A theoretical study of ultrafast phenomena in complex atoms

Resumen

Abstract The ultrafast movement of electrons is a driving force of chemical reactions, making it a highly desirable avenue for study. This thesis studies such move- ments, making use of pump-probe methods such as attosecond transient ab- sorption spectroscopy (ATAS) and reconstruction of attosecond beatings by in- terference of two-photon transitions (RABITT), in complex atomic systems. The main approach used to solve the time-dependent Schr¨odinger equation (TDSE) was exact, attosecond, full-electron, ab-initio calculations. Firstly, helium was probed above the second ionisation threshold, where several ionisation channels are open, using accurate ab-initio calculations. Here, the ATAS method was employed to predict beatings between the autoionising 3snp 1 P o resonances and nearby 1 Se and 1 De states. More surprisingly, two- photon beatings between the doubly-excited 3s3p state and the 1 P o contin- uum were also observed, demonstrating control of the correlated, two-electron, multi-channel wave packet. Secondly, two studies of neon were carried out below the second ionisation threshold. The first makes use of ATAS calculations to probe beatings between the autoionising neon states. Using a two-colour, mixed extreme-ultraviolet (XUV) near-infrared (NIR) pump, one-photon beatings between the 2s− 13p 1 P o and the nearby 2s− 13s 1 Se and 2s− 13d 1 De resonances are observed. Further, one- and two-photon beatings between the autoionising 2s− 13`, ` ∈ { 0,1 } and the 1 P o continuum are predicted. iThe second uses the RABITT method to probe the atomic phase in the vicin- ity of multiple resonances. This is far from trivial, and interferometric methods have until now been restricted to simpler energy-regions, due to the difficulty of accurately describing the electron correlation associated with the more com- plex case, making accurate ab-initio calculations needed to guide experiments unavailable. Despite the complex energy-dependence of the phase when sev- eral resonances are present, presented results from experiment and ab-initio theory are in excellent agreement. Further, using a simple extension of the Fano model for resonant continua, the contributions of the different involved resonances are disentangled. Such simple models are highly desirable in more advanced systems, where accurate ab-initio calculations are inaccessible. The ab-initio results of both neon studies were carried out using the newly developed XCHEM methodology, which is thus further validated by the excel- lent agreement with presented experiments and previous studies. Finally, a RABITT study of argon in the vicinity of the 3s− 1 n` resonances was performed. Angularly resolved, experimental results are presented, show- ing the anisotropy of the atomic phase in smooth continua as well as the vicin- ity of resonances. Due to the complexity of the system, no ab-initio results are present. Instead, simpler interferometric models are used to successfully explain the anisotropic behaviour of the phase.