Vibrationally Resolved Absorption and Luminescence Spectra of Mass-Selected Free-Base and Zinc Phthalocyanine Radical Cations Isolated in Solid Ne.

We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H2Pc+) and zinc phthalocyanine (ZnPc+) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging.

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non-emissive VOPc⋅ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.

Nanoscale patterning at the Si/SiO2/graphene interface by focused He+ beam.

We have studied the capability of He+ focused ion beam (He+-FIB) patterning to fabricate defect arrays on the Si/SiO2/Graphene interface using a combination of atomic force microscopy (AFM) and Raman imaging to probe damage zones. In general, an amorphized 'blister' region of cylindrical symmetry results upon exposing the surface to the stationary focused He+ beam. The topography of the amorphized region depends strongly on the ion dose, DS , (ranging from 103 to 107ions/spot) with craters and holes observed at higher doses. Furthermore, the surface morphology depends on the distance between adjacent irradiated spots, LS . Increasing the dose leads to (enhanced) subsurface amorphization and a local height increase relative to the unexposed regions. At the highest areal ion dose, the average height of a patterned area also increases as ∼1/LS . Correspondingly, in optical micrographs, the µm2-sized patterned surface regions change appearance. These phenomena can be explained by implantation of the He+ ions into the subsurface layers, formation of helium nanobubbles, expansion and modification of the dielectric constant of the patterned material. The corresponding modifications of the terminating graphene monolayer have been monitored by micro Raman imaging. At low ion doses, DS , the graphene becomes modified by carbon atom defects which perturb the 2D lattice (as indicated by increasing D/G Raman mode ratio). Additional x-ray photoionization spectroscopy (XPS) measurements allow us to infer that for moderate ion doses, scattering of He+ ions by the subsurface results in the oxidation of the graphene network. For largest doses and smallest LS values, the He+ beam activates extensive Si/SiO2/C bond rearrangement and a multicomponent material possibly comprising SiC and silicon oxycarbides, SiOC, is observed. We also infer parameter ranges for He+-FIB patterning defect arrays of potential use for pinning transition metal nanoparticles in model studies of heterogeneous catalysis.

Optical Spectroscopy of Small Carbon Clusters from Electron-Impact Fragmentation and Ionization of Fullerene-C60.

A series of cationic molecular fragments (C n+, n = 11, 12, 15, 16, 18, and 21), produced by electron-impact ionization of C60 in the gas phase, were each mass-selected and accumulated in cryogenic Ne matrices. Optical absorption measurements in the UV-vis and IR spectral ranges reveal linear carbon chain structures. In particular, we have observed the known electronic transitions of linear C11, C15, and C21. The NIR transitions of linear C15-, C16-, and C18- have also been detected, indicating that soft-landing of the corresponding cations can also involve charge-changing. Newly observed electronic absorptions at 410.3 and 429.9 nm have been assigned to linear C18 absorptions at 438.2, 443.5, 422.3, and 433.7 nm, to linear C15+, and absorption at 395.5 nm, to linear C16. Increasing deposition energy leads to fragmentation upon impact. This is indicated by absorptions of C10 (313, 316.3 nm), when depositing C n+ ( n = 11, 15, 16) as well as C12 (332 nm) or C14 (347.4, 356.6 nm), when depositing C15+ or C16+, respectively. These were previously assigned to cyclic isomers. We reassign them to linear isomers here on the basis of plausibility arguments. The observations have been supported by time-dependent density functional theory calculations for ring and chain isomers of C n+/-/0, 10 ≤ n ≤ 20 up to the vacuum-UV range. The electronic absorptions of carbon chains are at least 1 order of magnitude stronger than all NIR electronic absorptions of C60+, which have recently been attributed to several of the diffuse interstellar bands. Considering that fullerene multifragmentation yields long carbon chains that have very strong absorptions both in the UV-vis and IR spectral regions, these systems appear to be good candidates to be observed in regions of space containing fullerenes.

Near- and Mid-IR Gas-Phase Absorption Spectra of H2@C60+-He.

Near- and mid-IR absorption spectra of endohedral H2@C60+ have been measured using He-tagging. The samples have been prepared using a "molecular surgery" synthetic approach and were ionized and spectroscopically characterized in the gas phase. In contrast to neutral C60 and H2@C60, the corresponding He-tagged cationic species show distinct spectral differences. Shifts and line splittings in the near- and mid-IR regions indicate the influence of the caged hydrogen molecule on both the electronic ground and excited states. Possible relevance to astronomy is discussed.

Near-IR Photoluminescence of C60.

We have observed that C60+ ions isolated in cryogenic matrices show distinct near-IR photoluminescence upon excitation in the near-IR range. By contrast, UV photoexcitation does not lead to measurable luminescence. Near-IR C60+ photoluminescence is a one-photon process. The emission is mainly concentrated in one band and corresponds to 2A1u ← 2E1g relaxation. We present experimental data for the Stokes shift, power, and temperature dependencies as well as the quantum efficiency of the photoluminescence. Our findings may be relevant for astronomy, considering recent unequivocal assignment of five diffuse interstellar bands to near-IR absorption bands of C60+.

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.

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 D5d C60(+) and D3d C60(-) ground states. The new results may be of interest regarding the presence of C60 and C70 (as well as ions thereof) in Space.

Vibrationally Resolved Absorption, Fluorescence, and Preresonance Raman Spectroscopy of Isolated Pyronin Y Cation at 5 K.

Exploring how charge-changing affects the photoluminescence of small organic dyes presents challenges. Here, helium tagging photodissociation (PD) action spectroscopy in the gas phase and dispersed laser-induced fluorescence (DF) spectroscopy in the solid Ne matrix are used to compare the intrinsic photophysical properties of pyronin Y cation [PY]+ and its one electron-reduced neutral radical [PY]• at 5 K. Whereas the cation shows efficient visible photoluminescence, no emission from the neutral, in line with theoretical predictions, was detected. B3LYP/aug-cc-pVDZ calculations based on the TD-DFT/FCHT method allow for unambiguous assignment of recorded vibrationally resolved absorption and emission spectra. Surprisingly, our experimental sensitivity was high enough to also observe electronic preresonance Raman (ePR-Raman) spectra of [PY]+, with a significant efficiency factor (EF). These characteristics of the [PY]•/[PY]+ pair suggest that appropriately functionalized derivatives may open new perspectives in the area of in vivo bioimagining microscopy and find applications in various sophisticated stimulated-Raman spectroscopies.

Incorporating C2 into C60 films.

The material formed by depositing C(2)(-) anions onto/into thin C(60) films (on graphite) at room temperature has been studied by means of thermal desorption mass spectroscopy, ultraviolet photoionization spectroscopy, atomic force microscopy (AFM), and surface enhanced Raman spectroscopy. As-prepared, C(2)/C(60) films manifest thermal desorption behaviour which differs significantly from pure C(60) films. Whereas the latter can be fully sublimed, we observe decomposition of C(2)/C(60) films to a high-temperature-stable material while predominantly C(60), C(62), and C(64) are desorbed in parallel. Deposition of C(2)(-) also leads to significantly modified electronic and vibrational properties. Based on DFT model calculations of the Raman spectra, we suggest that as-prepared C(2)/C(60) films contain appreciable amounts of polymeric networks comprising -C(2)-C(60)-C(2)-C(60)- chains. Detection of sublimed C(62) and C(64) upon heating implies that thermal decomposition of C(2)/C(60) films involves addition/uptake of C(2) units into individual fullerene cages. Correspondingly, annealing films up to various intermediate temperatures results in significant modifications to valence-band UP spectra as well as to surface topographies as imaged by AFM. The novel carbonaceous material obtained by heating to T > 950 K has a finite density of states at the Fermi level in contrast to as-prepared C(2)/C(60). It comprises fused fullerene cages.

15 cm-1 to 12 000 cm-1 spectral coverage without changing optics: Diamond beam splitter adaptation of an FTIR spectrometer.

In order to facilitate IR absorption measurements, we have upgraded our Bruker IFS66v/S Fourier-transform infrared (FTIR) spectrometer. A synthetic diamond beam splitter without compensator plate and UHV diamond viewports was installed. We have also modified the IR detector chamber to allow measurements with 5 different detectors. As a result we can now obtain FT absorption spectra from 12 000 cm-1 to 15 cm-1 with the same sample held under ultrahigh vacuum conditions, simply by switching between appropriate IR detectors. We demonstrate the performance of the upgraded FTIR spectrometer by presenting measurements of matrix isolated fullerene ions and an adhesive tape.

IR, NIR, and UV Absorption Spectroscopy of C60(2+) and C60(3+) in Neon Matrixes.

C60(2+) and C60(3+) 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 C60(2+) were identified. C60(3+) was observed to absorb in the NIR region close to the known vibronic bands of C60(+). UV spectra of C60, C60(+), and C60(2+) were almost indistinguishable, consistent with a plasmon-like nature of their UV absorptions. The measurements were supported by DFT and TDDFT calculations, revealing that C60(2+) has a singlet D5d ground state whereas C60(3+) forms a doublet of Ci symmetry. The new results may be of interest regarding the presence of C60(2+) and C60(3+) in space.

Infrared and ultraviolet absorptions of matrix isolated C(6)O(2).

In the course of our research into carbon chains trapped in matrices of molecular oxygen, we encountered an IR absorption line at 2180.4 cm(-1), which we tentatively assigned to linear C(6)O(2). In this article, we describe our attempts to confirm the assignment by partial isotopic substitution of carbon by (13)C and oxygen by (18)O. A detailed analysis of the IR vibration pattern allowed the unambiguous identification of the carrier of the IR absorption as C(6)O(2). The identification work was very much facilitated by the observation that it is possible to produce and destroy C(6)O(2) in a controlled fashion by suitable laser exposures. With the help of this feature, most of the confusing spectral background could be removed. Two infrared absorptions at 2180.4 (nu(5)) and 1817.7 cm(-1) (nu(6)) and ultraviolet absorption at 252 nm were assigned to the C(6)O(2) molecule--all figures are valid for oxygen matrices. The obtained spectral data are compared with results of quantum chemical calculations. DFT B3LYP/6-31G(d) and semiempirical PM3 methods were used for geometry optimization and calculation of vibrational frequencies. CIS and TD-DFT were used to calculate the electronic absorption spectrum.

Assignment of carbon chain molecules in cryogenic matrices by selective laser-induced oxidation.

We tested and applied a laser-induced oxidation method for identifying the IR-active stretching mode absorptions of linear C(2)(n)(+1) molecules which have known strong UV-vis absorptions. For this purpose, we trapped the molecules of carbon vapor in non-inert matrices (pure O(2) and Ar-O(2) mixtures) at temperatures providing molecular growth. The matrix was exposed to laser light tuned to the wavelength of the UV-vis transition of a specific carbon species. We observed a bleaching of that UV-vis absorption and a correlated decrease of lines in the IR spectra for C(9), C(11), C(13), C(15), and C(21). The data suggest that the strongest IR absorptions of the C(2)(n)(+1) (n > or = 4) chains form a regular pattern with increasing n. To obtain information about IR absorptions of carbon chain oxides, oxygen matrices with replaced (16)O --> ( 18)O isotopes were applied. Our data revise some of the IR assignments existing in the literature.