Principal Investigator: Thomas Wolf (Stanford Profile, Google Scholar Profile)
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. To achieve this, we combine femtosecond time-resolved gas phase VUV and soft x-ray spectroscopy to investigate ultrafast changes in the electronic structure with electron diffraction to get exclusive access to the correlated dynamics of the nuclear wavepacket. Furthermore, we closely work together with our colleagues from theory on the interpretation of our experimental findings.
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.
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).
T. J. A. Wolf, R. M. Parrish, R. H. Myhre, T. J. Martínez, H. Koch, M. Gühr, Observation of Ultrafast Intersystem Crossing in Thymine by Extreme Ultraviolet Time-resolved Photoelectron Spectroscopy, J. Phys. Chem. A, 123, 6897 (2019).
Ultrafast soft x-ray spectroscopy at LCLS
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).
Megaelectronvolt ultrafast electron diffraction
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).
K. Wilkin, R. Parrish, J. Yang, T. J. A. Wolf, P. Nunes, M. Guehr, R. Li, X. Shen, Q. Zheng, X. Wang, T. J. Martinez, M. Centurion, Diffractive imaging of dissociation and ground state dynamics in a complex molecule, Phys. Rev. A, 100, 023402 (2019).
X. Shen, J. Pedro F. Nunes, J. Yang, K. Jobe, R. Li, M.-F. Lin, B. Moore, M. Niebuhr, S. Weathersby, T. J. A. Wolf, C. Yoneda, M. Gühr, M. Centurion, X. Wang, Femtosecond gas-phase mega-electron-volt ultrafast electron diffraction, Struct. Dyn., 6, 054305 (2019).