Notizen: Strahlen aus kondensiertem Helium im Hochvakuum

Becker, E. W.; Klingelhöfer, R.; Lohse, P.

doi:10.1515/zna-1961-1134

Cited by 41 publications

(20 citation statements)

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“…Alle bislang erhaltenen Molekülspektren in flüssigem Helium wurden mit Heliumtröpfchen erhalten, die durch Freistrahlexpansion hauptsächlich von gasförmigem Helium, in einigen Fällen auch der Flüssigkeit, erzeugt wurden. Die Erzeugung von Heliumtröpfchen in Gasexpansionen wurde erstmals 1961 von Becker et al beschrieben 116. Strahlen aus 4 He‐ und 3 He‐Tröpfchen sowie gemischte Tröpfchen mit einigen hundert bis 10 4 Atomen werden heute leicht durch Expansion des Gases bei hohen Quellendrücken von P 0 =20–100 bar und niedrigen Quellentemperaturen von T 0 =20–5 K durch eine schmale Düse von einigen Mikrometern Durchmesser oder durch eine enge dünnwandige Blende ins Vakuum erzeugt (Abbildung 5 a).…”

Section: Experimentelle Methodeunclassified

Suprafluide Heliumtröpfchen: außergewöhnlich kalte Nanomatrices für Moleküle und molekulare Komplexe

Toennies

Vilesov

2004

Angewandte Chemie

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Dieser Aufsatz gibt einen Überblick über neuere Experimente zur Spektroskopie und zu chemischen Reaktionen von Molekülen und Komplexen in Heliumtröpfchen. Im Inneren solcher Matrices wird eine ähnlich hohe spektroskopische Auflösung wie in der Gasphase erzielt, und durch Verdampfungskühlung lässt sich eine isotherme Tieftemperaturumgebung von T=0.37 K (4He‐Tröpfchen) oder 0.15 K (3He‐Tröpfchen) aufrechterhalten – kälter als in den meisten Festkörpermatrices möglich ist. Die Heliumtröpfchentechnik kombiniert somit die Vorteile von Gasphasen‐ und klassischen Matrixisolationstechniken. Ein wichtiger Punkt ist, dass die suprafluide Heliumumgebung binäre Stöße erleichtert und die bei der Rekombination freigesetzte Bindungsenergie absorbiert. Die Tröpfchen lassen sich folglich als isotherme nanoskopische Reaktoren betrachten, in denen Einzelmoleküle, Cluster oder sogar einzelne reaktive Stöße bei ultratiefen Temperaturen isoliert werden können.

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Section: Experimentelle Methodeunclassified

Suprafluide Heliumtröpfchen: außergewöhnlich kalte Nanomatrices für Moleküle und molekulare Komplexe

Toennies

Vilesov

2004

Angewandte Chemie

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“…Helium-droplet production in gas expansions was first demonstrated by Becker et al in 1961. [116] Beams containing either 4 He and 3 He droplets or mixed droplets with between several hundred and 10 4 atoms are readily produced today by expanding the gas at a high source pressure P 0 = 20-100 bar and a low source temperature T 0 = 20-5 K, through a tiny nozzle of several microns diameter or a narrow thin-walled orifice into vacuum ( Figure 5 a). [9,11,117,118] The properties of droplets formed in continuous beams obtained by expansion through a 5-mm diameter orifice are now well characterized.…”

Section: Superfluid Helium Dropletsmentioning

confidence: 99%

Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes

2004

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Herein, recent experiments on the spectroscopy and chemical reactions of molecules and complexes embedded in helium droplets are reviewed. In the droplets, a high spectroscopic resolution, which is comparable to the gas phase is achieved, while an isothermal low-temperature environment is maintained by evaporative cooling at T =0.37 K (4He droplets) or 0.15 K (3He droplets), lower than possible in most solid matrices. Thus the helium-droplet technique combines the benefits of both the gas phase and the classical matrix-isolation techniques. Most important, the superfluid helium facilitates binary encounters, and absorbs the released binding energy upon recombination. Thus the droplet can be viewed as an isothermal nanoscopic reactor, which isolates single molecules, clusters, or even a single reactive encounter at ultralow temperatures.

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“…7 Two years later the first clusters of helium were formed and analyzed by time of flight mass spectrometry. 8 It was hoped that the scattering of Cs atoms from He clusters would be sensitive to the superfluid phase. 9 4 He clusters were expected to cool below their bulk superfluid temperature, 2.18 K, while 3 He clusters were not, due to the much lower transition temperature, 0.002 K. It was hoped that this difference would result in observable differences in the scattering of cesium from 4 He and 3 He clusters.…”

Section: Introductionmentioning

confidence: 99%

Capture and ionization of argon within liquid helium droplets

Callicoatt

Förde

Ruchti

et al. 1998

The Journal of Chemical Physics

124

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Communication: The formation of helium cluster cations following the ionization of helium nanodroplets: Influence of droplet size and dopant Liquid helium droplets of initial mean cluster size, ͗N͘, ranging from 600 to 8000 atoms are doped with argon using the pick-up technique. The doped clusters are ionized by electron impact, and the resulting fragment ions are monitored as a function of argon pressure in the pick-up volume. Analysis of the pressure dependent ion signals is used to determine ͑1͒ the probability for charge transfer from He ϩ to the Ar atoms within the droplet, and ͑2͒ the probability for fragmentation of the Ar k subclusters upon ionization. The measured charge transfer probability from He ϩ to Ar ranges from 0.05Ϯ0.02 for clusters of mean original size ͗N͘ϭ8000 to 0.26Ϯ0.05 for ͗N͘ϭ600. Charge transfer to the Ar k constituent results in the following qualitative trends; a single Ar atom yields He n Ar ϩ ions; Ar 2 mainly yields Ar 2 ϩ , and Ar 3 mainly fragments to yield Ar 2 ϩ .Simulations of the results are performed to extract information on how the charge transfer and fragmentation processes within the ionized droplet dependent on the size of the helium droplet and the number of argon atoms captured. We use the positive-hole resonant-hopping mechanism to determine that the He ϩ hops 3-4 times prior to localization with either the Ar dopant or another He atom to form He 2 ϩ . This corresponds to a time scale for He 2 ϩ formation of 60-80 fs.

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Notizen: Strahlen aus kondensiertem Helium im Hochvakuum

Cited by 41 publications

References 0 publications

Suprafluide Heliumtröpfchen: außergewöhnlich kalte Nanomatrices für Moleküle und molekulare Komplexe

Suprafluide Heliumtröpfchen: außergewöhnlich kalte Nanomatrices für Moleküle und molekulare Komplexe

Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes

Capture and ionization of argon within liquid helium droplets

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