Crystalline phase geminate recombination of triiodide investigated with ultrafast time-resolved electron diffraction

XIAN, R.; HAYES, S. A.; CORTHEY, G.; MARX, A.; LU, C.; MILLER, R. J. D.

Congreso; 45th World Chemistry Congress (IUPAC-2015); 2015

IUPAC

Ultrafast time-resolved electron diffraction (UED) has emerged as a key technique to resolve molecular dynamics with atomic details on femtosecond time scales [1]. Photodissociation and the subsequent geminate recombination of polyatomic molecules involve large and rapid atomic motion and UED is best-suited for their studies. Here we investigate such reactions in a model triiodide system. Previously studied in gas phase [2] and in solution [3], the fast reaction processes and the partial reversibility pose great challenges for investigations using stroboscopic pump-probe techniques in crystalline phase [4]. In this study we initiated the reactions by 90 fs, 400 nm optical pump pulses in thin crystalline samples of tetra-n-butylammonium triiodide (TBAT). We cooled the sample and optimized the laser repetition rate so as not to alter the reaction kinetics too much while significantly reducing the deleterious effects caused by repeated laser excitation. We were able to map out global changes of the crystalline structure in the process of the reaction by looking at time-resolved intensity changes of various Bragg peaks (see Fig.1b-c). Within the limits of reaction-induced sample degradation, we observed coherent oscillation of a reaction-associated librational mode (~ 18 /cm) [5] that persisted for almost 7 ps. Since the linear triiodide chains are arranged along a primary crystalline axis in the static structure ([6] and Fig.1a), the coherent libration of them can directly modulate the inter-chain distance and therefore the potential barrier in recombination dynamics of fragmented species following the fast dissociation occurring within 100 fs. References 1. R. J. D. Miller, Annu. Rev. Phys. Chem. 65, 583 (2014). 2. A. A. Hoops, J. R. Gascooke, A. E. Faulhaber, K. E. Kautzman, D. M. Neumark, J. Chem. Phys. 120, 7901 (2004); R. Nakanishi, N. Saitou, T. Ohno, S. Kowashi, S. Yabushita, T. Nagata, J. Chem. Phys. 126, 204311 (2007). 3. A. Baratz, S. Ruhman, Chem. Phys. Lett., 461, 211 (2008); T. Kühne, R. Küster, P. Vöhringer, Chem. Phys. 233, 161 (1998). 4. P. Poulin, K. Nelson, Science, 313, 1756 (2006). 5. J. S. Zambounis, E. I. Kamitsos, A. P. Patsis and G. C. Papavassiliou, J. Raman Spectrosc., 23, 81 (1992). 6. F. H. Herbstein, M. Kaftory, M. Kapon, W. Saenger, Z. Kristallog., 154, 11 (1981).