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Register a new userCollective enhancement of above threshold ionization by resonantly excited helium nanodroplets
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Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared to He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.
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Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with subsequent decay paths which have been scarcely investigated to date. Here, we show using helium nanodroplets as an example that these systems can decay by a new type of process, named collective autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances much greater than that of direct single-photon ionization. This novel collective ionization process is expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed to high intensity radiation fields.
The ionization dynamics of helium droplets in a wide size range from 220 to 10^6 He atoms irradiated with intense femtosecond extreme ultraviolet (XUV) pulses of 10^9 {div} 10^{12} W/cm2 power density is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited in the photon energy range from ~ 21 eV (corresponding to the atomic 1s2s state) up to the atomic ionization potential (IP) at ~ 25 eV. A complex evolution of the electron spectra as a function of droplet size and XUV intensity is observed, ranging from atomic-like narrow peaks due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of numerical simulations based on rate equations taking into account various processes such as multi-step ionization, interatomic Coulombic decay (ICD), secondary inelastic collisions, desorption of electronically excited atoms, collective autoionization (CAI) and further relaxation processes.
Acene molecules (anthracene, tetracene, pentacene) and fullerene (C$_{60}$) are embedded in He nanodroplets (He$_N$) and probed by EUV synchrotron radiation. When resonantly exciting the He nanodroplets, the embedded molecules M are efficiently ionized by the Penning reaction $mathrm{He}_N^*+mathrm{M}rightarrowmathrm{He}_N + mathrm{M}^+ + e^-$. However, the Penning electron spectra are broad and structureless -- showing no resemblance neither with those measured by binary Penning collisions, nor with those measured for dopants bound to the He droplet surface. The similarity of all four spectra indicates that electron spectra of embedded species are substantially altered by electron-He scattering. Simulations based on elastic binary electron-He collisions qualitatively reproduce the measured spectra, but require the assumption of unexpectedly large He droplets.
Helium tagging in action spectroscopy is an efficient method for measuring the absorption spectrum of complex molecular ions with minimal perturbations to the gas phase spectrum. We have used superfluid helium nanodroplets doped with corannulene to prepare cations of these molecules complexed with different numbers of He atoms. In total we identify 13 different absorption bands from corannulene cations between 5500 {AA} and 6000 {AA}. The He atoms cause a small, chemically induced redshift to the band positions of the corannulene ion. By studying this effect as a function of the number of solvating atoms we are able to identify the formation of solvation structures that are not visible in the mass spectrum. The solvation features detected with the action spectroscopy agree very well with the results of atomistic modeling based on path-integral molecular dynamics simulations. By additionally doping our He droplets with D$_2$, we produce protonated corannulene ions. The absorption spectrum of these ions differs significantly from the case of the radical cations as the numerous narrow bands are replaced by a broad absorption feature that spans nearly 2000 {AA} in width.
A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion have been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large dataset allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.
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