Long-lasting XUV activation of helium nanodroplets for avalanche ionization

Abstract

We study the dynamics of avalanche ionization of pure helium nanodroplets activated by a weak extreme-ultraviolet (XUV) pulse and driven by an intense near-infrared (NIR) pulse. In addition to a transient enhancement of ignition of a nanoplasma at short delay times ~200 fs, long-term activation of the nanodroplets lasting up to a few nanoseconds is observed. Molecular dynamics simulations suggest that the short-term activation is caused by the injection of seed electrons into the droplets by XUV photoemission. Long-term activation appears due to electrons remaining loosely bound to photoions which form stable `snowball' structures in the droplets. Thus, we show that XUV irradiation can induce long-lasting changes of the strong-field optical properties of nanoparticles, potentially opening new routes to controlling avalanche-ionization phenomena in nanostructures and condensed-phase systems.

XUV fluorescence as a probe of laser-induced helium nanoplasma dynamics

“…The capability of larger He droplets to trap ions and electrons in stable complexes and quasifree states, respectively, thereby accumulating multiple charge carriers over longer periods of time, promotes TBR in the laser-driven nanoplasma. Our findings complement recent studies on the XUV activation of He nanodroplets, where long-lasting changes of the strong-field optical properties of nanoparticles have been observed and attributed to electrons remaining loosely bound to photoions forming stable snowball structures in the droplets [41].…”

“…This leads to the trapping of quasifree electrons in illuminated He clusters. The created quasifree electrons are driven by the laser field releasing an avalanche of secondary electrons mainly by electron-impact ionization of the surrounding neutral atoms and single charged ions [41]. Note, the presence of ions in the vicinity of He atoms facilitates this ionization cascade, because it lowers their ionization threshold by 10-17 eV [42].…”

Update on MAC End-Station at ELI Beamlines Facility

“…It is operational and open for user access. Several experiments at MAC have already revealed intricate fast processes in atoms [9,18], molecules, helium nanodroplets [14,15], and nanoparticles [19]. We have presented ongoing upgrades of the MAC end-station, i.e., a B4MAC chamber for complex beam manipulation, an MBES for efficient and wellresolved electron detection, and a TPX3CAM on VMI spectrometer for simultaneous time and position data acquisition.…”

“…High harmonic generation (HHG) is a wellestablished technique to produce coherent extreme ultraviolet (XUV) radiation with excellent properties, such as high temporal and spatial coherence, femtosecond pulse duration, high intensity, synchronization with other laser beams, and the possibility to select a single harmonic by a monochromator [1][2][3][4]. The applications of femtosecond high harmonics include time-resolved studies of chemical processes [5][6][7], tracking and controlling ultrafast electron dynamics in atoms [8,9], probing chiral molecules in the gas phase [10], or investigations of transient phenomena in nanoscale quantum systems [11][12][13][14][15].…”

“…We address the user end-station MAC at the HHG beamline -a multipurpose station for atomic, molecular, and optical sciences (AMO) and coherent diffractive imaging (CDI) [17]. At MAC, the HHG beam synchronized with an auxiliary laser beam is used to unveil ultrafast electron and ion dynamics in low-density targets, such as atoms [9,18], molecules, helium nanodroplets [14,15], or nanoparticles [19]. Moreover, CDI studies on solid targets are possible at MAC [20].…”