IR and UV-NIR Absorption Spectroscopy of Matrix-Isolated C₇₀⁺ and C₇₀^–

C₇₀⁺ ions were mass-selectively deposited into a neon or an argon matrix at 5 K. Like in the case of C₆₀⁺ deposition, soft landing into a rare gas matrix is associated with some charge-exchange processes such that C₇₀⁺ as well as resulting C₇₀ and C₇₀^– can be probed simultaneously. In contrast with a very good coincidence of the experimental and DFT-calculated IR spectra of C₆₀^±/2+, DFT predictions for C₇₀^± IR absorptions strongly deviate from our measurements. A possible explanation for this could be low-lying electronically excited states of C₇₀^± in the vicinity of vibrational energy levels. The corresponding non-Born–Oppenheimer case is likely of significant interest to theory

Thermally Activated D₂ Emission upon Decomposition of Thin Deuterofullerene Films on Au(111)

We have studied the formation and thermal properties of thin, deuterofullerene-containing films on Au(111) under ultrahigh vacuum conditions. The films were prepared in situ by exposure of predeposited C₆₀ layers to a flux of atomic deuterium. With increasing deuterium dose, a D + C₆₀ → C₆₀D_<i>x</i> reaction front propagates through the fullerene film toward the gold surface. Heating the resulting deuterofullerene-containing films to >600 K leads to desorption of predominantly C₆₀ and C₆₀D_<i>x</i>. Interestingly, some D₂ is also evolved while a significant fraction of the carbon initially deposited is left on the surface as nondesorbable residue. This is in contrast to analogous deuterofullerene-containing films prepared on graphite, which sublime completely but do not measurably evolve D₂, suggesting that the gold surface can act as a catalyst for D₂ formation. To explore this further, we have systematically studied (i) the thermal properties of C₆₀/Au­(111) reference films, (ii) the reaction of C₆₀/Au­(111) films with D atoms, and (iii) the heating-induced degradation of deuterofullerene-containing films on Au(111). In particular, we have recorded temperature-resolved mass spectra of the desorbing species (sublimation maps) as well as performed ultraviolet photoionization spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning tunneling microscopy measurements of the surfaces at various stages of study. We infer that heating deuterofullerene-containing films generates mobile deuterium atoms which can recombine to form molecular deuterium either at the gold surface or on fullerene oligomers in direct contact with it

IR, NIR, and UV Absorption Spectroscopy of C₆₀²⁺ and C₆₀³⁺ in Neon Matrixes

C₆₀²⁺ and C₆₀³⁺ were produced by electron-impact ionization of sublimed C₆₀ and charge-state-selectively codeposited onto a gold mirror substrate held at 5 K together with neon matrix gas containing a few percent of the electron scavengers CO₂ or CCl₄. This procedure limits charge-changing of the incident fullerene projectiles during matrix isolation. IR, NIR, and UV–vis spectra were then measured. Ten IR absorptions of C₆₀²⁺ were identified. C₆₀³⁺ was observed to absorb in the NIR region close to the known vibronic bands of C₆₀⁺. UV spectra of C₆₀, C₆₀⁺, and C₆₀²⁺ were almost indistinguishable, consistent with a plasmon-like nature of their UV absorptions. The measurements were supported by DFT and TDDFT calculations, revealing that C₆₀²⁺ has a singlet <i>D</i>_5<i>d</i> ground state whereas C₆₀³⁺ forms a doublet of <i>C</i>_<i>i</i> symmetry. The new results may be of interest regarding the presence of C₆₀²⁺ and C₆₀³⁺ in space

IR Absorptions of C₆₀⁺ and C₆₀^– in Neon Matrixes

C₆₀⁺ ions were produced by electron-impact ionization of sublimed C₆₀, collimated into an ion beam, turned 90° by an electrostatic deflector to separate them from neutrals, mass filtered by a radio frequency quadrupole, and co-deposited with Ne on a cold 5 K gold-coated sapphire substrate. Infrared absorption spectroscopy revealed the additional presence of C₆₀ and C₆₀^– in the as-prepared cryogenic matrixes. To change the C₆₀⁺/C₆₀^– ratio, CCl₄ or CO₂ electron scavengers were added to the matrix gas. Also taking into account DFT calculations, we have identified nine new previously unpublished IR absorptions of C₆₀⁺ and seven of C₆₀^– in Ne matrixes. Our measurements are in very good agreement with DFT calculations, predicting <i>D</i>_5<i>d</i> C₆₀⁺ and <i>D</i>_3<i>d</i> C₆₀^– ground states. The new results may be of interest regarding the presence of C₆₀ and C₇₀ (as well as ions thereof) in Space

From Planar to Cage in 15 Easy Steps: Resolving the C₆₀H₂₁F₉^– → C₆₀^– Transformation by Ion Mobility Mass Spectrometry

A combination of mass spectrometry, collision-induced dissociation, ion mobility mass spectrometry (IM-MS), and density functional theory (DFT) has been used to study the evolution of anionic species generated by laser-desorption of the near-planar, fluorinated polycyclic aromatic hydrocarbon (PAH), C₆₀H₂₁F₉ (s). The dominant decay process for isolated, thermally activated C₆₀H₂₁F₉^– species comprises a sequence of multiple regioselective cyclodehydrofluorination and cyclodehydrogenation reactions (eliminating HF and H₂, respectively, while forming additional pentagons and/or hexagons). The DFT calculations allow us to set narrow bounds on the structures of the resulting fragment ions by fitting structural models to experimentally determined collision cross sections. These show that the transformation of the precursor anion proceeds via a series of intermediate structures characterized by increasing curvature, ultimately leading to the closed-shell fullerene cage C₆₀^– as preprogrammed by the precursor structure

Photoluminescence Spectroscopy of Mass-Selected Electrosprayed Ions Embedded in Cryogenic Rare-Gas Matrixes

An apparatus is presented which combines nanoelectrospray ionization for isolation of large molecular ions from solution, mass-to-charge ratio selection in gas-phase, low-energy-ion-beam deposition into a (co-condensed) inert gas matrix and UV laser-induced visible-region photoluminescence (PL) of the matrix isolated ions. Performance is tested by depositing three different types of lanthanoid diketonate cations including also a dissociation product species not directly accessible by chemical synthesis. For these strongly photoluminescent ions, accumulation of some femto- to picomoles in a neon matrix (over a time scale of tens of minutes to several hours) is sufficient to obtain well-resolved dispersed emission spectra. We have ruled out contributions to these spectra due to charge neutralization or fragmentation during deposition by also acquiring photoluminescence spectra of the same ionic species in the gas phase

Thermal Decomposition of the Fullerene Precursor C₆₀H₂₁F₉ Deposited on Graphite

Specially fluorinated polycyclic aromatic hydrocarbons (F-PAHs) are of interest as precursors for transition metal catalyzed CVD growth of chiral-index pure single-walled carbon nanotubes as well as for the rational synthesis of fullerenes. Laser desorption/ionization of a prototypical F-PAH has recently been shown to lead to C₆₀ via a sequence of regioselective intramolecular cyclodehydrofluorination steps: C₆₀H₂₁F₉ → C₆₀H₂₀F₈ + HF → C₆₀H₁₉F₇ + HF ... → C₆₀ (Kabdulov et al. <i>Chem.–Eur. J.</i> <b>2013</b>, <i>19</i>, 17262). We have studied the thermal stability of solid C₆₀H₂₁F₉ films on graphite under UHV conditions toward exploring the extent to which such intramolecular dehydrofluorination can also occur on a hot chemically inert surface and to what extent intermolecular interactions influence such transformation processes. C₆₀H₂₁F₉ films were probed in situ by ultraviolet photoionization, X-ray ionization, Raman spectroscopy, and thermal desorption mass spectrometry, as well as by ex situ atomic force microscopy. Heating multilayer films results first in C₆₀H₂₁F₉ emission from the bulk (peaked at ∼630 K) followed at higher temperatures by desorption from the interface region (in the range 750–850 K). Sublimation from the interface region is also associated with some on-surface cyclo-dehydrofluorination as indicated by C₆₀H_{21–<i>n</i>}F_9–<i>n</i>, <i>n</i> = 1, 2, 3 emission. C₆₀ was not observed in the desorbed material suggesting that complete cage closure cannot be achieved on HOPG. Furthermore, C₆₀H₂₁F₉ deposits cannot be fully removed from HOPG. Instead, competing on-surface polycondensation of reactive intermediates yields a fluorinated carbon phase, which remains stable up to at least ∼1000 K. To complement these studies we have also used mass selective ion beam soft-landing to probe the desorption properties of monodispersed films consisting of mass-selected C₆₀H_{21–<i>n</i>}F_9–<i>n</i> fragments, <i>n</i> = 1, 2