Sivarama Krishnan

Photoionization process of acetylene doped in helium nanodroplets is studied with EUV synchrotron radiation with photon energies between 20 and 26 eV by Photoelectron-Photoion Coincidence (PEPICO) experiment by detecting photoelectrons in coincidence with the photoions using electron velocity map imaging (VMI) spectrometer and ion time of flight (TOF) spectrometer. Acetylene is ionized in the droplet via Penning ionization at 21.6 eV photon energy. For photon energy of 23.9 eV and above the photoionization threshold of He, charge transfer ionization occurs in acetylene following autoionization and direct ionization in the droplet respectively.

Helium nanodroplets irradiated by intense near-infrared laser pulses ignite and form highly ionized nanoplasmas even at laser intensities where helium is not directly ionized by the optical field, provided the droplets contain a few dopant atoms. We present a combined theoretical and experimental study of the He nanoplasma ignition dynamics for various dopant species. We find that the efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies and mostly depends on (i) the number of free electrons each dopant donates upon ionization, (ii) the pick-up process, and (iii) the hitherto unexplored effect of the dopant location in or on the droplet.

We present a combined experimental and theoretical study of the charging dynamics of helium nanodroplets doped with atoms of different species and irradiated by intense near-infrared laser pulses (≤1015 W cm−2). In particular, we elucidate the interplay of dopant ionization inducing the ignition of a helium nanoplasma, and the charging of the dopant atoms driven by the ionized helium host. Most efficient nanoplasma ignition and charging is found when doping helium droplets with xenon atoms, in which case high charge states of both helium (He2+) and of xenon (Xe21+) are detected. In contrast, only low charge states of helium and dopants are measured when doping with potassium and calcium atoms. Classical molecular dynamics simulations which include focal averaging generally reproduce the experimental results and provide detailed insights into the correlated charging dynamics of guest and host clusters.