VUV photoabsorption spectroscopy of sulfur dioxide ice

Condensed Matter Systems Exposed to Radiation: Multiscale Theory, Simulations, and Experiment

This roadmap reviews the new, highly interdisciplinary research field studying the behavior of condensed matter systems exposed to radiation. The Review highlights several recent advances in the field and provides a roadmap for the development of the field over the next decade. Condensed matter systems exposed to radiation can be inorganic, organic, or biological, finite or infinite, composed of different molecular species or materials, exist in different phases, and operate under different thermodynamic conditions. Many of the key phenomena related to the behavior of irradiated systems are very similar and can be understood based on the same fundamental theoretical principles and computational approaches. The multiscale nature of such phenomena requires the quantitative description of the radiation-induced effects occurring at different spatial and temporal scales, ranging from the atomic to the macroscopic, and the interlinks between such descriptions. The multiscale nature of the effects and the similarity of their manifestation in systems of different origins necessarily bring together different disciplines, such as physics, chemistry, biology, materials science, nanoscience, and biomedical research, demonstrating the numerous interlinks and commonalities between them. This research field is highly relevant to many novel and emerging technologies and medical applications.

Viability of Bacillus subtilis Spores Exposed to Ultraviolet Light at Ocean World Surface Temperatures

This work investigated microorganism survival under temperature and ultraviolet (UV) radiation conditions found at the surface of ice-covered ocean worlds. These studies were motivated by a desire to understand the ability of resilient forms of life to survive under such conditions as a proxy for potential endogenic life and to inform planetary protection protocols for future missions. To accomplish this, we irradiated Bacillus subtilis spores with solar-like UV photons at temperatures ranging from room temperature down to 11 K and reported survival fractions with respect to fluence. We observed an increase in survival at low temperatures and found that the inactivation rate follows an Arrhenius-type behavior above 60 K. For solar-photon fluxes and surface temperatures at Europa and Enceladus, we found that Bacillus subtilis spores would be inactivated in less than an hour when in direct sunlight.

Development of an experimental set-up for low-temperature spectroscopic studies of matrix-isolated molecules and molecular ices using synchrotron radiation

An experimental set-up for studying photophysics and photochemistry of molecules in an inert gas medium (matrix-isolated) and in the ice phase at low temperatures has been developed and commissioned at the Photophysics beamline, Indus-1 synchrotron radiation source. This end-station uses an in-house-developed closed-cycle cryostat for achieving cryo-temperatures (∼10 K). Synchrotron radiation from the Photophysics beamline is used as the source of UV-VUV photons and the system is equipped with a Fourier transform infrared spectrometer for characterization of the molecular species formed at low temperature. Various individual components of the end-station like closed-cycle cryostat, experimental chamber, gas mixing and deposition systems are tested to ascertain that the desired performance criteria are satisfied. The performance of the composite system after integration with the Photophysics beamline is evaluated by recording IR and UV-VUV photoabsorption spectra of sulfur dioxide at low temperatures (10 K), both in the ice phase as well as isolated in argon matrices. Results obtained are in good agreement with earlier literature, thus validating the satisfactory performance of the system. As an off-shoot of the study, the VUV absorption spectrum of matrix-isolated SO₂ in argon matrix up to 10.2 eV is reported here for the first time. This experimental end-station will provide new opportunities to study photon-induced reactions in molecules of environmental, astrochemical and industrial importance. Details of the design, development and initial experimental results obtained are presented.

Vacuum ultraviolet photoabsorption spectroscopy of crystalline and amorphous benzene

We present the first high resolution vacuum ultraviolet photoabsorption study of amorphous benzene with comparisons to annealed crystalline benzene and the gas phase. Vapour deposited benzene layers were grown at 25 K and annealed to 90 K under conditions pertinent to interstellar icy dust grains and icy planetary bodies in our solar system. Three singlet-singlet electronic transitions in solid benzene correspond to the ¹B_2u, ¹B_1u and ¹E_1u states, redshifted by 0.05, 0.25 and 0.51 eV respectively with respect to the gas phase. The symmetry forbidden ¹B_2u ← ¹A_1g and ¹B_1u ← ¹A_1g transitions exhibit vibronic structure due to vibronic coupling and intensity borrowing from the allowed ¹E_1u ← ¹A_1g transition. Additionally the ¹B_2u ← ¹A_1g structure shows evidence of coupling between intramolecular vibrational and intermolecular lattice modes in crystalline benzene with Davydov crystal field splitting observed. The optically forbidden 0-0 electronic origin is clearly visible as a doublet at 4.69/4.70 eV in the crystalline solid and as a weak broadened feature at 4.67 eV in amorphous benzene. In the case of the ¹B_1u ← ¹A_1g transition the forbidden 0-0 electronic origin is only observed in crystalline benzene as an exciton peak at 5.77 eV. Thicker amorphous benzene samples show diffuse bands around 4.3, 5.0 and 5.4 eV that we tentatively assign to spin forbidden singlet-triplet ³B_2u ← ¹A_1g, ³E_1u ← ¹A_1g and ³B_1u ← ¹A_1g transitions respectively, not previously reported in photoabsorption spectra of amorphous benzene. Furthermore, our results show clear evidence of non-wetting or 'islanding' of amorphous benzene, characterised by thickness-dependent Rayleigh scattering tails at wavelengths greater than 220 nm. These results have significant implications for our understanding of the physical and chemical properties and processes in astrochemical ices and highlight the importance of VUV spectroscopy.

SO3 formation from the X-ray photolysis of SO2 astrophysical ice analogues: FTIR spectroscopy and thermodynamic investigations

In this combined experimental-theoretical work we focus on the physical and chemical changes induced by soft X-rays on sulfur dioxide (SO₂) ice at a very low temperature, in an attempt to clarify and quantify its survival and chemical changes in some astrophysical environments. SO₂ is an important constituent of some Jupiter moons and has also been observed in ices around protostars. The measurements were performed at the Brazilian Synchrotron Light Source (LNLS/CNPEM), in Campinas, Brazil. The SO₂ ice sample (12 K) was exposed to a broadband beam of mainly soft X-rays (6-2000 eV) and in situ analyses were performed by IR spectroscopy. The X-ray photodesorption yield (upper limit) was around 0.25 molecules per photon. The values determined for the effective destruction (SO₂) and formation (SO₃) cross sections were 2.5 × 10^-18 cm² and 2.1 × 10^-18 cm², respectively. The chemical equilibrium (88% of SO₂ and 12% of SO₃) was reached after the fluence of 1.6 × 10¹⁸ photons cm^-2. The SO₃ formation channels were studied at the second-order Møller-Plesset perturbation theory (MP2) level, which showed the three most favorable reaction routes (ΔH < -79 kcal mol^-1) in simulated SO₂ ice: (i) SO + O₂ → SO₃, (ii) SO₂ + O → SO₃, and (iii) SO₂ + O⁺ → SO₃⁺ + e^- → SO₃. The amorphous solid environment effect decreases the reactivity of intermediate species towards SO₃ formation, and ionic species are even more affected. The experimentally determined effective cross sections and theoretical reaction channels identified in this work allow us to better understand the chemical evolution of certain sulfur-rich astrophysical environments.

Morphological study into the temperature dependence of solid ammonia under astrochemical conditions using vacuum ultraviolet and Fourier-transform infrared spectroscopy

The authors present the results of a morphological study of solid ammonia using both Fourier-transform infrared and vacuum ultraviolet (VUV) spectroscopy. Dramatic changes in the VUV and infrared spectra at temperatures between 65 and 85 K provide a deeper insight into the structure of ammonia ice particularly with the observation of an exciton transition at 194 nm (6.39 eV) in the VUV spectrum, revealing a structure that is composed of crystallites. A complementary structure is observed in the IR spectrum at 1100 cm(-1) which is assigned to the symmetric deformation of ammonia molecules at the surfaces of the crystallites. Such spectral signatures may be used to identify the environment within which the ammonia ice is formed and provide a new route for obtaining information on the physical and chemical conditions occurring within the interstellar medium, on the surfaces of planetary bodies, and in Kuiper belt objects.