Research activities XUV Physics
The main themes of our research during the past six years have been coherent control of atomic and molecular properties and the generation, characterization and application of high harmonic radiation, particularly in the time domain, where this technique provides a means to generate attosecond laser pulses. A constant theme throughout this work, which links to many of the projects we have completed, is the response of electrons to strong (time-varying) electric fields. Our main research highlights during the past five years have been:
The first observation of field-free molecular alignment following impulsive excitation by an intense laser pulse: we demonstrated that excitation of molecules with an intense, short (compared to the rotational period of the molecule) laser pulse leads to the excitation of a rotational wave packet that transiently aligns itself along the laser polarization axis after the laser has been turned off. We subsequently embarked on an effort to apply optimal control techniques to this problem and have developed a technique for single-shot characterization of molecular alignment.
The first observation of semiclassical interferences in photoelectron imaging near an ionization threshold: we observed how electron wave packets produced during an ionization event interfere constructively or destructively, depending on the quantum mechanical pathlength difference between the associated trajectories. In related work, we observed a nondipolar photoelectron emission that displays a strong asymmetry along the propagation axis of the laser.
The first characterization of attosecond laser pulses using angleresolved photoelectron spectroscopy: we demonstrated that in a RABBIT (Reconstruction of Attosecond Beats By Interfering Two-photon Transitions) experiment, attosecond pulses can be characterized on the basis of both the energy spectrum and the angular distribution of the photoelectrons. This is important for the characterization of isolated attosecond pulses.
The first application of attosecond pulse trains to studies of atomic and molecular dynamics (with L'Huillier, University of Lund): we have performed a two-colour XUV+IR ionization experiment, that - depending on the time delay between the XUV and the IR pulses - provides information on the laser pulses and the momentum space electronic wavefunction.
The first observation of a carrier-envelope phase dependence to control a molecular dissociation (with Krausz, Max-Planck Institut for Quantenoptik in Garching): we demonstrated that the localization of an electron in the dissociative ionization of hydrogen molecules can be controlled using the carrier-envelope phase. This is the first demonstration of carrier-phase control in molecules. Prior to this measurement, we performed experiments on carrier-phase control in the radio-frequency ionization of high Rydberg states.
The first application of optimal control techniques to control the explosion of large clusters irradiated by intense femtosecond laser pulses: we experimentally observed and numerically confirmed that the cluster explosions are optimized by sequences of pulses that exploit the occurrence of enhanced ionization and/or plasma resonances in the cluster.
Future directions
In the coming years our research will focus on the study of the motion of
electrons in atoms and molecules, using laser pulses with a duration of several hundred attoseconds. In strong laser fields, we will induce electron motion using an intense infrared laser, and investigate the motion of bound electrons (in molecules undergoing dynamic alignment) and continuum electrons (in molecules undergoing ionization). Next, we plan to study photo-absorption processes in weak laser fields, where we will probe how the electronic excitation couples to other degrees of freedom, i.e., other electrons, and the rotational and vibrational degrees of the nuclei. We plan to study electron transfer and dissociation in large molecules, as well as the excitation and decay of giant plasmon resonances in clusters and large molecules such as C60. As experimental tools, we intend to use techniques for measuring the emission of
electrons with both angular and kinetic energy resolution, including fully
coincident ion and electron detection schemes. This work will provide a deeper insight into the motions of electrons in molecules and pave the way for the exploitation of attosecond laser pulses in physics, chemistry and photobiology.
