The Bristol & Oxford Chemical Dynamics Group

Multimass VMI to examine mechanisms of neutral molecule and ion fragmentation

Velocity-map imaging is well established within the chemical reacton dynamics community as a means of studying molecular photofragmentation. The technique provides detailed insight into the energetics and dynamics of the fragmentation process, yielding bond strengths, information on energy partitioning between the various accessible translational and internal degrees of freedom, and photofragment angular distributions, which often elucidate the identities of the electronic state or states through which the dissociation proceeds. To date, most studies have been carried out on very small molecules, and a key goal is to generalise VMI methods and extend their use to studies of much larger molecules of more general interest to the broader chemical community. Such molecules often possess numerous fragmentation channels and yield a variety of fragmentation products, which ideally should all be probed in a single experiment. Much of the groundwork for these studies has already been completed, with the development of universal ionization sources (VUV laser photoionization and electron-impact ionization) and the PImMS multimass imaging sensor, and we are now well placed to study the fragmentation dynamics of a variety of neutral and charged molecules of importance in areas such as atmospheric and interstellar chemistry, organic photochemistry and photobiology, and mass spectrometry.

To give a few examples, we are currently studying a number of gas-phase organic photofragmentation reactions, including McLafferty-type rearrangements and retro-cycloaddition reactions, as well as investigating the fragmentation dynamics of model peptide compounds of relevance to peptide mass spectrometry. Over the course of the Programme Grant, we will be developing new molecular sources for large-molecule gas-phase studies, as well as exploiting covariance and coincidence techniques to reveal correlations between the scattering distributions of molecular fragments from the same parent molecule.

Velocity-map images and kinetic energy release distributions for products of the photoinitiated retro-Diels-Alder reaction of cyclohexene: (a) ene product; (b) diene product (note that most of these fragments go on to lose a proton and are detected one mass unit lower in the time-of-flight spectrum); (c) momentum matching between the fragments, confirming that they are formed in the same fragmentation process.

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