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EIM: Excited States in Isolated Molecules

Principal Investigator: Thomas Wolf

Introduction

How do electrons and atoms move in a molecule undergoing an ultrafast photochemical reaction? Answering this question will help us in better understanding processes like photosynthesis, photodamage in DNA, or human vision. Ultrafast photoinduced dynamics involve correlated electronic and nuclear motion in the vicinity of conical intersections between different electronic states in the molecule. To understand in detail, what drives a molecule like cyclohexadiene to undergo an electrocyclic reaction within less than 100 femtoseconds after absorption of an ultraviolet photon or a nucleobase like thymine to instead dissipate the absorbed energy into heat on the same timescale, we require direct access on the electronic and nuclear dynamics on the timescale of those processes. We therefore investigate isolated molecules in the gas phase and closely work together with our colleagues from theory on the interpretation of our experimental findings. We combine gas phase spectroscopy in the VUV and soft x-ray regime to investigate ultrafast changes in the electronic structure with electron diffraction to get exclusive access to the correlated dynamics of the nuclear wavepacket.

Time-resolved photoelectron spectroscopy with VUV pulses from high harmonic generation

Time-resolved photoelectron spectroscopy is a well-established method to investigate ultrafast excited state dynamics of molecules in the gas phase. The molecules are photoexcited by a femtosecond pump pulse. The molecular response is probed by photoionizing a valence electron from the molecule. The kinetic energy of the photoelectron contains information about changes in the electronic structure and vibrational dynamics of the molecule. The probe step is traditionally conducted with femtosecond laser pulses in the ultraviolet regime. However, the ultraviolet photon energy is not always sufficient to photoionize the molecule from any excited state. Therefore, not the whole relaxation process is observable. To be able to follow molecular dynamics all the way back to the ground state, we developed a combination of a magnetic bottle photoelectron spectrometer with a high harmonic generation beamline producing VUV pulses well above the typical ground state ionization potentials of organic molecules. We have used it to conduct experiments on the nucleobase thymine and the fluorescence dye perylene.

Publications:

M. Koch, T. J. A. Wolf, J. Grilj, E. Sistrunk, M. Gühr, Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses. Journal of Electron Spectroscopy and Related Phenomena. 197, 22–29 (2014).

M. Koch, T. J. A. Wolf, M. Gühr, Understanding the modulation mechanism in resonance-enhanced multiphoton probing of molecular dynamics. Phys. Rev. A. 91, 31403 (2015).

Ultrafast soft x-ray spectroscopy at LCLS

The 1s electron binding energies of the most abundant elements in organic chemistry and biochemistry lie in the soft x-ray spectral range. Since 1s binding energies of different elements are several 10s to 100s of electronvolts apart, element-specific spectroscopic information can be gained with soft x-ray spectroscopy. Additionally, 1s electrons are strongly localized at their respective atomic core and are therefore strongly sensitive to the local bonding and electronic structure environment. The environments of different sites of the same element in a molecule are often different enough to be distinguishable, making soft x-ray spectroscopy a site-specific probe of molecular structure. With the upcoming of x-ray free electron lasers like linac coherent light source (LCLS) at SLAC, well-established steady-state soft x-ray spectroscopic methods can now be used to investigate ultrafast dynamics in gas phase molecules. We have performed investigations of the excited state dynamics of the nucleobase thymine at LCLS using time-resolved Auger electron spectroscopy and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. We could demonstrate that Auger electron spectroscopy provides high sensitivity to local changes in the nuclear structure during the ultrafast dynamics and that it gives the opportunity to measure time-dependent inter-fragment distances in photoinduced fragmentation reactions. Time-resolved NEXAFS spectroscopy yields unprecedented selective sensitivity to local changes in the valence electronic structure. In the case of our thymine study, we could for example show that the molecule undergoes internal conversion through a conical intersection between an electronic state with ππ* character to a state with nπ* character within 70 fs.

Publications:

T. J. A. Wolf, R. H. Myhre, J. P. Cryan, S. Coriani, R. J. Squibb, A. Battistoni, N. Berrah, C. Bostedt, P. Bucksbaum, G. Coslovich, R. Feifel, K. J. Gaffney, J. Grilj, T. J. Martinez, S. Miyabe, S. P. Moeller, M. Mucke, A. Natan, R. Obaid, T. Osipov, O. Plekan, S. Wang, H. Koch, M. Gühr, Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption, Nat. Commun. 8, 29 (2017).

T. J. A. Wolf, F. Holzmeier, I. Wagner, N. Berrah, C. Bostedt, J. Bozek, P. Bucksbaum, R. Coffee, J. Cryan, J. Farrell, R. Feifel, T. J. Martinez, B. McFarland, M. Mucke, S. Nandi, F. Tarantelli, I. Fischer, M. Gühr, Observing femtosecond fragmentation using ultrafast x-ray induced Auger spectra , Appl. Sci. 7, 681 (2017).

T. J. A. Wolf, M. Gühr, Gas Phase Photochemistry Probed by Free Electron Lasers, in X-Ray Free Electron Lasers: Applications in Materials, Chemistry and Biology, eds. U. Bergmann, V. Yachandra, J. Yano, Royal Society of Chemistry (2017).

R. H. Myhre, T. J. A. Wolf, L. Cheng, S. Nandi, S. Coriani, M. Gühr, H. Koch, A theoretical and experimental benchmark study of core-excited states in nitrogen, J. Chem. Phys, 148, 064106 (2018).

T. J. A. Wolf, M. Gühr, Photochemical pathways in nucleobases measured with an X-ray FEL, Philos. Trans. Royal Soc. A, 377, 2145 (2019).

Megaelectronvolt ultrafast electron diffraction

Time-resolved diffraction is complementary to above spectroscopic methods, since it almost exclusively provides information about nuclear structure changes. The megaelectronvolt ultrafast electron diffraction facility at SLAC offers a unique combination of femtosecond termporal resolution and sub-Angstrom spatial resolution for diffraction experiments in the gas phase. In a recently published study of the ultrafast photochemical ring-opening of 1,3-cyclohexadiene, we used ultrafast electron diffraction to investigate how the nuclei rearrange during this reaction. We could follow the ring-opening in real time by observing transient changes of the atomic distances in the molecule. Furthermore, we observed a substantial acceleration of the ring-opening motion, when the molecule accessed the electronic ground state through a conical intersection. The ring-opening motion transforms into rotation of the ends of the carbon chain, which is generated as a reaction product. This rotation can be followed up to at least one picosecond.  

Publications:

T. J. A. Wolf, D. M. Sanchez, J. Yang, R. M. Parrish, J. P. F. Nunes, M. Centurion, R. Coffee, J. P. Cryan, M. Gühr, K. Hegazy, A. Kirrander, R. K. Li, J. Ruddock, X. Shen, T. Veccione, S. P. Weathersby, P. M. Weber, K. Wilkin, H. Yong, Q. Zheng, X. J. Wang, M. P. Minitti, T. J. Martínez, The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction, Nat. Chem., 10.1038/s41557-019-0252-7.

J. Yang, X. Zhu, T. J. A. Wolf, Z. Li, J. P. F. Nunes, R. Coffee, J. Cryan, M. Gühr, K. Hegazy, T. F. Heinz, K. Jobe, R. Li, X. Shen, T. Veccione, S. Weathersby, K. J. Wilkin, C. Yoneda, Q. Zheng, T. J. Martinez, M. Centurion, X. Wang, Imaging CF3I conical intersection and photodissociation dynamics by ultrafast electron diffraction, Science, 361, 64 (2018).