Photoionization spectroscopy of small mercury clusters in the energy range from vacuum ultraviolet to soft x ray

Photoabsorption markers of pressure-induced phase changes in small mercury clusters. A case study on Hg8

Photoabsorption spectra of Hg₈ have been calculated at various cluster temperatures and external pressures. A diatomics-in-molecules method has been used to model cluster electronic structure and classical isothermal–isobaric Monte Carlo simulations have been employed for sampling representative cluster configurations. Contributions of different structural isomers of the Hg₈ cluster have been analyzed and related to structural transitions in the cluster, particularly those induced by an increased pressure.

Evolution of low-temperature Hg₈ absorption spectrum with increased external pressure.

Liquid-drop model for fragmentation of multiply charged mercury clusters

The fragmentation of doubly and triply charged mercury clusters is theoretically studied to analyze an experiment performed by Katakuse's group at Osaka University [T. Satoh et al., J. Mass Spectrom. Soc. Jpn. 51, 391 (2003)]. The fission barrier is calculated using a liquid-drop model proposed by Echt et al. In the decay of doubly charged clusters, the barrier height is found to take the minimum value for nearly symmetric fission. On the other hand, in the decay of triply charged clusters, the barrier is the lowest for strongly asymmetric fission. These results well explain the product size distribution observed in the experiment. The appearance size for multiply charged clusters measured in the experiment is found to be the size where the fission barrier is equal to the monomer evaporation energy. These findings provide evidence that small mercury clusters behave like van der Waals clusters in the process of fragmentation.

Electron photodetachment dissociation for structural characterization of synthetic and bio-polymer anions

Tandem mass spectrometry (MS-MS) is a generic term evoking techniques dedicated to structural analysis, detection or quantification of molecules based on dissociation of a precursor ion into fragments. Searching for the most informative fragmentation patterns has led to the development of a vast array of activation modes that offer complementary ion reactivity and dissociation pathways. Collisional activation of ions using atoms, molecules or surface resulting in unimolecular dissociation of activated ions still plays a key role in tandem mass spectrometry. The discovery of electron capture dissociation (ECD) and then the development of other electron-ion or ion/ion reaction methods, constituted a significant breakthrough, especially for structural analysis of large biomolecules. Similarly, photon activation opened promising new frontiers in ion fragmentation owing to the ability of tightly controlled internal energy deposition and easy implementation on commercial instruments. Ion activation by photons includes slow heating methods such as infrared multiple photon dissociation (IRMPD) and black-body infrared radiative dissociation (BIRD) and higher energy methods like ultra-violet photodissociation (UVPD) and electron photo detachment dissociation (EPD). EPD occurs after UV irradiation of multiply negatively charged ions resulting in the formation of oxidized radical anions. The present paper reviews the hypothesis regarding the mechanisms of electron photo-detachment, radical formation and direct or activated dissociation pathways that support the observation of odd and even electron product ions. Finally, the value of EPD as a complementary structural analysis tool is illustrated through selected examples of synthetic polymers, oligonucleotides, polypeptides, lipids, and polysaccharides.