Dr. Oleg Kornilov
University of California - Berkeley
Thursday, January 28, 2010
Femtosecond Photoionization Dynamics of Pure 4He Droplets
Fascinating properties of superfluid helium remain in the focus of research for more than 70 years since their discovery. Over the last decade the research concentrated on helium clusters, which once again proved being very different from clusters of other substances. New observations such as microscopic superfluidity, ultracold spectroscopy and chemistry inside droplets, nucleation dynamics have called for new experimental and theoretical approaches. In line with a number of rotational and vibrational spectroscopic techniques applied to doped helium droplets we introduce a novel time-domain experiments on photoionization dynamics of pure helium droplets. These experiments follow up on the recent observation of ultraslow (E <1 meV)photoelectrons emitted upon ionization by synchrotron radiation. The dynamics of photoionization is studied by exciting droplets with femtosecond VUV pulses (23.7 eV photons) generated using the novel high-order harmonic generation technique. The droplets subsequently interact with IR pulses (1.6 eV photons) and emit photoelectrons from transient electronic states before droplet autoionization. Femtosecond and picosecond relaxation dynamics and competition between droplet ionization and formation of long-lived excited states are observed. Probe pulses also lead to droplet "re-excitation" and production of even more ultraslow electrons. First results on detection of photoions using time and position sensitive detector will also be presented.