Ultrafast XUV photoelectron spectroscopy of materials for solar energy conversion
Objectives. The study of photoinduced electron dynamics in light-harvesting materials represents the key to understanding the mechanisms of solar energy conversion, which is ultimately needed for the development of novel materials with better conversion performance. Following photoexcitation, the early time electron dynamics occur typically on a femtosecond time scale, requiring application of ultrafast transient spectroscopies to track the electron pathways.
Method. In our studies we apply transient XUV photoemission spectroscopy (PES). In pump-probe experiments, a femtosecond laser pulse in the UV/visible spectral range interacts with the sample first and promotes electrons from the ground state of the light-harvesting molecular compound to an excited electronic state. Applied at a variable time delay, an ultrashort XUV probe pulse ionizes the excited sample and thereby maps the electron density distribution among bound states into the continuum spectrum. By recording the transient kinetic energy spectra of ionized electrons at different pump-probe time delays, one can follow the electron population dynamics on the absolute energy scale with a femtosecond temporal resolution.
Experimental setup. A femtosecond Ti: sapphire laser system, delivering pulses of 25 fs minimum duration at 800 nm wavelength with a repetition rate of 5kHz, is used to generate both the optical pump and XUV probe beams. XUV light is produced via upconverting the laser frequency in the process of high-order harmonic generation (HHG) induced in a gas medium. A zone-plate monochromator is used to select a desired harmonic. Typically the 21st harmonic of 31.5 eV photon energy is applied in experiments. The monochromator is designed to minimize the temporal broadening of XUV pulses, yielding a pulse duration of 45 fs . The optical pump beam is generated with the use of an optical parametric amplifier, which facilitates tuning of the pump wavelength in a wide range between 250 and 2500 nm. The pump and probe beams are focused in the experimental chamber on the sample positioned in front of a time-of-flight electron spectrometer. Solid as well as liquid samples can be studied with the use of our setup. In the latter case, a liquid micro-jet technique is applied to facilitate the high-vacuum conditions required for electron detection.
Schematic view of the experimental setup including generation and monochromatization of the XUV probe beam and its delivery optics, laser pump beam, time-of-flight electron spectrometer, and liquid micro-jet.
While the pump and probe pulses are intrinsically synchronized, an optical delay stage is used in the pump beam path to vary the pump-probe time delay.
Our laboratory includes also a second experimental setup, enabling to conduct studies with the use of transient absorption spectroscopy. For details, contact Dr. Merschjann.
 J. Metje, M. Borgwardt, A. Moguilevski, A. Kothe, N. Engel, M. Wilke, R. Al-Obaidi, D. Tolksdorf, A. Firsov, M. Brzhezinskaya, A. Erko, I. Yu. Kiyan, E. F. Aziz. Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates. Optics Express 22, 10747 - 10760 (2014). DOI: 10.1364/OE.22.010747
We greatly appreciate collaboration with the theory group of Prof. Kühn (University of Rostock, Germany), with the chemist group of Dr. Simonov (Monash University, Australia), and with Prof. Zhang (Tianjin University, China).