Dissociative Ionization Research Articles

Vibrational spectroscopy and structural characterization of neutral and cationic hydroxyacetone

In this study, we utilized the infrared (IR)–vacuum-ultraviolet (VUV) non-resonant ionization and fragmentation detected IR spectroscopy (NRIFD-IR) technique to explore the IR spectra (2700–7200 cm-1) and the structures of both neutral and cationic hydroxyacetone. By comparing the experimentally obtained IR spectra with the anharmonic frequencies calculated at the B2PLYP-D3/cc-pVTZ and B3LYP-D3(BJ)/def2-TZVPP level of theory, we assigned the structures of hydroxyacetone and its cation. Notably, we observed a discrepancy in the neutral spectrum within the CH stretch region, depending on whether the parent or fragment channel was monitored. This observation suggests that the near-threshold dissociative ionization process exhibits mode-selectivity, implying that certain vibrational modes play a crucial role in either facilitating or inhibiting the dissociation event.

Electrolyte Formulation with Improved Ion Desolvation and Diffusion Kinetics, and Superior Anti-Corrosion Properties for Ultrawide-Temperature Supercapacitors (–70∼100°C)

Electrochemical energy storage (EES) devices operating over a wide temperature range are crucial for extreme applications like near-space exploration (–70 °C) and desert exploitation (80 °C). However, their electrochemical performance is still hindered by the high-energy-barrier desolvation process at low temperature and the parasitic redox corrosion reactions at high temperature. In this work, an ultrawide-operating-temperature-window supercapacitor is proposed by simultaneously regulating ion kinetics of electrolyte and modulating anti-corrosion property of current collector. The optimized kinetics at low temperatures (–70 °C) refer to the enhanced ion dissociation and accelerated desolvation process, which is precisely regulated by the medium dielectric constant and low donor number of both methyl propyl ketone (MPK) and methyl ethyl ketone (MEK) cosolvents. Meanwhile, the remarkable cycling stability at high temperatures (100 °C) is realized by the strengthened corrosion resistance of carbon cloth current collector. The combination of MEK/MPK based electrolyte and carbon cloth current collector enables the supercapacitor to achieve a superior low-temperature capacitance retention (91.13 % @ –60 °C and 75.4 % @ –70 °C), extended high-temperature cycling stability (69.2 % @ 100 °C, 10,000 cycles) and excellent power and energy densities (13,820 W kg–1 @ 20.68 W h kg–1). This research is expected to provide valuable references for the advanced design of EES devices with wide-operating-temperature range in future studies.

The Orbital-Resolved Dissociative Ionization of the Molecular IBr in a Near-Infrared Femtosecond Laser Field

The Orbital-Resolved Dissociative Ionization of the Molecular IBr in a Near-Infrared Femtosecond Laser Field

Improved model for the dissociative sequential double ionization of nitrogen molecules by an intense few-cycle infrared laser pulse

Improved model for the dissociative sequential double ionization of nitrogen molecules by an intense few-cycle infrared laser pulse

Metal-organic frameworks Derived frustrated Lewis acid-base pairs accelerate ion dynamics to dendrite-free solid-state lithium batteries

The Poly(ethylene oxide) (PEO)-based polymer electrolytes exhibit the drawbacks of high crystallinity and low ionic conductivity (<10−5 S m−1), which significantly restrict their application in solid-state lithium metal batteries. This study presents the bismuth metal-organic frameworks (MOFs) as a PEO-based electrolyte filler, incorporating frustrated Lewis acid-base pairs, which is formed in-situ through the coordination of ellagic acid (EA) and bismuth ions (Bi3+). The simultaneous presence of Lewis acids (unsaturated Bi3+) and bases (free nitrate, NO3−) in the MOFs, which effectively transfers and transports the lithium ions through the synergistic interaction based on the polymer matrix and the lithium salts, resulting in the high durable solid-state lithium batteries. Meanwhile, NO3− can further promote the dissociation of lithium ion (Li+) and the formation of inorganic solid electrolyte interfaces (SEIs) with lithium metal, which is conducive to the dynamic repair of SEIs. Characterizations and calculations show that Bi-MOFs promote the increase of composite ionic conductivity (6.4 × 10−4 S cm−1, 60 °C) and lithium transference number (0.52, 60 °C). At a current density of 0.4 mA cm−2, the assembled Li–Li symmetric battery can be continuously cycled for more than 500 h. This strategy opens a new window for improvement of solid-state electrolyte performance.

Performance evaluation of a low power Hall thruster with carbon dioxide propellant

Carbon dioxide (CO2) is an attractive candidate for condensable propellants because its availability and storability. Although several studies have been reported on adopting CO2 as a propellant for Hall thrusters, there have been few studies on CO2 standalone operation, and none of them have focused on physical phenomena in plasmas. This study experimentally evaluated, thrust performance and plume characteristics of a 100 W-class Hall thruster under CO2 operation. CO2 operation was compared to xenon operation under the same discharge power conditions to determine its unique properties. As typical thrust performance under this experimental condition, a specific impulse of 727 s, thrust-power-ratio of 19.8 mN/kW, and anode efficiency of 0.0711 were achieved at a discharge voltage of 200 V and a discharge power of 352 W under a background pressure of 3.4 × 10−2 Pa. Plume measurements showed that mass utilization efficiency was 0.228 under CO2 operation, which was particularly poor compared to under xenon operation. The first reason for this is that the ionization mean free path of CO2 is 5.18 mm, about eight times longer than that of xenon. The second reason is that the energy is used for other processes, such as dissociative ionization and dissociation etc., considering the reaction process of the particles. Therefore, the results suggest narrowing the channel width to increase the channel density, and increasing the magnetic field strength to suppress the discharge current as ways to improve thrust efficiency.

Deciphering the enhanced oxygen reduction reaction activity of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ via constructing negative thermal expansion offset for high-performance solid oxide fuel cell

Solid oxide fuel cells (SOFCs) are one of the most efficient energy conversion devices. However, the sluggish oxygen reduction reaction (ORR) kinetics at low temperatures significantly challenge the performance and commercialization of SOFCs. Introducing negative expansion coefficient materials has been recognized as an effective approach to enhancing the ORR catalytic properties, but a clear understanding of this enhanced electrochemical performance is still lacking. In this work, the composite cathode of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) with different amounts of negative-thermal-expansion material Sm0.85Zn0.15MnO3 (SZM) is prepared, and in-depth analysis the effect of SZM on the catalytic activity of cathodic ORR is systematically investigated. Simultaneously, the mechanistic studies verify that the enhanced ORR activity might be attributed to the constructed compression strain during sintering, which significantly improves the adsorption, dissociation, and oxygen ion exchange process of the PBSCF cathode.

Determination of product ions emerged from dications by varying retarding electric field in reflectron time-of-flight mass spectrometer supported by ion trajectory simulations

Investigation of the metastable dissociations clarifies the characteristics of ionic states. Tandem mass spectrometry using an ion trap mass spectrometer, a time-of-flight mass spectrometer (TOF-MS), and the combination of those spectrometers were used for the analysis of metastable dissociation. However, investigation of the metastable dissociations of multiply charged ions is not straightforward because collision-induced charge transfer reactions to lose charges should be avoided. TOF-MS equipped with a reflectron (refTOF-MS) maintained under high vacuum condition is one of the suitable instruments. However, another difficulty arises due to the working principle of refTOF-MS: The product ions whose m/z is greater than that of multiply charged precursor ions can pass through a reflectron under the experimental conditions for detecting the product ions of a singly charged precursor ion. In this study, we report the ionization of decafluorobiphenyl (DFB) by femtosecond laser pulses and the determination of the product ions emerged from doubly charged precursor ions using a refTOF-MS. Detection of ions behind a reflectron by varying the retarding potential of a reflectron enables us to identify the m/z of product ions in two ways: measurement of the threshold retarding potential reflecting product ion; comparing the relative flight time of the product ions obtained by experiments and ion trajectory simulations. We have reported three and one metastable dissociation channels of DFB+ and DFB2+, respectively, by a conventional product ion analysis, that is the selection of a precursor ion and its product ions using an ion gate followed by the reflection and separation of them by a reflectron. In this study, we further identified the products that passed through a reflectron: charge transfer product of C2+; the product ion of C4F22+ which is a secondary product of DFB ion; the product ion of DFB2+. The detection of the product ions that passed through a reflectron expanded the measurable m/z range of product ions emerged from doubly charged precursor ions.

A Solvent-Free Covalent Organic Framework Single-Ion Conductor Based on Ion–Dipole Interaction for All-Solid-State Lithium Organic Batteries

HighlightsA class of solvent-free covalent organic framework (COF) single-ion conductors (Li-COF@P) has been designed via ion–dipole interaction as opposed to traditional ion–ion interaction, promoting ion dissociation and Li+ migration through directional ionic channels.The Li-COF@P enabled long cycle life (88.3% after 2000 cycles) in all-solid-state Li organic batteries (ASSLOBs) under ambient operating conditions, which outperformed those of previously reported ASSOLBs.This Li-COF@P strategy holds promise as a viable alternative to the currently prevalent inorganic solid electrolytes.

Characterization of a Monoclonal Antibody by Native and Denaturing Top-Down Mass Spectrometry.

Established in recent years as an important approach to unraveling the heterogeneity of intact monoclonal antibodies, native mass spectrometry has been rarely utilized for sequencing these complex biomolecules via tandem mass spectrometry. Typically, top-down mass spectrometry has been performed starting from highly charged precursor ions obtained via electrospray ionization under denaturing conditions (i.e., in the presence of organic solvents and acidic pH). Here we systematically benchmark four distinct ion dissociation methods─namely, higher-energy collisional dissociation, electron transfer dissociation, electron transfer dissociation/higher-energy collisional dissociation, and 213 nm ultraviolet photodissociation─in their capability to characterize a therapeutic monoclonal antibody, trastuzumab, starting from denatured and native-like precursor ions. Interestingly, native top-down mass spectrometry results in higher sequence coverage than the experiments carried out under denaturing conditions, with the exception of ultraviolet photodissociation. Globally, electron transfer dissociation followed by collision-based activation of product ions generates the largest number of backbone cleavages in disulfide protected regions, including the complementarity determining regions, regardless of electrospray ionization conditions. Overall, these findings suggest that native mass spectrometry can certainly be used for the gas-phase sequencing of whole monoclonal antibodies, although the dissociation of denatured precursor ions still returns a few backbone cleavages not identified in native experiments. Finally, a comparison of the fragmentation maps obtained under denaturing and native conditions strongly points toward disulfide bonds as the primary reason behind the largely overlapping dissociation patterns.

Boosting Lithium Ion Conduction of Carboxylate‐Type Single‐Ion Conducting Polymers through Complexation with Ionic Liquids for Lithium‐Ion Batteries

AbstractSingle‐ion conducting (SIC) polymers, wherein anions are covalently bonded to the polymer chain, selectively facilitate the movement of lithium ions, making them promising polymer electrolytes for lithium‐ion batteries because only lithium ion acts as a charge carrier. Carboxylate‐type SIC polymers offer cost advantages and are facilely prepared compared to conventional types like sulfonylamide‐type SIC polymers. However, they are known to suffer from inefficient dissociation of lithium ions due to their high basicity. In this study, enhancement of lithium‐ion dissociation in carboxylate‐type SIC polymers is investigated by complexing them with ionic liquid (IL). Incorporation of IL with the SIC polymer significantly improves electrochemical properties, achieving a high ionic conductivity close to 2 mS cm−1 and a critical current density of 2.1 mA cm−2 in a Li symmetric cell at 25 °C. The interaction between the SIC polymer and IL is confirmed through infrared and Raman spectroscopy. Electrochemical tests of the Li/LiFePO4 cell exhibit a high reversible capacity of 128.6 mAh g−1 with a cycle retention of 89.9 % after 100 cycles at 1 C (1 C=170 mA g−1) and an excellent recuperation even after rate tests at 6 C.

Utilization of Matrix Effect for Enhancing Resolution in Cation Exchange Chromatography.

In ion chromatography studies, the matrix effect of other inorganic ions present in the sample is a well-known phenomenon. In this work, the behavior of inorganic and organic ions was studied in a system overloaded with ammonium ions. The ammonium ions came from a solution of ammonium hydroxide in various concentrations (0.25-1.25%). In this system, which was significantly overloaded with ammonium ions, the behavior of three ions were tested (lithium, tris, and sodium cations). The measurements were performed at different eluent concentrations (6-17 mM), chromatographic column temperatures (25-40 °C), and injected volumes (15-40 µL). The retention times of sodium and lithium ions increased with increasing amounts of injected ammonium, while tris remained essentially unchanged, indicating that the resolution of these ions can be influenced by varying the concentration of the matrix. The results suggested that the observed effect was due to a combination of the pH change caused by the injected matrix, the dissociation of tris ions, the dissociation of the carbocylic ion-exchange groups of stationary phase, the change in buffer capacity, and the amount of ammonium ion introduced. It has been shown that in a well-designed experiment, the addition of ammonium hydroxide to the sample at concentrations greater than 1% can improve the efficiency of organic and inorganic cation separation. It was found that 8 mM methanesulfonic acid eluent, 30 °C, 1% ammonium hydroxide matrix concentration, and 25 µL injection were optimal for the baseline separation of tris and sodium ions on the high-capacity Dionex CS16 column. These ions could not be separated on this column without the presence of the ammonium matrix.

Dissociative Ionization of the CHBr2Cl Molecule in 800 nm and 400 nm Femtosecond Laser Fields

The dissociative ionization of CHBr2Cl molecules in femtosecond laser fields at 800 nm and 400 nm is investigated to enhance the comprehension of ultrafast dynamics phenomena. The kinetic energy distribution of the resulting ions following photo-dissociation is analyzed using time-of-flight mass spectrometry in combination with DC-sliced ion velocity map imaging. The findings from the experimental study indicate that the presence of low kinetic energy components is attributed to the dissociative ionization processes of CHBr2Cl molecules. The complexity of individual dissociation pathways remains unaffected by the laser fields but is determined by factors such as bond energy, ionization energy of neutral groups, and charge distribution. In the case of 400 nm laser fields, distinct elimination channels enable CHBr2Cl+ ions to circumvent the transition state, leading to the formation of BrCl+ and Br2+ fragments.

Deciphering and Integrating Functionalized Side Chains for High Ion‐Conductive Elastic Ternary Copolymer Solid‐State Electrolytes for Safe Lithium Metal Batteries

AbstractA critical challenge in solid polymer lithium batteries is developing a polymer matrix that can harmonize ionic transportation, electrochemical stability, and mechanical durability. We introduce a novel polymer matrix design by deciphering the structure‐function relationships of polymer side chains. Leveraging the molecular orbital‐polarity‐spatial freedom design strategy, a high ion‐conductive hyperelastic ternary copolymer electrolyte (CPE) is synthesized, incorporating three functionalized side chains of poly‐2,2,2‐Trifluoroethyl acrylate (PTFEA), poly(vinylene carbonate) (PVC), and polyethylene glycol monomethyl ether acrylate (PEGMEA). It is revealed that fluorine‐rich side chain (PTFEA) contributes to improved stability and interfacial compatibility; the highly polar side chain (PVC) facilitates the efficient dissociation and migration of ions; the flexible side chain (PEGMEA) with high spatial freedom promotes segmental motion and interchain ion exchanges. The resulting CPE demonstrates an ionic conductivity of 2.19×10−3 S cm−1 (30 °C), oxidation resistance voltage of 4.97 V, excellent elasticity (2700 %), and non‐flammability. The outer elastic CPE and the inner organic–inorganic hybrid SEI buffer intense volume fluctuation and enable uniform Li+ deposition. As a result, symmetric Li cells realize a high CCD of 2.55 mA cm−2 and the CPE‐based Li||NCM811 full cell exhibits a high‐capacity retention (~90 %, 0.5 C) after 200 cycles.

Deciphering and Integrating Functionalized Side Chains for High Ion-Conductive Elastic Ternary Copolymer Solid-State Electrolytes for Safe Lithium Metal Batteries.

A critical challenge in solid polymer lithium batteries is developing a polymer matrix that can harmonize ionic transportation, electrochemical stability, and mechanical durability. We introduce a novel polymer matrix design by deciphering the structure-function relationships of polymer side chains. Leveraging the molecular orbital-polarity-spatial freedom design strategy, a high ion-conductive hyperelastic ternary copolymer electrolyte (CPE) is synthesized, incorporating three functionalized side chains of poly-2,2,2-Trifluoroethyl acrylate (PTFEA), poly(vinylene carbonate) (PVC), and polyethylene glycol monomethyl ether acrylate (PEGMEA). It is revealed that fluorine-rich side chain (PTFEA) contributes to improved stability and interfacial compatibility; the highly polar side chain (PVC) facilitates the efficient dissociation and migration of ions; the flexible side chain (PEGMEA) with high spatial freedom promotes segmental motion and interchain ion exchanges. The resulting CPE demonstrates an ionic conductivity of 2.19×10-3 S cm-1 (30 °C), oxidation resistance voltage of 4.97 V, excellent elasticity (2700 %), and non-flammability. The outer elastic CPE and the inner organic-inorganic hybrid SEI buffer intense volume fluctuation and enable uniform Li+ deposition. As a result, symmetric Li cells realize a high CCD of 2.55 mA cm-2 and the CPE-based Li||NCM811 full cell exhibits a high-capacity retention (~90 %, 0.5 C) after 200 cycles.

Anchored Weakly‐Solvated Electrolytes for High‐Voltage and Low‐Temperature Lithium‐ion Batteries

AbstractElectrolytes endowed with high oxidation/reduction interfacial stability, fast Li‐ion desolvation process and decent ionic conductivity over wide temperature region are known critical for low temperature and fast‐charging performance of energy‐dense batteries, yet these characteristics are rarely satisfied simultaneously. Here, we report anchored weakly‐solvated electrolytes (AWSEs), that are designed by extending the chain length of polyoxymethylene ether electrolyte solvent, can achieve the above merits at moderate salt concentrations. The −O−CH2−O− segment in solvent enables the weak four‐membered ring Li+ coordination structure and the increased number of segments can anchor the solvent by Li+ without largely sacrificing the ionic dissociation ability. Therefore, the single salt/single solvent AWSEs enable solvent co‐intercalation‐free behavior towards graphite (Gr) anode and high oxidation stability towards high‐nickel cathode (LiNi0.8Co0.1Mn0.1O2‐NCM811), as well as the formation of inorganic rich electrode/electrolyte interphase on both of them due to the anion‐rich solvation shells. The capacity retention of Gr||NCM811 Ah‐class pouch cell can reach 70.85 % for 1000 cycles at room‐temperature and 75.86 % for 400 cycles at −20 °C. This work points out a promising path toward the molecular design of electrolyte solvents for high‐energy/power battery systems that are adaptive for extreme conditions.

Anchored Weakly-Solvated Electrolytes for High-Voltage and Low-Temperature Lithium-ion Batteries.

Electrolytes endowed with high oxidation/reduction interfacial stability, fast Li-ion desolvation process and decent ionic conductivity over wide temperature region are known critical for low temperature and fast-charging performance of energy-dense batteries, yet these characteristics are rarely satisfied simultaneously. Here, we report anchored weakly-solvated electrolytes (AWSEs), that are designed by extending the chain length of polyoxymethylene ether electrolyte solvent, can achieve the above merits at moderate salt concentrations. The -O-CH2-O- segment in solvent enables the weak four-membered ring Li+ coordination structure and the increased number of segments can anchor the solvent by Li+ without largely sacrificing the ionic dissociation ability. Therefore, the single salt/single solvent AWSEs enable solvent co-intercalation-free behavior towards graphite (Gr) anode and high oxidation stability towards high-nickel cathode (LiNi0.8Co0.1Mn0.1O2-NCM811), as well as the formation of inorganic rich electrode/electrolyte interphase on both of them due to the anion-rich solvation shells. The capacity retention of Gr||NCM811 Ah-class pouch cell can reach 70.85 % for 1000 cycles at room-temperature and 75.86 % for 400 cycles at -20 °C. This work points out a promising path toward the molecular design of electrolyte solvents for high-energy/power battery systems that are adaptive for extreme conditions.

Computer-aided drug design supporting sunscreen research: a showcase study using previously synthesized hybrid UV filter-antioxidant compounds.

Although quantum mechanical calculations have proven effective in accurately predicting UV absorption and assessing the antioxidant potential of compounds, the utilization of computer-aided drug design (CADD) to support sustainable synthesis research of new sunscreen active ingredients remains an area with limited exploration. Furthermore, there are ongoing concerns about the safety and effectiveness of existing sunscreens. Therefore, it remains crucial to investigate photoprotection mechanisms and develop enhanced strategies for mitigating the harmful effects of UVR exposure, improving both the safety and efficacy of sunscreen products. A previous study conducted synthesis research on eight novel hybrid compounds (I-VIII) for use in sunscreen products by molecular hybridization of trans-resveratrol (RESV), avobenzone (AVO), and octinoxate (OMC). Herein, time-dependent density functional theory (TD-DFT) calculations performed in the gas phase on the isolated hybrid compounds (I-VIII) proved to reproduce the experimental UV absorption. Resveratrol-avobenzone structure-based hybrids (I-IV) present absorption maxima in the UVB range with slight differences between them, while resveratrol-OMC structure-based hybrids (V-VIII) showed main absorption in the UVA range. Among RESV-OMC hybrids, compounds V and VI exhibited higher UV absorption intensity, and compound VIII stood out for its broad-spectrum coverage in our simulations. Furthermore, both in silico and in vitro analyses revealed that compounds VII and VIII exhibited the highest antioxidant activity, with compound I emerging as the most reactive antioxidant within RESV-AVO hybrids. The study suggests a preference for the hydrogen atom transfer (HAT) mechanism over single-electron transfer followed by proton transfer (SET-PT) in the gas phase. With a strong focus on sustainability, this approach reduces costs and minimizes effluent production in synthesis research, promoting the eco-friendly development of new sunscreen active ingredients. The SPARTAN'20 program was utilized for the geometry optimization and energy calculations of all compounds. Conformer distribution analysis was performed using the Merck molecular force field 94 (MMFF94), and geometry optimization was carried out using the parametric method 6 (PM6) followed by density functional theory (DFT/B3LYP/6-31G(d)). The antioxidant behavior of the hybrid compounds (I-VIII) was determined using the highest occupied molecular orbital (εHOMO) and the lowest unoccupied molecular orbital (εLUMO) energies, as well as the bond dissociation enthalpy (BDE), ionization potential (IP), and proton dissociation enthalpy (PDE) values, all calculated at the same level of structural optimization. TD-DFT study is carried out to calculate the excitation energy using the B3LYP functional with the 6-31G(d) basis set. The calculated transitions were convoluted with a Gaussian profile using the Gabedit program.

Investigations of atomic and molecular processes of NBI-heated discharges in the MAST Upgrade Super-X divertor with implications for reactors

This experimental study presents an in-depth investigation of the performance of the MAST-U Super-X divertor during NBI-heated operation (up to 2.5 MW) focussing on volumetric ion sources and sinks as well as power losses during detachment. The particle balance and power loss analysis revealed the crucial role of Molecular Activated Recombination and Dissociation (MAR and MAD) ion sinks in divertor particle and power balance, which remain pronounced in the change from ohmic to higher power (NBI heated) L-mode conditions. The importance of MAR and MAD remains with double the absorbed NBI heating. MAD results in significant power dissipation (up to ∼20% of PSOL ), mostly in the cold ( Te<5 eV) detached region. Theoretical and experimental evidence is found for the potential contribution of D− to MAR and MAD, which warrants further study. These results suggest that MAR and MAD can be relevant in higher power conditions than the ohmic conditions studied previously. Post-processing reactor-scale simulations suggests that MAR and MAD can play a significant role in divertor physics and synthetic diagnostic signals of reactor-scale devices, which are currently underestimated in exhaust simulations. This raises implications for the accuracy of reactor-scale divertor simulations of particularly tightly baffled (alternative) divertor configurations.

Electron-induced ligand loss from iron tetracarbonyl methyl acrylate.

We probe the separation of ligands from iron tetracarbonyl methyl acrylate (Fe(CO)4(C4H6O2) or Fe(CO)4MA) induced by the interaction with free electrons. The motivation comes from the possible use of this molecule as a nanofabrication precursor and from the corresponding need to understand its elementary reactions fundamental to the electron-induced deposition. We utilize two complementary electron collision setups and support the interpretation of data by quantum chemical calculations. This way, both the dissociative ionization and dissociative electron attachment fragmentation channels are characterized. Considerable differences in the degree of precursor fragmentation in these two channels are observed. Interesting differences also appear when this precursor is compared to structurally similar iron pentacarbonyl. The present findings shed light on the recent electron-induced chemistry of Fe(CO)4MA on a surface under ultrahigh vacuum.