Ionization of Methane Clusters in Helium Nanodroplets

Leidlmair, Christian; Bartl, Peter; Schöbel, Harald; Denifl, Stephan; Yang, Shengfu; Ellis, Andrew M.; Scheier, P.

doi:10.1002/cphc.201100880

Cited by 25 publications

(32 citation statements)

References 50 publications

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“…The yield of the parent cations Val + n increases from about 1% of the neighboring peak Val n H + at n = 1 to more than 20% at n = 30. A similar trend was observed previously for methanol and methane cluster ions [35,36].…”

Section: Experimental Set-upsupporting

confidence: 89%

Ion formation upon electron collisions with valine embedded in helium nanodroplets

et al. 2016

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Abstract. We report here experimental results for the electron ionization of large superfluid helium nanodroplets with sizes of about 10 5 atoms that are doped with valine and clusters of valine. Spectra of both cations and anions were monitored with high-resolution time-of-flight mass spectrometry (mass resolution >4000). Clear series of peaks with valine cluster sizes up to at least 40 and spaced by the mass of a valine molecule are visible in both the cation and anion spectra. Ion efficiency curves are presented for selected cations and anions at electron energies up to about 40 eV and these provide insight into the mode of ion formation. The measured onset of 24.59 eV for cations is indicative of valine ionization by He + whereas broad resonances at 2, 10 and 22 eV (and beyond) in the formation of anions speak to the occurrence of various modes of dissociative electron attachment by collisions with electrons or He* − and the influence of droplet size on the relative importance of these processes. Comparisons are also made with gas phase results and these provide insight into a matrix effect within the superfluid helium nanodroplet.

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Section: Experimental Set-upsupporting

confidence: 89%

Ion formation upon electron collisions with valine embedded in helium nanodroplets

et al. 2016

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“…This scales with cluster size and influences the production of intermediate adducts, notably protonated methane CH + 5 . Such adducts have been observed in other type of experiments probing the ion-molecule chemistry, for example electron impact ionization on methane clusters [30] and heliumdroplets doped with methane clusters [31]. These experiments used electrons of 20-70 eV to fragment the clusters and have observed a series of protonated and unprotonated cluster ions.…”

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confidence: 71%

Explosion, ion acceleration, and molecular fragmentation of methane clusters in the pulsed beam of a free-electron laser

et al. 2012

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X-ray lasers offer new possibilities for creating and probing extreme states of matter. We used intense and short X-ray pulses from the FLASH soft X-ray laser to trigger the explosions of CH4 and CD4 molecules and their clusters. The results show that the explosion dynamics depends on cluster size, and indicate a transition from Coulomb explosion to hydrodynamic expansion in larger clusters. The explosion of CH4 and CD4 clusters shows a strong isotope effect: the heavier deuterons acquire higher kinetic energies than the lighter protons. This may be due to an extended inertial confinement of deuterons vs. protons near a rapidly charging cluster core during exposure. [5,6] and solid samples [7][8][9]. In this paper we follow the X-ray-induced explosion of methane (CH 4 ) and heavy methane (CD 4 ) clusters and compare the results to the explosion of the individual molecules. Clusters are eminently suitable to study size-dependent phenomena in the explosion dynamics, and we employed clusters, containing 1000 to 250000 molecules and corresponding to sizes of 4 -32 nm diameter. Molecular clusters bridge the gap between isolated molecules and bulk condensed material. An understanding of ultrafast explosion dynamics in clusters of various dimensions has relevance to imaging by "diffraction before destruction" [10], plasma physics, and fusion research [11,12].The target response to an intense X-ray pulse depends on the wavelength, the intensity of the field, and the size of the sample. Photoionization liberates electrons, which can escape from small samples, and these positively charged objects undergo a Coulomb explosion. The probability of electrons escaping from a cluster depends on its size [13][14][15]. In larger objects, photoelectrons elicit secondary electron cascades and these create additional ionizations. As the overall positive charge of the sample increases during the pulse, photoelectrons will not be able to escape the growing positive potential [13,16]. Trapped photoelectrons create more cascades and the electrons move inwards to neutralize the core of the cluster, leaving behind a positively charged outer layer, which then peels off [17][18][19][20][21]. As a result, large samples burn from the outside inwards. The core of such large objects expands hydrodynamically due to the growing electron pressure within [17,18]. Theoretical studies predict a * Electronic address: nicusor@xray.bmc.uu.se transition from Coulomb explosion to hydrodynamic expansion with increasing sample size [13].The study of Coulomb explosion of molecular clusters with intense infrared lasers has shown the advantage of using heteronuclear molecules, such as CH 4 or CD 4 , to analyze the dynamics of the explosion [22,23]. In a heteronuclear cluster, the light ions can gain additional energy during the explosion by interacting with multiply charged heavy ions (C n+ ). This kinematic effect would be indicative of a Coulomb process, while in a hydrodynamics expansion all the cluster constituents are expected to gain the same energy. We ...

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“…64 In another recent study, methane clusters in helium were ionized by electron ionization; doubly charged (CH 4 ) n 2+ were observed above a critical size of 70 with an energy threshold of 44 eV. 69 The findings were attributed to formation of He * (2 3 S 1 ) + He + . In this model, a singly charged dopant ion is formed by charge transfer from He + ; Penning ionization of the ion in a collision with He * is then facilitated by the large attraction between the dopant ion and the highly polarizable He * .…”

Section: B Doubly Charged Ionsmentioning

confidence: 98%

Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

Kaiser

Leidlmair

Bartl

et al. 2013

The Journal of Chemical Physics

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A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu The Journal of Chemical Physics 132, 154104 (2010) Helium droplets are doped with fullerenes (either C 60 or C 70 ) and hydrogen (H 2 or D 2 ) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H 2 ) n HC m + where m = 60 or 70. Another series of even-numbered ions, (H 2 ) n C m + , is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H 2 ) n + is barely detectable. The ion series (H 2 ) n HC m + and (H 2 ) n C m + exhibit abrupt drops in ion abundance at n = 32 for C 60 and 37 for C 70 , indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H 2 are adsorbed on C 60 + ; the corresponding value for C 70 + is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H 2 /D 2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C 60 and C 70 , respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C 60 -hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H 2 is 57 meV for H 2 C 60 + and (H 2 ) 2 C 60 + , and slightly above 70 meV for H 2 HC 60 + and (H 2 ) 2 HC 60 + . The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H 2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

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Ionization of Methane Clusters in Helium Nanodroplets

Cited by 25 publications

References 50 publications

Ion formation upon electron collisions with valine embedded in helium nanodroplets

Ion formation upon electron collisions with valine embedded in helium nanodroplets

Explosion, ion acceleration, and molecular fragmentation of methane clusters in the pulsed beam of a free-electron laser

Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

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