# Few-electron systems, strong eld interactions, numerical simulations

Prof. A. Scrinzi,Faculty of Physics Computational & Plasma Physics, LMU

Main research areas: ultrafast dynamics of electrons, electron-nucluear correlations, attosecond measurement techniques.

Fully controlled light fifields and ultrashort pulses allow the observation and control of electronic motion on the atomic time scale. Our group develops theory for the newly observable phenoma: generation and propagation of ultrashort pulses, sub-femtosecond measurements, few-electron dynamics and electron-nuclear correlation.

Our traditional way of thinking of atoms and molecules as being in well-defined electronic states with sudden transitions between them fails when the time scales become very short or when time-dependent external fifields disrupt the stationary structures. Electron motion is governed by the electric fifields of the nuclei, by interactions between the electrons, and by external laser fifields, which may rival or exceed the internal fifields. As new experimental techniques allow observation and control of electronic motion, we revisit atomic and molecular physics on the ultra-short time scale for an appropriate time-dependent picture of electronic transitions and relaxation and its interplay with evolutions of the slower time scale of nuclear motion. How does the electron density evolve while an atom is ionized? Can we observe the tunneling process? How does an electron leave the system and in which state is the ion left behind? We address these questions using simple models and numerical ab initio calculations.

Time-dependent detachment of electrons by a strong laser field: the momentum distribution of electrons right outside the atom. Negative momenta indicate that electrons are directed back to the ion when the laser field reverses sign.

Ultrashort pulses with durations of a fraction of a femtosecond can probe electronic motion on the characteristic time scale of valence electrons in atoms and molecules. A precisely timed sequence of measurements provides us with an "atomic movie" of electronic rearrangement and relaxation. For the design and interpretation of experiments we must understand which parts of an atom or molecule in motion are probed, what is the theoretical maximum information that can be obtained from a given measurement, and which parameters set the limits of a measurement. Our group develops observation techniques for the new sources and analyzes them from a fundamental point of view and with respect to experimental realization. We relate the observed high harmonic radiation, electron energy spectra, and electron diffraction patterns to the underlying microscopic dynamics.

The "atomic transient recorder": a distribution of electrons in time and momentum is probed by a laser field at different time delays, providing a set of projections of the distribution onto measurable spectra.

A third area of research is the interplay of linear and non-linear processes for the generation of high frequency radiation and attosecond pulses. Sub-femtosecond pulses are generated during the propagation of a strong short laser pulse through a gas. An extremely non-linear response of the medium returns high frequencies that are phase-locked into short bursts of radiation. These "attosecond pulses" (1 attosecond = 10^{-18} seconds) inherit their time structure from the electric field of the generating laser pulse: the number attosecond pulses corresponds to the number of the strongest electric field peaks in the laser pulse, their chirp can be related to the rise time of the electric field during their generation. As the laser field is controlled precisely, attoscecond pulses can be timed and shaped with the same precision. The work involves the simultaneous solution of Maxwell's equations for the propagation and of the Schrödinger equation for the atom's response to the field. The goal is to determine optimal conditions for generation of high frequency radiation and the optimal design of attosecond pulses.

An attosecond pulse: after filtering a range of frequencies from the complex high frequency response of the atoms in a gas volume to a short laser pulse a smooth single pulse results.

### Contribution to IMPRS curriculum:

### Attosecond physics/ theory

#### Lecturer: Armin Scrinzi (2 hours/week)

Content:

Our theoretical understanding of key phenomena of attosecond physics such as strong- eld tunnel ionization, high-harmonic generation, ATI electron spectra, attosecond electronic dynamics, and measurement of pulse duration and shape is presented. The most important calculational techniques for classical, semi-classical (stationary phase), and quantum models are introduced.

### The Schrödinger equation with strong external fields

#### Lecturer: Armin Scrinzi (2 hours/week)

Content:

An overview of techniques for computational solution of the time-dependent Schrödinger equation in strong external fields is given. The lectures introduce single- and few- electron models, grid- and basis-set discretization techniques, and the most important computational methods for solution of discrete equations.