Photochemical Dissociation Research Articles

Photodissociation and induced chemical asymmetries on ultra-hot gas giants

Context. Recent observations have resulted in the detection of chemical gradients on ultra-hot gas giants. Notwithstanding their high temperature, chemical reactions in ultra-hot atmospheres may occur in disequilibrium, due to vigorous day-night circulation and intense UV radiation from their stellar hosts. Aims. The goal of this work is to explore whether photochemistry is affecting the composition of ultra-hot giant planets, and if it can introduce horizontal chemical gradients. In particular, we focus on hydrogen cyanide (HCN) on WASP-76 b, as it is a photochemically active molecule with a reported detection on only one side of this planet. Methods. We used a pseudo-2D chemical kinetics code to model the chemical composition of WASP-76 b along its equator. Our approach improved on chemical equilibrium models by computing vertical mixing, horizontal advection, and photochemistry. Results. We find that the production of HCN is initiated through the thermal and photochemical dissociation of CO and N2 on the day side of WASP-76 b. The resulting radicals are subsequently transported to the night side via the equatorial jet stream, where they recombine into different molecules. This process results in an HCN gradient with a maximal abundance on the planet’s morning limb. We verified that photochemical dissociation is a necessary condition for this mechanism, as thermal dissociation alone proves insufficient. Other species produced via night-side disequilibrium chemistry are SO2 and S2. Conclusions. Our model acts as a proof of concept for chemical gradients on ultra-hot exoplanets. We demonstrate that even ultra-hot planets can exhibit disequilibrium chemistry and recommend that future studies do not neglect photochemistry in their analyses of ultra-hot planets.

Rapid Photoacoustic Exhaust Gas Analyzer for Simultaneous Measurement of Nitrogen Dioxide and Sulfur Dioxide.

A rapid photoacoustic (PA) exhaust gas analyzer is presented for simultaneous measurements of nitrogen dioxide (NO2) and sulfur dioxide (SO2). A laser diode (LD) emitting at 450 nm and a light-emitting diode (LED) with a peak wavelength of 275 nm operated simultaneously, producing PA signals of NO2 and SO2, respectively. The LD and LED were modulated at different frequencies of 2568 and 2570 Hz, and their emission light beams were transmitted through two resonant tubes in a differential PA cell (DPAC), respectively. A self-made dual-channel digital lock-in amplifier was used to realize the simultaneous detection of dual-frequency PA signals. Cross interference between the PA signals at the two different frequencies was reduced to 0.02% by using a lock-in amplifier. In order to achieve a rapid dynamic measurement, gas sampling was accelerated by an air pump. The use of mufflers and the differential PA detection technique significantly reduced the gas sampling noise. When the gas flow rate was 1000 sccm, the response time of the PA dual-gas analyzer was 8 and 17 s for NO2 and SO2, respectively. The minimum detection limits of NO2 and SO2 were 1.7 and 26.1 ppb when the averaging time of the system was 10 s, respectively. Due to the wide spectral bandwidth of the LED, NO2 produced an interference to the detection of SO2. The interference was reduced by the precise detection of NO2. Since the radiations of the LD and LED passed through two different PA tubes, the impact of NO2 photochemical dissociation caused by UV LED luminescence on NO2 gas detection was negligible. The sharing of the PA cell, the gas lines, and the signal processing modules significantly reduced the size and cost of the PA dual-gas analyzer.

Unraveling segregation behavior of inactive secondary phase driven by ion-competition reaction for perovskite-2D PbI2 heterojunction solar cells

The undesired segregation of inactive secondary phase that randomly distributed across the perovskite film is detrimental to charge-carrier transport dynamics and light stability. However, in-depth studies for understanding the underlying chemical reaction mechanism and the relationship between phase structural configuration and physical properties are still lacking. Herein, the segregation behavior of the inactive secondary phase and the underlying microcosmic mechanism regarding ionic chemical replacement reaction and lattice reconfiguration process are systematically elaborated. A high-quality perovskite-PbI2 heterojunction film is constructed via converting the three-dimensional PbI2 photoactive phase into a two-dimensional inactive phase, alleviating the energetic disorder of carrier dynamics and photochemical dissociation. The resultant perovskite solar cells achieve an efficiency of 24.23% with excellent light-resistance. The dissociation energetics of phase dimensional structure is theoretically analyzed. This work provides new insights into crystal structure design for constructing high-quality perovskite heterojunction devices.

Reaction of ammonia and dioxygen in solid neon excited with far-ultraviolet radiation investigated with electronic and vibrational spectra

Irradiation of dilute mixtures of NH3 and O2 dispersed in solid Ne at 3.6 K with light of wavelengths 200, 192.5, 173, and 143 nm from a synchrotron initiated photochemical dissociation of those precursors and the production of N, NH, NH2, O, O3, HO2, NO, NO2, N2O, H2O, HONO in both s-cisoid and s-transoid rotational isomers, and HONO2, detected according to their spectra in mid-infrared absorption from 450 to 4000 cm−1 and in absorption and emission from 200 to 1100 nm.

MODERN METHODS FOR WASTEWATER TREATMENT

The article considers the photochemical degradation of the phenol solution in the presence of TiO2 nanoparticles, which synthesized in moderate conditions and dimensions vary between 10-30 nm and the 100 mg/ml NH4OH solution. The appearance of photochemical dissociation was confirmed on the basis of the absorption curves taken before and after the process and Abs curves were drawn on the “Varian” device. Phenol's photochemical dissociation in UV-visible area has been confirmed by experiments.

PHOTODEGRADATION OF PHENOL USING N/TiO2 SYSTEM

In the article the photochemical dissociation of phenol in the participation of TiO2 nano-particles and methyl-3-aminocrotonate was done for the first time, the period was taken as 60 minutes, the processing of photochemical dissociation was verified by the curves drawn for reaction solution in the UV radiation device and the 60% decomposition of phenol was defined. Light absorption of a system with TiO2 is observed only in the UV area, whereas absorption of a system with N/TiO2 falls on the visible area of the spectrum .

Investigation of photocatalytic properties of TiO2 nanoparticles belonging to rutile phase

In this article, photochemical dissolution of phenol solution was carried out by ALDKIT001 reactor in the presence of TiO2 nanoparticles possessing rutile phase, the physical and chemical properties of these nanoparticles have been investigated. Photochemical dissociation in the UV field was performed using a Varian device. Photochemical decomposition of phenol was confirmed by gas chromatographic mass spectroscopy.Therefore, we carried out the decomposition of phenol by photochemical reactions using rutile phase TiO2 nanoparticles to purify phenol from wastewater. After photolysis process, 99% phenol decomposition has been occurred.

Photochemical Ring Opening of Oxirane Modeled by Constrained Density Functional Theory.

A constrained density functional theory/classical trajectory surface hopping study of the photochemical dissociation of oxirane (CH2)2O is presented. The calculations confirm the Gomer–Noyes mechanism for the initial reaction and agree largely with experimental photolysis data including reaction yields. The calculated yields, however, depend both on temperature and its modeling. The timescales of the various reaction steps are well below 100 fs, similar to previous time-dependent density functional calculations. At variance with those, however, the present calculations obey Kasha’s rule, i.e., the photoreaction is initiated in the energetically lowest excited state.

Energy Landscapes in Photochemical Dissociation of Small Peroxides.

Organic peroxides are known to have important roles in many chemical and biochemical processes such as intermediates in the oxidation of various hydrocarbons, as initiators of free-radical polymerization and cross-linking agents, etc. Consequently, the study of the organic peroxides and their radicals are of fundamental interest and importance. Although several reaction pathways after dissociation of organic peroxides have been successfully identified using time-resolved optical absorption spectroscopy, interpretation of the data can be complicated due to spectral overlap of parent molecules, intermediates, and products. Therefore, a reliable theoretical framework is necessary in case of complex or less studied systems. In this study, we investigated the plausible thermal dissociation pathways of diethyl peroxide, ditert butyl peroxide, and dicumyl peroxide by density functional theory with M06-2X hybrid functional and compared its results to coupled cluster single double and perturbative triple, CCSD(T), level energies. Our results indicate that methyl radical elimination is the main dissociation mechanism for all of the studied peroxides after O-O bond cleavage which has been also observed in experiments. The resulting relative energies of the M06-2X functional were found to have reasonable accuracy in comparison with the CCSD(T) method. We also show that time-dependent density functional theory (TD-DFT) with the M06-2X functional provides a suitable guide for interpretation of time-resolved optical absorption spectra of peroxides. The experimental transient absorption spectra of dicumyl peroxide are interpreted using the theoretically predicted pathways and transient radical species. Both results agree within experimental resolution and accuracy. We propose that the traditionally assigned visible absorption is not due to the cumuloxyl radical and the photodissociation of dicumyl peroxide involves other pathways with extremely short-lived radicals.

Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors.

Photochemistry of glyoxylate embedded in sodium chloride clusters, a laboratory model for tropospheric sea-salt aerosols.

Although marine aerosols undergo extensive photochemical processing in the troposphere, a molecular level understanding of the elementary steps involved in these complex reaction sequences is still missing. As a defined laboratory model system, the photodissociation of sea salt clusters doped with glyoxylate, [NanCln-2(C2HO3)]+, n = 5-11, is studied by a combination of mass spectrometry, laser spectroscopy and ab initio calculations. Glyoxylate acts as a chromophore, absorbing light below 400 nm via two absorption bands centered at about 346 and 231 nm. Cluster fragmentation dominates, which corresponds to internal conversion of the excited state energy into vibrational modes of the electronic ground state and subsequent unimolecular dissociation. Photochemical dissociation pathways in electronically excited states include CO and HCO elimination, leading to [Nan-xCln-x-2HCOO]+ and [NanCln-2COO˙]+ with typical quantum yields in the range of 1-3% and 5-10%, respectively, for n = 5. The latter species contains CO2˙- stabilized by the salt environment. The comparison of different cluster sizes shows that the fragments containing a carbon dioxide radical anion appear in a broad spectral region of 310-380 nm. This suggests that the elusive CO2˙- species may be formed by natural processes in the troposphere. Based on the photochemical cross sections obtained here, the photolysis lifetime of glyoxylate in a dry marine aerosol is estimated as 10 h. Quantum chemical calculations show that dissociation along the C-C bond in glyoxylic acid as well as glyoxylate embedded in the salt cluster occurs after reaching the S1/S0 conical intersection, while this conical intersection is absent in free glyoxylate ions.

UV photochemistry of pyridine-water and pyridine-ammonia complexes trapped in cryogenic matrices

In the quest to reach the “Photochemical Graal”, i.e. the photochemical homolytic water dissociation, we have carried out UV irradiation on the pyridine-water and pyridine-ammonia complexes trapped in cryogenic matrices (Ar, Ne and N2). The UV irradiation effects have been followed by FTIR spectroscopy in order to check the solid solvation effect. The experimental results have been compared systematically with theoretical ones, carried out at the UB3LYP and B3LYP/6-311 + G(d) levels of theory. The most salient result is that whatever the matrix and the pyridine partner, a UV irradiation, inducing uniquely pyridine electronic excitation, leads unambiguously to pyridine isomerisation toward the Dewar form while no water/ammonia dissociation and, concomitantly, no transfer toward pyridine of a H species has been observed unlike what we observed in molecular beam [Jouvet et al., PCCP, 2016, 18, 25637–25644]. From native pyridine or its Dewar form, no further photodissociation toward cyclobutadiene and hydrogen cyanide, has been observed.

Photoactivated Transition-Metal Triggers for Ambient Temperature Enediyne and Dienyne Cyclization: Ruthenium-η6-Naphthalene Complexes

A persistent challenge confronting potential applications of the Bergman cycloaromatization reaction is the development of methods for spatiotemporal control of diradical formation. Photochemical variants (photo-Bergman cycloaromatizations) have thus far met with limited success, failing completely in the case of acyclic enediynes. Here we describe the development of efficient photoactivated transition-metal complexes that allow for spatiotemporal control of enediyne cycloaromatization at ambient temperatures. This strategy relies on air- and moisture-stable ruthenium naphthalene complexes that undergo photochemical dissociation of the naphthalene ligand, thereby generating coordination sites for enediyne binding and cycloaromatization. The same ruthenium naphthalene complexes also trigger dienyne cyclization under photochemical conditions.

Photochemical Water-Splitting with Organomanganese Complexes.

Certain organometallic chromophores with water-derived ligands, such as the known [Mn(CO)3(μ3-OH)]4 (1) tetramer, drew our attention as possible platforms to study water-splitting reactions. Herein, we investigate the UV irradiation of various tricarbonyl organomanganese complexes, including 1, and demonstrate that dihydrogen, CO, and hydrogen peroxide form as products in a photochemical water-splitting decomposition reaction. The organic and manganese-containing side products are also characterized. Labeling studies with 18O-1 suggest that the source of oxygen atoms in H2O2 originates from free water that interacts with 1 after photochemical dissociation of CO (1-CO) constituting the oxidative half-reaction of water splitting mediated by 1. Hydrogen production from 1 is the result of several different processes, one of which involves the protons derived from the hydroxido ligands in 1 constituting the reductive half-reaction of water splitting mediated by 1. Other processes that generate H2 are also operative and are described. Collectively the results from the photochemical decomposition of 1 provide an opportunity to propose a mechanism, and it is discussed within the context of developing new strategies for water-splitting reactions with organomanganese complexes.

Specifications of nanosecond laser ablation with solid targets, aluminum, silicon rubber, and polymethylmethacrylate (PMMA)

The ablation parameters such as threshold fluence, etch depth, ablation rate and the effect of material targets were investigated under the interaction of laser pulse with low intensity. The parameters of the laser system are: laser pulse energy in the range of 110–140 mJ, wavelength 1064 nm and pulse duration 20 ns. By macroscopic estimation of the outward images of the ablation and data obtained, we can conclude that the photothermal and photoionization processes have more influence for aluminum ablation. In contrast, for polymer samples, from the macroscopic observation of the border pattern at the irradiated spot, and also the data obtained from the experiment results, we deduce that both chemical change due to heating and photochemical dissociation were effective mechanisms of ablation. However, concerning the two polymer samples, apart from considering the same theoretical ablation model, it is conceived that the photomehanical specifications of PMMA are involved in the ablation parameters. The threshold fluence for an ablation rate of 30 laser shots were obtained as 12.4, 24.64, and 11.71 J cm−2, for aluminum, silicon rubber and polymethylmethacrylate (PMMA) respectively. The ablation rate is exponentially decreased by the laser-shot number, especially for aluminum. Furthermore, the etch depth after 30 laser shots was measured as 180, 630 and 870 μm, for aluminum, silicon rubber and PMMA, respectively.

Disentangling Multichannel Photodissociation Dynamics in Acetone by Time-Resolved Photoelectron-Photoion Coincidence Spectroscopy.

For the investigation of photoinduced dynamics in molecules with time-resolved pump-probe photoionization spectroscopy, it is essential to obtain unequivocal information about the fragmentation behavior induced by the laser pulses. We present time-resolved photoelectron-photoion coincidence (PEPICO) experiments to investigate the excited-state dynamics of isolated acetone molecules triggered by two-photon (269 nm) excitation. In the complex situation of different relaxation pathways, we unambiguously identify three distinct pump-probe ionization channels. The high selectivity of PEPICO detection allows us to observe the fragmentation behavior and to follow the time evolution of each channel separately. For channels leading to fragment ions, we quantitatively obtain the fragment-to-parent branching ratio and are able to determine experimentally whether dissociation occurs in the neutral molecule or in the parent ion. These results highlight the importance of coincidence detection for the interpretation of time-resolved photochemical relaxation and dissociation studies if multiple pathways are present.

Photochemical dissociation of HOBr. A nonadiabatic dynamics study

Abstract A number of single-reference post Hartree-Fock methods, namely CIS, symmetry adopted cluster configuration interaction (SA C-CI), equation of motion coupled cluster (EOM-CCSD) and time-dependent density functional theory (TDDFT), were used to investigate the excited state properties and photochemical dissociation of hypobromous acid, HOBr. Results are in good agreement with experimental data. The decay of the excited states of HOBr in the gas phase was examined by simulating the UV photoabsorption spectrum and nonadiabatic dynamics at the TDDFT M06-2X level of theory. The spectrum, composed of 10 excited states, was simulated with the nuclear ensemble approximation, sampling a Wigner distribution with 500 points. Dynamics simulations were done with the surface hopping method, started in two separate spectral windows, at 4.0 ± 0.25 eV and 8.0 ± 0.25 eV, to be compared to experimental UV spectral data. Three hundred trajectories were considered from each of these windows according to their excitation probabilities. The excited-state lifetime was determined. The main photochemical channel observed were HO and Br eliminations, representing 67% of all processes.

Deciphering the cryptic role of a catalytic electron in a photochemical bond dissociation using excited state aromaticity markers.

The breaking of bonds by catalytic electrons has gained prominence very recently but has been limited to cases where electrons from external sources have been used. Here, we show that upon photoexcitation, an electron of intramolecular origin is transferred from one part of a molecule to another followed by bond cleavage and then returns to its original moiety on completion of its catalytic function. By a proper assessment of the dramatic changes in aromaticity in excited-state intermediates along the photoreaction coordinate captured by the magnetically induced current density (MICD) technique, we show that in 5-phenyltetrazole, an excited electron, which migrates from the phenyl ring to the tetrazole ring, induces bond cleavage catalytically. Using the MICD technique, we establish for the first time a link between the phenomenon of excited-state electron/charge transfer among aromatic rings and the intricate interplay of aromatic, antiaromatic and non-aromatic states.

Mechanism of Formation of Copper(II) Chloro Complexes Revealed by Transient Absorption Spectroscopy and DFT/TDDFT Calculations.

Copper(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 10(6)-10(9) M(-1) s(-1). As a result, it is often difficult to identify the actual formation mechanism of these complexes. In this work, using UV-vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper(II) CuCl4(2-) chloro complexes from copper(II) CuCl3(-) trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl4(2-) upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 10(7) and (5.3 ± 0.4) × 10(8) M(-1) s(-1) in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where [CuCl3](-) reacts with the solvent to form a [CuCl3CH3CN](-) complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a [CuCl4CH3CN](2-) intermediate. In both solvents, the recombination reaction is potential energy controlled.

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study.

This work is a mechanistic study of total and partial methanol photooxidation using operando FTIR coupled to gas phase analysis techniques (gas-IR and MS). Methoxy and formate/formyl species play a key role in the reaction. Methoxy species are formed by thermal and photochemical dissociation of methanol. The formation of methylformate is favored by a high surface coverage by methoxy species. Surface and/or bridged oxygen atoms are also important actors. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments showed that the limiting step is the conversion of chemisorbed formyl/formate and that methylformate is a secondary product from a reaction between methoxy and neighboring formyl species. Methanol concentration, among other reaction parameters, influences greatly the selectivity of photooxidation.