Strong-field Ionisation of Atomic Hydrogen

Abstract

The past two decades have seen advancements in laser technologies wherein laser pulses are routinely generated with durations on the femtosecond ( 10ð€€€15 s) timescale. Such pulses allow only a few cycles (oscillations) of the laser electric eld for lasers with wavelengths centered in the visible to near-infrared (IR). Not only can there be marked variations in the peak electric eld value, caused by varying the pulse's carrier-envelope phase (CEP), but the compression of the pulse energy to within such short durations readily allows the production of strong laser elds with peak intensities that are well into the petawatt ( 1015 W/cm2) regime. The strong- eld ionisation of atoms and molecules via light{matter interactions is a fundamental process across many research elds. Such interactions have previously been used to reveal the structure and control the dynamics of atoms, molecules and solids, and to drive processes such as lamentation and to generate attosecond ( 10ð€€€18 s) pulses. These interactions are highly non-linear and are di cult to model theoretically, bringing into question the ability of current theoretical models to capture all of the signi cant physical processes occurring during said light{matter interactions. Not surprisingly, there is often a discord between theoretical predictions and experimental data for strong- eld processes, hampering experiment-theory comparison and, more worryingly, obscuring the correct physical interpretation of the experimental results. Therefore, an obvious need for accurate experimental data exists. The gold standard for target species in atomic physics is atomic hydrogen (H). It is the simplest atomic system and is the only species for which highly-precise (limited by numerical accuracy) solutions to the three-dimensional time-dependent Schr odinger (3DTDSE) equation are possible. Atomic H serves as a test bed for various theoretical models of ionisation, however it is di cult to produce experimentally. The author of this thesis was in a unique position of having access to both a source of atomic H and a few-cycle laser with which to undertake investigations. This thesis details two investigations of strong- eld ionisation of atomic H using fewcycle light pulses. The rst investigation explores the e ect that the CEP of a few-cycle laser pulse has on the directional yield of photoelectrons that are ejected from atomic H atoms undergoing strong- eld ionisation. These experimental photoelectron yields have been compared to predictions obtained from solving the 3D-TDSE and good experimenttheory agreement has been achieved without the use of any free t parameters. The results, which are the rst of their kind in atomic H, pave the way for obtaining CEP-calibrated reference data across a range of gas species and experimental parameters. The second investigation explores the e ect that changing the laser peak intensity in the 1{5 1014 W/cm2 regime has on the yield of photoions arising from strong- eld ionisation of atomic H atoms. These photoion yields, again the rst of their kind in atomic H, have been compared to predictions obtained from solving the 3D-TDSE. Quantitative experiment-theory agreement has been achieved at the few-percent level, proving accurate knowledge of the photoion yields and permitting the theory-certi ed retrieval of the true laser peak intensity to better than 1.1 % accuracy. This demonstrably accurate experimental method has been exploited to measure the photoion yields of the noble gases argon (Ar), krypton (Kr), and xenon (Xe); such gases are more readily available to other experimental laboratories. Comparison of the noble-gas yields with solutions to the 3DTDSE under the single-active electron approximation result in marked discrepancies which challenge the accuracy of the aforementioned ionisation models. However, a phenomenological model was developed to t to the noble-gas photoion yields, permitting a transferrable second-order laser peak intensity calibration standard that is accurate to 1.3 %, 1.5 %, and 2.5 % in Ar, Kr, and Xe respectively. Not only does the model allow any laboratory around the world with a few-cycle near-IR laser to now calibrate their laser peak intensity to the few-percent level, the data presented here also provides a benchmark for the testing of theoretical models of ionisation in noble-gas atoms.