Molecular Physics seminar

Absorption spectra of chromophore ions obtained from storage ring experiments

by Dr Steen Brøndsted Nielsen (Aarhus University)

FA31 ()


The electronic structure of ions strongly depends on their environment, and gas-phase studies are therefore needed to reveal intrinsic properties. A complication, however, is too few absorbing species for a traditional light transmission experiment. In Aarhus we have developed state-of-the-art apparatus to record gas-phase absorption spectra of macromolecular ions. The technique is based on the combination of an electrospray ion source, an electrostatic ion storage ring, and pulsed tunable lasers, and it relies on the measurement of delayed dissociation of photoexcited ions (action spectroscopy). I will present some of our recent results for chromophores of relevance in materials science and photobiology and discuss how the transition energies depend on the spatial delocalization of the pi electrons, the character of the electronic transition, nearby charges, and, not least, solvation by polar molecules. Our results are used to benchmark quantum chemistry calculations. Finally, if time allows, I will address the electronic coupling between bases in DNA revealed by synchrotron radiation circular dichroism experiments on DNA samples in aqueous solutions.