Solvated Electrons in Liquid Jets of Water
Final Report Abstract
In the framework of this fellowship, the dynamics of the photoionization process of pure helium nanodroplets was investigated applying VUV / IR two color femtosecond pumpprobe techniques. The 15th harmonic of a Ti:Sa laser system was used to excite the droplets into a broad band centered at 23.8 eV followed by a second IR-pulse to probe the state of the system in dependence of the time delay between both pulses. Detecting the delay time dependent ion and electron momenta yields a real-time picture of the droplet relaxation dynamics. A new ion spectrometer was added to the existing setup to measure the pump-probe time delay dependent mass spectrum and momenta of the ions. Different ion clusters show distinct dynamics. Two timescales determine the observed data: 300 fs and ∼3 ps. They are tentatively assigned to a direct and an indirect ionization process respectively. The direct ionization process is connected to the rapid desorption of helium atoms excited on the surface of the droplet. In the first 300 fs, the excited atoms are still very close to the surface, leading to cluster formation when they are ionized. Later on, they have left the droplet and can be detected as He+ ions. The indirect ionization process leads to the formation of dimer and trimer ions. Here the molecules are ionized by an auto- or tunnel ionization mechanism and not by the probe pulse. Hence, this process is first suppressed by the IR-photon at short time delays. However, for longer time delays the probe pulse enhances the formation of dimers and trimers. These results confirm the repopulation mechanism suggested before. After the droplet is excited it may relax via a variety of pathways. One of them is the indirect ionization process but by far the most probeable pathway is relaxation into the lower electronic band(s). Adding an additional 1.58 eV re-excites the droplet back into the higher band and opens the indirect ionization channel again, leading to an enhancement of the signal. Further analysis and theoretical modeling will be mandatory to interpret the discussed data and to get a complete picture of the involved mechanisms.
Publications
- Ultrafast dynamics in helium nanodroplets probed by femtosecond time-resolved EUV photoelectron imaging, J. Phys. Chem. A 114, 1437 (2010)
O. Kornilov, C. C. Wang, O. Bünermann, A. T. Healy, M. Leonard, C. Peng, S. R. Leone, D. M. Neumark, and O. Gessner
