Charging dynamics of dopants in helium nanoplasmas

Abstract

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 (NIR) laser pulses (≤10 15 Wcm -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 both of helium (He 2+ ) and of xenon (Xe 21+ ) 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.

Dopant-induced ignition of helium nanoplasmas—a mechanistic study

“…This links to our previous work where the interplay between dopant and He ionization during nanoplasma formation was studied [13,14]. We shall show that a partial loss of seed electrons plays an important role in the opening of the EII channel of He in Ca doped droplets.…”

“…The intensity maxima of the Gaussian pulses are in the range M = 10 13 − 10 14 Wcm -2 , their temporal width is FWHM = 200 fs (FWHM of the Gaussian electric field envelope τ = 283 fs) which is the pulse width in our previous work. Since in this paper we are interested in the dynamics of doped droplets at distinct pulse peak intensities rather than in a comparison with experiments, MD results are not focally averaged, unlike in our preceding work [13,14]. However, whenever indicated in the text, MD results are averaged over a set of 50 or 100 trajectories with the same dopant size and pulse parameters but different initial conditions (slightly different initial atomic coordinates and a different seed for the random number generator for TI).…”

“…Subsequently, at ≈ 40 fs, rises steeply, constituting the onset of avalanche-like EII of the He droplet. As described in our previous papers [13,14], He ionization is always induced by EII; the role of the dopant is to provide the seed electrons and to assist EII by lowering the Coulomb barrier at He by the field of the dopant cations.…”

“…We choose He2171 as a sample of the droplet size distribution under experimental conditions [13,14]. The dopant clusters are densest packed assemblies of tetrahedrons and form, as far as possible, spherical shapes.…”

“…In two recent studies [13,14], we have elucidated the roles of the electronic structure as well as the location of dopants inside (rare-gas) or on the surface (alkali, alkaline earth metals) of the host He droplet in triggering avalanche ionization for relatively long pulses of FWHM = 200 fs (FWHM of the Gaussian intensity envelope) with peak laser intensities in the range 10 13 -10 15 Wcm -2 . Both the ability of dopant atoms to provide seed electrons as well as their location in close contact with the He droplet were found to be crucial parameters.…”