Fielding, HH

UCL Discovery

		                             sciencemag.org      SCIENCE    VOL. xxx  • galley printed 14 September, 2018  • •  For Issue Date: ???? 1  INSIGHTS  |  PERSPECTIVES SCIENCE	GALLEY 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59    C H E M I C A L  P H Y S I C S  Molecular movies filmed at intersections Electron diffraction maps atomic motions that result from photoexcitation of CF3I mole-cules  By Helen H. Fielding How molecules respond to ultraviolet (UV) light is not only a fundamental issue, but has relevance in nature (e.g. photosynthesis) and technology (e.g. solar cells). Many spectro-scopic methods have been developed for studying photoexcited molecules in the gas-phase, free from interactions with any envi-ronment, because these conditions allow the intrinsic properties of the molecule to be stud-ied in exquisite detail and to be compared di-rectly with theory. Many spectroscopic meth-ods rely on measuring observables from which it is possible to infer the positions of the indi-vidual atoms of the molecule but do not di-rectly measure molecular structure. On p. xxx of this issue, Yang et al. (1) report the use of ultrafast gas-phase electron diffraction (GED) and high-level calculations to probe the movements of individual atoms in the proto-typical molecule CF3I after the absorption of UV light.  When a molecule absorbs UV light, it is promoted to an electronically excited state in which the individual atoms are no longer in their equilibrium positions. Their resulting ex-cess potential energy can be redistributed in a variety of ways. For example, a molecular bond may break, or the atoms may rearrange to form a new molecular structure in which the excited electronic state and another elec-tronic state become degenerate (a conical in-tersection of potential energy surfaces). Such conical intersections provide particularly effi-cient “funnels” for transferring molecules from an electronically excited state to a lower lying excited electronic state or back to the ground state.  A molecular bond typically breaks or relax through a conical intersection is a few tens of femtoseconds. Spectroscopic measurements on these time scales became possible in the late 1980s with the advent of femtosecond la-sers. Zewail and co-workers carried out pio-neering experiments using femtosecond lasers to record spectroscopic signatures of molecu-lar bonds breaking and rearranging. (2). In these experiments, a femtosecond pulse of UV light (pump pulse) initiated a photochemical reaction, and a subsequent femtosecond pulse of light (probe pulse) at a series of precisely timed intervals after the pump probed the structure of the electronically excited mole-cule.  The information that can be retrieved from pump-probe experiment depends on the spectroscopic measurement being made. Femtosecond time-resolved photoelectron spectroscopy has proved to be a powerful de-tection technique for probing ultrafast elec-tronic relaxation processes in neutral mole-cules (3–5) and molecular anions (6, 7). The related techniques of femtosecond pump-probe velocity-map ion imaging (8) and time-resolved photoelectron-photoion imaging (9) have also proved valuable for monitoring the fragments of dissociation reactions. However, these techniques probe the evolution of the electronic wave function or the products of dissociation reactions and rely on complemen-tary quantum chemistry calculations to de-duce the corresponding molecular geometries after the absorption of the photon or after dis-sociation.  Time-resolved x-ray and electron diffrac-tion techniques, on the other hand, are now emerging as exciting new techniques for real-time measurement of molecular structure fol-lowing absorption of UV light (molecular mov-ies) (10, 11). Yang et al. used the ultrafast GED setup at the SLAC National Accelerator Labora-tory to record a molecular movie of photoex-cited CF3I molecules. A femtosecond UV pump pulse promoted the molecules to electronical-ly excited states. Ultrashort pulses of electrons were then fired at the photoexcited molecules at a series of precisely timed intervals after the UV pump pulse. The resulting interference patterns arising from electron waves being scattered from the nuclei and electron clouds of the molecule were recorded at each time interval. Each diffraction pattern effectively provides a snapshot of the movie of molecular structures at the time each was recorded. Yang et al. used a single UV photon to ini-tiate C-I bond dissociation on an excited elec-tronic state. This process has been the subject of several femtosecond pump-probe experi-ments and the electronic states involved and time scale for dissociation are well-characterized (12). However, the ultrafast GED measurements could also follow the molecu-lar motions of the CF3 fragment after dissocia-tion (see the figure, bottom). This fragment underwent a repeated opening and closing of the F-C-F bond angle (an umbrella motion). This motion was accompanied by an elonga-tion of the C-F bonds, and there was a delay between the initial F-C-F bond opening and the C-F bond elongation.  If the CF3I molecules absorbed two UV photons, they were excited to a different set of higher lying excited electronic states. Ultra-fast GED measurements revealed that the nu-clei rearranged in the first 100 fs to the molec-ular structure associated with a conical intersection between this high lying electronic state and one lying just below it. The popula-tion of molecules was observed to bifurcate at the conical intersection (see the figure, top). Some of the population was transferred through the conical intersection to lower lying electronically excited states, and the nuclei re-turned to their initial geometry (before photo-excitation) after ~500 fs. The remaining popu-lation underwent a C-I stretching motion with a period of ~200 fs on the higher lying excited electronic state, but each time the nuclei re-turned to the geometry corresponding to the conical intersection, more of the population was transferred down to the lower lying states. This process resulting in quantum me-chanical interference patterns on the lower ly-ing electronically excited stare. This is the first time that the molecular structure of a mole-cule has been measured as it passed through a conical intersection.   The results of Yang et al. show that ultra-fast GED can measure molecular structure af-ter photoexcitation of molecules in exquisite detail. Combining the information obtained about the nuclear coordinates with that ob-tained about the electronic wave function from femtosecond pump-probe spectrosco-pies and time-averaged experiments has the potential to provide the most stringent tests for theory. This understanding will advance our understanding of photochemistry and photobiology to the point where we can de-sign molecular systems with a particular func-tion from first principles.   REFERENCES:   1.  J. Yang, Science XXX, xxxx (2018).  2.  A. H. Zewail, Angew. Chem. Int. Ed. 39, 2586 (2000). Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK. Email: h.h.fielding@ucl.ac.uk                               sciencemag.org      SCIENCE    VOL. xxx  • galley printed 14 September, 2018  • •  For Issue Date: ???? 2  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59     3. V. Blanchet, M. Z. Zgierski, T. Seideman, A. Stolow, Nature 401, 52 (1999).  4. R. Spesyvtsev, O. M. Kirkby, M. Vacher, H. H. Fielding, Phys. Chem. Chem. Phys. 14, 9942 (2012).  5. T. Horio et al., J. Chem. Phys. 145, 44306 (2016).  6. C. R. S. Mooney et al., Chem. Sci. 4, 921 (2013).  7. M. A. Yandell, S. B. King, D. M. Neumark, J. Am. Chem. Soc. 135, 2128 (2013).  8. G. M. Roberts et al., J. Am. Chem. Soc. 134, 12578 (2012).  9. O. Geßner et al., Science 311, 219 (2006).  10. M. P. Minitti et al., Phys. Rev. Lett. 114, 255501 (2015).  11. A. A. Ischenko, P. M. Weber, R. J. Dwayne Miller, Chem. Rev. 117, 11066 (2017).  12. W. G. Roeterdink, M. H. M. Janssen, Phys. Chem. Chem. Phys. 4, 601 (2002).  10.1126/science.aat6002  Responding to excitation. Potential energy sur-faces of CF3I illustrating the dissociative state ac-cessed using a single UV photon and the higher lying excited states accessed using two UV pho-tons and the conical intersection connecting the higher lying excited electronic states.  

Molecular movies filmed at conical intersections

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Molecular movies filmed at conical intersections

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