A novel optical tool for controlling and probing ultrafast surface dynamics

Ultrashort pulse laser sources have been greatly developed over the past few decades. The available pulse duration has been reduced to the single-cycle pulse regime. The discovery of high harmonic generation has freed us from the limitation of the laser wavelength. Moreover, the demonstration of isolated attosecond pulse generation has indicated the advent of the attosecond science era. Attosecond pulses undoubtedly allow one to study ultrafast dynamics with unprecedented time resolution. However, physical systems with genuine attosecond time scale dynamics are rather challenging to find. Ultrafast surface charge transfer, which is an important process in photochemistry and electrochemistry, is a good candidate experimental system exhibiting attosecond electronic dynamics. Specifically, the ultrafast surface charge transfer on the c(4 x 2)S/Ru(0001) surface was previously studied and the charge transfer time inferred to be 320 as using core-hole clock spectroscopy at a synchrotron facility. In order to measure this benchmark attosecond electronic dynamics with real time-resolving methods, pump pulses centered at 160 eV and probe pulses centered at 40 eV are required. To this end, a dedicated attosecond experimental beamline including an ultrashort laser pulse source and an attosecond pulse generation and characterization setup has been designed and is being developed. The author of this thesis was responsible for the construction of the attosecond experimental beamline which will be used ultrafast surface charge transfer studies. In this thesis, a completely functional attosecond extreme ultraviolet (XUV) beamline, which includes a few-cycle laser pulse source, an attosecond pulse generation and characterization setup, is described. A commercial Ti:sapphire-based chirped-pulse amplification (CPA) laser system is the overall source of the beamline. The laser system is actively carrier-envelope phase (CEP) stabilized and the output pulse duration is ∝35 fs. The laser pulse spectrum is then broadened via self-phase modulation by means of propagation through a gas-filled hollow-core fiber. The output pulses, spectrally broadened and temporally stretched, are recompressed with double-chirped mirrors. The compressed few-cycle pulses are characterized with SHG-FROG. The pulse characterization difficulties due to the broad bandwidth associated with few-cycle pulses have been tackled by exploiting the FROG trace frequency marginal correction. The pulse duration is measured to be 5.3 fs. The few-cycle pulses are then used for HHG in the attosecond pulse generation setup. The generated XUV pulses are characterized with an XUV spectrometer, which is equipped with varied line space gratings and an XUV CCD camera as detector. An XUV continuum, indicating the generation of isolated attosecond pulses, has been obtained with the attosecond pulse generation setup via double optical gating. The XUV continuum supports pulse durations shorter than 300 as. Meanwhile, obvious CEP effects on the XUV continuum strongly support the generation of isolated attosecond pulses. Full characterization of the attosecond pulses requires measurement of a photoelectron energy spectrogram in the presence of both the streaking IR and XUV fields. The XUV beam is focused by a gold coated toroidal mirror with grazing incidence. The delay between the XUV pulses and the IR pulses is controlled by varying the difference between the arm lengths of a Mach-Zehnder interferometer. A FROG-CRAB photoelectron spectrogram, which is used for attosecond pulse characterization, has been measured. The highly demanding interferometric stability for attosecond streaking experiments is achieved passively thanks to an elaborate vibration decoupling design of our vacuum system. In contrast to the broad bandwidth of the XUV continuum required for generating attosecond pulses, narrow bandwidth XUV sources with tunability are important for photoelectron spectroscopy experiments requiring high spectral resolution. To obtain an XUV source with narrow bandwidth and tunability for our experiments, a multilayer mirror monochromator has been designed and developed by our collaboration partners from Prof. T. Uphues group. The multilayer mirror monochromator is able to select out a desired spectral region with bandwidth narrower than 0.5 eV in the range between 90 eV and 98 eV. In this thesis, the characterization of the multilayer mirror monochromator has been carried out in collaboration with Prof. T. Uphues group. Some more efforts are still necessary on implementing the attosecond pulse reconstruction algorithm and completing the setup automatization. Eventually, the beamline will become a powerful tool for time-resolved attosecond dynamics studies and also XUV related spectroscopy studies in the near future.