, both $CDO+$ and $CHO+$ ions are formed at high flux, indicative of a hydrogen shift during the dissociative ionization. Extensive fragmentation is found for all species, with the fragmentation pattern highly flux dependent. Simple integer power law behavior for the fragment dependence on flux is not generally found. Total ion signals are compared in both the collision-free (mass spectrometer) and binary collision (parallel-plate) regime. Al though at low flux a $F3$ dependence is found for both experiments, reflecting the lowest energy 2+1 photonionization at higher flux, ion signal variations differ. These differences, as well as the complex fragmentation behaviour, can be explained using a rate equation model for the fragmentation which properly includes the effect of spatial averaging over the ion collection region. Conditions for isotopically selective fragmentation are also discussed