Electron Spectroscopy at Intense Laser Fields

Research project P 14447 Electron Spectroscopy at Intense Laser Fields Matthias LEZIUS 09.10.2000 The development of a laser technology that delivers intense laser light at a pulse duration below 10 femtosecond has given access to a research domain that opens several interesting applications. The pulse duration is so short that it consists of only a few optical cycles. The whole energy of the laser beam is squeezed into this time window. Extremely high light intensities are created, corresponding to electromagnetic fields having millions of millions of Volts/cm. Physicists refer to this area as strong field physics. Under the influence of such a laser pulse matter shows a distinct behavior, because the electric field is larger than the force that holds it together. Electrons leave the atoms and they are driven towards high velocities by the intense light. This is called strong field ionization or above threshold ionization. Because the electric field reverses its direction every half-optical cycle, electrons sometimes re-collide with their original atom and produce x-rays. This phenomenon was named high harmonic radiation and it has found increasing interest because it represents a convenient way to produce coherent x-ray radiation. For future applications these light sources are especially interesting ig the photon energies are exceeding several 100 eV. Such energies have already been demonstrated by the Technical University of Vienna. Today it is desired to optimize the efficiency and to understand in detail the behavior of electrons in strong light fields. The present proposal aims to aims to carry out the most complete experimental measurements to reveal the underlying physics. It is planned to collect data on many different aspects of the photoelectron spectra, with special attention to multi-electron effects that are presently less well understood. We will investigate the influence of the laser intensity, wavelength, polarization orientation and pulse duration on the photoelectron spectra produced from a number of atoms and molecules. A specialized photoelectron spectrometer will be designed at the University of Innsbruck, which is dedicated to measure photoelectron energies up to 1000 eV with an accuracy better than 1.01%. Our data acquisition will be performed at several collaborating laser laboratories situated in Vienna, Ottawa (Canada), and Saclay (France). We plan to work in close discussion with theorists, we aim to test and to test and to improve their most recent models which make an effort to describe matter under such extreme conditions.

Recent developments in laser technology have opened access to light pulses with a duration below 5 fs (1 fs = 10- 15 s). Such pulses consist of only a few optical cycles. When the energy of the laser light is squeezed into such a small time window, extremely high fields of several million Volts/cm are created. Atoms and molecules in such fields are instantly ionised, and electrons are liberated. The oscillating field then accelerates these electrons to high kinetic energies. Using precise control over the laser light-wave the electron trajectories can, in principle, be controlled at wish. In addition, if the phase between the light-wave and the pulse envelope is also controlled, the timing of the electron movement becomes even better and reaches the attosecond timescale (1 as = 10-18 s). Using so-called carrier-envelope phase control, which has recently become accessible, it is now possible to produce attosecond bursts of electrons, and attosecond ultraviolet and x-ray emission. This has opened a new research area in which the accurate temporal determination of the movement of electrons in atoms and molecules seems within reach. The results of the present research project have strongly contributed to this kind of research. Using high- precision photoelectron spectroscopy it has been discovered, that depending on the carrier-envelope phase the liberated electrons will leave the focal region in different directions and with different energies. This can with advantage be used to determine the carrier-envelope phase and also to phase-stabilize the laser, so that only laser pulses with a well-defined phase are produced. Such kind of measurement of the carrier-envelope phase has been one of the major goals in femtosecond science within the recent years, because it opens the door to attosecond physics. Via control and precision measurement of the laser phase we will now be able to install precision clocks into our laboratories, by which we can measure timed events with an outstanding accuracy of 10-17 seconds.