Dr. Prashant Jain (GT) will present the talk:

The Physical Nature of Coupling between Noble Metal Plasmons

The unique optical attributes of a noble metal nanocrystal are determined by its plasmon resonance - the collective oscillation of the nanocrystal free electrons in resonance with an electromagnetic wave. However, when nanocrystals are assembled together, the plasmon oscillations on the neighboring particles couple with each other, resulting in a dramatic change in the optical properties of the nanostructure. Plasmon coupling forms the basis for the utility of assembled metal nanostructures in surface-enhanced spectroscopy, sub-diffraction waveguiding, and biological probing. I will discuss the physical nature of plasmon coupling as revealed by the optical spectroscopy of colloidal and lithographic nanoparticle assemblies and supported by electrodynamic models. It is observed from our studies that plasmon coupling is qualitatively quite analogous to electronic orbital hybridization showing "bonding" and "anti-bonding" interactions. Furthermore, the inter-plasmon bonding interaction shows an interesting distance dependence with a universal size-scaling behavior, independent of the plasmonic metal, nanoparticle shape, or the surrounding medium. The universal scaling model has emerged as a very generalized and fundamental picture for understanding and designing a wide range of functional metal nanostructures. This I will show using three examples: the plasmon ruler (Alivisatos and Liphardt) developed for measuring nanoscale distances in biomacromolecular systems, the tunable metal nanoshell structure (Halas and Nordlander) used in biomedicine, and noble metal nanostructures for chemical and biological sensing.

Dr. Ioannis Kerkines (Emory University) will present the talk:

Polynitrogen Chemistry

Polynitrogen (Nx) compounds have recently been the focus of much theoretical and experimental work. These compounds are generally predicted to be unstable with respect to dissociation to N2 molecules, however, many of these dissociations are "protected" by barriers of several kcal/mol, rendering some of these polynitrogen species so-called "metastable". Theoretical research has focused on understanding the factors which affect their stability as well as suggesting possible pathways of formation of these species. If we can find a way to synthesize such compounds in the laboratory, this could possibly open the way for production of new "clean" fuels, as the Nx dissociation products are already present in Earth's atmosphere.

Some exciting new chemistry is also the result of this interesting research "outbreak". In this talk focus will be given to two of the smallest polynitrogen species, N3 and N5+. The bent pentanitrogen cation N5+ was first synthesized in 1999 by Christe and coworkers [1], and an effort will be made to describe and understand its interesting structural properties and stability using theoretical and computational means. On the other hand, in azide photolytic experiments, cyclic-N3 was observed among the photodissociation products of HN3 and ClN3 [2-4]. Furthermore, in a very recent such experiment on ClN3 by Wodtke and collaborators using 157 nm radiation, cyclic-N3 was the exclusive N3 photoproduct [4]. The mechanism of why this product is exclusively formed at this particular wavelength remains a challenge. Study of the photodissociation dynamics of ClN3 gives some insights about what takes place in this high energy region and why and how cyclic-N3 is formed.

References
1. K. O. Christe, W. W. Wilson, J. A. Sheehy, J. A. Boatz, Angew. Chem. Int. Ed. 38, 2004 (1999).
2. J. Zhang, Y. Chen, K. Yuan, S. A. Harich, X. Wang, X. Yang, P. Zhang, Z. Wang, K. Morokuma, and A. M. Wodtke, Phys. Chem. Chem. Phys. 8, 1690 (2006), and references therein.
3. S. J. Goncher, N. E. Sveum, D. T. Moore, N. D. Bartlett and D. M. Neumark, J. Chem. Phys. 125, 224304 (2006).
4. P. C. Samartzis, J. J.-M. Lin, T.-T. Ching, C. Chaudhuri, S.-H. Lee, and A. M. Wodtke, J. Chem. Phys. 126, 041101 (2007), and references therein.

Contact: Dr. Christine Payne (404-385-3125) for more information.