Item

Abstract

Studying the dipole response of the excited quantum system could provide insights into the extreme ultraviolet (XUV) light-matter interaction. Transient absorption spectroscopy uses the dipole response field’s interaction with incoming light to leave a signature on the transmitted light. This thesis uses high-harmonic generation (HHG) and free electron lasers (FEL) as XUV light sources, with helium serving as the subject of interest. We study the absorption lineshape of singly excited state resonances of helium in a delayresolved manner using a delay stage, which we can detect in single-shot mode on our XUV spectrometer. The study aimed to understand the perturbations in HHG-induced atomic dipole responses when there is interaction with intense FEL pulses. The XUV HHG pulses first excite the absorption lineshape, and intense XUV FEL pulses further modify it. It leads to a change in the absorption feature. The linewidth broadening indicates the dynamics of the interaction of the XUV pulses with the excited states in helium. The perturbed dipole response function can be used to determine how the XUV-driven excited state dynamics evolve in real-time. The femtosecond-level time-delay-resolved absorption spectra have been recorded at different helium target gas pressures, FEL intensities, and FEL photon energies. The experiment was conducted at beamline FL26 in the free electron laser (FEL) FLASH 2 facility in DESY Hamburg, which generated tunable intense SASE FEL pulses of 20 μJ, 50 fs of pulse length, and a single bunch pulse train with a repetition rate of 10 Hz. In this work, an attempt has been made to adapt the dipole control model to the FEL interaction and the resulting plasma environment.