Molecular symmetry can influence the photochemical fate of molecules by controlling excited-state lifetimes and, consequently, the time window available for secondary reactions. We have investigated the primary photodynamics of aqueous fumarate and maleate, the trans- and cis-isomers of 2-butenedioate, using femtosecond transient infrared absorption spectroscopy. Following π* ← π excitation at 200 nm, both isomers undergo rapid decarboxylation to form CO2 and acrylate with identical quantum yields of maximum Φ = 30 ± 10%, independent of symmetry. Strikingly, excited-state lifetimes differ by more than an order of magnitude: fumarate remains excited for 5.9 ps, while maleate returns to its ground state in less than 0.5 ps. We attribute this disparity to the higher symmetry of fumarate, which may restrict nonradiative decay pathways. These results demonstrate that even when primary photoproducts are unaffected by symmetry, excited-state lifetimes - and thus the potential for subsequent bimolecular reactions - can be strongly impacted.

Near-UV photolysis of pyruvic acid results in decarboxylation within 0.8 ps. In contrast, pyruvate anions excited by near-UV light relax back to the ground state within 50 ps with no detectable photolysis. The sharply different behavior of the acid and its conjugate base provides a mechanistic explanation for the puzzling pH dependence of the pyruvic acid photolysis.

Per- and polyfluoroalkyl substances (PFAS) are widely detected in the water environment at levels posing significant risks to the ecosystem and human health. While these "forever chemicals" are considered highly resistant to natural photolysis, this study demonstrates the unexpected decomposition of perfluoroalkyl carboxylic acids (PFCAs) and hexafluoropropylene oxide dimer acid (GenX) under simulated solar light in a catalyst-free environment, with GenX exhibiting up to 49.1% degradation and 21.2% defluorination within 5 h. The probe experiment and electron spin resonance spectroscopy, together with a subpicosecond transient absorption spectrometer, confirm the production of hydrogen radicals from the photoexcitation of water at acidic and neutral pH. The hydrogen radical-driven reactions are inconsistent with the widely proposed hydrated electron-dominated defluorination, and we have theoretically elaborated the PFAS decomposition pathways supported by the identified decarboxylated shorter-chain PFCAs and hydrodefluorinated intermediates in mass spectrometry analysis. It is also concluded that light above 300 nm may degrade PFCAs through a long, continuous photolysis treatment (e.g., >24 h) but is still insufficient for defluorination; instead, the hydrogen radical-driven defluorination is primarily attributed to UV wavelengths below 300 nm. This study contributes a comprehensive and fundamental perspective on PFAS photolysis, and the obtained results will inform new strategies by applying simulated solar light for sustainable PFAS remediation.

Atomic motion in solids is conventionally driven by elastic collisions between ionizing particles and atoms, which transfer momentum and induce lattice displacements. In this work, we demonstrate a different mechanism for atomic displacement based on optical excitation of scintillating ionic crystals. Ionic crystals are unique systems because of the closed-shell electronic configuration of their constituent ions. In these materials, excitation above the band gap generates a hole that strongly distorts the lattice, resulting in the formation of a self-trapped hole (STH). The STH is Coulomb-attracted to the electron, thereby forming a self-trapped exciton (STE). Here, we demonstrate that in BaF2 - one of the fastest scintillators - the STE structure promotes the formation of long-lived electron and hole traps that persist in the lattice at room temperature. Such trapped electron-hole pairs occupy vacancy-interstitial fluorine pair positions, and can be created indiscernibly using optical or ionizing radiation excitation, as long as the STH is formed. Further, we demonstrate that it is possible to control the defect evolution with light. Selective optical stimulation of the trapped electrons or holes enables the regeneration of the STE at later times. This light-controlled defect engineering allows us to increase the yield of the STE signal appearing as optically stimulated luminescence (OSL) and to image the spatial distribution of the initial energy deposition, holding strong potential for ionizing-radiation detection. These findings provide a common framework underlying scintillation and OSL in ionic crystals of the fluorite structure, allowing for optical manipulation of atomic vacancies-interstitial pairs in similar systems.

We apply transient absorption spectroscopy supported by 2D-IR spectroscopy and density functional theory calculations to determine the primary photolysis of acrylate excited via the transition at 200 nm. Upon photoexcitation, about half of the excited acrylate anions return to the ground state and relax to equilibrium in 5 ps primarily through intermolecular coupling between the carboxylate group and the surrounding water. The rest of the excited acrylate anions dissociate. Three dissociation channels have been identified. In one reaction, decarboxylation of acrylate forms CO2 and CH2CH-. CH2CH- is protonated by water and forms ethene, C2H4, in <0.8 ps. In the second reaction, the excited acrylate anions dissociate to H2CCHO- and CO. In about 20 ps, H2CCHO- picks up a proton from water to produce vinyl alcohol, H2CCHOH. A third dissociation channel forms H2CCHO˙ and CO-. H2CCHO˙ abstracts a hydrogen atom from water and forms vinyl alcohol. Vinyl alcohol will tautomerize to acetaldehyde, but this occurs on a time scale longer than the experimental observation time of 0.56 ns.

The dominant source of methane (CH4) on Earth is biological and thus a sign of life. Therefore, the discovery of CH4 in the Martian atmosphere was sensational and attracted a lot of attention both within the science community and in the public. Since its discovery, we have learned that the concentration of CH4 on Mars is very dynamic and follows an annual cycle with relatively high concentration during Martian summer and low concentration during winter. Until now the drivers behind this dynamic pattern remain enigmatic as photochemistry which stands behind most atmospheric processes on Mars is too slow to explain the rapid decline. We studied wind-driven erosion as known from dust-devils and/or sand storms and explored whether it can work as a rapid CH4 sink. The outcome of this study will be reported in my presentation.

The presence or absence of methane in the atmosphere of Mars has been a matter of intense investigations and debate for decades. Current theories and observations require some as-yet unidentified mechanism that can remove methane from the lower martian atmosphere on a timescale of a few weeks or less. In this work, we experimentally tested if methane sequestration by wind-driven saltation of martian surface minerals can explain the observations. Triboelectric charging of sand particles during the frequent martian dust storms could potentially provide the energy needed to chemically sequester methane and thereby act as a sink for methane on Mars. We performed laboratory experiments with basaltic martian mineral analog sand from Gufunes, Iceland, which was abraded by tumbling end-over-end in a container made from a monolithic block of the same mineral. In this way, wind-driven saltation was simulated in an all-basalt environment with minimal interference from wall-effects. The results show that methane is not affected during more than 100 terrestrial days of simulated saltation in the all-basalt environment. This stands in contrast to similar experiments using quartz or glass simulation containers. Furthermore, methane remains unaffected by saltation in the presence of excess amounts of Marsrelevant oxidants, such as oxygen and perchlorate salt, which again contrasts to experiments performed in glass containers. However, methane is oxidized to carbon dioxide in the presence of reactive hypochlorite salt. Our results are discussed in the context of recent reports on the chemistry of oxychlorine species on Mars, and they highlight the need to account for wall-effects in experimental simulations of wind-driven saltation in planetary environments.

The deep ultraviolet photochemistry of aqueous pyruvate is believed to have been essential to the origin of life, and near ultraviolet excitation of pyruvate in aqueous aerosols is assumed to contribute significantly to the photochemistry of the Earth’s atmosphere. However, the primary photochemistry of aqueous pyruvate is unknown. Here we study the susceptibility of aqueous pyruvate to photodissociation by deep ultraviolet and near ultraviolet irradiation with femtosecond spectroscopy supported by density functional theory calculations. The primary photo-dynamics of the aqueous pyruvate show that upon deep-UV excitation at 200 nm, about one in five excited pyruvate anions have dissociated by decarboxylation 100 ps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340 nm, the dissociation yield of aqueous pyruvate 200 ps after the excitation is insignificant and no products are observed. The experimental results are explained by our calculations, which show that aqueous pyruvate anions excited at 200 nm have sufficient excess energy for decarboxylation, whereas excitation at 340 nm provides the aqueous pyruvate anions with insufficient energy to overcome the decarboxylation barrier.

Urea is believed to have been essential to the synthesis of prebiotic nucleotides and thereby the RNA or DNA of the first lifeforms. Models suggesting that life began in wet-dry cycles around shallow aquatic ponds imply that reactants such as urea were exposed to deep ultraviolet irradiation from the young sun. Detrimental photodissociation of urea induced by deep UV excitation potentially challenges these models. We here follow the primary deep ultraviolet photochemistry of aqueous urea. The data show that urea is barely excited at 200 nm due to weak ultraviolet absorption. The likelihood of photodissociation is further reduced by strong intra-molecular coupling of the CN and CO stretch vibrations accompanied by an efficient dissipation of the excitation energy to the surrounding water molecules mitigated by urea-water hydrogen bonds. We find that 54±5 % of the excited urea molecules dissociate. Reactions between the photoproducts and surrounding solvent molecules form carbamic acid or the carbamate anions within 0.6 ps. The molecules that do not dissociate return to the electronic ground state in 2 ps. Interestingly, the photodissociation processes of urea in the aqueous phase is different from earlier reported reactions observed following the VUV photolysis of urea in noble gas matrices and highlight the potential influence of water on the prebiotic photochemistry.

The susceptibility of aqueous dipeptides to photodissociation by deep ultraviolet irradiation is studied by femtosecond spectroscopy supported by density functional theory calculations. The primary photodynamics of the aqueous dipeptides of glycyl-glycine (gly-gly), alalyl-alanine (ala-ala), and glycyl-alanine (gly-ala) show that upon photoexcitation at a wavelength of 200 nm, about 10% of the excited dipeptides dissociate by decarboxylation within 100 ps, while the rest of the dipeptides return to their native ground state. Accordingly, the vast majority of the excited dipeptides withstand the deep ultraviolet excitation. In those relatively few cases, where excitation leads to dissociation, the measurements show that deep ultraviolet irradiation breaks the Cα-C bond rather than the peptide bond. The peptide bond is thereby left intact, and the decarboxylated dipeptide moiety is open to subsequent reactions. The experiments indicate that the low photodissociation yield and in particular the resilience of the peptide bond to dissociation are due to rapid internal conversion from the excited state to the ground state, followed by efficient vibrational relaxation facilitated by intramolecular coupling among the carbonate and amide modes. Thus, the entire process of internal conversion and vibrational relaxation to thermal equilibrium on the dipeptide ground state occurs on a time scale of less than 2 ps.

Mechanical activation of quartz grains in a dry atmosphere causes triboelectric charging that can capture CO2 quantitatively (Thogersen et al., https://doi.org/10.1016/j.cplett.2021.139069). Based on electron structure calculations, we propose a mechanism for this process. According to the mechanism, CO2 is inserted in the quartz lattice to form an anchored CO3 moiety. Predicted 13C chemical shifts of this product agree with solid-state 13C NMR measurements.

We study the primary photolysis dynamics of aqueous carbonate, CO32-(aq), and hydrogen carbonate, HCO3-(aq), when they are excited at λ = 200 nm. The photolysis is recorded with sub-picosecond time resolution using UV pump-Vis probe and UV pump-IR probe transient absorption spectroscopy and interpreted with the aid of density functional theory calculations. When CO32- is excited via single photon absorption at λ = 200 nm, Φ(t = 20 ps) = 82 ± 5% of the excited di-anions either detach an electron or dissociate. The electron detachment takes place from the excited state in t < 1 ps and forms ground state CO3˙- and eaq-. Dissociation occurs from both the electronic ground and excited states of CO32-. Dissociation from the CO32- excited state is assisted by water molecules and forms CO2˙-, OH˙ and OH-. The dissociation occurs both directly from the Franck-Condon region in t < 1 ps and indirectly with a time constant of τ = 13.9 ± 0.5 ps as the excited state relaxes. Dissociation of vibrationally excited CO32- molecules in the electronic ground state is also assisted by water molecules and forms CO2 and two OH- anions. The dissociation and subsequent vibrational relaxation of CO2 occur with a time constant of τ = 10.2 ± 0.5 ps. The residual 1 - Φ(t = 20 ps) = 18 ± 5% of the excited CO32- di-anions return by internal conversion to the equilibrated CO32- ground state with a time constant of τ = 4.0 ± 0.4 ps. The extinction coefficient of aqueous hydrogen carbonate HCO3-(aq) at λ = 200 nm is an order of magnitude smaller than that of carbonate, so even though the hydrogen carbonate anions dominate the carbonate di-anions in the hydrogen carbonate solution, the primary photolysis of hydrogen carbonate is obscured by the photo-products of carbonate. Hence, we are unable to assess the primary photolysis of hydrogen carbonate. However, the weak one-photon absorption facilitates two-photon ionization of water, which forms hydronium, H3O+, cations. The sudden increase in the acidity induced by two-photon ionization protonates the ground state hydrogen carbonate molecules, thus offering a rare spectroscopic glimpse of aqueous carbonic acid.

Mechanical activation (i.e., tumbling) of quartz sandin a carbondioxide (CO2) atmosphere leads to triboelectric chargingof the sand grains, driving a process that ultimately sequesters andremoves CO2 from the ambient atmosphere. Supported by diffusereflectance FTIR and C-13 solid-state NMR experiments anddensity functional theory (DFT) calculations, we propose that CO2 is inserted into the quartz lattice to form an anchored CO3 species, a process that otherwise requires high temperatureand pressure to proceed synthetically. The products of the reactionare stable for at least 6 months at ambient temperature and pressure,but CO2 is liberated at temperatures above 150 degrees C.The prospect of using this method to remove CO2 from theatmosphere and, ultimately, to help mitigate the adverse effects ofgreenhouse gases and global warming is briefly discussed.

Abstract The deep ultraviolet photochemistry of aqueous pyruvate is believed to have been essential to the origin of life, and near ultraviolet excitation of pyruvate in aqueous aerosols is assumed to contribute significantly to the photochemistry of the Earth’s atmosphere. However, the primary photochemistry of aqueous pyruvate is unknown. Here we study the susceptibility of aqueous pyruvate to photodissociation by deep ultraviolet and near ultraviolet irradiation with femtosecond spectroscopy supported by density functional theory calculations. The primary photo-dynamics of the aqueous pyruvate show that upon deep-UV excitation at 200 nm, about one in five excited pyruvate anions have dissociated by decarboxylation 100 ps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340 nm, the dissociation yield of aqueous pyruvate 200 ps after the excitation is insignificant and no products are observed. The experimental results are explained by our calculations, which show that aqueous pyruvate anions excited at 200 nm have sufficient excess energy for decarboxylation, whereas excitation at 340 nm provides the aqueous pyruvate anions with insufficient energy to break the decarboxylation barrier.

Since the Viking Labeled Release experiments were carried out on Mars in the 1970s, it has been evident that the martian surface regolith has a strong oxidizing capacity that can convert organic compounds into CO2 and probably water. While H2O2 was suggested originally for being the oxidizing agent responsible for the outcome of the Viking experiments, recent analyses of the martian regolith by the Phoenix lander and by consecutive missions point toward radiation-mediated decomposition products of perchlorate salts as the primary oxidant. In a series of experiments, we have shown that abrasion and triboelectric charging of basalt by simulated saltation could be an additional way of activating regolith. We have also shown that abraded basalt with a chemical composition close to that of martian regolith is toxic to several bacterial species and thus may affect the habitability of the martian surface. In the present study, we investigated the effect of the quantitatively most important minerals (olivine, augite, and plagioclase) and iron oxides (hematite, magnetite, and maghemite) on the survival of bacterial cells to elucidate whether a specific mineral that constitutes basalt is responsible for our observations. We observed that suspensions of iron-containing minerals olivine and augite in phosphate-buffered saline (1 × PBS) significantly reduce the number of surviving cells of our model organism Pseudomonas putida after 24 h of incubation. In contrast, the iron-free mineral plagioclase showed no effect. We also observed that suspending abraded olivine and augite in 1 × PBS led to a dramatic increase in pH compared to the pH of 1 × PBS alone. The sudden increase in pH caused by the presence of these minerals may partly explain the observed cytotoxicity. The cytotoxic effect of augite could be relieved when a strong buffer (20 × PBS) was used. In contrast, olivine, despite the stronger buffer, maintained its cytotoxicity. Iron oxides per se have no negative effect on the survival of our test organism. Overall, our experiments confirm the cytotoxicity of basalt and show that no single constituent mineral of the basalt can account for its toxicity. We could show that abraded iron-containing minerals (olivine and augite) change the pH of water when brought into suspension and thereby could affect the habitability of martian regolith.

Experiments in quartz ampoules that simulate the saltation of quartz grains in a methane atmosphere show that the solid phase acquires a reddish color, reminiscent of the color observed on Triton, Pluto, and Charon. Reflection spectroscopy of the coated grains show a wide, continuous absorption spectrum peaking at near-UV wavelengths, in line with the reddish color. X-ray photoelectron spectroscopy indicates the grains are coated with a substance containing C-C and C-O bonds, and estimates the average thickness of the surface layer to 2.3 angstrom. Solid state Raman measurements of the coating shows a transition at 1540 cm(-1). A model is proposed to describe these measurements. The model is a polydiene, anchored to the quartz surface. Electron structure calculations show that a polydiene with around 8 CH units reproduces the measurements. AFM-IR experiments support this result. Our findings suggest a pathway for synthesis of complex molecules with C = C bonds on planets and moons with a solid surface and a methane-containing atmosphere.

We study the primary dissociation dynamics of aqueous formamide (HCONH2) and dimethylformamide (HCON(CH3)2) induced by photo-excitation at λ = 200 nm. The photolysis is recorded with sub-picosecond time resolution by UV pump-IR probe transient absorption spectroscopy. Formamide dissociates with a quantum yield of Φ(t = 20 ps) = 0.30 ± 0.05, t = 20 ps after the excitation. The rest of the excited formamide molecules return to the ground state within t = 1 ps and vibrationally relax towards equilibrium in t ≈ 10 ps. The only product observed is NH3. NH3 is produced with a yield of Φ(NH3) = 0.23 ± 0.10 on a timescale of τ = 3 ± 1 ps and likely constitutes the dominating product. The CO counter product to NH3 is not observed. Dimethylformamide is photolysed with a quantum yield of Φ(t = 30 ps) = 0.29 ± 0.05, t = 30 ps after the excitation. The photolysis of dimethylformamide produces CO on a time scale of τ ≈ 30 ps. The data indicate that dimethylamine and the N(CH3)2 radical are likely photoproducts.

AstrobiologyVol. 22, No. 9 News & ViewsSpore Survival During Abrasive Saltation on Mars: A Reply to the Comment by Minns et al.Ebbe Norskov Bak, Mikkel Bregnhøj, Per Nørnberg, Svend J. Knak Jensen, Jan Thøgersen, and Kai FinsterEbbe Norskov BakDepartment of Biology, Microbiology section, Aarhus University, Aarhus, Denmark.Search for more papers by this author, Mikkel BregnhøjDepartment of Biology, Microbiology section, Aarhus University, Aarhus, Denmark.Department of Chemistry, Aarhus University, Aarhus, Denmark.Search for more papers by this author, Per NørnbergDepartment of Biology, Microbiology section, Aarhus University, Aarhus, Denmark.Search for more papers by this author, Svend J. Knak JensenDepartment of Chemistry, Aarhus University, Aarhus, Denmark.Search for more papers by this author, Jan ThøgersenDepartment of Chemistry, Aarhus University, Aarhus, Denmark.Search for more papers by this author, and Kai FinsterAddress correspondence to: Kai Finster, Department of Biology, Microbiology section, Ny Munkegade 114–116, Aarhus University, DK-8000 Aarhus C, Denmark E-mail Address: Kai.Finster@bio.au.dkDepartment of Biology, Microbiology section, Aarhus University, Aarhus, Denmark.Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark.Search for more papers by this authorPublished Online:5 Sep 2022https://doi.org/10.1089/ast.2022.0085AboutSectionsView articleView Full TextPDF/EPUB Permissions & CitationsPermissionsDownload CitationsTrack CitationsAdd to favorites Back To Publication ShareShare onFacebookTwitterLinked InRedditEmail View article"Spore Survival During Abrasive Saltation on Mars: A Reply to the Comment by Minns et al.." Astrobiology, 22(9), pp. 1032–1033FiguresReferencesRelatedDetails Volume 22Issue 9Sep 2022 InformationCopyright 2022, Mary Ann Liebert, Inc., publishersTo cite this article:Ebbe Norskov Bak, Mikkel Bregnhøj, Per Nørnberg, Svend J. Knak Jensen, Jan Thøgersen, and Kai Finster.Spore Survival During Abrasive Saltation on Mars: A Reply to the Comment by Minns et al..Astrobiology.Sep 2022.1032-1033.http://doi.org/10.1089/ast.2022.0085Published in Volume: 22 Issue 9: September 5, 2022Online Ahead of Print:August 10, 2022PDF download

Abstract The iconic Viking Landers that landed on Mars in 1976 demonstrated that the Martian surface is an extreme place, dominated by high UV fluxes and regolith chemistry capable of oxidizing organic molecules. From follow-on missions, we have learned that Mars was much warmer and wetter in its early history, and even some areas of Mars (such as crater lakes, possibly with sustained hydrothermal activity) were habitable places (e.g. Grotzinger et al. (2014). Science (New York, N.Y.) 343; Mangold et al. (2021). Science (New York, N.Y.). However, based on the Viking results we have learnt that the search for life and its remains is challenged by abiotic breakdown and alteration of organic material. In particular, the harsh radiation climate at the Martian surface that directly and indirectly could degrade organics has been held accountable for the lack of organics in the Martian regolith. Recent work simulating wind-driven erosion of basalts under Mars-like conditions has shown that this process, comparable to UV- and ionizing radiation, produces reactive compounds, kills microbes and removes methane from the atmosphere. and thereby could equally jeopardize the success of life-seeking missions to Mars. In this review, we summarize and discuss previous work on the role of physical and chemical mechanisms that affect the persistence of organics, and their consequences for the detection of life and/or its signatures in the Martian regolith and in the atmosphere.

We study the primary photolysis dynamics of lactic acid induced by excitation at λ = 200 nm with the aim of elucidating how simple aqueous carboxyl acids react to the deep ultraviolet exposure on the prebiotic Earth. UV-IR transient absorption spectroscopy shows a photolysis quantum yield of Φ(100 ps) = 100 ± 5%. The primary products are CO2, CO2˙- and their counter products CH3CHOH˙ and CH3CHOH-. DFT calculations suggest that the dissociation takes place from the strongly acidic nπ* excited state. Dehydroxylation of lactic acid is not observed.