Development of an ultrashort deep- and extreme ultraviolet beamline and its application to photoelectron holography

Abstract

Motion in the microcosm is extremely rapid. For instance, a chemical reaction can be completed in a few femtoseconds. Due to the extremely short time scale at which chemical reactions occur, only the initial and final products are known; the reaction pathways and intermediate products, on the other hand, remain unknown. To observe these ultrafast processes on their time scales, science requires measurement technology that is faster than the processes to be observed. Development of ultrashort pulsed laser technology enabled the detection of these processes in real time. For example, the so-called ”pump-probe” spectroscopy technique enables the tracking of ultrafast dynamics in atoms and molecules following the excitation with a short laser pulse called the pump pulse. A delayed second pulse, also known as the probe pulse, probes the current state of the evolving system. In this manner, by varying the time interval between the pump-probe pulses with multiple repetitions, a dynamical evolution of the system can be obtained. The available shortest pulse duration, however, was limited to capturing only nuclear dynamics with this technique. A new regime in ultrafast optical science called attosecond physics emerged at the turn of the 21st century. In the extreme ultraviolet regime, the high-order harmonic generation method has enabled previously unattainable ultrashort pulse durations of a few hundred attoseconds. Attosecond pulses enable us to probe dynamics beyond the Born-Oppenheimer approximation, in which electronic states are not stationary and influential. We need to investigate how nonstationary electronic states are coupled to nuclear motion and then understand how manipulating the electronic motion can be used to trigger the nuclei to move selectively, thereby implementing a chemical reaction pathway. To aid in our knowledge and control of ultrafast dynamics in a new paradigm, an intense few-cycle deep-ultraviolet beamline was built in conjunction with ultrashort near-infrared and extreme ultraviolet pulses enabling time-resolved studies using various combinations of these light sources.