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Thisperspective is concerned with laboratory measurements of the electronicspectra of carbon chains, rings, and their ions, including derivativesterminated by hydrogen and nitrogen atoms. The selected-species haverelevance to astronomical observations through diffuse clouds, absorptionfeatures known as diffuse interstellar bands (DIBs). Two indicationsto decide which molecules should be studied are the observations ofpolar carbon chains in dense clouds by rotational spectroscopy andthe knowledge that a certain number of these have electronic transitionsin the DIB region. This information has been obtained initially bymeasurements of the electronic absorptions in 6 K neon matrixes usingmass-selection. This was followed by the gas-phase observations usingcavity ringdown and resonance enhanced techniques in combination withpulsed-supersonic discharge sources or via laser vaporization. Thegas-phase spectra were then compared with DIB data, all with negativeresults, except for the detection of C3, but leading toupper limits of their column densities <1012cm–2. By reference to mm-wave absorption measurementsin the diffuse medium, it is shown that, although species such asH2C3are present there, the product of the expectedcolumn densities and oscillator strength of the transitions will leadto only very weak DIBs. The significant conclusion is that carbonchains and their derivatives containing hydrogen or nitrogen comprisingup to a dozen atoms cannot be responsible for stronger DIBs.However, chains with an odd-number of carbon atoms, C17, C19, ···, have very intense transitions in the regionabove 4400 Å and remain attractive candidates. An uncertaintyis the excited electronic state lifetime; if this is less than 70fs, then the resulting absorptions would be too broad to be astronomicallyrelevant. The electronic absorptions of some of the species studiedbear a striking resemblance to DIB data. The two peaked rotationalcontour of the origin band in the electronic transition of dicyanoacetylenecation is superimposable on a DIB absorption when shifted by 1 Å.The band profiles of cyclic C18at 100 or 20 K aresimilar to DIBs but differ in wavelength. This suggests that anotherset of potential candidates are the carbon rings of sizes up to ahundred of atoms, including ions and heavy atoms, with the requirementof a large oscillator strength. Observations on the absorptionsof propadienylidene C3H2and C60+are discussed. [ABSTRACT FROM AUTHOR]

The origin band in the electronic transition to the dipole bound excited state of C5H(D)¯ anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X3∑¯ and excited A3∏ electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C5H¯: T00 = 19248.0(1), B′ = 0.0835(1), B″ = 0.0826(2), A′SO = -11.96(1), λSS′ = 1.97(8)λSS″ =0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C5H¯ is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C5H¯. [ABSTRACT FROM AUTHOR]

Four electronic systems with origin bands at 759.5, 559.3, 476.3, and 385.5 nm are detected in a 6 K neon matrix following deposition of mass-selected protonated fluoranthene C16H11+ produced from a reaction of neutral vapor and ethanol in a hot-cathode ion source. Two cationic isomers are identified as the carriers of these band systems. The 559.3, 476.3, and 385.5 nm absorptions are assigned to 4,3,2 1A' ↑ X 1A' transitions of isomer E+ (γ-) and the 2 1A' ↑ X 1A' system at 759.5 nm is of isomer C+ (α-) of protonated fluoranthene on the basis of theoretical predictions. The electronic spectrum of E+ was also recorded in the gas phase using a resonant 1 + 1 two-photon excitation-dissociation technique in an ion trap at vibrational and rotational temperatures of 10 K. The 3,2 1A' ↑ X 1A' transitions have origin band maxima at 558.28 ± 0.01 and 474.92 ± 0.01 nm. Both the 2 1A' and 3 1A' excited states have a distinct vibrational pattern with lifetimes on the order of 1 ps. [ABSTRACT FROM AUTHOR]

Theelectronic spectrum of C17H12N+,phenanthrene with a side chain, was measured in the gas phase ata vibrational and rotational temperature of ∼15 K in an iontrap using a resonant multiphoton dissociation technique. The C17H12N+structure was produced in a chemicalionization source and identified by a comparison with theoreticalcalculations of stable structures and excitation energies. The (3), (2), (1) 1A ← X 1A electronic transitions of this nitrogen-containing aromatic specieswith 30 atoms have origin band maxima at 23 586 ± 1 cm–1, 16 120 ± 50 cm–1,and 14 519 ± 30 cm–1. Distinct vibrational structure in the (3) 1A state is observed, and assignments are made. Astronomical aspectsare considered. [ABSTRACT FROM AUTHOR]

Electronic absorption spectra of C6Cl, C6Cl+, C5Cl, and C5Cl+ have been recorded in 6 K neon matrices. The bands observed are assigned to the B 2Π ← X 2Π electronic transition of C6Cl, 3Σ- ← X 3Σ- of C6Cl+, and 1Σ+ ← X 1Σ+ of C5Cl+ with band origins at 545.8, 527.4, and 226.1 nm, respectively. Two electronic band systems are apparent for C5Cl and these are tentatively assigned to a 2Π ← X 2Π transition with a band origin at 247.1 nm and to Σ+ ← X 2Π at 532.3 nm. For each of the four molecules, several transitions due to the excitation of vibrational modes in the excited electronic states are observed. The spectral assignments in each case are based upon the observation of clear, vibronic progressions with appropriate spacing for C-C and C-Cl stretching modes and by comparison with the absorption spectra of the isoelectronic sulfur-terminated carbon chains. [ABSTRACT FROM AUTHOR]