Above Threshold Ionisation of Atomic Hydrogen using Few-Cycle Pulses

Abstract

The interaction of intense few-cycle infrared laser pulses with matter is the fundamental process at the heart of attosecond science, which is the study of phenomena with duration below 10−15 seconds. Few-cycle laser pulses, which have durations in the femtosecond regime, have previously been used to reveal and control the structure and dynamics of atoms, molecules and solids. The physical interpretation of such experiments relies on the accurate simulation of the complex, highly nonlinear material dynamics during the few-cycle laser pulse. The future of attosecond science thus depends on the correct interpretation of few-cycle laser interactions with matter. The question of whether the current theoretical models completely capture all of the significant interaction physics would be answered by an experiment showing quantitative agreement between theoretical predictions and experimental data in the regime necessary for the study of attosecond science. In this thesis, experimental data which shows quantitative agreement with theory in this regime is presented. The ionised photoelectron spectrum from few-cycle above threshold ionisation is directly compared with theoretical predictions1. The method by which the predictions are compared to experimental data is presented and it includes detailed simulations of the electron detection system and takes into account the finite spot size of the focussed laser beam. The target atomic species is atomic hydrogen (H), the simplest of all atomic systems, and the only species for which ab initio simulations can be calculated in this regime. The data show an unprecedented level of quantitative agreement with the advanced ab initio time dependent Schr¨odinger equation simulation over an order of magnitude range of electron energies and laser intensities. The strong-field approximation fails to quantitatively agree with the data which confirms that this commonly used model is not adequate for experiments requiring significant agreement between data and predictions. These results mark the first quantitative experiment in the regime of attosecond science and as such should provide the basis for future comparisons of experiment and theory.