Photoionisaton of pure and doped helium nanodroplets

Mudrich, M.; Stienkemeier, F.

doi:10.1080/0144235x.2014.937188

Cited by 114 publications

(120 citation statements)

References 220 publications

(464 reference statements)

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“…These latest experiments utilized the new "LAMP" soft x-ray end station [22][23]. Experiments with beams of helium nanodroplets have been extensively documented elsewhere [24][25][26][27]. Helium droplets with radii R = 300 -1000 nm (N He = 10 9 -10 11 ) were produced upon fragmentation of liquid 4 He expanding continuously into vacuum through a 5 μm diameter nozzle at a temperature of 5 K and a backing pressure of 20 bar [24,[28][29].…”

Section: Methodsmentioning

confidence: 99%

Shapes of rotating superfluid helium nanodroplets

et al. 2017

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Rotating superfluid He droplets of approximately 1 μm in diameter were obtained in a free nozzle beam expansion of liquid He in vacuum and were studied by single-shot coherent diffractive imaging using an x-ray free electron laser. The formation of strongly deformed droplets is evidenced by large anisotropies and intensity anomalies (streaks) in the obtained diffraction images. The analysis of the images shows that, in addition to previously described axially symmetric oblate shapes, some droplets exhibit prolate shapes. Forward modeling of the diffraction images indicates that the shapes of rotating superfluid droplets are very similar to their classical counterparts, giving direct access to the droplet angular momenta and angular velocities. The analyses of the radial intensity distribution and appearance statistics of the anisotropic images confirm the existence of oblate metastable superfluid droplets with large angular momenta beyond the classical bifurcation threshold.3

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Section: Methodsmentioning

confidence: 99%

Shapes of rotating superfluid helium nanodroplets

et al. 2017

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“…It has been demonstrated that it suffices to generate a small number of seed electrons in nanoclusters to trigger avalanche EII at laser intensities two orders of magnitude below the threshold intensity of TI of the atomic constituents. This can be achieved either by irradiating the pristine nanoclusters by additional weak XUV pulses [8] or by inserting dopant atoms with low ionization threshold intensity into the host clusters [9][10][11][12]. The triggering of avalanche-like ionization we refer to as ignition.…”

Section: Introductionmentioning

confidence: 99%

Dopant-induced ignition of helium nanoplasmas—a mechanistic study

Heidenreich¹,

Schomas²,

Mudrich³

2017

J. Phys. B: At. Mol. Opt. Phys.

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Abstract. Helium (He) nanodroplets irradiated by intense near-infrared laser pulses form a nanoplasma by avalanche-like electron impact ionizations even at lower laser intensities where He is not directly field ionized, provided that the droplets contain a few dopant atoms which provide seed electrons for the electron impact ionization avalanche. In this theoretical paper on calcium and xenon doped He droplets we elucidate the mechanism which induces ionization avalanches, termed ignition. We find that the partial loss of seed electrons from the activated droplets starkly assists ignition, as the Coulomb barrier for ionization of helium is lowered by the electric field of the dopant cations, and this deshielding of the cation charges enhances their electric field. In addition, the dopant ions assist the acceleration of the seed electrons (slingshot effect) by the laser field, supporting electron impact ionizations of He and also causing electron loss by catapulting electrons away. The dopants' ability to lower the Coulomb barriers at He as well as the slingshot effect decrease with the spatial expansion of the dopant, causing a dependence of the dopants' ignition capability on the dopant mass. Here, we develop criteria (impact count functions) to assess the ignition capability of dopants, based on (i) the spatial overlap of the seed electron cloud with the He atoms and (ii) the overlap of their kinetic energy distribution with the distribution of Coulomb barrier heights at He. The relatively long time delays between the instants of dopant ionization and ignition (incubation times) for calcium doped droplets are determined to a large extent by the time it takes to deshield the dopant ions.

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“…He nanodroplets are generally considered as an ultracold, inert spectroscopic matrix for embedded, isolated molecules and clusters [12,13]. Upon ionization by intense or energetic radiation, however, He droplets turn into a highly reactive medium, inducing reactions and secondary ionization processes of the embedded species [14]. Their homogeneous quantum liquid density profile, and the simple structure of atomic constituents, make He droplets particularly beneficial as benchmark systems for elucidating correlated decay processes.…”

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confidence: 99%

Interatomic Coulombic decay in helium nanodroplets

et al. 2017

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Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n = 2 excited state of He + using XUV synchrotron radiation. By recording multiple coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic He + ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow He + ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of He + k complexes.Isolated atoms or molecules excited by energetic radiation typically decay through intramolecular processes such as the emission of an electron or photon. In contrast, in weakly bound complexes, locally generated electrons can additionally interact with neighboring atoms or molecules, leading to new interatomic or intermolecular interactions. Interatomic Coulombic decay (ICD) is a particularly interesting decay process which occurs when local electronic decay is energetically forbidden [1]. Thus, ICD offers a new, ultrafast decay path where energy is exchanged with a neighboring atom leading to its ionization. Since its discovery, ICD has been observed in a wide variety of weakly-bound systems from He dimers [2,3] and rare-gas clusters to biologically relevant systems such as water clusters; for reviews see [4,5]. Today, the focus is on condensed-phase systems where ICD is involved in complex relaxation mechanisms [6][7][8], which can generate genotoxic low-energy electrons and radical cations [9]. Recently, it was suggested to utilize this property of ICD for cancer treatment [10,11].Here we present the first study of ICD in helium (He) nanodroplets. He nanodroplets are generally considered as an ultracold, inert spectroscopic matrix for embedded, isolated molecules and clusters [12,13]. Upon ionization by intense or energetic radiation, however, He droplets turn into a highly reactive medium, inducing reactions and secondary ionization processes of the embedded species [14]. Their homogeneous quantum liquid density profile, and the simple structure of atomic constituents, make He droplets particularly beneficial as benchmark systems for elucidating correlated decay processes. Recent examples include the collective autoionization of multiply excited pure He droplets [15,16] and the creation of doubly charged species by one-photon ionization of doped He droplets [17]. In this work we fully characterize the product states generated by ICD and secondary processes in He nanodroplets using coincidence imaging techniques.The experiments were performed using a He droplet machine attached to a velocity map imaging photoelectron-photoion coincidence (VMI-PEPICO) detector at the GasPhase beamline of Elettra-Sincrotrone Trieste, Italy. The apparatus has been described in detail elsewhere [18,19]. Briefly, a beam of He nanodroplets is produced by continuously expanding pressurized He (50 bar) of high purity He out of a c...

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Photoionisaton of pure and doped helium nanodroplets

Cited by 114 publications

References 220 publications

Shapes of rotating superfluid helium nanodroplets

Shapes of rotating superfluid helium nanodroplets

Dopant-induced ignition of helium nanoplasmas—a mechanistic study

Interatomic Coulombic decay in helium nanodroplets

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