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Doped Helium Nanodroplets in Intense Few-Cycle Infrared Pulses


Krishnan,  Siva Rama
Division Prof. Dr. Joachim H. Ullrich, MPI for Nuclear Physics, Max Planck Society;

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Krishnan, S. R. (2011). Doped Helium Nanodroplets in Intense Few-Cycle Infrared Pulses. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg, Germany.

In this dissertation, experimental studies on the interaction of intense few-cycle pulses with doped helium nanodroplets are reported. In single-pulse measurements, the dopant induced ignition of He nanodroplets by 10 fs near-infrared (NIR) pulses of peak intensities in the range of 1014-1015 Wcm-2 is demonstrated. This results in the complete ionization of all the 104 He atoms in the droplet triggered by less than 10 dopant atoms residing at the center. These experimental observations aided by a theoretical model demonstrate for the first time, a very efficient absorption and resonant coupling of intense NIR laser fields to cluster nanoplasmas on a 10 fs timescale. In pump-probe studies performed with two 10 fs pulses, the effects of doping these large He nanodroplets with few atoms (1....50) on the previously known nanoplasma resonance due to ionic expansion are investigated. The crucial roles played by the highly-charged dopant ions at the center and the surrounding by helium ions on this expansion occurring on sub- or few-picosecond timescales are studied. This highlights the prominent dynamical role of the fast expanding shells of He ions, which was not apparent from previous experiments. Hence, the pumpprobe studies reported in this work call for a revised view of the expansion-induced resonance in such composite nanoplasmas.