Deep-Ultraviolet Photoexcitation of Aqueous Urea Forms Carbamic Acid/Carbamate in Less Than One Picosecond.

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.

Aqueous pyruvate partly dissociates under deep ultraviolet irradiation but is resilient to near ultraviolet excitation.

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.

Investigation of the Cytotoxicity of Mars-Relevant Minerals upon Abrasion.

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.

The primary photolysis of aqueous carbonate di-anions.

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.

Peptide Bond of Aqueous Dipeptides Is Resilient to Deep Ultraviolet Irradiation.

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.

The primary photo-dissociation dynamics of aqueous formamide and dimethylformamide.

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.

The primary photo-dissociation dynamics of lactic acid: decarboxylation as CO2 and CO2˙.

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.

Insects use lubricants to minimize friction and wear in leg joints.

A protein-based lubricating substance is discovered in the femoro-tibial joint of the darkling beetle Zophobas morio (Insecta). The substance extrudes to the contacting areas within the joint and appears in a form of filiform flows and short cylindrical fragments. The extruded lubricating substance effectively reduces the coefficient of sliding friction to the value of 0.13 in the tribosystem glass/lubricant/glass. This value is significantly lower than 0.35 in the control tribosystem glass/glass and comparable to the value of 0.14 for the tribosystem glass/dry PTFE (polytetrafluoroethylene or Teflon). The study shows for the first time that the friction-reducing mechanism found in Z. morio femoro-tibial joints is based on the lubricant spreading over the contacting surfaces rolling or moving at low loads and deforming at higher loads, preventing direct contact of joint counterparts. Besides Z. morio, the lubricant has been found in the leg joints of the Argentinian wood roach Blaptica dubia.

The primary photolysis dynamics of oxalate in aqueous solution: decarboxylation.

We study the primary reaction dynamics of aqueous oxalate following photo-excitation of the nO → πCO* transition at λ = 200 nm. After the excitation, some of the oxalate molecules return to the electronic ground state on two very different time scales: a fast component of τ = 1.1 ± 0.5 ps comprising 40% of the excited molecules and a much slower component of τ = 0.28 ± 0.05 ns accounting for 15% of the excited molecules. The remaining 45% of the excited molecules do not return to the ground state during the first 500 ps, because they either detach an electron, dissociate or stay excited for hundreds of picoseconds. Dissociation and electron detachment of oxalate predominantly produces CO2 molecules with only minor yields of CO2˙- radical anions. The CO2 formation is accompanied by the ejection of electrons.

The primary photo-dissociation dynamics of lactate in aqueous solution: decarboxylation prevents dehydroxylation.

We study the primary photolysis dynamics of aqueous lactate induced by photo-excitation at λ = 200 nm. Our calculations indicate that both decarboxylation and dehydroxylation are energetically possible, but decarboxylation is favoured dynamically. UV pump - IR probe transient absorption spectroscopy shows that the photolysis is dominated by decarboxylation, whereas dehydroxylation is not observed. Analysis of the transient IR spectrum suggests that photo-dissociation of lactate primarily produces CO2 and CH3CHOH- through the lowest singlet excited state of lactate, which has a lifetime of τ = 11 ps. UV pump - VIS probe transient absorption spectroscopy of electrons from the dissociating lactate anion indicates that the anionic electron from the CO2˙- fragment is transferred to the CH3CHOH˙ counter radical during the decarboxylation process, and CO2˙- is consequently only observed as a minor photo-product. The photo-dissociation quantum yield after the full decay of the excited state is Φ(100ps) = 38 ± 5%.

Remotely Triggered Liquefaction of Hydrogel Materials.

Adaptable behavior such as triggered disintegration affords a broad scope and utility for (bio)materials in diverse applications in materials science and engineering. The impact of such materials continues to grow due to the increased importance of environmental considerations as well as the increased use of implants in medical practices. However, examples of such materials are still few. In this work, we engineer triggered liquefaction of hydrogel biomaterials in response to internal, localized heating, mediated by near-infrared light as external stimulus. This adaptable behavior is engineered into the readily available physical hydrogels based on poly(vinyl alcohol), using gold nanoparticles or an organic photothermal dye as heat generators. Upon laser light irradiation, engineered biomaterials underwent liquefaction within seconds. Pulsed laser light irradiation afforded controlled, on-demand release of the incorporated cargo, successful for small molecules as well as proteins (enzymes) in their biofunctional form.

X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame.

We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV (λ ≈ 130 pm) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the Cornell-SLAC pixel array detector, and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to be within 5%. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data, which should be readily available at upcoming high-repetition-rate facilities.

The primary photo-dissociation dynamics of amino acids in aqueous solution: breaking the Cα-bond.

We report a study of the primary photo-dissociation dynamics of aqueous alanine, isoleucine and proline by 200 nm UV pump-IR probe transient absorption spectroscopy. Photo-dissociation of the three amino acids predominantly results in decarboxylation, and 38 ± 7% of the excited alanine, 35 ± 10% of the excited isoleucine and 47 ± 10% of the excited proline zwitterions remain dissociated 100 picoseconds after the excitation. The decarboxylation occurs from a transient intermediate with a lifetime of ∼20 picoseconds to which we assign the excited state of the amino acids based on comparison of the measured and calculated IR spectra, and calculated excited state energy surfaces.

The primary photo-dissociation dynamics of carboxylate anions in aqueous solution: decarboxylation.

We study the primary photo-dissociation of aqueous anions of formic, acetic and propionic acids induced by photo-excitation at 200 nm. The photo-dissociation dynamics are recorded with sub-picosecond time resolution by UV pump-IR probe transient absorption spectroscopy. Hundred picoseconds after the excitation, 47 ± 5% of the excited formate anions, 35 ± 5% of the excited acetate anions and 27 ± 5% of the excited propionate anions are dissociated, while the rest of the excited molecules return to the electronic ground state of the parent anion. Photo-dissociation of the three anions produces CO2(aq) through a precursor with a lifetime of approximately 20 ps. The precursor is assigned to the excited state of the parent anion based on comparison with calculated IR spectra and isotope shifts. Of the molecules that remain dissociated after 100 ps, the percentage leading to the production of CO2(aq) is 7 ± 3%, 48 ± 20% and 92 ± 30% for formate, acetate and propionate, respectively, while photo-dissociation of formate in addition leads to formation of CO2-. Decarboxylation is thus the dominating reaction channel in acetate and propionate, and this suggests formation of CH4 and C2H6 when the initially formed anions are protonated by water.

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization.

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera.

Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths.

Transient IR Spectroscopic Observation of Singlet and Triplet States of 2-Nitrofluorene: Revisiting the Photophysics of Nitroaromatics.

The dynamics of 2-nitrofluorene (2-NF) in deuterated acetonitrile is studied using UV pump, IR probe femtosecond transient absorption spectroscopy. Upon excitation to the vibrationally excited S1 state, the excited-state population of 2-NF branches into two different relaxation pathways. One route leads to intersystem crossing (ISC) to the triplet manifold within a few hundred femtoseconds and the other to internal conversion (IC) to the ground state. The experiments indicate that after relaxation to the energetic minimum on S1, 2-NF undergoes internal conversion to the ground state in about 15 ps. IC within the triplet manifold is also observed as the initially populated triplet state relaxes to T1 in about 6 ps. Rotational anisotropy measurements corroborate the assignment of the transient IR frequencies and indicate a rotational diffusion time of 2-NF in the solvent of about 14 ps. The combined set of results provides a unified picture of the dynamics in photoexcited 2-NF. This to our knowledge is the first example using femtosecond vibrational spectroscopy for the study of the fundamental photoinduced processes in nitroaromatic compounds.

Pushing the limit: investigation of hydrodynamic forces on a trapped particle kicked by a laser pulse.

We introduce a new optical technique where a train of short optical pulses is utilized to disturb a trapped microscopic particle. Using fast (250 kHz) and accurate (nm) detection of the position of the particle, accurately synchronized to the repetition rate of the laser pulses, we can coherently superimpose the displacement caused by each individual laser pulse. Thereby we are able to both bypass the influence from the Brownian motion of the trapped particle and to simultaneously increase the ability to localize its average trajectory by √n, where n is the number of repetitive pulses. In the results presented here we utilize a train of 1200 pulses to kick a 5 µm polystyrene sphere and obtain a spatial resolution corresponding to 0.09 nm and a time resolution of 4 µs. The magnitude of the optical force pushing the particle corresponds to ∼ 10(4)g and enables an investigation of both the hydrodynamical drag and the inertial effects caused by the particle and the surrounding liquid. Our results enables a more accurate testing of the existing extended models for the hydrodynamic drag and we discuss the observed agreement between experiments and theory.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.