Jan Fulara

ELECTRONIC ABSORPTION SPECTRA OF LINEAR CARBON CHAINS IN NEON MATRICES. III. HC2N+1H

Absorption spectra observed between 400 and 2500 nm in 5 K neon matrices have been assigned to electronic transitions of linear C−2n (n=2–10): 2Π←X 2Π, C2n (n=3–7): 3Σ−u←X 3Σ−g, and C2nH (n=3–8): 2Π←X 2Π chains. The species have been produced by mass selected deposition of cations or anions produced in a hot cathode discharge source (C2n, C−2n, C2nH) and by laser vaporization of graphite (C2n, C−2n). In addition to experimental and chemical evidence, the dependence of the absorption wavelength on the number of carbon atoms was used to assign the electronic transitions. Infrared absorptions which were recorded around 2000 cm−1 are attributed to asymmetric stretching frequencies of C8, C10, and C12. This is based on correlation of their intensities with the identified electronic bands. The possible relevance of the electronic spectra of these carbon chains to astrophysical observations of diffuse interstellar bands is discussed.

Electronic and infrared spectra of C+60 and C−60 in neon and argon matrices

Abstract The electronic transitions of C+60 and C−60 isolated in 5 K neon matrices were detected with well-resolved vibrational structure after NeI irradiation of C60 during deposition. On the basis of a C+60 spectrum obtained by growing a matrix with a mass-selected C+60 ion beam, overlapping peaks in the electronic spectra were assigned to either C+60 or C−60. The vibrational frequencies 1406 and 1332 cm−1 for C+60, and 1386 and 1202 cm−1 for C−60, were identified in a neon matrix from the infrared spectrum. C−60 was also detected after codepositing Na and K with C60, without irradiation. Relevance of the data to interstellar observations is discussed.

Matrix isolation studies of cytosine: The separation of the infrared spectra of cytosine tautomers

Abstract The i.r. spectral studies of cytosine and its d3-deuterated derivative isolated in Ar and Ne matrices are reported. The amino-hydroxy and the amino-oxo tautomeric forms dominate in matrices. Separation of the i.r. spectra of the two tautomers was based on observation of spectral changes following u.v. irradiation of the matrix. Detailed analysis and interpretation of the spectra was performed.

IDENTIFICATION OF H2CCC AS A DIFFUSE INTERSTELLAR BAND CARRIER

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450 A are caused by the B 1 B1 ← X 1 A1 transition of H2CCC (l-C3H2). The large widths of the bands are due to the short lifetime of the B 1 B1 electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450 A DIB leads to a l-C3H2 column density of ∼5 × 10 14 cm −2 toward HD 183143 and ∼2 × 10 14 cm −2 to HD 206267. Despite similar values of E(B −V), the 4881 and 5450 A DIBs in HD 204827 are less than one-third their strength in HD 183143, while the column density of interstellar C3 is unusually high for HD 204827 but undetectable for HD 183143. This can be understood if C3 has been depleted by hydrogenation to species such as l-C3H2 toward HD 183143. There are also three rotationally resolved sets of triplets of l-C3H2 in the 6150–6330 A region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the α-band of telluric O2.

Absorption spectra of conjugated hydrocarbon cation chains in neon matrices

Abstract Absorption spectra for a series of mass selected C k H 2 + systems ( k = 4–16) isolated in neon matrices at 5 K have been measured in the visible and near infrared region. Application of the free-electron molecular-orbital model reveals two homologous series corresponding to the deposition of systems containing even or odd numbers of carbon atoms. With the aid of previous gas-phase data, even-numbered carbon systems are identified as cations of polyacetylenic chains. Positions of the A 2 Π←X 2 Π origin bands as well as fundamental vibrational frequencies in the excited state for the systems H-(CC) n -H + ( n = 4–8) have been obtained. For the deposition of ions containing an odd number of carbons, absorption transitions are tentatively assigned to HC 2 n +1 H + where n = 2–7.

Origin of diffuse interstellar bands: spectroscopic studies of their possible carriers

Abstract The results of astrophysical observations that provide evidence for the molecular origin of the diffuse interstellar bands (DIBs) are reviewed. It is established that molecular systems with a linear, planar or spherical carbon skeleton are good candidates as DIB carriers, and laboratory spectroscopic data concerning such systems are discussed. The characteristic features of the electronic spectra of fullerenes, polycyclic aromatic hydrocarbons, and of linear carbon chains and their ions are found to be consistent with the principal spectroscopic features of DIBs. Analysis of the astrophysical electronic spectra of several simple molecules that have so far been assigned indicates that polar molecules should be easier to detect in this way than those without a permanent electric dipole moment.

Electronic spectra of the C70 molecule and C70+, C70− ions in neon matrices

Abstract The long-wavelength part of the electronic absorption spectrum of C 70 isolated in a 5 K neon matrix was observed with well-resolved vibrational structure. Its origin lines at 15690 cm −1 . The electronic transitions of C 70 + and C 70 − were detected after Ne I irradiation of C 70 during deposition. C 70 − has a broad absorption (fwhm ≈ 330 cm −1 ) centered at 7336 cm −1 . The absorption of C 70 + is found in the range of 12400 to 14000 cm −1 and assigned to the 2 E 1 ′← 2 E 1 ″ transition. The spectrum is also discernible by growing a matrix with mass selected C 70 + ion beam.

Electronic absorption spectra of protonated pyrene and coronene in neon matrixes.

Protonated pyrene and coronene have been isolated in 6 K neon matrixes. The cations were produced in the reaction of the parent aromatics with protonated ethanol in a hot-cathode discharge source, mass selected, and co-deposited with neon. Three electronic transitions of the most stable isomer of protonated pyrene and four of protonated coronene were recorded. The strongest, S(1) ← S(0) transitions, are in the visible region, with onset at 487.5 nm for protonated pyrene and 695.6 nm for protonated coronene. The corresponding neutrals were also observed. The absorptions were assigned on the basis of ab initio coupled-cluster and time-dependent density functional theory calculations. The astrophysical relevance of protonated polycyclic aromatic hydrocarbons is discussed.

Electronic absorption spectra of cyano‐substituted polyacetylene cations in neon matrices

The A 2Π←X 2Π transitions and vibrational frequencies in the excited electronic states have been determined for the cyanopolyacetylene cations H–(C≡C)n–CN+ (n=2–6) and NC–(C≡C)n–CN+ (n=1–5). The spectroscopic information was derived by absorption measurements after cations from a mass selected beam were condensed together with excess of neon to form a 5 K matrix. Spectra have been also obtained for ions with odd number of π‐centers and are tentatively assigned to H–C2n−1–CN+ (n=3–6) and NC–C2n−1–CN+ (n=3–5). The assignments are based on trends observed for the homologous series of these cations, known gas phase data on three of the smaller species, and the free electron molecular orbital model.

Electronic transitions of C3− above the photodetachment threshold

The A 2Δu←X 2Πg, B 2Σu−←X 2Πg, and C 2Σu+←X 2Πg electronic transitions of C3− were observed in a neon matrix and in the gas phase, although the energy of the excited electronic states involved in these transitions is 1–1.5 eV above the photodetachment threshold. The excited Feshbach states are sufficiently long-lived that some of the bands in the gas-phase photodetachment spectrum exhibit rotational structure. Assignment of the transitions is made on the basis of rotational analysis or profile simulations and theoretical calculations. The b 4Πu←X 2Πg transition is also weakly observed. The presence of such discrete bands, though in the continuum, provides a means of detection for anions in the interstellar medium.