`Attoclock' experiments on atomic and molecular hydrogen

Abstract

The current thesis aims at benchmarking strong- eld physics with the help of precision measurements performed on the simplest atomic (H) and molecular (H2) systems. The importance of H in validating strong- eld models is demonstrated through the rst set of experimental data. It aims at calibrating the absolute Carrier-envelope phase (CEP) of few-cycle laser pulses using H against complete ab initio solution of the three dimensional time-dependent Schr odinger equation (3D-TDSE). Subsequent set of measurements with noble gases against widely used strong- eld models based on single-active electron (SAE) approximation, is shown to reveal a systematic o set of 0:25 radians in tagging CEP, questioning the validity of such models. The second experimental study forms the main result of this thesis, that attempts to resolve the ongoing debate on tunnelling times (tunnelling delays in the context of strong- eld physics). We address this by employing the `attoclock' technique with 6 fs pulses on H and validating the results against full numerical solutions of ab initio 3D-TDSE. The validated numerical codes are then used to arti cially screen the parent ion-electron interaction, concluding that the tunnelling time 1.8 as. The nal experimental results presented in this dissertation are the alignmentdependent attoclock measurements using both few-cycle (7 fs) and multi-cycle (28 fs) pulses on H2. The measured attoclock observable for various molecular orientations (in laser polarisation frame) shows a strong modulation with a periodicity of . Initial ab initio simulations for few-cycle pulses under the frozen-nuclei and SAE approximations, fail to explain these observations. Further experimental studies with H2/D2 (50:50 mixed gases) show no signi cant relative di erences among the attoclock observables, suggesting a prominent role of the electron-electron correlations at play. The ongoing study is believed to have far reaching implications in applications such as studying molecular dissociation processes and tomography.