Mathias Steglich

Tunable graphene antennas for selective enhancement of THz-emission

In this paper, we will introduce THz graphene antennas that strongly enhance the emission rate of quantum systems at specific frequencies. The tunability of these antennas can be used to selectively enhance individual spectral features. We will show as an example that any weak transition in the spectrum of coronene can become the dominant contribution. This selective and tunable enhancement establishes a new class of graphene-based THz devices, which will find applications in sensors, novel light sources, spectroscopy, and quantum communication devices.

Abundances of PAHs in the ISM: confronting observations with experimental results

Context. The identification of the carriers of the diffuse interstellar bands (DIBs) is the longest standing problem in the study of the interstellar medium. Here we present recent UV laboratory spectra of various polycyclic aromatic hydrocarbons (PAHs) and explore the potential of these molecules as carriers of the DIBs. Whereas, in the near IR range, the PAHs exhibit vibrational bands that are not molecule-specific, their electronic transitions occurring in the UV/vis provide characteristic fingerprints. The comparison of laboratory spectra calibrated in intensity with high signal-to-noise observational data in the UV enables us to establish new constraints on PAH abundances. Aims. From a detailed comparison of the gas-phase and Ne-matrix absorption spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, benzo[ghi]perylene, and hexabenzocoronene with new interstellar spectra, we aim to infer the abundance of these PAHs in the interstellar medium. Methods. We present spectra of PAHs measured at low temperature in the gas phase and in an Ne matrix, and present methods to derive absolute absorption cross sections for the matrix and gas phase spectra. We have obtained high signal to noise (S/N > 100) absorption spectra toward five lines of sight with reddenings of EB−V = 1−1.6 mag. The spectra cover the 3050−3850 A wavelength region where the PAHs studied here show prominent absorption features. Results. From the observations, we infer upper limits in the fractional abundances of the PAHs studied here. Upper limits in the column densities of anthracene of 0.8−2.8 × 10 12 cm −2 and of pyrene and 2,3-benzofluorene ranging from 2−8 × 10 12 cm −2 are inferred. Upper limits in the column densities of benzo[ghi]perylene are 0.9−2.4 × 10 13 and 10 14 cm −2 for phenanthrene. The measurements indicate fractional abundances of anthracene, pyrene, and 2,3-benzofluorene of a few times 10 −10 . Upper limits in the fractional abundance of benzo[ghi]perylene of a few times 10 −9 and of phenanthrene of few times 10 −8 are inferred. Toward CPD −32 ◦ 1734, we found near 3584 A an absorption line of OH + , which was discovered in the interstellar medium only very recently. Conclusions. The fractional abundances of PAHs inferred here are up to two orders of magnitude lower than estimated total PAH abundances in the interstellar medium. This indicates that either neutral PAHs are not abundant in translucent molecular clouds or that a PAH population with a wide variety of molecules is present.

ELECTRONIC SPECTROSCOPY OF MEDIUM-SIZED POLYCYCLIC AROMATIC HYDROCARBONS: IMPLICATIONS FOR THE CARRIERS OF THE 2175 Å UV BUMP

Mixtures of polycyclic aromatic hydrocarbons (PAHs) have been produced by means of laser pyrolysis. The main fraction of the extracted PAHs was primarily medium-sized, up to a maximum size of 38 carbon atoms per molecule. The use of different extraction solvents and subsequent chromatographic fractionation provided mixtures of different size distributions. UV-VIS absorption spectra have been measured at low temperature by matrix isolation spectroscopy and at room temperature with PAHs as film-like deposits on transparent substrates. In accordance with semi-empirical calculations, our findings suggest that large PAHs with sizes around 50-60 carbon atoms per molecule could be responsible for the interstellar UV bump at 217.5 nm.

CAN NEUTRAL AND IONIZED POLYCYCLIC AROMATIC HYDROCARBONS BE CARRIERS OF THE ULTRAVIOLET EXTINCTION BUMP AND THE DIFFUSE INTERSTELLAR BANDS

Up to now, no laboratory-based study has investigated polycyclic aromatic hydrocarbon (PAH) species as potential carriers of both the diffuse interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed correlation between these two features by applying experimental and theoretical techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and hexabenzocoronene C42H18) in their neutral and cationic states. It was already shown that mixtures of sufficiently large, neutral PAHs can partly or even completely account for the UV bump. We investigated how the absorption bands are altered upon ionization of these molecules by interstellar UV photons. The experimental studies presented here were realized by performing matrix isolation spectroscopy with subsequent far-UV irradiation. The main effects were found to be a broadening of the absorption bands in the UV combined with slight red shifts. The position of the complete pi - pi* absorption structure around 217.5 nm, however, remains more or less unchanged which could explain the observed position invariance of the interstellar bump for different lines of sight. This favors the assignment of this feature to the interstellar PAH population. As far as the DIBs are concerned, neither our investigations nor the laboratory studies carried out by other research groups support a possible connection with this class of molecules. Instead, there are reasonable arguments that neutral and singly ionized cationic PAHs cannot be made responsible for the DIBs.

Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice - II. Near UV/VIS spectroscopy and ionization rates

Context. Mid-infrared emission features originating from polycyclic aromatic hydrocarbons (PAHs) are observed towards photon dominated regions in space. Towards dense clouds, however, these emission features are quenched. Observations of dense clouds show that many simple volatile molecules are frozen out on interstellar grains, forming thin layers of ice. Recently, observations have shown that more complex non-volatile species, presumably including PAHs, also freeze out and contribute to the ongoing solid-state chemistry. Aims. The study presented here aims at obtaining reaction rate data that characterize PAH photochemistry upon vacuum ultraviolet (VUV) irradiation in an interstellar H2O ice analogue to explore the potential impact of PAH:H2O ice reactions on overall interstellar ice chemistry. To this end, the experimental results are implemented in a chemical model under simple interstellar cloud conditions. Methods. Time-dependent near-UV/VIS spectroscopy on the VUV photochemistry of anthracene, pyrene, benzo[ghi]perylene and coronene containing interstellar H2O ice analogs is performed at 25 and 125 K, using an optical absorption setup. Results. Near-UV/VIS absorption spectra are presented for these four PAHs and their photoproducts including cationic species trapped in H 2O ice. Oscillator strengths of the cation absorption bands are derived relative to the oscillator strength of the neutral parent PAH. The loss of the parent and growth of PAH photoproducts are measured as a function of VUV dose, yielding solid state reaction constants. The rate constants are used in an exploratory astrochemical model, to assess the importance of PAH:H2 Oi ce photoprocessing in UV exposed interstellar environments, compared with the timescales in which PAH molecules are incorporated in interstellar ices. Conclusions. All four PAHs studied here are found to be readily ionized upon VUV photolysis when trapped in H2O ice and exhibit similar rates for ionization at astronomically relevant temperatures. Depending on the relative efficiency of H2O photodesorption and PAH photoionization in H2O ice, the latter may trigger a charge induced aromatic solid state chemistry, in which PAH cations play a central role.

UV/visible spectroscopy of matrix-isolated hexa-peri-hexabenzocoronene: Interacting electronic states and astrophysical context

Absorption spectra of hexa-peri-hexabenzocoronene isolated in rare-gas matrices are reported for the wavelength range between 200 and 500 nm. Measurements were carried out in neon and in argon at 5.8 and 12.0 K, respectively. Calculations based on semiempirical models and on density-functional theory were performed to assign the observed features. The electronically excited states involved in Clars alpha- and p-bands are identified as S(1)(B(2u)) and S(2)(B(1u)), respectively. Although the upper state associated with the beta-band is found to be a (1)E(1u) state, it remains undetermined whether it is S(3) or S(4). Structures in the beta-band are interpreted as resulting from the interaction between the (1)E(1u) state and the e(2g) vibrational manifold of S(2)(B(1u)). The new measurements are used to narrow down the wavelength ranges where the bands of hexa-peri-hexabenzocoronene should be found in the gas phase. A previous estimate of the interstellar abundance of this polycyclic aromatic hydrocarbon is discussed.

ELECTRONIC SPECTROSCOPY OF FUV-IRRADIATED DIAMONDOIDS: A COMBINED EXPERIMENTAL AND THEORETICAL STUDY

Irradiation with high-energy photons (10.2-11.8 eV) was applied to small diamondoids isolated in solid rare gas matrices at low temperature. The photoproducts were traced via UV absorption spectroscopy. We found that upon ionization the smallest of these species lose a peripheral H atom to form a stable closed-shell cation. This process is also likely to occur under astrophysical conditions for gas phase diamondoids and it opens the possibility of detecting diamond-like molecules using their rotational spectrum since the dehydrogenated cations possess strong permanent dipole moments. The lowest-energy electronic features of these species in the UV were found to be rather broad, shifting to longer wavelengths with increasing molecular size. Calculations using time-dependent density functional theory support our experimental findings and extend the absorption curves further into the vacuum ultraviolet. The complete σ-σ* spectrum displays surprisingly strong similarities to meteoritic nanodiamonds containing 50 times more C atoms.

IR, Raman, and UV/Vis Spectra of Corannulene for Use in Possible Interstellar Identification

The spectroscopic characterization of corannulene (C(20)H(10)) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography-mass spectrometry (GC-MS). During a high-performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time-dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.

ON THE RELEVANCE OF POLYYNYL-SUBSTITUTED POLYCYCLIC AROMATIC HYDROCARBONS TO ASTROPHYSICS

We report on the absorption spectra of the polycyclic aromatic hydrocarbon (PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl (-C2H) or a butadiynyl (-C4H) group. Measurements were carried out in the mid infrared at room temperature on grains embedded in CsI pellets and in the near ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The infrared measurements show that interstellar populations of polyynyl-substituted PAHs would give rise to collective features in the same way non-substituted PAHs give rise to the aromatic infrared bands. The main features characteristic of the substituted molecules correspond to the acetylenic CH stretching mode near 3.05 mum and to the almost isoenergetic acetylenic CCH in- and out-of-plane bending modes near 15.9 mum. Sub-populations defined by the length of the polyynyl side group cause collective features which correspond to the various acetylenic CC stretching modes. The ultraviolet spectra reveal that the addition of an ethynyl group to a non-substituted PAH molecule results in all its electronic transitions being redshifted. Due to fast internal energy conversion, the bands at shorter wavelengths are significantly broadened. Those at longer wavelengths are only barely affected in this respect. As a consequence, their relative peak absorption increases. The substitution with the longer butadiynyl chain causes the same effects with a larger magnitude, resulting in the spectra to show a prominent if not dominating pi-pi* transition at long wavelength. After discussing the relevance of polyynyl-substituted PAHs to astrophysics, we conclude that this class of highly conjugated, unsaturated molecules are valid candidates for the carriers of the diffuse interstellar bands.We report on the absorption spectra of the polycyclic aromatic hydrocarbon (PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl (–C2H) or a butadiynyl (–C4H) group. Measurements were carried out in the mid-infrared at room temperature on grains embedded in CsI pellets and in the near-ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The infrared measurements show that interstellar populations of polyynyl-substituted PAHs would give rise to collective features in the same way as non-substituted PAHs give rise to the aromatic infrared bands. The main features characteristic of the substituted molecules correspond to the acetylenic CH stretching mode near 3.05 μm and to the almost isoenergetic acetylenic CCH in- and out-of-plane bending modes near 15.9 μm. Sub-populations defined by the length of the polyynyl side group cause collective features which correspond to the various acetylenic CC stretching modes. The ultraviolet spectra reveal that the addition of an ethynyl group to a non-substituted PAH molecule results in all of its electronic transitions being redshifted. Due to fast internal energy conversion, the bands at shorter wavelengths are significantly broadened. Those at longer wavelengths are only barely affected in this respect. As a consequence, their relative peak absorption increases. The substitution with the longer butadiynyl chain causes the same effects with a larger magnitude, resulting in the spectra showing a prominent if not dominating π-π* transition at long wavelengths. After discussing the relevance of polyynyl-substituted PAHs to astrophysics, we conclude that this class of highly conjugated, unsaturated molecules represents valid candidates for the carriers of the diffuse interstellar bands.

The smoothness of the interstellar extinction curve in the UV - Comparison with recent laboratory measurements of PAH mixtures

Context. As revealed by high-resolution spectral investigations in the wavelength range between 300 and 400 nm, the interstellar extinction curve does not display any of the sharp electronic absorption bands that are characteristic for large polyatomic molecules, such as polycyclic aromatic hydrocarbons (PAHs), which belong to the most abundant interstellar molecules. Aims. We aim to verify whether the absorption curves of mixtures of medium-sized PAHs produced in the laboratory can explain the astronomical observations. Methods. The PAH mixtures were synthesized by infrared laser pyrolysis and subsequent chemical extraction and size separation. The matrix isolation technique was used to study the absorption spectra of isolated molecules at low temperature. Results. Our experimental results demonstrate that the UV-visible absorption curves of PAH mixtures can be very smooth, displaying no sharp bands, if the molecular diversity is sufficiently high. Conclusions. In view of the absence of sharp electronic features on the interstellar extinction curve for 300 <λ< 400 nm, we conclude from our experimental findings that the interstellar PAH population must be very diverse. The low fractional abundances of individual species prevent their detection on the basis of spectral fingerprints in the UV.