Electron-correlation effects in laser-driven helium

The interaction of atomic systems with strong ultra-short laser pulses poses challenges to theory. The role of the interaction (correlation) between the electrons is, in many cases, not understood. Since the advent of strong lasers in the femtosecond region and the availability of XUV pulses in the attosecond region, the interaction of strong and short laser pulses with matter has become a rapidly growing field in the physics community. Using attosecond pulses, it is possible to experimentally access the time scales of electronic motion in atoms, allowing for direct time-dependent studies of correlation effects. We will theoretically study helium where full numerical simulations of the interaction with external fields is within reach of presently available super-computers. We propose to perform the following studies: i) Two-photon above-threshold double-ionization, where preliminary results indicate that correlation effects can be clearly resolved by using ultra-short pulses. ii) Pump-probe scenarios, i.e., simulations of exciting and probing the motion of wave packets among doubly-excited states by means of ultrashort XUV-pulses. iii) Probing of resonances excited by means of infrared fields. The single ionization yield can give information both on the phases of the states and on the time-dependent build-up of Fano resonances. iv) Time-dependent imaging of shake-up states by forcing the electrons ionized to coherently rescatter on the ion using an infrared field. The detailed knowledge gained from helium will be relevant for more complex atoms where ab initio simulations are not yet feasible and may lead to the development of experimental proposals for time-dependent studies of correlation effects.

During the last decade, the development of laser sources has been stunning. The shortest laser pulses present are shorter than the typical time for electrons in atoms to circle around the nucleus. At the same time, they are so strong, that they directly can rip of the electrons from an atom. The development of such pulses has opened up a wealth of novel possibilities to drive atoms, and to probe the time-dependence of the processes. In this project, we have relied on large scale computer simulations to explain and propose experiments utilizing the novel laser sources. Our largest simulations have involved the two-electron atom helium. In one case studied, using ultra-short pulses makes a huge difference. For long pulses, the two electrons leave the atom at different times during the pulse, the repulsion between the electrons plays only a minor role, and the direction the second electron leaves the atom in is independent of that of the first one. For ultra-short pulses, the electrons are forced to leave the atom almost at the same time. This means that the electrons repel each other when leaving the atom implying that they leave the atom in opposite directions. Hence, by controlling the length of the pulse, one can decide how important the repulsion between the electrons is for the outcome of the interaction of the laser pulse with the atom. We have also considered driving atoms with infrared laser light consisting of two different colors. Here, the relative phase of the two colors plays a decisive role. By tuning the phase, one can decide in which direction the electron leaves the atom. In present day experiments, the fully angularly resolved momentum distribution can be measured. Such two-dimensional distributions display a complicated interference pattern, the shape of which is strongly dependent on the relative phase of the two colors. Our studies have revealed that some structures in the interference pattern stem from processes taking place on a time scale much shorter than the length of the driving pulse. This example shows that, by carefully analyzing the interference patterns, one can study processes being much faster than the length of the laser pulse.