Time-resolved studies on the collapse of magnesium atom foam in helium nanodroplets

Göde, S.; Irsig, Robert; Tiggesbäumker, J.; Meiwes‐Broer, Karl‐Heinz

doi:10.1088/1367-2630/15/1/015026

Cited by 28 publications

(33 citation statements)

References 55 publications

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“…However, we speculate that larger organic molecules, for which the change in electronic and molecular structure upon electronic excitation is weaker and therefore vibronic spectra are less perturbed [14], may still exhibit vibronic dynamics on time scales accessible to fs spectroscopy, similar to molecules embedded in heavier rare gas matrices [157]. Apart from these studies by our group, only Mg-doped He nanodroplets have been studied by fs spectroscopy in the group of K.-H. Meiwes-Broer, J. Tiggesbäumker in Rostock [109,158]. Based on linear absorption spectra and on fs REMPI transients of multiply doped He droplets, it was concluded that Mg atoms aggregate in He nanodroplets in an unusual way to form a foam-like structure where the metal atoms arrange themselves in a regular 10Å-spaced network separated by He atoms.…”

Section: Time-resolved Photoionizationmentioning

confidence: 99%

“…These cluster mass progressions can extended out to masses of thousands of dopant monomers, in particular when the binding of the neutral cluster is weak thereby causing only little shrinkage of the He droplets due to cluster aggregation [107][108][109]. Therefore, small Ak metal clusters are presumably formed in weakly bound high-spin configurations, whereas large clusters observed in mass spectra of heavily doped large He droplets are assumed to be metallic [69,91,110].…”

Section: Photoion Mass Spectramentioning

confidence: 99%

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Photoionisaton of pure and doped helium nanodroplets

Mudrich

Stienkemeier

2014

International Reviews in Physical Chemistry

118

114

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Helium nanodroplets, commonly regarded as the "nearly ideal spectroscopic matrix", are being actively studied for more than two decades now. While they mostly serve as cold, weakly perturbing and transparent medium for high-resolution spectroscopy of embedded molecules, their intrinsic quantum properties such as microscopic superfluidity still are subject-matter of current research. This article reviews recent work on pure and doped He nanodroplets using PI spectroscopy, an approach which has greatly advanced in the past years. While the notion of the ideal spectroscopic matrix mostly no longer holds in this context, photoionization techniques provide detailed insights into the photo-physical properties of pure and doped He nanodroplets and their relaxation dynamics following electronic excitation. Exploiting nowadays available high laser fields, even highly ionized states of matter on the nanoscale can be formed. Our particular focus lies on recent experimental progress including fs time-resolved spectroscopy, photoion and electron imaging, and novel sources of highly energetic radiation.

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Section: Time-resolved Photoionizationmentioning

confidence: 99%

Section: Photoion Mass Spectramentioning

confidence: 99%

Photoionisaton of pure and doped helium nanodroplets

Mudrich

Stienkemeier

2014

International Reviews in Physical Chemistry

118

114

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“…[2][3][4][5][6][7] Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets.…”

mentioning

confidence: 99%

On subthreshold ionization of helium droplets, ejection of He⁺, and the role of anions

Renzler

Daxner

Weinberger

et al. 2014

Phys. Chem. Chem. Phys.

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The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He + ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He + and He2 + as a function of electron energy, electron emission current, and droplet size reveals that metastable He *-anions play a crucial role in the formation of free He + at subthreshold energies. The proposed model is testable. Research into helium nanodroplets, originally a scientific niche driven by curiosity about the minimum droplet size that supports superfluidity, 1 has matured to a point where 4 He droplets provide a novel method to synthesize and characterize unusual molecules, large aggregates in unusual morphologies, metallic foam, or nanowires from a wide range of materials.2-7 Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets. 7,8 A topic that has been of interest ever since large helium droplets were efficiently produced in supersonic jets 9 is the mechanism by which droplets become charged by ionizing radiation. How do small Hen + ions containing as few as two atoms emerge from a very large neutral, undoped droplet? 10,11 How do monomer or dimer ions form when the energy of the ionizing radiation is below their thermodynamic threshold? [12][13][14] How do large Hen -cluster anions form upon electron impact? [15][16][17][18] What role do metastable electronically excited species play? 12,19 What is the local structure near a positive or negative charge in undoped helium droplets, and how does it compare to that in bulk helium or helium films? [20][21][22][23] Our present work addresses the formation and subsequent ejection of bare He + from undoped droplets. For ionizing radiation exceeding the ionization threshold of atomic He (24.59 eV) small Hen + cluster ions (n > 1) are thought to result from a two-step mechanism. 12,22,[24][25][26][27][28] The process commences with the formation of He + in the droplet. Direct formation of Hen + cluster ions (n > 1) is disfavored by very small Franck-Condon factors. The hole will hop, on the time scale of femtoseconds, by resonant charge exchange with adjacent helium atoms. After about 10 hops the charge will localize by forming a vibrationally excited He2 + . Its excess energy will be large given the large (about 2.4 eV) dissociation energy of He2 + . 29 This energy would be sufficient to boil off thousands of helium atoms (the bulk cohesive energy of helium is 0.62 meV) but a thermal process appears unlikely; evaporation of even thousands of helium atoms from a primary droplet containing »10 4 helium atoms would still result in a helium cluster ion whose size lies outside the range of...

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“…In the ultimate case multiple particle doping may thus produce numerous individual particles inside one helium droplet shielded from each other by a helium solvation layer. In contrast to the formation of a large cluster inside the helium droplet this phenomenon is addressed as foam (Przystawik et al, 2008; Goede et al, 2013). …”

Section: Discussionmentioning

confidence: 99%

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

et al. 2014

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The empirical model explaining microsolvation of molecules in superfluid helium droplets proposes a non-superfluid helium solvation layer enclosing the dopant molecule. This model warrants an empirical explanation of any helium induced substructure resolved for electronic transitions of molecules in helium droplets. Despite a wealth of such experimental data, quantitative modeling of spectra is still in its infancy. The theoretical treatment of such many-particle systems dissolved into a quantum fluid is a challenge. Moreover, the success of theoretical activities relies also on the accuracy and self-critical communication of experimental data. This will be elucidated by a critical resume of our own experimental work done within the last ten years. We come to the conclusion that spectroscopic data and among others in particular the spectral resolution depend strongly on experimental conditions. Moreover, despite the fact that none of the helium induced fine structure speaks against the empirical model for solvation in helium droplets, in many cases an unequivocal assignment of the spectroscopic details is not possible. This ambiguity needs to be considered and a careful and critical communication of experimental results is essential in order to promote success in quantitatively understanding microsolvation in superfluid helium nanodroplets.

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Time-resolved studies on the collapse of magnesium atom foam in helium nanodroplets

Cited by 28 publications

References 55 publications

Photoionisaton of pure and doped helium nanodroplets

Photoionisaton of pure and doped helium nanodroplets

On subthreshold ionization of helium droplets, ejection of He⁺, and the role of anions

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

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Time-resolved studies on the collapse of magnesium atom foam in helium nanodroplets

Cited by 28 publications

References 55 publications

Photoionisaton of pure and doped helium nanodroplets

Photoionisaton of pure and doped helium nanodroplets

On subthreshold ionization of helium droplets, ejection of He+, and the role of anions

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy

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On subthreshold ionization of helium droplets, ejection of He⁺, and the role of anions