Perspective: C60+ and laboratory spectroscopy related to diffuse interstellar bands

Ion Spectroscopy in the Context of the Diffuse Interstellar Bands: A Case Study with the Phenylacetylene Cation

Identification of the molecular carriers of diffuse interstellar bands (DIBs) requires gas phase electronic spectra of suitable candidate structures. Recording the spectra of these in the laboratory is challenging because they include large, carbon-rich molecules, many of which are likely to be ionic. The electronic spectra of ions are often obtained using action spectroscopy methods, which can induce small perturbations to the absorption characteristics and hinder comparison with astronomical observations. In this contribution, the appropriateness of helium-tagging and two-color resonant-enhanced photodissociation spectroscopy as suitable techniques to obtain the requisite laboratory data for comparison to DIBs is explored. As a proof-of-concept, the C̃ 2B1 ← X̃ 2B1 electronic transition of the phenylacetylene cation (PA+, C8H6 +), obtained by helium-tagging and two-color photodissociation, is compared to the direct absorption spectrum recorded using cavity ring-down spectroscopy. The results indicate that for DIBs with typical widths of a few ångströms, the wavelengths, bandwidths, and relative intensities from action spectroscopy are obtained with sufficient precision to facilitate accurate comparisons to catalogued DIBs.

Electronic Transition of the l-C6+ Cation at 417 nm

A new electronic transition is reported for the linear C6+ cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. (J. Chem. Phys. 123, 044305 (2005)), which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk (Rev. Sci. Instrum. 90, 103101 (2019)), which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C6+. The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hotbands at lower energies. The hotbands provide the first gas phase information on ground state vibrational frequencies. The vibronic structure of this excited state of C6+ provides a severe challenge to computational chemistry.

Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture

Carbon cluster cations (C_n⁺) produced by laser vaporization are mass selected and photodissociated at 355 nm. Multiphoton dissociation of smaller ions leads to the elimination of neutral C₃, as in previous work, whereas larger clusters exhibit more varied fragmentation channels. Photofragment velocity-map imaging detects significant kinetic energy release (KER) in the various n - 3 cation fragments. Small cations (n = 6 or 7) with linear structures produce moderate KER, whereas larger cations (n = 10, 11, 12, 15, or 20) having monocyclic ring structures produce much higher KER values. Such high KER values are unanticipated, as optical excitation should produce a wide distribution of internal energies. These carbon clusters have a surprising ability to absorb multiple photons of ultraviolet radiation, achieving a state of extreme excitation prior to dissociation. The remarkable nonstatistical distribution of energy is apparently influenced by the significant ring strain that can be released upon photodissociation.

Electronic Spectroscopy of Monocyclic Carbon Ring Cations for Astrochemical Consideration

Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C_2n⁺–He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N₂ tagging by Buntine et al. (J. Chem. Phys.2021, 155, 21430234879679). The set of data presented here further constrains the spectral characteristics inferred for the bare C_2n⁺ ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.

Electronic Spectroscopy of cis- and trans-meta-Vinylbenzyl Radicals

The D₀(²A″)-D₁(²A″) electronic transition of resonance-stabilized radical C₉H₉ isomers cis- and trans-meta-vinylbenzyl (MVB) has been investigated using resonant two-color two-photon ionization (R2C2PI) and laser-induced fluorescence. The radicals were produced in a discharge of m-vinyltoluene diluted in Ar and probed under jet-cooled conditions. The origin bands of the cis and trans conformers are at 19 037 and 18 939 cm^-1, respectively. Adiabatic ionization energies near 7.17 eV were determined for both conformers from two-color ion-yield scans. Dispersed fluorescence (DF) was used to conclusively identify the cis-conformer: ground-state cis-MVB eigenvalues calculated for a Fourier series fit of a computed vinyl torsion potential are in excellent agreement with torsional transitions in the 19 037 cm^-1 DF spectrum. R2C2PI features arising from cis- or trans-MVB were distinguished by optical-optical hole-burning spectroscopy and vibronic assignments were made with guidance from density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. There is a notable absence of mirror symmetry between excitation and emission spectra for several totally symmetric modes, whereby modes that are conspicuous in emission are nearly absent in excitation, and vice versa. This effect is largely ascribed to interference between Franck-Condon and Herzberg-Teller contributions to the electronic transition moment, and its pervasiveness a consequence of the low symmetry (C_s) of the molecule, which permits intensity borrowing from several relatively bright electronic states of A″ symmetry.

Energetics of Formation of Cyclacenes from 2,3-Didehydroacenes and Implications for Astrochemistry

The carriers of the diffuse interstellar bands (DIBs) are still largely unknown although polycyclic aromatic hydrocarbons, carbon chains, and fullerenes are likely candidates. A recent analysis of the properties of n-acenes of general formula C4n+2 H2n+4 suggested that these could be potential carriers of some DIBs. Dehydrogenation reactions of n-acenes after absorption of an interstellar UV photon may result in dehydroacenes. Here the reaction energies and barriers for formation of n-cyclacenes from 2,3-didehydroacenes (n-DDA) by intramolecular Diels-Alder reaction to dihydro-etheno-cyclacenes (n-DEC) followed by ejection of ethyne by retro-Diels-Alder reactions are analyzed using thermally assisted occupation density functional theory (TAO-DFT) for n=10-20. It is found that the barriers for each of the steps depend on the ring strain of the underlying n-cyclacene, and that the ring strain of n-DEC is about 75 % of that of the corresponding n-cyclacene. In each case, ethyne extrusion is the step with the highest energy barrier, but these barriers are smaller than CH bond dissociation energies, suggesting that formation of cyclacenes is an energetically conceivable fate of n-acenes after multiple absorption of UV photons.

INFRA-ICE: An ultra-high vacuum experimental station for laboratory astrochemistry

Laboratory astrochemistry aims at simulating, in the laboratory, some of the chemical and physical processes that operate in different regions of the universe. Amongst the diverse astrochemical problems that can be addressed in the laboratory, the evolution of cosmic dust grains in different regions of the interstellar medium (ISM) and its role in the formation of new chemical species through catalytic processes present significant interest. In particular, the dark clouds of the ISM dust grains are coated by icy mantles and it is thought that the ice-dust interaction plays a crucial role in the development of the chemical complexity observed in space. Here, we present a new ultra-high vacuum experimental station devoted to simulating the complex conditions of the coldest regions of the ISM. The INFRA-ICE machine can be operated as a standing alone setup or incorporated in a larger experimental station called Stardust, which is dedicated to simulate the formation of cosmic dust in evolved stars. As such, INFRA-ICE expands the capabilities of Stardust allowing the simulation of the complete journey of cosmic dust in space, from its formation in asymptotic giant branch stars to its processing and interaction with icy mantles in molecular clouds. To demonstrate some of the capabilities of INFRA-ICE, we present selected results on the ultraviolet photochemistry of undecane (C₁₁H₂₄) at 14 K. Aliphatics are part of the carbonaceous cosmic dust, and recently, aliphatics and short n-alkanes have been detected in situ in the comet 67P/Churyumov-Gerasimenko.

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.

On the Existence of HeHe Bond in the Endohedral Fullerene Hе2 @C60

Twenty years have already been passed since the endohedral fullerene's void ceaselessly attracts attention of both, experimentalists and theoreticians, computational chemists and physicists in particular, who direct their efforts on computer simulations of encapsulating atoms and molecules into fullerene void and on unraveling the arising bonding patterns. We review recent developments on the endohedral He₂ @C₆₀ fullerene, on its experimental observation and on related computational works. The two latter are the main concerns in the present work: on the one hand, there experimentally exists the He dimer embedded into C₆₀ void. On the other, computational side, each He atom exhibits a negligible charge transfer to C₆₀ resulting in that altogether, the He dimer exists as a fractionally charged (He^+δ )₂ . Whether there exists a bond between these two helium atoms is the key question of the present work. Since a bond is a two-body creature, we assert that it suffices to define the bond on the basis of Löwdin's postulate of a molecule which we invoke to investigate such formation of the He dimer in a given C₆₀ void in terms of the HeHe potential energy well. It is analytically demonstrated that this well enables to maintain at least one bound (ground) state, and therefore, according to Löwdin's postulate which is naturally anticipated within quantum theory, we infer that (He^+δ )₂ is a molecule, a diatomic, where two heliums are bonded to each other. Using these arguments, we also propose to extend the concept of stability of endohedral fullerenes. © 2017 Wiley Periodicals, Inc.