Thomas Pino

Context. Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. Aims. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3 µm in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. Methods. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program “PDRs4All.” Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. Results. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1–4.5 µm. The continuum at wavelengths longer than 2.7 µm shows good correlations with the 3.3 µm AIB, while the correlation of the continuum at 1.2 µm is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27 µm, which are attributed to the presence in the spectra of water ice and CO ₂ ice. Conclusions. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

Context . Interstellar dust made of siliceous and carbonaceous grains is exposed to cosmic ray ions in the diffuse and dense medium. This highly energetic interaction not only affects the morphology of such systems, but also plays a role in altering the surface and bulk chemistry. These two effects are both key points in understanding the evolution of the interstellar medium matter cycle throughout which we observe variations in the dust’s spectroscopic signatures. Aims . The goal is to better understand, at the nanoscale, the effect of impinging ions on the physical and chemical structure of irradiated dust grains. Methods . To experimentally investigate this phenomenon, two types of dust analogs were produced in the laboratory: an organic fraction made of hydrogenated amorphous carbon (a-C:H) samples and polystyrene (PS) samples to which an inorganic component (SiO ₂ ) was added. Their irradiation was then conducted at the GSI UNILAC accelerator using a 945.4 MeV ¹⁹⁷ Au ²⁵⁺ beam, simulating the irradiation by Galactic cosmic rays. In situ infrared spectra were acquired during irradiation to follow the sample’s chemical evolution, and ex situ transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) micrographs were recorded to investigate the post-irradiation nanostructure and composition. Results . The radiolysis cross sections of the different dust grain analogs were determined. The TEM and EDX images allowed us to constrain the size of tracks produced by the heavy ions in these matrices and visualize the irradiation effects on the morphology of the different samples. The results are extrapolated in an astrophysical context to quantify the tracks produced in interstellar dust under Galactic cosmic ray irradiation.

Context. In the interstellar medium (ISM) and circumstellar environments, photoionization, or the photoelectric effect, emerges as a prevalent phenomenon. In regions exposed to UV photons, either stellar or secondary photons induced by cosmic rays, polycyclic aromatic hydrocarbons (PAHs) liberate electrons through the photoelectric effect, efficiently contributing to the heating budget of the gas. In contrast to shielded areas, neutral and cationic PAHs can scavenge electrons and retain a significant portion of the cloud’s electronic charge. This dual behavior of PAHs not only influences the gas thermal and dynamical behavior but also plays a pivotal role in shaping the chemistry of the environments. Aims. Our aim was to measure the photoelectron kinetic energy distribution (KED) of PAHs of varying sizes, symmetries, and C:H ratios to describe the photoelectron KED with a law that can be implemented in astrophysical photoelectric models that describe gas heating. Methods. We used a double-imaging photoelectron photoion coincidence spectrometer coupled with the DESIRS VUV beamline at the SOLEIL synchrotron to record the gas-phase spectra of a series of sublimated PAHs with different sizes and structures in the 13–20 C atom range. We then compared our data to current astrophysical dust photoelectric models used to describe the PAH charge and gas photoelectric heating in the ISM. In particular, we extended the Kimura 2016, (MNRAS, 459, 2751) model (eK16) to take into account the KED of the photoelectron and its interaction with the grain. Results. We show that although subtle differences between the molecules in our dataset arise from individual electronic structures, the photoelectron KED of PAHs of different sizes and symmetry display remarkable similarities. A general law can thus be implemented in sophisticated ISM astrochemical models to describe their photoelectron KED behavior. We find that the eK16 photoelectric model closely reproduces the present photoionization cross sections of neutral, small PAHs as well as literature data obtained on cationic PAHs. It is noteworthy that the eK16 model, unlike former models, matches the absolute scale of the measured photoionization cross sections. We show that the eK16 model predicts a maximum photoelectric efficiency significantly lower than the previous models, implying a lower interstellar gas temperature and less emission.

We have prepared lycopene aggregates with low scattering in an acetone-water suspension. The aggregates exhibit highly distorted absorption, extending from the UV up to 568 nm, as a result of strong excitonic interactions. We have investigated the structural organization of these aggregates by resonance Raman and TEM, revealing that the lycopene aggregates are not homogeneous, containing at least five different aggregate species. Transient absorption measurements upon excitation at 355, 515, and 570 nm, to sub-select these different species, reveal significant differences in dynamics between each of the aggregate types. The strong excitonic interactions produce highly distorted transient electronic signatures, which do not allow an unequivocal identification of the excited states at times shorter than 60 ps. However, these experiments demonstrate that all the lycopene aggregated species form long-living triplets via singlet fission.

We provide direct evidence of singlet fission occurring with water-soluble compounds. We show that perylene-3,4,9,10-tetracarboxylate forms dynamic dimers in aqueous solution, with lifetimes long enough to allow intermolecular processes such as singlet fission. As these are transient dimers rather than stable aggregates, they retain a significant degree of disorder. We performed a comprehensive analysis of such dynamic assemblies using time-resolved absorption and fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, and theoretical modelling, allowing us to observe the characteristic signatures of singlet fission and develop a model to characterize the different species observed. Our findings reveal that structure fluctuations within perylene-3,4,9,10-tetracarboxylate associations are key in favoring either singlet fission or charge separation. The efficiency of triplet formation is higher than 100%, and the disordered system leads to triplets living in the nanosecond time range.

A zinc porphyrin photosensitizer (ZnF ₂₀ ) is investigated for photoinduced charge separation with different electron donors. Probes of optical light absorption and resonance Raman scattering are used to track the evolution of transient species.

Context. The so-called bump spectral signature observable on interstellar extinction curves, peaking at 217.5 nm, is commonly assigned to π^* ← π transitions from carbonaceous carriers, but the exact nature of the carbonaceous carriers remains debated. Aims. To constrain the chemical structures associated with the bump carriers, we record and compare the UV spectra of a large variety of carbonaceous molecules to this interstellar feature. Methods. Large carbonaceous molecules, such as polycyclic aromatic hydrocarbons (PAHs), were produced by a combustion process stabilized at low pressure under rich flame conditions. Species were extracted and probed through resonance enhanced multiphoton ionization spectroscopy coupled to a time-of-flight mass spectrometer. Masses and absorption profiles of the carbonaceous molecules were measured, and their spectra were compared to the bump feature. Results. Species showing a specific mass progression starting at mass 128 u visible up to mass 394 u with a characteristic progression of +14 u present a common electronic absorption band profile peaking asymptotically around 220 nm. The first masses were assigned to a naphthalene C₁₀H₈ molecule and two of its derivatives: C₁₀H₇CH₃ and C₁₀H₇C₂H₅. The mass progression of +14 u is explained by successive H atom substitutions by CH₃ functional groups. This mass distribution was thus assigned to naphthalene derivatives with large aliphatic carbon substitution. This derivative family shows an electronic band assigned to S₃ ← S₀ transitions involving electron promotion within the π aromatic orbitals of the naphathlene chromophore. More importantly, after a few substitutions, the position of the band converges asymptotically to a value close to the interstellar bump signature, independent of the molecule size. Conclusions. Based on the asymptotic behavior of the larger members in the species distribution, a similar band position is expected from double aromatic ring substructures within hydrogenated amorphous carbons (HACs). Similar to the conclusions of previous works, we find substituted naphthalene units as substructures of interstellar HACs to be good candidates as carriers of the bump feature.

Light-induced charge accumulation is at the heart of biomimetic systems aiming at solar fuel production in the realm of artificial photosynthesis. Understanding the mechanisms upon which these processes operate is a necessary condition to drive down the rational catalyst design road. We have built a nanosecond pump-pump-probe resonance Raman setup to witness the sequential charge accumulation process while probing vibrational features of different charge-separated states. By employing a reversible model system featuring methyl viologen (MV) as a dual electron acceptor, we have been able to watch the photosensitized production of its neutral form, MV⁰, resulting from two sequential electron transfer reactions. We have found that, upon double excitation, a fingerprint vibrational mode corresponding to the doubly reduced species appears at 992 cm^-1 and peaks at 30 μs after the second excitation. This has been further confirmed by simulated resonance Raman spectra which fully support our experimental findings in this unprecedented buildup of charge seen by a resonance Raman probe.