Analysis and control of transient spectra using time-dependent density functional theory.

Abstract

Attosecond-transient absorption spectroscopy and attosecond time-resolved photoelectronspectroscopy based on pump-probe setups have recently become possibledue to the availability of ultra-fast pulsed laser sources. These experiments allow tofollow the electronic motion on its natural time-scale, allowing the characterizationof electronic excited states and transitions while neglecting the influence of thenuclear degrees of freedom. Molecular absorption and photo-electron spectra can beefficiently predicted with real-time time-dependent density-functional theory, whichis normally used for the description of systems in their ground state. In this thesis, Ishow how these techniques can be extended to study time-resolved pump-probeexperiments in an excited state providing a tool for the interpretation of fastevolving attosecond time-resolved spectroscopic experiments. I show how the extradegrees of freedom (pump pulse duration, intensity, frequency, and time-delay),which are absent in a conventional steady state experiment, provide additionalinformation about the electronic structure and the dynamics, which can be used toimprove a system characterization. As an extension of this approach, timedependent2D spectroscopies can also be simulated, in principle, for large-scalestructures and extended systems. In addition to numerical modelling, a theoreticalunderstanding of the processes would be useful. In this thesis, I reexamine theLehmann representation of the response function of a pump-driven system in thenon-overlapping regime, deriving some general properties of the excited statespectra, including position and shape of their resonance peaks. I demonstrate, howas a consequence, this Lehmann representation can be used to understand laserinducedline shape changes. Measurement and control of ultrafast processes areinherently intertwined. In this work, I assess the possibility of using tailored pumpsin order to enhance (or reduce) some given features of the probe absorption (forexample, absorption in the visible range of otherwise transparent samples). Thistype of manipulation of the system response could be helpful for its fullcharacterization, since it would allow to visualize transitions that are obscured whenusing un-shaped pulses. In order to investigate this possibility, I firstly combine theLehmann representation with a simple numerical model of the Hydrogen atom toshow, how Hydrogen can be manipulated to loose its transparency in the visible. Ithen proceed to investigate the feasibility of using time-dependent densityfunctionaltheory as a means to implement, theoretically, this absorptionoptimizationidea, for more complex atoms or molecules.