Vibrationally resolved geminate recombination of solid state triiodide using ultrafast electron diffraction

XIAN, R.; HAYES, S. A. ; CORTHEY, G.; MARX, A.; CHENG, L.; MILLER, R. J. D.

Hamburg

Congreso; 12th Femtochemistry Conference (FEMTO12); 2015

Triiodide is a model system suited for studying dissociation and recombination processes in chemistry. We use ultrafast electron diffraction to probe the structural changes in the partially reversible geminate recombination of triiodide initiated by 400 nm optical pump pulses in thin crystalline samples of tetra-n-butylammonium triiodide (TBAT). We observed a large-amplitude librational mode (~ 18 cm-1) that resembles the behavior of rotationally hot photoproducts in the reorientational relaxation following triiodide dissociation in EtOH [1]. By exploring dynamics on different time scales using electron bunches with different charges, we are able to further resolve the high frequency vibrations (> 100 cm-1) that modulate the librational amplitude. These oscillations come from recombination dynamics that trail the fast dissociation occurring within 100 fs. The frequencies that we can extract from these time-resolved measurements match well with those recorded in static Raman and IR spectroscopic studies [2]. The diffraction method also allows us to distinguish the dynamics from two crystallographically distinct types of triiodide chains (straight and bent) due to their differing mode frequencies, therefore complementing the spectrally-resolved optical pump-probe study such as ref 3.References:[1] T. Kühne, P. Vöhringer, J. Phys. Chem. A, 102, 4177 (1998). See also the comment, J. Phys. Chem. A, 103, 5621 (1999) and reply, J. Phys. Chem. A, 103, 5623 (1999).[2] J. S. Zambounis, E. I. Kamitsos, A. P. Patsis and G. C. Papavassiliou, J. Raman Spectrosc., 23, 81 (1992).[3] P. Poulin, K. Nelson, Science, 313, 1756 (2006). Note that the optical pump in this study is at 300 nm, which excites the ground state reactants to a higher band than excitation at 400 nm can reach. Nevertheless, both kinds of excitations lead to dissociation via repulsive energy surfaces. They differ in the reaction branching ratio.