Nuclear motion in molecular ions studied with synchrotron radiation and multicoincidence momentum imaging spectrometry

Sammanfattning: A momentum imaging spectrometer optimized for photoionization experiments with synchrotron radiation was built in 2008. The spectrometer is equipped with an electrostatic lens, that focuses the charged dissociation products on to a position-sensitive multi-hit delay line anode. The possibility of focusing is essential for most applications, where fragment momentum is to be extracted. The apparatus is equipped with a micro-channel-plate detector opposite to the delay line anode, enabling electron-ion coincidence experiments and thereby mass-resolved ion spectroscopy independent of the time structure of the synchrotron radiation. Calibration and characterization of the instrument is explained. Results for the photo-fragmentation of several molecules are presented, including measurements of kinetic and angular distributions as well as momentum correlations between coincident fragments. Various data analysis methods are described. High sensitivity of the instrument enables detection and analysis of weak processes such as isomerization. Full three-dimensional momentum information for each coincidence event is achieved which allows for in depth understanding of geometry, dynamics and energetics. In Ethyne, molecules with a cis-bending motion has been identified. In carbonyl sulfide and carbon dioxide, the Renner-Teller splitting has been analyzed. For the water molecule, transient isomerization has been found to be induced by strong bending vibrations in core-excited states. Also studies have been made on clusters, displaying the capability of the instrument to measure a large range of masses and kinetic energies.