Relaxation dynamics of pure and doped He nanodroplets probed by EUV-UV pump-probe ionization at the free-electron laser FERMI
Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
Final Report Abstract
The experiment carried out during one beamtime at Fermi@Elettra was carried out very successfully and turned out to ideally exploit the capabilities of this new FEL in terms of tunability, intensity, and synchronization with an optical laser. At low intensities (~1010 Wcm-2) the relaxation dynamics of He nanodroplets excited to the droplet-equivalent state of the 1s2p1P excited He atom was characterized by EUV (~21.5eV)+UV(4.8eV) pump-probe measurements of photoelectrons and ions. Photoelectron spectra reveal fast (<0.5ps) relaxation into the 1s2s1S droplet state followed by the appearance of a sharp peak at the energy of free 1s2s He atoms at 1ps delay; Photoion mass spectra contain mostly He+ and He2+ signals which have a rise time of about and 0.5 and 1ps, respectively. These signals give complementary information about the dynamics of ejection of excited He* atoms out of the droplets and of the formation of He2* excimers. A sharp peak at about 18eV in the photoelectron spectra (PES), which is indicative for He*-He* Penning or ICD-like ionization of doubly excited He droplets shows no dependence on the presence or time delay of the UV laser pulse. In the case of He droplets doped with lithium (Li) atoms, a sharp peak in the PES at about 16 eV, which is indicative for He*-Li Penning or ICD-like ionization by the transfer of excitation from He droplets to the dopants, shows no dependence on the presence or time delay of the UV laser pulse. From these observations we tentatively conclude that Penning or ICD-like ionization proceeds on a timescale much shorter than the resolution of the pump-probe experiment (~100 fs). At intermediate intensities (~1012 Wcm^-2) the broadening and shifting toward zero-energy of the sharp ICD peak in the PES in the absence of the UV laser marks the transition from doubly excited to multiply excited He droplets. These systematic measurements complement previous experiments exploring collective autoionization. The addition of UV laser pulses to EUV-excited He droplets close to the onset of CAI is found to induce the appearance of a distribution of low-energy electrons which is indicative for nanoplasma formation. In the regime of EUV-induced CAI at the highest intensities (~1013 Wcm^-2), the UV pulse is found to significantly enhance the yield of nanoplasma-electrons. Finalization of the data analysis and detailed modeling, which are still ongoing, will finally allow to draw the complete picture of the rich ionization dynamics of excited He nanodroplets.
