Gas-phase Absorption Research Articles

Under the magnifying glass: A combined 3D model applied to cloudy warm Saturn-type exoplanets around M dwarfs

Warm Saturn-type exoplanets orbiting M dwarfs are particularly suitable for an in-depth cloud characterisation through transmission spectroscopy because the contrast of their stellar to planetary radius is favourable. The global temperatures of warm Saturns suggest efficient cloud formation in their atmospheres which in return affects the temperature, velocity, and chemical structure. However, a consistent modelling of the formation processes of cloud particles within the 3D atmosphere remains computationally challenging. We explore the combined atmospheric and micro-physical cloud structure and the kinetic gas-phase chemistry of warm Saturn-like exoplanets in the irradiation field of an M dwarf. The combined modelling approach supports the interpretation of observational data from current (e.g. JWST and CHEOPS) and future missions (PLATO, Ariel, and HWO). A combined 3D cloudy atmosphere model for HATS-6b was constructed by iteratively executing the 3D general circulation model (GCM) expeRT/MITgcm and a detailed kinetic cloud formation model, each in its full complexity. The resulting cloud particle number densities, particle sizes, and material compositions were used to derive the local cloud opacity which was then used in the next GCM iteration. The disequilibrium H/C/O/N gas-phase chemistry was calculated for each iteration to assess the resulting transmission spectrum in post-processing. We present the first model atmosphere that iteratively combines cloud formation and 3D GCM simulation and applied it to the warm Saturn HATS-6b. The cloud opacity feedback causes a temperature inversion at the sub-stellar point and at the evening terminator at gas pressures higher than 10$^ $ bar. Furthermore, clouds cool the atmosphere between $10^ $ bar and 10 bar, and they narrow the equatorial wind jet. The transmission spectrum shows muted gas-phase absorption and a cloud particle silicate feature at $ The combined atmosphere-cloud model retains the full physical complexity of each component and therefore enables a detailed physical interpretation with JWST NIRSpec and MIRI LRS observational accuracy. The model shows that warm Saturn-type exoplanets around M dwarfs are ideal candidates for a search for limb asymmetries in clouds and chemistry, for identifying the cloud particle composition by observing their spectral features, and for identifying in particular the cloud-induced strong thermal inversion that arises on these planets.

Prediction of Vacuum Ultraviolet/Ultraviolet Gas-Phase Absorption Spectra Using Molecular Feature Representations and Machine Learning.

Ultraviolet (UV) absorption spectroscopy is a widely used tool for quantitative and qualitative analyses of chemical compounds. In the gas phase, vacuum UV (VUV) and UV absorption spectra are specific and diagnostic for many small molecules. An accurate prediction of VUV/UV absorption spectra can aid the characterization of new or unknown molecules in areas such as fuels, forensics, and pharmaceutical research. An alternative to quantum chemical spectral prediction is the use of artificial intelligence. Here, different molecular feature representation techniques were used and developed to encode chemical structures for testing three machine learning models to predict gas-phase VUV/UV absorption spectra. Structure data files (.sdf) and VUV/UV absorption spectra for 1397 volatile and semivolatile chemical compounds were used to train and test the models. New molecular features (termed ABOCH) were introduced to better capture pi-bonding, aromaticity, and halogenation. The incorporation of these new features benefited spectral prediction and demonstrated superior performance compared to computationally intensive molecular-based deep learning methods. Of the machine learning methods, the use of a Random Forest regressor returned the best accuracy score with the shortest training time. The developed machine learning prediction model also outperformed spectral predictions based on the time-dependent density functional theory.

The role of solute polarizability and microenvironment in the spectroscopic behaviour of the local anesthetic drug - bupivacaine

The solvent dependent ground- and excited-state photophysical and photochemical properties of the local anesthetic drug – bupivacaine (BUP) – have been studied using a steady-state spectroscopic technique. The Kamlet-Taft solvent scale was used to describe the solvatochromism of BUP in terms of non-specific (dipole-dipole) and specific (hydrogen bonding) solute-solvent interactions. The performed analysis indicates that the major factor responsible for the shift of the absorption and fluorescence bands is solvent polarity, while the solvent acidity and basicity have a less significant effect. Finally, it has also been shown how the solvatochromic equations can be used, in principle, to obtain estimates for the ground- and excited-state (Franck-Condon and relaxed) dipole moments, the difference thereof and the polarizability of BUP from a simple analysis of spectral shift data (solvatochromic methods of Lippert and Mataga, McRae, Bakhshiev and Mataga and Kubota) and multiple linear regression analysis (model of solvatochromism developed by Bayliss, McRae and Ooshika). In addition, an innovative physical treatment was used, consisting of a non-linear fit of the absorption and emission spectral shift data using the theoretical model of solvatochromism developed by Bilot and Kawski and visualisation of the least squares coefficient (χ2) on a 3D map as a function of the solute polarizability and the gas phase absorption or fluorescence energies. Since the solute polarizability can affect both the electronic characteristics and the reactivity of the molecule, our investigations may have significant implications for the development of the pharmaceutical industry (understanding of the influence of the environment on intermolecular processes occurring in the drug-medium system).

Exploring how molluscan purple has survived throughout the Ages: The excited state dynamics of 6,6′-dibromoindigotin

The color purple has long been used as a symbol of royalty and power in art, fashion, and architecture. This correlation with aristocracy can be traced back to the use of molluscan purple, a rich purple dye that is still presently more expensive than gold, by royalty as early as the 18th century BCE. While the molluscan purple dye is composed of a mixture of various proteins, its color is derived from indigotin, indirubin and their brominated derivatives, including the molecule, 6,6′-dibromoindigotin, an analogue of the indigotin dye which is also widely used in various modern industries. Both dyes can exhibit high stability, in particular against photodamage from UV and Visible radiation. In this study, we present the gas phase absorption spectrum and excited-state lifetimes of 6,6′-dibromoindigotin combined with static calculations of the excited and ground-state potential energy surfaces. The lifetime measurements reveal that molluscan purple has nearly the same relaxation rate as indigotin providing new insights in the possible relaxation mechanisms of the indigotin family of dye molecules.

Spectroscopy of helium-tagged C60 anions

We present measurements of the electronic absorption spectrum of He-tagged buckminsterfullerene anions, C60−. Using a technique that allows for the efficient tagging of complex molecular anions with He, we achieve atomically resolved action spectra that provide an accurate determination of the gas-phase absorption spectrum in the near-infrared regime. In total we identify 27 absorption bands between 895 and 1057 nm. Published by the American Physical Society 2024

Computational investigation of molecular structure, spectral analysis, PES study and molecular docking studies of 4-(butan-2-ylideneamino) benzenesulfonamide

The compound 4-(butan-2-ylideneamino)benzenesulfonamide (EMKNI) was subjected to spectroscopic investigations, both experimental and theoretical in nature. The research study was conducted at two distinct levels. The initial stage involved the acquisition of experimental data, including FT-IR, NMR (1H, 13C), absorption (UV) and fluorescence spectral data. The subsequent stage involved the execution of theoretical calculations (UV) at the TD-DFT/WB97XD/cc-pVDZ basis set. The vibrational analysis was done for each vibrational modes using potential energy vibrational analysis (PED). A comparison was made between the theoretical NMR and the corresponding experimental data. In NLO study the water solvent showed highest NLO property, due to solvent effect. The emission study was done and showed two different wavelengths. A TD-DFT was made between water, DMSO and gas phase and experimental absorption wavelengths (λ) made in DMSO solvent. Furthermore, the FMO and MEP study were made using the different solvents such as water, DMSO and gas phase. The topological analysis studies were done such as ELF, LOL and RDG. The docking study was done and identify the highest binding energy score.

Applications of time‐resolved step‐scan Fourier‐transform infrared spectroscopy in revealing the light‐initiated reactions in condensed phases

AbstractStep‐scan Fourier‐transform infrared spectroscopy (ssFTIR) simultaneously provides the spectroscopic and kinetic information of a given reaction. ssFTIR has been extensively employed to acquire the transient absorption and emission spectra in gas phase for identifying unstable species, for example, various Criegee intermediates, and elucidating the dynamics and kinetics of the reaction, such as the molecular elimination dynamics of haloalkenes and the bimolecular reactions involving chlorine atoms and singlet oxygen atoms. In addition to gaseous studies, ssFTIR has been also utilized to record the time‐resolved difference spectra of the photochemical reactions in condensed phases, such as the photolysis of metal–ligand complexes, photocycles of the retinal proteins, coordination capability of solvents to unstable transient species, chemical reactions of atmosphere‐related molecules in aqua, and the exciplex dynamics of organic light emitting materials. Moreover, my group has pioneered the recording of the transient thermal infrared emission of gold nanostructures upon photoexcitation. The experimental setups and the working principles for probing the time‐resolved infrared absorption and emission in condensed phases will be revealed and a number of studies on chemical, biological, and materials systems will be described. These reported results demonstrate that ssFTIR is a versatile tool for exploring the properties of novel materials and photoreactions in condensed phases.

Cryogenic Absorption and Emission Spectroscopy of the Oxyluciferin Anion In Vacuo.

Bioluminescence from fireflies, click beetles, and railroad worms ranges in color from green-yellow to orange to red. The keto form of oxyluciferin is considered a key emitter species in the proposed mechanisms to account for color variation. To establish the intrinsic photophysics in the absence of a microenvironment, we present experimental and theoretical gas-phase absorption and emission spectra of the 5,5-dimethyloxyluciferin anion (keto form) at room and cryogenic temperatures as well as lifetime measurements based on fluorescence. The theoretical model includes all 75 vibrational modes. The spectral impact of the large number of excited states at elevated temperatures is captured by an effective state distribution. At low temperature, spectral congestion is greatly reduced, and the observed well-resolved vibrational features are assigned to multiple Franck-Condon progressions involving different vibrational modes. An in-plane ∼60 cm-1 scissoring mode is found to be involved in the dominant progressions.

Vibrationally resolved electronic spectroscopy of rhodamine B cation at less than 5 K: combining gas phase absorption and matrix isolation fluorescence spectroscopy

Helium tagging photodissociation action spectroscopy in the gas phase together with laser-induced fluorescence (LIF) spectroscopy in neon matrix has been used to probe the photophysics of electrosprayed rhodamine B cation cooled to < 5 K and studied in two different minimally perturbing environments ([RhB•He]+ as well as [RhB]+ in solid neon). Helium tagging experiments were performed using a newly developed apparatus He-TAG. LIF measurements were carried out using an improved matrix-isolation spectroscopy machine Depo-II. The resulting, partially vibrationally resolved electronic spectra of the S0→S1, resp. S1→S0 transition provide benchmarks for gauging the theoretical description of this prototypical organic dye. As a first approach, we have compared our spectra to TD-DFT-based predictions of vibronic structure at the Franck-Condon (FC) and Franck-Condon Herzberg-Teller (FCHT) levels. Depletion action and fluorescence spectroscopy of rhodamine B cation at < 5 K

Structural Sampling and Solvation Models for the Simulation of Electronic Spectra: Pyrazine as a Case Study

The impact of sampling methods on spectral broadening in the gas phase and on the convergence of spectra in aqueous solution when using microsolvation, continuum solvation, and hybrid models is studied using pyrazine as a test case. For the sake of comparing classical Maxwell-Boltzmann and Wigner samplings in the gas phase, static and time-resolved X-ray absorption spectra after photoexcitation to the lowest 1B2u(ππ*) state, as well as the static UV-vis absorption spectrum, are considered. In addition, the UV-vis absorption spectrum of pyrazine in aqueous solution is also computed in order to systematically investigate its convergence with the number of explicitly included solvent shells with and without taking bulk solvation effects into account with the conductor-like screening model to represent implicit water beyond such explicit solute complexes. Concerning the static and time-resolved X-ray absorption spectra of pyrazine at the carbon K-edge as well as its UV-vis absorption spectrum in the gas phase, we find that these spectra obtained with Wigner and Maxwell-Boltzmann samplings substantially agree. For the UV-vis absorption spectrum in the aqueous solution, only the first two energetically low-lying bands converge quickly with the size of the explicitly included solvation shells, either with or without an additional continuum solvation taken into account. In stark contrast, calculations of the higher-lying excitations relying on finite microsolvated clusters without additional continuum solvation severely suffer from unphysical charge-transfer excitations into Rydberg-like orbitals at the cluster/vacuum interface. This finding indicates that computational UV-vis absorption spectra covering sufficiently high-lying states converge only if continuum solvation of the explicitly microsolvated solutes is included in the models.

Efficient implementation of time-dependent auxiliary density functional theory.

The random phase approximation of time-dependent auxiliary density functional theory (TDADFT) is rederived from auxiliary density perturbation theory. Our exhaustive validation of TDADFT reveals an upshift of the excitation energies by ∼0.1eV with respect to standard time-dependent density functional theory. For the computationally efficient implementation of TDADFT, floating point operation optimized three-center electron repulsion integral recurrence relations and their double asymptotic expansions are implemented into the Davidson solver. The computational efficiency of TDADFT is benchmarked with four sets of molecules comprising alkanes, fullerenes, DNA fragments, and zeolites. The results show that TDADFT has a computational scaling between 1.3 and 1.9 with respect to the number of basis functions, which is lower than the scaling of standard time-dependent density functional theory. Due to its computational simplifications, TDADFT is particularly well suited for Born-Oppenheimer molecular dynamics simulations. As illustrative examples, we present the temperature effects on the gas-phase absorption spectra of benzene, naphthalene, and anthracene.

Long path gas-phase absorption detector using a 235 nm deep-UV LED source for the determination of nitrite, nitrate and total dissolved nitrogen in waters

Long path gas-phase absorption detector using a 235 nm deep-UV LED source for the determination of nitrite, nitrate and total dissolved nitrogen in waters

Rotationally resolved gas-phase spectrum of the [formula omitted]2Σ+–[formula omitted]2Π3/2 electronic transition for the cyanogen halide radical cation ICN+

A rotationally resolved gas-phase absorption spectrum of the A∼2Σ+–X∼2Π3/2 electronic transition of the cyanogen iodide radical cation ICN+ was observed by cavity ring-down spectroscopy for the first time. This cation was produced in a supersonic planar discharge jet through a mixture of ICN in helium. By the aid of a program for rotational, vibrational and electronic spectra PGOPHER, the rotational constants were determined to be 0.10700(12) and 0.11002(12) cm−1 for the A∼2Σ+ and X∼2Π3/2 electronic states, respectively, and the band origin to be 18262.083(3) cm−1. The rotational constant ratio β {= (B′−B″)/B″} was determined to be −2.8%. The β values for FCN+, ClCN+, BrCN+, and ICN+ were also evaluated theoretically by CAM-B3LYP/CEP-121G using Gaussian 09W. The rotational profile of the absorption band and its temperature dependence for ClCN+, one of the important candidates for Diffuse Interstellar Bands, were simulated, aiding us in the identification of this cation as in interstellar space.

Theoretical analysis of ADP molecule in gas phase and aqueous solution: a DFT and PCM/TD-DFT study

This paper has reported some theoretical results for the geometry optimization in ground state (S0) and the first excited state (S1), the frontier molecular orbitals, the global chemical reactivity descriptor, the excited states, the absorption and emission spectra in gas phase and aqueous solution of protonated Adenosine diphosphate molecule. Also, the excited state intramolecular proton transfer process was reported between O37-H38∙∙∙N13 bonds. Then, the results obtained have been compared with the experimental data reported and, therefore, available in literature. All calculations were carried out using density functional theory and time-dependent density functional theory method with B3LYP functional with 6-311+G (d, p) basis set. As well as, the solvent effects were investigated using Polarizable Continuum Model. As a conclusion, it has been indicated that the theoretical results obtained in this work are all in well agreement with experimental counterparts taken from literature.

Experimental and theoretical gas-phase absorption spectra of thionated uracils

We present a comparative study of the gas-phase UV spectra of uracil and its thionated counterparts (2-thiouracil, 4-thiouracil and 2,4-dithiouracil), closely supported by time-dependent density functional theory calculations to assign the transitions observed. We systematically discuss pure gas-phase spectra for the (thio)uracils in the range of 200–400 nm (∼3.2–6.4 eV), and examine the spectra of all four species with a single theoretical approach. We note that specific vibrational modelling is needed to accurately determine the spectra across the examined wavelength range, and systematically model the transitions that appear at wavelengths shorter than 250 nm. Additionally, we find in the cases of 2-thiouracil and 2,4-dithiouracil, that the gas-phase spectra deviate significantly from some previously published solution-phase spectra, especially those collected in basic environments.

Designing of small organic non-fullerene (NFAs) acceptor molecules with an A−D−A framework for high-performance organic solar cells: A DFT and TD-DFT method

Abstract The main objective of this research was to design non-fullerene acceptors (NFAs) A–D–A framework, using carbazole and benzothiazole derivatives. Density functional theory (DFT) was used to calculate the geometry optimized structures and electronic properties at B3LYP functional with a 6-311G basis set in the gas and solvent phase. The frontier molecular orbitals (FMO), bandgap, open-circuit voltage (VOC) and dipole moments of these developed acceptors have been calculated. The theoretical UV absorption spectra were calculated from time-dependent DFT with the same level of theory used DFT method. They show a suitable bandgap (2.24–2.93 eV) and dipole moment (1.8–10.8 Debye). The maximum wavelength (λmax) for all studied molecules in the range is 665.17–679.97 in both gas and solvent. A slight redshift was observed in all acceptors selected for chlorobenzene compared with gas phase absorption. The NFA A11 has the lowest bandgap energy (2.24 eV), gas-phase excitation energy (1.86 eV) and chlorobenzene excitation energy (1.86 eV). As a result, A11 is predicted to be a good contender for organic NFAs in the future. The open-circuit voltage (VOC) values range from 1.53 to 2.56 eV. Consequently, the optoelectronic, molecular orbital distribution and A11 and A12 molecules were suitable acceptors for NFAs.

Designing small organic non-fullerene acceptor molecules with diflorobenzene or quinoline core and dithiophene donor moiety through density functional theory

The non-fullerene acceptors A1–A5 with diflourobenzene or quinoline core (bridge) unit, donor cyclopenta[1,2-b:3,4-b′]dithiophene unit and 2-(2-methylene-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as acceptor unit with additional phenyl, fulvene or thieno[3,2-d]pyrimidinyl 5-oxide groups have been designed through DFT calculations. The optimization of molecular geometries were performed with density functional theory (DFT) at B3LYP 6-31G (d,p) level of theory. The frontier molecular orbital (FMO) energies, band gap energies and dipole moments (ground and excited state) have been calculated to probe the photovoltaic properties. The band gap (1.42–2.01 eV) and dipole moment values (5.5–18. Debye) showed that these designed acceptors are good candidates for organic solar cells. Time-Dependent Density Functional Theory (TD-DFT) results showed λmax (wave length at maximum absorption) value (611–837 nm), oscillator strength (f) and excitation energies (1.50–2.02 eV) in gas phase and in CHCl3 solvent (1.48–1.89 eV) using integral equation formalism variant (IEFPCM) model. The λmax in CHCl3 showed marginal red shift for all designed acceptors compared with gas phase absorption. The partial density of states (PDOS) has been plotted by using multiwfn which showed that all the designed molecules have more electronic distribution at the donor moiety and lowest at the central bridge. The reorganization energies of electron (λe) (0.0007 eV to 0.017 eV), and the hole reorganization energy values (0.0003 eV to − 0.0403 eV) were smaller which suggested that higher charged motilities. The blends of acceptors A1–A5 with donor polymer D1 provided open circuit voltage (Voc) and ∆HOMO off-set of the HOMO of donor and acceptors. These blends showed 1.04 to 1.5 eV values of Voc and 0 to 0.38 eV ∆HOMO off set values of the donor–acceptor bends which indicate improved performance of the cell. Finally, the blend of D1–A4 was used for the study of distribution of HOMO and LUMO. The HOMO were found distributed on the donor polymer (D1) while the A4 acceptor was found with LUMO distribution. Based on λmax values, and band gap energies (Eg), excitation energies (Ex), reorganization energies; the A3 and A4 will prove good acceptor molecules for the development of organic solar cells.

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}.

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i)an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii)a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i)further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Rotational and vibrational spectroscopy of 1-cyanoadamantane and 1-isocyanoadamantane

Because of their high stability, the presence of diamond-type molecules has long been suspected in the interstellar medium, a hypothesis supported by the extraction of diamond nanocrystal from some meteorites. We report the rotational and vibrational investigation of two polar derivatives of adamantane (C10H16), 1-cyanoadamantane (C10H15–CN) and 1-isocyanoadamantane (C10H15–NC), using room temperature gas phase absorption spectroscopy. Pure rotational spectra have been recorded at millimeter wavelengths (75–220 GHz) while vibrational spectra were obtained in the far- and mid-infrared domains (50–3500 cm−1). Quantum chemical calculations have been performed on these two C3v rotors to support the spectral analysis enabling the assignment, for both species, of more than 7000 pure rotational transitions in the ground (A1 symmetry) and first vibrationally excited (E symmetry) states, and of most of the infrared active bands. The pure rotational lines were fit to their experimental accuracy using a symmetric-top Hamiltonian. Our study provides all necessary information for an active search of these species in space.

Instrumentation for THz Spectroscopy in the Laboratory and in Space

Spectroscopic measurements in the millimeter, submillimeter, or THz range, with resolutions exceeding a MHz, provide for highly specific detections of gas-phase absorption and emission by atoms and molecules. Due to relatively low excitation energies involved in the transitions, multiple features are observable in most physical systems, and thus such observations dominate the scientific discovery of molecules in space and contribute significantly to remote sensing of the Earth and planetary bodies. The methods and techniques of THz spectroscopy continue to evolve as capabilities and technologies expand. In this article, we review the genesis of THz spectroscopy in both the laboratory and in space, and follow its development to date, providing background on the challenges, and context for the current developments that promise to extend both remote and in-situ gas composition sensing.