Higher excited electronic transitions of polyacetylene cations HC2nH+ n = 2-7 in neon matrixes

Gas-phase electronic spectra of HC2n+1H+ (n = 2-6) chains

Highly unsaturated carbon chains are generated in combustion processes and electrical discharges, and are confirmed constituents of the interstellar medium. In hydrogen-rich environments smaller carbon clusters tend to exist as linear chains, capped on each end by hydrogen atoms. Although the HC2nH+ polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HC2n+1H+ chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HC2n+1H+ (n = 2-6) chains contained in a cryogenically cooled quadrupole ion trap. The HC2n+1H+ chains are formed either top-down by ionizing and fragmenting pyrene molecules using pulsed 266 nm radiation, or bottom-up by reacting cyclic carbon cluster cations with acetylene. Ion mobility measurements confirm that the HC2n+1H+ species are linear, consistent with predictions from electronic structure calculations. The HC2n+1H+ electronic spectra exhibit three band systems in the visible/near infrared spectral range, which each shifts progressively to longer wavelength by ≈90 nm with the addition of each additional CC subunit. The strongest visible HC11H+ band has a wavelength (λ = 545.1 nm) and width (1.5 nm) that match the strong λ 5450 diffuse interstellar band (DIB). However, other weaker HC11H+ bands do not correspond to catalogued DIBs, casting doubt on the role of HC11H+ as a carrier for the λ 5450 DIB. There are no identifiable correspondences between catalogued DIBs and bands for the other HC2n+1H+ chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.

An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N₂ mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N₂), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters (C_n ⁺) and polyacetylene cations (HC_2nH⁺) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected Au₂ ⁺ and Au₂Ag⁺ clusters.

Electronic Spectrum and Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH. J Phys Chem A 2020;124:2366-2371. [PMID: 32119779 DOI: 10.1021/acs.jpca.9b11810]

The B̃¹A' ← X̃¹A' electronic spectra of the 1-butyn-3-yl cation (H₃CCHCCH⁺) and the H₃CCHCCH⁺-Ne and H₃CCHCCH⁺-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm^-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H₃C₄H₂⁺) and propargyl (H₂C₃H⁺) cations. The photofragment ions are C₂H₃⁺ (major) and C₄H₃⁺ (minor), with the preference for C₂H₃⁺ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

Electronic Spectra of the Triacetylene Cation (HC6H+) and Protonated Triacetylene (HC6H2+) Tagged with Ar

Polyacetylene cations (HC2nH+) play important roles in combustion processes and in the chemistry of planetary atmospheres and interstellar clouds. Here we report the electronic spectrum for the triacetylene cation (HC6H+) recorded over the 300–610nm range by photodissociating mass-selected ions tagged with argon atoms in a tandem mass spectrometer. The spectrum shows three band systems that are assigned to (origin transition 16665cm−1), (origin transition 23916cm−1), and (origin transition 29920cm−1). Although the band system is well known, the and band systems are observed for the first time in the gas phase. In addition, the electronic spectrum of the protonated triacteylene cation tagged with an argon atom (HC6-Ar) is reported, providing the first gas-phase spectrum for this species.

Hydrocarbon growth via ion-molecule reactions: computational studies of the isomers of C4H2+, C6H2+ and C6H4+ and their formation paths from acetylene and its fragments

Structures, vibrational and electronic spectra, and AIMD trajectories of formation paths for C₄H₂⁺, C₆H₂⁺ and C₆H₄⁺ from acetylene ion and its fragments are reported in this article.

Spectroscopic characterization of C7H3+ and C7H3˙: electronic absorption and fluorescence in 6 K neon matrices

Electronic absorption spectra of mass-selected C₇H₃⁺ and C₇H₃˙ isomers in a neon matrix have been identified for the first time.

Electronic Spectroscopy of a C7H4+ Isomer in a Neon Matrix: Methyltriacetylene Cation

Absorptions commencing at 602.6 nm are detected following deposition of mass-selected C7H4+ in a 6 K neon matrix produced from a 1 : 1 mixture of diacetylene and propyne in an ion source. The 602.6 nm system, and a weaker one near 421.1 nm, are assigned to the A 2E ← X 2E and B 2E ← X 2E electronic transitions of methyltriacetylene cation (C3V symmetry), based on mass-selection, spectroscopic analysis of the vibrational structure, and the excitation energies calculated with CASPT2. Structured fluorescence is detected in the 600–760 nm range upon laser excitation at wavelengths of the CH3C6H+ absorptions. The vibrational bands observed in the absorption and fluorescence spectra are assigned with the aid of calculated frequencies of the totally symmetric (a1) vibrations of methyltriacetylene cation.

Electronic spectra of carbon chains and rings: Astrophysical relevance?

Our research has focused on the measurement of the electronic spectra of unstable molecules which are presumed to be of relevance to astrophysical observations. Among these are the carbon chains and their ions. Thus we have been using and developing a number of spectroscopic methods to determine their spectra in the gas phase, including absorption via cavity ring-down and REMPI methods. The species are produced in supersonic jets coupled with discharge and laser ablation sources. With the successful laboratory detection of the electronic spectra of a number of key species, such as bare carbon chains Cnn=4,5, comparisons with astrophysical data could be made which lead to interesting implications for the future search for the species which could be responsible for the diffuse interstellar bands. Among the recent relevant observations in the laboratory have been the electronic spectra of carbon rings, Cnn=14,18,22, the development of a method to study transitions in mass-selected ions collisionally relaxed to 20 K and held in a 22-pole radiofrequency trap, and the study of metal containing carbon chains.