Non-sequential ionization

The processes responsible for the experimentally observed enhanced production of doubly charged ions for intensities below the appearance of the knee-like structure are summarized

as non-sequential ionization mechanisms [62]. In the literature several processes which explain the non-sequential double ionization (NSDI) were proposed. All non-sequential processes involve electron correlations. The dynamic of the second electron is influenced by the first electron. Potential processes of non-sequential ionization are:

Shake-off mechanism

The first electron is removed by ionization very quickly (’sudden approximation’). The wave function of the remaining bound electrons relaxes to the new eigenstates of the modified potential. Some of these new states are located in the continuum (’shake-off’) or are still bound but excited (’shake-up’), which means that a second electron can be ’shaken off’ in the course of the relaxation process [68]. A ’shake-up’ electron can be tunnel ionized within the next half-cycle [69, 62]. It becomes the dominant mechanism of double ionization at very high incident photon energies in the keV range [68]. Its importance to non-sequential double ionization by strong-field ionization in the tunneling regime is negligible since the underlying ’sudden approximation’ is strictly speaking not fulfilled.

Collective two-electron tunneling

Another possible mechanism for NSDI is the quantum mechanical process in which two electrons tunnel through the potential barrier created by the Coulomb potential and the strong external electric field. This scenario is only possible if the two electrons have the same distance from the nucleus. If not, the delayed electron is recaptured with a high probability [70]. However it was found that this ionization rate on its own cannot explain the high experimentally observed ionization rate. Possibly it is a contributing channel to NSDI but might become dominant for very high field strengths or sub-cycle pulses. Rescattering mechanism

In experiments where NSDI is observed with ultashort laser pulses, the importance of tun- neling in the non-sequential mechanism is been proven [65]. In the rescattering model, on which also the high-order harmonic generation is based [59], the first electron is tunnel or over the barrier ionized close to an electric field maximum and afterwards accelerated and finally driven back to its parent ion. If the recollision energy is larger than the ionization potential of the singly charged ion, the recolliding electron can directly knock off the second electron (for HHG the recolliding electron recombines). This is known as the recollision- induced direct ionization (RIDI) process. If the return energy of the driven first electron is not sufficient to lead to double ionization, it can collisionally excite a still bound electron to an excited state which then can rapidly field ionize in a so called recollision-induced excitation plus tunneling process (RIET) [62, 71].

In both processes the two electrons in the continuum are correlated with each other and therefore the recollision process is still classified as a non-sequential ionization process. Recollision induced ionization is the dominant process which explains the observed non- sequential doubly charged ion yield which is by orders of magnitude higher than the se-

quential contribution for intensities of 1014 _{to 10}16 W

cm2. Recent experiments indicate that

the role of the rescattering mechanism for ionization, and any non-sequential ionization is strongly suppressed for intensities between 1016 _{to 10}18 W

cm2 [72].

The applicability of the proposed ionization mechanisms and their relative contribution to the final ion yield cannot only be charged upon the integrated measurement of the ion counts after the strong field laser pulse. A technique which measures the momentum of every electron or ion in coincidence, namely measurements addressing electron-electron and electron-ion coincidences which are based on the COLTRIMS (cold target recoil ion momentum spectroscopy) technique have shown to deliver additional insight into the dom- inant processes responsible for NSDI [65, 73].

The presented attosecond transient absorption spectroscopy in this thesis provides first evidence that it can access ionization dynamics in form of a pure pump-probe experiment in real time and therefore will add the required temporal aspect of the ongoing ioniza- tion mechanism. The sub-cycle resolved ionization dynamics, which were up to now only been limited to phenomenological modeling based on a time-integrated detection, might be studied soon for several different atomic systems and parameter regimes very precisely in a time-resolved fashion within the generating laser pulse. First proof-of-principle exper- iments are reported in chapter 6.

Generation and metrology of isolated

attosecond XUV pulses

2.1

Introduction and requirements

Time-resolved studies of tracking and controlling electron dynamics in the interior of atoms as well as in molecules and solids, require attosecond temporal resolution. Experiments, mainly those which are based on the generation of high-order harmonic radiation are very sensitive to the field waveform rather than the envelope of the driving laser field and there- fore have very high demands on the precision, stability and reproducible control of the carrier envelope phase or more generally speaking of the waveform of the laser pulses. With driving pulse durations as short as 1.5 cycles of the carrier field, experiments become even more sensitive to variations of the carrier envelope phase as the interaction is con- fined to a small fraction of a cycle. Since several highly nonlinear processes are employed to generate isolated attosecond XUV light bursts, the overall generation of those pulses is also very sensitive to intensity fluctuations of the driving laser pulses. Thus, a high degree of shot-to-shot and long term laser pulse stability is essential for the performance of attosecond pump-probe experiments.

This chapter will introduce the state-of-the-art tools for attosecond metrology from the perspective of laser requirements and measurement principles whereas chapter 3 describes in detail the experimental apparatus and implementation of diagnostics specifically tailored for attosecond streaking and for the first attosecond transient absorption experiments re- ported here.

Actuated mirror 2

Actuated mirror 1

Neon filled hollow core fiber f = 1.8 m f = - 0.5 m re-collimation Prism compressor Pulse duration CEP Positive dispersive mirrors Negative chirped mirror compressor IR photodiode Ti:Sapphire oscillator Stretcher SF57 + dispersion correction PP-MgO:LN dichroic mirror 400 µJ / <4 fs 750 nm Auto- correlator 2% Dispersion control f-to-2f interferometer PSD PSD BBO Short-pass filter Half-wave plate Glan-Thompson polarizer Spectrometer Feedback QS Nd:YLF pump laser

3kHz 9-pass Ti:Sa CPA amplifier

CW LBO pump laser Acousto-optic

modulator

Figure 2.1: Overview of the 3 kHz sub-1.5-cycle laser pulse system for attosecond ex- periments. It shows the commercially available front end consisting of a Ti:Sa oscillator seeding a 9-pass Ti:Sa amplifier system which was tailored for the special needs of at- tosecond experiments. Subsequent spectral broadening of the pulses is taking place in a neon filled hollow-core fiber. Pulses are dispersion controlled and compressed by a chirped mirror compressor yielding a pulse duration of sub-4fs laser pulses centered at 750 nm with 330 µJ pulse energy.