Methods
Femtosecond UV-VIS Laser Spectroscopy
Ultrafast UV-VIS (ultraviolet-visible) laser spectroscopy methods are pump-probe techniques which allow us to obtain first information about the reaction pathways and time scales involved after photoexcitation of a molecular system. The techniques utilize ~ 200-800 nm photons from pulsed femtosecond laser systems, i.e. e.g. from Titan:Sapphire lasers. The first laser pulse, the pump pulse, initiates a photoreaction in the sample, while the second pulse, the probe pulse, interrogates the state of the system after a certain time delay. One of the ultrafast laser spectroscopy methods is Transient Absorption Spectroscopy (TAS), an extension of conventional absorption spectroscopy. The technique measures the changes in the absorbance/transmittance of a sample after the pump beam excites valence shell electrons and this at a particular probe beam wavelength or range of wavelengths. The observable is the difference in the absorbance measured with and without pump beam exciting the sample, ΔA, as a function of time and wavelength. It reflects ground state bleaching, further excitation of the excited electrons to energetically even higher states, stimulated emission or product absorption. Bleaching of the ground state refers to depletion of the molecular ground state by electronic excitation to energetically higher states. Stimulated emission describes the potential lasing effect of the excited molecules under the intense probe light. Product absorption refers to any absorption changes due to formation of intermediate reaction products.
We focus on solvated molecular systems and utilize TAS to provide first evidence regarding their photoreaction pathways as well as the associated processes such as intersystem crossing, intermediate unstable electronic states etc. The projection of TAS data onto the wavelength axis provides us with information regarding the evolution/decay of various intermediate species involved in the chemical reaction and this for different wavelengths. The projection of TAS data onto the time axis contains information regarding the number of decay processes contributing to the signal at a given wavelength as well as on their time scales. The TAS results also serve as starting point for structural dynamics investigations on molecular systems which utilize ultrafast x-ray spectroscopy and scattering techniques.
Image AG Kubicek
The figure shows the simple case of a transient absorption spectroscopy experiment for a sample with a ground state (G) and two excited states (A and B), a pump laser with defined wavelength and a white light probe source. After the sample, the probe beam passes through a spectrometer (grating) allowing the ground and excited absorbance changes of the sample to be resolved for all wavelengths independently.
Ultrafast X-ray Spectroscopy and Scattering
X-ray spectroscopy techniques utilize photons with energies in the ~100 eV to some keV region to excite core shell electrons of atoms into unoccupied bound or continuum states. The techniques measure the absorption or emission of x-ray photons or the generated photoelectrons. As every element has characteristic binding energies, the inner shell transition energies of every element are unique. Since the inner shell electrons are localized close to the nucleus with well-defined symmetry and spatial extension, x-ray spectroscopy is chemically highly specific at atomic resolution. There exists a huge variety of x-ray spectroscopy methods, which can all tackle different aspects of the atomic structure, amongst others spin states, bond lengths, electronic correlations etc. Time-resolved x-ray spectroscopy is a pump-probe method which is based on classical x-ray spectroscopy: It uses the uniqueness of inner shell transitions to track the dynamics of transient states of matter after excitation with a (e.g. UV-VIS) pump beam.
Wide Angle X-ray Scattering (WAXS) uses photons in the hard X-ray range (above few keV). These are scattered elastically at the electrons of all atoms, whereby atoms with more electrons contribute more to the scattering signal. This scattering signal bears information about the global structure of the investigated system, in particular about the configuration of the nearest neighboring atoms, and can yield insight about the interaction with the solvent molecules and solvation. Time-resolved WAXS elucidates the dynamic changes in these quantities during light-induced reactions.
Hundred picosecond x-ray pulses can be generated by synchrotrons, and x-ray pulse durations in the femtosecond range can be reached by Free-Electron-Lasers. Femtosecond UV and soft x-ray pulses can also be produced with methods that rely on femtosecond laser sources. Ultrafast x-ray science is an exciting and rapidly developing field. In our work group we contribute with the objective to develop a new picture of photochemical processes on a microscopic level, while exploring new experimental techniques and spectral ranges.
Image AG Kubicek
Collection of complementary (ultra)fast time-resolved x-ray probe techniques following molecular excitation via a light-triggered vA → vC valence electronic transition (far left). The evolving molecular dynamics is probed by a time-delayed x-ray pulse, offering different observables: (a) XANES (X-ray Absorption Near Edge Structure) for changes in the lowest unoccupied molecular orbitals (LUMOs) and local oxidation states; (b) Resonant Inelastic X-ray Scattering (RIXS), also called Resonant X-ray Raman Spectroscopy (RXRS), for the occupancy and interactions of occupied/unoccupied orbitals in both core- and valence-excited states; (c) EXAFS (Extended X-ray Absorption Fine Structure) for the local structural changes (bond lengths) around the absorber; (d) XES (X-ray Emission Spectroscopy) for changes in occupied molecular orbitals and local spin states; (e) WAXS (Wide Angle X-ray Scattering) for global structural changes, and for the solvation shell around the reacting molecule; (f) Non-Resonant Inelastic X-ray Scattering (NIXS), also termed Non-Resonant X-Ray Raman Spectroscopy (NXRS), with bulk-penetrating hard x-rays, and tender and soft XAS (a,c) and XES (d) both address lighter ligand atoms (C, N, O, S…).