Photoprocessing of cationic triazacoronene: dissociation characteristics of polycyclic aromatic nitrogen heterocycles in interstellar environments.

Polycyclic aromatic nitrogen heterocycles (PANHs) are present in various astronomical environments where they are subjected to intense radiation. Their photodissociation pathways give crucial insights into the cycle of matter in the universe, yet so far only the dissociation characteristics of few PANHs have been investigated. Moreover, most experiments use single photon techniques that only reveal the initial dissociation step, and are thus unsuited to replicate astronomical environments and timescales. In this work, we use the Instrument for the Photodynamics of PAHs (i-PoP) at the Laboratory for Astrophysics to simulate the interstellar photodissociation of a model PANH, cationic triazacoronene (TAC˙+, C21H9N3). Comparing the observed fragments to similar PAHs such as the isoelectronic coronene can give mechanistic insight into PAH dissociation. For coronene the major photodissociation products were found to be C9H+, C10+, and C11+. In contrast, fragmentation in TAC˙+ is initiated by up to three HCN losses often in combination with H- or H2 losses. In the lower mass region, the fragments show similarities to comparable PAHs like coronene, but for TAC˙+ the inclusion of nitrogen atoms into the ionic fragments in the form of e.g. (di)cyanopolyynes is also observed. These nitrogen-containing species may be important tracers of regions in interstellar space where interstellar PANHs are being photodissociated.

Probing the electronic structure and ground state symmetry of gas phase C60+ via VUV photoionization and comparison with theory.

Recently, some of us reviewed and studied the photoionization dynamics of C60 that are of great interest to the astrochemical community as four of the diffuse interstellar bands (DIBs) have been assigned to electronic transitions in the C60+ cation. Our previous analysis of the threshold photoelectron spectrum (TPES) of C60 [Hrodmarsson et al., Phys. Chem. Chem. Phys. 22, 13880-13892 (2020)] appeared to give indication of D3d ground state symmetry, in contrast to theoretical predictions of D5d symmetry. Here, we revisit our original measurements taking account of a previous theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), obtained within a vibronic model parametrized on density functional theory/local-density approximation electronic structure involving all hg Jahn-Teller active modes, which couple to the 2Hu components of the ground state of the C60+ cation. By reanalyzing our measured TPES of the ground state of the C60 Buckminsterfullerene, we find a striking resemblance to the theoretical spectrum calculated in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), and we provide assignments for many of the hg modes. In order to obtain deeper insights into the temperature effects and possible anharmonicity effects, we provide complementary modeling of the photoelectron spectrum via classical molecular dynamics (MD) involving density functional based tight binding (DFTB) computations of the electronic structure for both C60 and C60+. The validity of the DFTB modeling is first checked vs the IR spectra of both species which are well established from IR spectroscopic studies. To aid the interpretation of our measured TPES and the comparisons to the ab initio spectrum we showcase the complementarity of utilizing MD calculations to predict the PES evolution at high temperatures expected in our experiment. The comparison with the theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), furthermore, provides further evidence for a D5d symmetric ground state of the C60+ cation in the gas phase, in complement to IR spectroscopy in frozen noble gas matrices. This not only allows us to assign the first adiabatic ionization transition and thus determine the ionization energy of C60 with greater accuracy than has been achieved at 7.598 ± 0.005 eV, but we also assign the two lowest excited states (2E1u and 2E2u) which are visible in our TPES. Finally, we discuss the energetics of additional DIBs that could be assigned to C60+ in the future.

Polycyclic Aromatic Hydrocarbons (PAHs) in Interstellar Ices: A Computational Study into How the Ice Matrix Influences the Ionic State of PAH Photoproducts.

It has been experimentally observed that water-ice-embedded polycyclic aromatic hydrocarbons (PAHs) form radical cations when exposed to vacuum UV irradiation, whereas ammonia-embedded PAHs lead to the formation of radical anions. In this study, we explain this phenomenon by investigating the fundamental electronic differences between water and ammonia, the implications of these differences on the PAH-water and PAH-ammonia interaction, and the possible ionization pathways in these complexes using density functional theory (DFT) computations. In the framework of the Kohn-Sham molecular orbital (MO) theory, we show that the ionic state of the PAH photoproducts results from the degree of occupied-occupied MO mixing between the PAHs and the matrix molecules. When interacting with the PAH, the lone pair-type highest occupied molecular orbital (HOMO) of water has poor orbital overlap and is too low in energy to mix with the filled π-orbitals of the PAH. As the lone-pair HOMO of ammonia is significantly higher in energy and has better overlap with filled π-orbitals of the PAH, the subsequent Pauli repulsion leads to mixed MOs with both PAH and ammonia character. By time-dependent DFT calculations, we demonstrate that the formation of mixed PAH-ammonia MOs opens alternative charge-transfer excitation pathways as now electronic density from ammonia can be transferred to unoccupied PAH levels, yielding anionic PAHs. As this pathway is much less available for water-embedded PAHs, charge transfer mainly occurs from localized PAH MOs to mixed PAH-water virtual levels, leading to cationic PAHs.

Carbon Atom Reactivity with Amorphous Solid Water: H2O-Catalyzed Formation of H2CO.

We report new computational and experimental evidence of an efficient and astrochemically relevant formation route to formaldehyde (H2CO). This simplest carbonylic compound is central to the formation of complex organics in cold interstellar clouds and is generally regarded to be formed by the hydrogenation of solid-state carbon monoxide. We demonstrate H2CO formation via the reaction of carbon atoms with amorphous solid water. Crucial to our proposed mechanism is a concerted proton transfer catalyzed by the water hydrogen bonding network. Consequently, the reactions 3C + H2O → 3HCOH and 1HCOH → 1H2CO can take place with low or without barriers, contrary to the high-barrier traditional internal hydrogen migration. These low barriers (or the absence thereof) explain the very small kinetic isotope effect in our experiments when comparing the formation of H2CO to D2CO. Our results reconcile the disagreement found in the literature on the reaction route C + H2O → H2CO.

Structural investigation of doubly-dehydrogenated pyrene cations.

The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C16H10˙+; Py+ and C16H8˙+; ddPy+) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py+ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy+ is dominated by absorption features from two isomers (4,5-ddPy+ and 1,2-ddPy+) with, at most, minor contributions from other isomers. These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results suggest that this process favours the loss of adjacent hydrogen atoms.

VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context.

We present the photoionization dynamics of the C60 buckminsterfullerene from threshold up to 14.0 eV recorded with VUV synchrotron radiation at the DESIRS beamline at the SOLEIL synchrotron. The recorded data is obtained using a double-imaging photoelectron photoion coincidence spectrometer and is presented as a two-dimensional photoelectron matrix which contains a wealth of spectroscopic data. We present these data in an astrophysical context which relates to (i) the threshold photoelectron spectrum which is compared to data relevant to the diffuse interstellar bands (DIBs), (ii) the kinetic photoelectron distribution at the Lyman-α line which is relevant to the dominant heating source in the ISM, and (iii) the absolute photoionization cross section of C60 up to approx. 10.5 eV. The photoelectron spectrum implies that the symmetry of the ground state is different than previous theoretical models have predicted, and this result is discussed in context of recent experimental and theoretical findings. Also presented are partial photoionization cross sections of the first two photoelectron bands and their anisotropy parameters. These data are compared with previous theoretical values and discussed where appropriate.

Stretching our understanding of C3: Experimental and theoretical spectroscopy of highly excited nν1 + mν3 states (n ≤ 7 and m ≤ 3).

We present the high resolution infrared detection of fifteen highly vibrationally excited nν1 + mν3 combination bands (n ≤ 7 and m ≤ 3) of C3 produced in a supersonically expanding propyne plasma, of which fourteen are reported for the first time. The fully resolved spectrum, around 3 µm, is recorded using continuous wave cavity ring-down spectroscopy. A detailed analysis of the resulting spectra is provided by ro-vibrational calculations based on an accurate local ab initio potential energy surface for C3 (X̃1Σg+). The experimental results not only offer a significant extension of the available data set, extending the observed number of quanta v1 to 7 and v3 to 3, but also a vital test to the fundamental understanding of this benchmark molecule. The present variational calculations give remarkable agreement compared to experimental values with typical accuracies of ∼0.01% for the vibrational frequencies and ∼0.001% for the rotational parameters, even for high energy levels around 10 000 cm-1.

High-Resolution Infrared Spectra of the ν1 Fundamental Bands of Mono-Substituted 13C Propyne Isotopologues.

We present a combined experimental and ab initio study on the jet-cooled high-resolution infrared spectra of the ν1 (acetylenic stretch) fundamental band for three isotopologues of propyne: 13CH312C≡12CH, 12CH313C≡12CH, and 12CH312C≡13CH. The experimental spectra are recorded in natural abundance using a continuous supersonic expansion of regular propyne diluted in argon and helium, in combination with continuous wave cavity ring-down spectroscopy (cw-CRDS). The fully rotationally resolved K' = 0 and 1 subbands of all three monosubstituted 13C isotopologues have been measured near 3330 cm-1, and their spectroscopic analysis is presented here for the first time. The assignment of the bands and perturbation analysis are assisted by high level ab initio calculations at the CCSD(T) level of theory, from which vibrational frequencies, rotational constants, and Fermi resonances are predicted for each isotopologue.

Theoretical investigation of the infrared spectrum of small polyynes.

The full cubic and semidiagonal quartic force fields of acetylene (C2H2), diacetylene (C4H2), triacetylene (C6H2), and tetraacetylene (C8H2) are determined using CCSD(T) (coupled cluster theory with single and double excitations and augmented by a perturbative treatment of triple excitations) in combination with the atomic natural orbital (ANO) basis sets. Application of second-order vibrational perturbation theory (VPT2) results in vibrational frequencies that agree well with the known fundamental and combination band experimental frequencies of acetylene, diacetylene, and triacetylene (average discrepancies are less than 10 cm-1). Furthermore, the predicted ground state rotational constants (B0) and vibration-rotation interaction constants (αi) are shown to be consistent with known experimental values. New vibrational frequencies and rotational parameters from the presented theoretical predictions are given for triacetylene and tetraacetylene, which can be used to aid laboratory and astronomical spectroscopic searches for characteristic transitions of these molecules.

The ESO Diffuse Interstellar Bands Large Exploration Survey: EDIBLES I. Project description, survey sample and quality assessment.

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the [Formula: see text] fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ~ 70 000 - 100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.

LABORATORY PHOTO-CHEMISTRY OF PAHS: IONIZATION VERSUS FRAGMENTATION.

Interstellar Polycyclic Aromatic Hydrocarbons (PAH) are expected to be strongly processed by Vacuum Ultra-Violet (VUV) photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8-40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium (ISM) are briefly discussed.

A fish scale-derived collagen matrix as artificial cornea in rats: properties and potential.

PURPOSE: A fish scale-derived collagen matrix (FSCM) is proposed as an alternative for human donor corneal tissue. Light scatter and light transmission of the FSCM were measured and compared with human cornea, and its short-term biocompatibility was tested in a rat model. METHODS: light scatter was determined with a straylight measuring device, whereas light transmission was measured using a broadband absorption spectrometer. for evaluation of the biocompatibiliy, three approaches were used: the FSCM was implanted as an anterior lamellar keratoplasty (ALK), placed in an interlamellar corneal pocket (IL), and placed subconjunctivally (SC). Transparency, neovascularization, and epithelial damage were followed for 21 days. Morphology and cellular infiltration were assessed histologically. RESULTS: The amount of scattered light was comparable to that seen in early cataract and the percentage of light transmission was similar to the transmission through the human cornea. Implantation of the FSCM as an ALK led to mild haziness only, not obscuring the pupil, despite the development of neovascularization around the sutures; IL placement led to a moderate haze, partly obscuring the pupil, and to (partial) melting of the anterior corneal lamella. The SC group exhibited local swelling and induration, which decreased over time. Histology showed a chronic inflammation varying from mild and moderate in the ALK and IL group, to severe in the SC group. CONCLUSIONS: In spite of technical difficulties, it was feasible to use the FSCM for ALK, whereas IL placement led to melting of the anterior lamella. Further studies are necessary for better understanding of its immunogenicity. The light scatter and transmission data show that the first version of this FSCM is comparable to human cornea tissue in this respect.

Water formation at low temperatures by surface O2 hydrogenation III: Monte Carlo simulation.

Water is the most abundant molecule found in interstellar icy mantles. In space it is thought to be efficiently formed on the surfaces of dust grains through successive hydrogenation of O, O2 and O3. The underlying physico-chemical mechanisms have been studied experimentally in the past decade and in this paper we extend this work theoretically, using Continuous-Time Random-Walk Monte Carlo simulations to disentangle the different processes at play during hydrogenation of molecular oxygen. CTRW-MC offers a kinetic approach to compare simulated surface abundances of different species to the experimental values. For this purpose, the results of four key experiments-sequential hydrogenation as well as co-deposition experiments at 15 and 25 K-are selected that serve as a reference throughout the modeling stage. The aim is to reproduce all four experiments with a single set of parameters. Input for the simulations consists of binding energies as well as reaction barriers (activation energies). In order to understand the influence of the parameters separately, we vary a single process rate at a time. Our main findings are: (i) The key reactions for the hydrogenation route starting from O2 are H + O2, H + HO2, OH + OH, H + H2O2, H + OH. (ii) The relatively high experimental abundance of H2O2 is due to its slow destruction. (iii) The large consumption of O2 at a temperature of 25 K is due to a high hydrogen diffusion rate. (iv) The diffusion of radicals plays an important role in the full reaction network. The resulting set of 'best fit' parameters is presented and discussed for use in future astrochemical modeling.

Note: cavity enhanced self-absorption spectroscopy: a new diagnostic tool for light emitting matter.

We introduce the concept of Cavity Enhanced Self-Absorption Spectroscopy (CESAS), a new sensitive diagnostic tool for analyzing light-emitting samples. The technique works without an additional light source and its implementation is straight forward. In CESAS, a sample (plasma, flame, or combustion source) is located in an optically stable cavity consisting of two high reflectivity mirrors, and here it acts both as light source and absorbing medium. A modest portion of the emitted light is trapped inside the cavity, making 10(4)-10(5) cavity round trips while crossing the sample and an artificial augmentation of the path length of the absorbing medium occurs as the light transverses the cavity. Light leaking out of the cavity simultaneously provides emission and absorption features. The performance is illustrated by CESAS results on supersonically expanding pulsed hydrocarbon plasma. We expect CESAS to become a generally applicable analytical tool for real time and in situ diagnostics.

Optomechanical shutter modulated broad-band cavity-enhanced absorption spectroscopy of molecular transients of astrophysical interest.

We describe a sensitive spectroscopic instrument capable of measuring broad-band absorption spectra through supersonically expanding planar plasma pulses. The instrument utilizes incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) and incorporates an optomechanical shutter to modulate light from a continuous incoherent light source, enabling measurements of durations as low as ∼400 µs. The plasma expansion is used to mimic conditions in translucent interstellar clouds. The new setup is particularly applicable to test proposed carriers of the diffuse interstellar bands, as it permits swift measurements over a broad spectral range with a resolution comparable to astronomical observations. The sensitivity is estimated to be better than 10 ppm/pass, measured with an effective exposure time of only 1 s.

UV photodesorption of interstellar CO ice analogues: from subsurface excitation to surface desorption.

Carbon monoxide is after H(2) the most abundant molecule identified in the interstellar medium (ISM), and is used as a major tracer for the gas phase physical conditions. Accreted at the surface of water-rich icy grains, CO is considered to be the starting point of a complex organic--presumably prebiotic--chemistry. Non-thermal desorption processes, and especially photodesorption by UV photons, are seen as the main cause that drives the gas-to-ice CO balance in the colder parts of the ISM. The process is known to be efficient and wavelength-dependent, but, the underlying mechanism and the physical-chemical parameters governing the photodesorption are still largely unknown. Using monochromatized photons from a synchrotron beamline, we reveal that the molecular mechanism responsible for CO photoejection is an indirect, (sub)surface-located process. The local environment of the molecules plays a key role in the photodesorption efficiency, and is quenched by at least an order of magnitude for CO interacting with a water ice surface.

The electronic spectrum of the C(s)-C11H3 radical.

The electronic gas-phase absorption spectrum of the bent carbon-chain radical, HC(4)CHC(6)H with C(s) symmetry, is recorded in the 595 nm region by cavity ring-down spectroscopy through an expanding hydrogen plasma. An unambiguous spectroscopic identification becomes possible from a systematic deuterium labeling experiment. A comparison of the results with recently reported spectra of the nonlinear HC(4)CHC(4)H and HC(4)C(C(2)H)C(4)H radicals with C(2v) symmetry provides a more comprehensive understanding of the molecular behavior of π-conjugated bent carbon-chain systems upon electronic excitation. We find that the electronic excitation in the bent carbon-chain HC(4)CHC(2n)H (n = 1-4) series exhibits a similar trend as in the linear HC(2n+1)H (n = 3-6) series, shifting optical absorptions towards longer wavelengths for increasing overall bent chain lengths. The π-conjugation in bent HC(4)CHC(2n)H (n = 1-4) chains is found to be generally smaller than in the linear HC(2n+1)H (n = 3-6) case for equivalent numbers of C-atoms. The addition of an electron-donating group to the bent chain causes a slight decrease of the effective conjugation.

C6H and C6D: electronic spectra and Renner-Teller analysis.

Rotationally resolved spectra of the B(2)Π - X(2)Π 0(0)(0) electronic origin bands and 11(1)(1) µ(2)Σ-µ(2)Σ vibronic hot band transitions of both C(6)H and C(6)D have been recorded in direct absorption by cavity ring-down spectroscopy through a supersonically expanding planar plasma. For both origin and hot bands accurate spectroscopic parameters are derived from a precise rotational analysis. The origin band measurements extend earlier work and the 11(1)(1) µ(2)Σ-µ(2)Σ vibronic hot bands are discussed here for the first time. The Renner-Teller effect for the lowest bending mode ν(11) is analyzed, yielding the Renner parameters ε(11), vibrational frequencies ω(11), and the true spin-orbit coupling constants A(SO) for both (2)Π electronic states. From the Renner-Teller analysis and spectral intensity measurements as a function of plasma jet temperature, the excitation energy of the lowest-lying 11(1) µ(2)Σ vibronic state of C(6)H is determined to be (11.0 ± 0.8) cm(-1).

Structure determination of the nonlinear hydrocarbon chains C9H3 and C11H3 by deuterium labeling.

A systematic deuterium labeling experiment is presented that aims at an unambiguous determination of the geometrical ground state structure of the C(9)H(3) and C(11)H(3) hydrocarbon chains. Cavity ring-down spectroscopy and special plasma expansions constituting C/H, C/D, and C/H/D are used to record optical transitions of both species and their (partially) deuterated equivalents in the 19,000 cm(-1) region. The number of observed bands, the quantitative determination of isotopic shifts, and supporting calculations show that the observed C(9)H(3) and C(11)H(3) spectra originate from HC(4)(CH)C(4)H and HC(4)[C(C(2)H)]C(4)H species with C(2v) symmetry. This result illustrates the potential of deuterium labeling as a useful approach to characterize the molecular structure of nonlinear hydrocarbon chains.