Tracking both ultrafast electrons and nuclei

doi:10.1126/science.abb9937

GSTDTAP > 气候变化

DOI	10.1126/science.abb9937
	Tracking both ultrafast electrons and nuclei
	Wolfgang Domcke; Andrzej L. Sobolewski
	2020-05-22
发表期刊	Science
出版年	2020
英文摘要	Excitation of polyatomic molecules with visible or ultraviolet (UV) light to a higher-energy electronic state results in a complex competition between radiative and radiationless electronic decay processes and photochemical reactions. Although the time evolution of the population probability of excited electronic states has been extensively explored with time-resolved laser spectroscopy in recent decades, the accompanying nuclear motion could so far not be resolved at the fastest (femtosecond) time scales. On page 885 of this issue, Yang et al. ([ 1 ][1]) report the simultaneous experimental detection of the excited-state decay and the associated deformation of the nuclear frame with subpicosecond time resolution and subnanometer structural resolution for the example of the pyridine molecule. The simplest framework for describing the motion of nuclei and electrons in a molecule is the Born-Oppenheimer (BO) approximation, which assumes that the much faster electronic motions can be calculated separately from the much slower nuclear motions. Traditionally, radiationless transitions in polyatomic molecules were described in a theoretical framework that assumes weak deviations of the nuclear motion from the BO approximation. This concept was appropriate for decay time scales on the order of nanoseconds, but femtosecond time-resolved laser spectroscopy provided ample evidence that radiationless transitions can occur on ultrafast time scales (just tens of femtoseconds) that approach the periods of high-frequency vibrations. This evidence required a profound conceptual revision of the description of radiationless transitions. The current understanding of ultrafast radiationless transitions is that they are driven by so-called conical intersections (CIs), which are manifolds of exact degeneracy of electronic potential-energy surfaces at which divergent non-BO couplings cause a complete breakdown of the BO approximation ([ 2 ][2]). For example, the first excited state and the ground state of a molecule have the same energy at a CI of these states, but the nuclear motion will be subject to different forces. Ultrafast excited-state deactivation through CIs plays an essential role in the protection of fundamental biological molecules (such as DNA and proteins) from photodamage by UV radiation ([ 3 ][3]). ![Figure][4] Photophysics of pyridine Yang et al. used ultrafast electron diffraction to reveal both the structural and electronic changes that occur when photoexcited pyridine relaxes back to the ground state. The excited state has a distorted (prefulvenic) structure. GRAPHIC: A. KITTERMAN/ SCIENCE FROM A. SOBOLEWSKI The photochemistry of benzene and of aza-arenes such as pyridine played a paradigmatic role for the understanding of radiationless transitions through the discovery, in the early 1970s, of the so-called “channel-three” phenomenon, which is a sudden increase in the radiationless decay rate at a certain excess energy in the lowest excited singlet (S1) state ([ 4 ][5]). Extensive spectroscopic studies attributed the channel-three effect to an abrupt onset of intrastate vibrational relaxation (IVR) and a sudden shortening of the lifetime of vibrational levels ([ 5 ][6]). The generic mechanism behind the channel-three effect was revealed by early ab initio calculations for benzene ([ 6 ][7], [ 7 ][8]). It involves a low-barrier reaction path to a biradical structure which was termed “prefulvene” ([ 8 ][9]) because it is geometrically related to the valence isomer fulvene. Along the reaction path to prefulvene, a low-lying CI exists at which ultrafast decay from the S1 energy surface to the energy surface of the electronic ground state (S) can occur. In pyridine, the lowest singlet excited state is of nπ* character, where n denotes a “nonbonding” orbital mainly localized on the nitrogen atom and π* denotes the lowest unoccupied orbital that is delocalized over the six-membered ring. Qualitative potential-energy profiles along the reaction path to the prefulvenic form of pyridine are displayed in the figure. The energy profile of the 1nπ* state is crossed by the energy profile of the 1ππ* state at a CI marked CI1. Beyond a plateau, the 1ππ* energy in turn crosses the energy profile of the S state at the CI marked CI2. This specific model proposed in the 1990s established a direct relation between a photophysical phenomenon (radiationless decay) and a photochemical reaction (photoisomerization to fulvene) ([ 9 ][10]). Although this general scenario of ultrafast radiationless decay is now widely accepted, it has not been confirmed so far by direct experimental observation. Time-resolved population probabilities of electronic states can now be measured routinely by femtosecond laser spectroscopy with a variety of detection schemes. Time scales of ultrafast radiationless transitions have been established for numerous molecular systems, but these measurements do not provide information on the nuclear motion driving the electronic transition. Molecular structure can be determined with diffraction methods, and electron diffraction (ED) can be applied to gas-phase samples. In the 1990s, Zewail and co-workers pioneered the development of nonstationary (time-resolved) ED ([ 10 ][11]). A femtosecond UV pump pulse excites the molecular sample, and diffraction of a time-delayed electron pulse provides structural information. However, the time resolution was limited to ∼10 ps. The recent development of brilliant pulsed x-ray sources has generated new opportunities for time-resolved diffraction experiments in the femtosecond regime. The experiment of Yang et al. is an ED experiment performed at the SLAC-MeV-UED facility ([ 11 ][12]) on a target gas of pyridine. The pump laser launches a nonstationary wave packet on the potential-energy surface of the S1(nπ) excited state. Large-angle elastic scattering encodes information on the nuclear structure, whereas small-angle inelastic scattering is sensitive to electron correlation. In the electronic ground state of pyridine, the localized n orbital is doubly occupied, which results in strong so-called dynamical electron correlation (the two electrons try to avoid each other). In the S1(nπ) excited state, these two electrons occupy spatially separated orbitals, which reduces dynamical electron correlation. The population of the S1(nπ) state can be detected by the reduction of the small-angle inelastic electron scattering signal in the excited state. Yang et al. extracted the nuclear structural dynamics from the simultaneously measured large-angle elastic ED, using algorithms that were developed earlier for stationary ED. Specifically, the main geometric parameters are the average bond length of the C5N ring and the dihedral angle representing the distortion of one of the atoms out of the plane of the six-membered ring. The transient structure confirms the prefulvenic distortion predicted earlier by ab initio calculations (see the figure). The time-resolved ED data unequivocally reveal that the decay of the population of the S1(nπ) state and the distortion of the ring occur on the same time scale of ∼300 fs, resolving a decades-old puzzle in molecular spectroscopy. This work of Yang et al. represents a milestone on the path toward the characterization of photochemical events with simultaneous complete resolution in time as well as in atomic structure. 1. [↵][13]1. J. Yang et al ., Science 368, 885 (2020). [OpenUrl][14][Abstract/FREE Full Text][15] 2. [↵][16]1. W. Domcke, 2. D. R. Yarkony, 3. H. Köppel 1. D. R. Yarkony , in Conical Intersections: Electronic Structure, Dynamics and Spectroscopy, W. Domcke, D. R. Yarkony, H. Köppel, Eds. (World Scientific, 2004), chap. 2. 3. [↵][17]1. S. Boldissar, 2. M. S. de Vries , Phys. Chem. Chem. Phys. 20, 9701 (2018). [OpenUrl][18] 4. [↵][19]1. J. H. Callomon, 2. J. E. Parkin, 3. R. Lopez-Delgado , Chem. Phys. Lett. 13, 125 (1972). [OpenUrl][20][CrossRef][21][Web of Science][22] 5. [↵][23]1. E. Riedle, 2. T. Knittel, 3. T. Weber, 4. H. J. Neusser , J. Chem. Phys. 91, 4555 (1989). [OpenUrl][24] 6. [↵][25]1. S. Kato , J. Chem. Phys. 88, 3045 (1988). [OpenUrl][26] 7. [↵][27]1. A. L. Sobolewski, 2. C. Woywod, 3. W. Domcke , J. Chem. Phys. 98, 5627 (1993). [OpenUrl][28][CrossRef][29] 8. [↵][30]1. D. Bryce-Smith, 2. H. C. Longuet-Higgins , Chem. Commun. 1966, 593 (1966). [OpenUrl][31] 9. [↵][32]1. I. J. Palmer, 2. I. N. Ragazos, 3. F. Bernardi, 4. M. Olivucci, 5. M. A. Robb , J. Am. Chem. Soc. 115, 673 (1993). [OpenUrl][33][CrossRef][34][Web of Science][35] 10. [↵][36]1. V. A. Lobastov et al ., J. Phys. Chem. A 105, 11159 (2001). [OpenUrl][37][CrossRef][38] 11. [↵][39]1. X. Shen et al ., Struct. Dyn. 6, 054305 (2019). [OpenUrl][40][CrossRef][41][PubMed][42] [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: pending:yes [5]: #ref-4 [6]: #ref-5 [7]: #ref-6 [8]: #ref-7 [9]: #ref-8 [10]: #ref-9 [11]: #ref-10 [12]: #ref-11 [13]: #xref-ref-1-1 "View reference 1 in text" [14]: {openurl}?query=rft.jtitle%253DScience%26rft.stitle%253DScience%26rft.aulast%253DYang%26rft.auinit1%253DJ.%26rft.volume%253D368%26rft.issue%253D6493%26rft.spage%253D885%26rft.epage%253D889%26rft.atitle%253DSimultaneous%2Bobservation%2Bof%2Bnuclear%2Band%2Belectronic%2Bdynamics%2Bby%2Bultrafast%2Belectron%2Bdiffraction%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.abb2235%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [15]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEyOiIzNjgvNjQ5My84ODUiO3M6NDoiYXRvbSI7czoyMjoiL3NjaS8zNjgvNjQ5My84MjAuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9 [16]: #xref-ref-2-1 "View reference 2 in text" [17]: #xref-ref-3-1 "View reference 3 in text" [18]: {openurl}?query=rft.jtitle%253DPhys.%2BChem.%2BChem.%2BPhys.%26rft.volume%253D20%26rft.spage%253D9701%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [19]: #xref-ref-4-1 "View reference 4 in text" [20]: {openurl}?query=rft.jtitle%253DChem.%2BPhys.%2BLett.%26rft.volume%253D13%26rft.spage%253D125%26rft_id%253Dinfo%253Adoi%252F10.1016%252F0009-2614%252872%252980059-9%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [21]: /lookup/external-ref?access_num=10.1016/0009-2614(72)80059-9&link_type=DOI [22]: /lookup/external-ref?access_num=A1972L890100010&link_type=ISI [23]: #xref-ref-5-1 "View reference 5 in text" [24]: {openurl}?query=rft.jtitle%253DJ.%2BChem.%2BPhys.%26rft.volume%253D91%26rft.spage%253D4555%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [25]: #xref-ref-6-1 "View reference 6 in text" [26]: {openurl}?query=rft.jtitle%253DJ.%2BChem.%2BPhys.%26rft.volume%253D88%26rft.spage%253D3045%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [27]: #xref-ref-7-1 "View reference 7 in text" [28]: {openurl}?query=rft.jtitle%253DJ.%2BChem.%2BPhys.%26rft.volume%253D98%26rft.spage%253D5627%26rft_id%253Dinfo%253Adoi%252F10.1063%252F1.464907%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [29]: /lookup/external-ref?access_num=10.1063/1.464907&link_type=DOI [30]: #xref-ref-8-1 "View reference 8 in text" [31]: {openurl}?query=rft.jtitle%253DChem.%2BCommun.%26rft.volume%253D1966%26rft.spage%253D593%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [32]: #xref-ref-9-1 "View reference 9 in text" [33]: {openurl}?query=rft.jtitle%253DJ.%2BAm.%2BChem.%2BSoc.%26rft.volume%253D115%26rft.spage%253D673%26rft_id%253Dinfo%253Adoi%252F10.1021%252Fja00055a042%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [34]: /lookup/external-ref?access_num=10.1021/ja00055a042&link_type=DOI [35]: /lookup/external-ref?access_num=A1993KJ68900042&link_type=ISI [36]: #xref-ref-10-1 "View reference 10 in text" [37]: {openurl}?query=rft.jtitle%253DJ.%2BPhys.%2BChem.%2BA%26rft.volume%253D105%26rft.spage%253D11159%26rft.atitle%253DJ%2BPHYS%2BCHEM%2BA%26rft_id%253Dinfo%253Adoi%252F10.1021%252Fjp013705b%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [38]: /lookup/external-ref?access_num=10.1021/jp013705b&link_type=DOI [39]: #xref-ref-11-1 "View reference 11 in text" [40]: {openurl}?query=rft.jtitle%253DStruct.%2BDyn.%26rft.volume%253D6%26rft.spage%253D054305%26rft_id%253Dinfo%253Adoi%252F10.1063%252F1.5120864%26rft_id%253Dinfo%253Apmid%252F31649964%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [41]: /lookup/external-ref?access_num=10.1063/1.5120864&link_type=DOI [42]: /lookup/external-ref?access_num=31649964&link_type=MED&atom=%2Fsci%2F368%2F6493%2F820.atom
领域	气候变化 ; 资源环境
URL	查看原文
引用统计	被引频次：3[WOS] [WOS记录] [WOS相关记录]
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/270481
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Wolfgang Domcke,Andrzej L. Sobolewski. Tracking both ultrafast electrons and nuclei[J]. Science,2020.
APA	Wolfgang Domcke,&Andrzej L. Sobolewski.(2020).Tracking both ultrafast electrons and nuclei.Science.
MLA	Wolfgang Domcke,et al."Tracking both ultrafast electrons and nuclei".Science (2020).