Photoionization of clusters in intense few-cycle near infrared femtosecond pulses

Abstract

In this article we present a perspective on the current state of the art in the photoionization of atomic clusters in few-cycle near-infrared laser pulses. Recently, several studies have reported intriguing phenomena associated with the photoionization of clusters by pulses as short as ~10 fs which approach the natural timescales of collective electronic motion in such nanoscale aggregates. In contrast to the dynamics occurring on few- and sub-picosecond timescales where ionic motion sets in and plays a key role marked by resonant plasmon oscillations, the few-cycle limit precludes cluster expansion due to the nuclear motion of ionic constituents. Thus, pulses lasting just a few optical cycles explore a new "impulsive" regime for the first time in cluster nanoplasmas wherein ions essentially remain "frozen". Along with the perspective on this new regime, we present first measurements of photoelectron distributions and temperatures.

Efficiency of dopant-induced ignition of helium nanoplasmas

“…To outline the microscopic processes evolving in the course of droplet ionization, figure 2 depicts MD simulation results for the time evolution of several characteristic quantities for selected dopant species. Shown are the intensity envelope of the laser pulse in panel (a) and, averaged over trajectory bundles, the charge per dopant atom (b) and per He atom (c), and the probability of igniting a He nanoplasma (d), for He 2171 droplets doped with K 8 and K 16 (both on the surface), Ca 8 (in a dimple on the surface) as well as Xe 8 (in the center). The pulse peak intensity is = I 10 14 W cm −2 .…”

“…In case of K dopants, a single ionization per K atom occurs early in the pulse at »t 250 fs. For K 16 , the incubation time is 100-150 fs until the average He charge begins to rise. During the incubation time, EII of He competes with a partial drain (outer ionization [41]) of the seed electrons.…”

“…For Xe 8 , the initial seed ionization begins much closer to the laser pulse peak ( »t 100 fs) than for K and Ca dopants. The K 8 and K 16 examples reveal a dopant cluster size effect: the ignition probability increases with the number of dopant atoms as a larger dopant cluster provides more seed electrons and a stronger electric field created by the sum of dopant ion charges, thereby assisting EII by reducing the Coulomb barrier at the adjacent He atoms. Moreover, the dopant cluster size effect is nonlinear: while for K 8 the average charge per K atom remains 1 during the incubation time ( figure 2(b)), for K 16 it rises gradually, making via EII also up to two inner shell electrons available for seed ionizations.…”

“…Note that even random placement of water or carbon disulfide molecules as dopants into argon host clusters has resulted in significant enhancement of photon or electron emission [6,7]. Novel resonant dynamics driven by ultrashort NIR laser pulses have been uncovered for superfluid helium (He) nanodroplets doped with a few xenon (Xe) atoms forming a dopant cluster in the droplet interior [8][9][10][11][12][13][14][15][16][17]. These reports have triggered a search for the optimal conditions for doping in such clusters.…”

Charging dynamics of dopants in helium nanoplasmas

“…Although a generic understanding of the intense NIR pulse dynamics of single component clusters from few-femtosecond to picosecond timescales has been achieved [6,7], the ignition of the nanoplasma as well as the charging of dopants embedded inside ionizing host clusters remains to be investigated in greater detail [9,10]. A series of experiments involving dopant clusters grown in He nanodroplets have led to similar conclusions in accordance with the generic nanoplasma dynamics of single component clusters [2,11] However, in certain cases the presence of a He shell surrounding the dopant cluster was found to distinctly influence the charging dynamics of the dopant atoms [12].…”