Iryna Garkusha

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.

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.

Higher energy electronic transitions of HC2n+1H+ (n=2–7) and HC2n+1H (n=4–7) in neon matrices

Electronic absorption spectra of linear HC(2n+1)H(+) (n=2-7) were recorded in 6 K neon matrices following their mass-selective deposition. Four new electronic band systems are identified; the strongest E (2)Pi(g/u)<--X (2)Pi(u/g) lies in the UV and the second most intense C (2)Pi(g/u)<--X (2)Pi(u/g) is located in the visible range. The known A (2)Pi(g/u)<--X (2)Pi(u/g) absorption is an order of magnitude weaker than C (2)Pi(g/u)<--X (2)Pi(u/g). Transitions to the B and D states are also discussed. The wavelengths of the HC(2n+1)H(+) (n=2-7) electronic systems obey a linear relation as a function of the size of the cations, similar to other carbon chains. The B (3)Sigma(u)(-)<--X (3)Sigma(g)(-) transition in the UV of neutral HC(2n+1)H (n=4-7) has also been identified upon photobleaching of the cations trapped in the matrices.

ELECTRONIC ABSORPTION SPECTRA OF PROTONATED ANTHRACENES AND PHENANTHRENES, AND THEIR NEUTRALS IN NEON MATRICES

Electronic spectra of three isomers of protonated anthracene and five isomers of protonated phenanthrene have been detected in 6 K neon matrices following deposition of mass-selected m/z = 179 cations produced from dihydro-anthracene or -phenanthrene. The cations exhibit moderately intense band systems in the 400-550 nm range. Corresponding neutrals have been observed in the UV. The absorptions are assigned to specific isomers of the protonated species on the basis of time-dependent density functional theory calculations. The astrophysical relevance of protonated anthracenes and phenanthrenes as candidates for carriers of diffuse interstellar bands is discussed.

On the Benzylium/Tropylium Ion Dichotomy: Electronic Absorption Spectra in Neon Matrices

Benzyl and tropyl in charge: Electronic spectra of mass-selected benzylium (Bz+) and tropylium (Tr+) cations embedded in solid neon are reported for the first time. They reveal a weak (1)1B1←equation image1A1 visible (see picture) and a much stronger (1)1A1←equation image1A1 ultraviolet transition for Bz+ (C2v symmetry). The lowest dipole-allowed 1A′′2←equation image1A′1 absorption in the ultraviolet region for Tr+ (D7h) is also observed.

Electronic Spectra and Reversible Photoisomerization of Protonated Naphthalenes in Solid Neon

Alpha- and beta-protonated naphthalenes (α- and β-HN(+)) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C(10)H(9)(+) ion beam. The absorption spectra reveal S(1)/S(2) ← S(0) transitions with onsets at 502.1 and 396.1 nm for α-HN(+), and 534.5 and 322.3 nm in the case of β-HN(+). Wavelength-dispersed fluorescence was detected for α-HN(+), starting at 504.4 nm. Light-induced α-HN(+) → β-HN(+) isomerization was observed upon S(2) ← S(0) excitation of α-HN(+), whereas β-HN(+) relaxed back into the more stable alpha form either upon excitation to S(1) or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN(+) ↔ β-HN(+) photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.

Electronic absorption spectra of C7O and C7O+ in 6 K neon matrices

Electronic absorption spectra of C7O, neutral and cationic, have been obtained after mass selection and deposition into 6 K neon matrices. The band system with origin at 305.1 nm is identified as the (2) 1Σ+ ← X 1Σ+ electronic transition of linear C7O, and the absorption, with onset at 276.0 nm, to (4) 2Σ+ ← X 2Σ+ of C7O+ on the basis of experimental observations and theoretical calculations.

Electronic spectra of C4H3Cl+ isomers

Two experimental methods were applied to identify the structure and electronic transitions of C4H3Cl+ isomers. The first is a direct absorption technique where mass-selected ions are embedded in 6 K neon matrices using a mass-selected ion beam and absorption spectra of different C4H3Cl+ isomers were thus observed. The second is a gas phase method on ions which have been collisional cooled with cryogenic helium inside of a 22-pole ion trap. The c-type (1)2 A′ ← X 2 A″ electronic transition of a C4H3Cl+ isomer could then be measured by a one-colour, two-photon technique at 20 and 50 K in the gas phase. The two sets of data, complemented by calculated excitation energies, allowed the assignment of particular isomers. Rotational structure in the gas phase spectra was resolved for C4H3 35Cl+ and C4H3 37Cl+ isomers of cis-1-chlorobutenynylium. The analysis leads to the spectroscopic constants: T 00 = 19 184.680(5), , , , , and (all in cm−1).

Electronic Spectra of Corannulenic Cations and Neutrals in Neon Matrices and Protonated Corannulene in the Gas Phase at 15 K

Abstract Three absorption systems starting at 624.1, 601.2, and 590.0 nm were detected in a 6 K neon matrix following deposition of mass selected m/z = 250 ions produced from corannulene vapour in a hot cathode ion source. The two latter systems were also observed after deposition of neutral corannulene in solid neon with concomitant bombardment of the matrix with argon ions. The features in the absorption spectrum are assigned to the 42A′′  ←  X2A′′ transition of cylobutadieno-benzo[ghi]fluoranthene cation and to the 32A′  ←  X2A′′ and 32A′′  ←  X2A′ transitions of two Jahn-Teller structures of bowl-shaped corannulene cations, respectively. The assignment is based on excitation energies calculated with the SAC-CI and CASPT2 methods. The electronic absorption spectrum of protonated corannulene has onsets at 515.1 and 398.8 nm in a neon matrix, following deposition of a mass-selected beam produced by reactions of corannulene with EtOH2+. The absorptions are assigned, on the basis of theoretical predictions, to the 3,41A  ←  X1A transitions. The electronic spectrum was also recorded in the gas phase using a resonant multiphoton fragmentation technique in an ion trap at vibrational and rotational temperatures of 15 K. The 3,41A  ←  X1A transitions are observed with origin bands at 521 ± 1 nm and 396.4 ± 0.1 nm. The 31A excited electronic state indicates fast internal conversion of ≈ 5 fs, while the 41A state has a lifetime of ≈ 0.2 ps. A distinct vibrational pattern is discernible in the 41A  ←  X1A transition.