Band Contours Research Articles

Precise determination of adiabatic ionisation energies of large polyatomic molecules: p-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized to large polyatomic molecules in supersonic beams. The method combines single-photon excitation from the electronic ground state using tunable vacuum-ultraviolet laser radiation, the delayed pulsed-field ionisation of high Rydberg states with sequences of pulsed electric fields, and the modelling of the lineshapes from calculations of the field-ionisation dynamics of high Rydberg states and of the rotational contours of the photoelectron bands. The method is illustrated by a measurement of the adiabatic ionisation energy of p-diaminobenzene (p-DB), for which we obtain the value of 54,617.47(15) cm , which represents a more than 20-fold accuracy improvement over previous determinations.

Gas-phase optical absorption spectra of the indene cation (C9H8 +)

The electronic transition spectra of the indene molecular cation (In, ) in the 550–580 nm region has been measured in the gas phase in direct absorption. The spectra were experimentally recorded using sensitive cavity ring-down spectroscopy in a supersonically expanding planar plasma. Two lifetime-broadened vibronic bands are assigned to the - electronic transition system of In. A set of indicative spectroscopic parameters have been obtained from a band contour analysis of the 0 band in combination with supporting ab-initio calculations. The inferred Lorentz line width of indicates an upper state lifetime of . The laboratory data are compared to the diffuse interstellar bands (DIBs), and no matches can be found. Upper limits for In column densities in the diffuse interstellar medium have also been estimated from six sightline spectra.

Optical Spectrum of the Diamantane Radical Cation

Similarities exist between the infrared spectra of diamondoids and unidentified infrared emission bands seen in the spectra of young stars with circumstellar disks. Due to their low ionization energy and absorption in the visible range, the radical cations of these highly stable molecules are also suggested to contribute to the well-known but largely unassigned diffuse interstellar bands. However, thus far only the optical spectrum of the adamantane cation (Ada+) has been measured in the laboratory, which is required for astronomical identification. Herein, we present the optical spectrum of the diamantane radical cation (C14H20 +, Dia+, D 3d) between 400 and 1000 nm in the gas phase. The spectra are obtained by electronic photodissociation (EPD) of mass-selected ions generated by electron ionization and trapping in a cryogenic trap at 5 K. The most intense fragmentation channels are the losses of H and C4H9. The optical spectrum reveals two broad and unresolved bands centered near 760 and 450 nm that are assigned to the D 2(2 E u) ← D 0(2 A 1g) and D 5(2 A 2u) ← D 0(2 A 1g) transitions using density functional theory. Despite a vibrational temperature below 20 K, no vibrational structure is resolved. Franck–Condon simulations of the D 5 ← D 0 transition predict intense vibronic progressions that become indiscernible from the band contour at spectral widths above 350 cm−1. Thus, the lack of resolved spectral features is attributed to lifetime broadening, Franck–Condon congestion arising from geometric changes, and possibly vibronic coupling. In addition to the EPD spectra, we characterize the ground state of Dia+ by analysis of a remeasured photoelectron spectrum and a predicted infrared spectrum.

Exploration of the СH3CN…HF complex in the gas phase with high-resolution FTIR spectroscopy and computational methods

This paper presents a comprehensive study of the IR absorption spectrum of the СH3CN … HF complex in the gas phase, including computation methods. The high resolution IR absorption spectra of the gaseous mixtures of the title complex were recorded in the region of 4000–2000 cm−1 at a resolution from 0.1 to 0.005 cm−1 with a Bruker IFS-120 HR vacuum Fourier-transform infrared spectrometer. The formation mechanism of the ν(HF) stretching band contour was explained. By utilizing DFT quantum chemical calculations we determined the equilibrium configuration and the geometric parameters, the harmonic vibrational frequencies, the dipole moments, and the binding energies of the СH3CN … HF complex in the mp2//311++G(3df,3pd) approximation. The three 3 D potential energy surfaces of the complex were evaluated, and the constructed graphs allowed us to reveal a link between the structural changes and the potential energies of the СH3CN … HF complex.

Extended application of deep learning combined with 2DCOS: Study on origin identification in the medicinal plant of Paris polyphylla var. yunnanensis.

Medicinal plants are very important to human health, and ensuring their quality and rapid evaluation are the current research concerns. Deep learning has a strong ability in recognition. This study extended it to the identification of medicinal plants from the perspective of spectrum. In order to realise the rapid identification and provide a reference for the selection of high-quality resources of medicinal plants, a combination of deep learning and two-dimensional correlation spectroscopy (2DCOS) was proposed. For the first time, Fourier transform mid-infrared (FT-MIR) and near-infrared (NIR) spectroscopy 2DCOS images combined with residual neural network (ResNet) was used for the origin identification of Paris polyphylla var. yunnanensis. In total 1593 samples were collected and 12821 2DCOS images were drawn. The climate of different origins was briefly analysed. The xishuangbanna, puer, lincang, honghe and wenshan are the five regions with more ecological advantages. The synchronous 2DCOS models of FT-MIR and NIR could realise origin identification with the accuracy of 100%. The synchronous images were suitable for the identification of medicinal plants with complex systems. The full band, feature band and different contour models had no big difference in distinguishing ability, so they were not the key factors affecting the discrimination results. The ResNet models established were stable, reliable, and robust, which not only solved the problem of origin identification, expanded the application field of deep learning, but also provided practical reference for the related research of other medicinal plants.

Electronic structure of two-dimensional electron gases at differently prepared indium arsenide surfaces

Using angle-resolved photoelectron spectroscopy we study band structures of two dimensional electron gases (2DEGs) formed at surfaces of InAs(001) due to the band bending effect. We find that, for surfaces prepared by nondestructive methods very clear, model 2DEG bands are obtained. In contrast, for ion bombarded surfaces features looking as if 2DEG band contours were uniformly filled with signal are seen. We show that these observations may be explained within the one-electron Poisson-Schrödinger scheme, however, it is necessary to relax the standard boundary conditions with adding a transition zone in which the crystal potential may interpolate between its vacuum and bulk values in nontrivial way. In reality such a zones do exist and correspond to non zero thick surface/interface layers. For ion bombarded surfaces it is also necessary to take into account diffusion and complex reactions of point defects induced with an ion beam. These observations are discussed in the context of electronics devices that often contain similar 2DEGs.

Mechanisms of the Formation of the ν3 Band Contour in Absorption and Raman Scattering Spectra of Tetrafluoromethane in Condensed Phases

Mechanisms of the Formation of the ν3 Band Contour in Absorption and Raman Scattering Spectra of Tetrafluoromethane in Condensed Phases

Reflection-absorption FTIR study of SF6 thin films in combinational modes region

Reflection-absorption IR (RAIR) spectra of sulfur hexafluoride (SF6) crystalline thin films in 10–100 K temperature range and absorption FTIR spectra of thin layer (90 μm) of liquid SF6 near the melting point (Tm = 222 K) are recorded. Special attention is paid to combinational bands region. The three types of band behavior upon phase transitions are identified and illustrated by 3ν6, ν1+ ν3, ν2+ ν3, ν1+ν4 and ν5+ν4 bands. The contours of the latter bands uncharacteristically change upon phase transitions. The different behavior of those bands is explained by the various contributions of different band forming mechanisms, evaluated by the mathematical apparatus of spectral moments.

A new 4Σ − a4Π electronic transition of ScC in the near infra-red

A new 4Σ − a4Πi electronic transition of ScC with (0,0) band near 12,700 cm−1 has been discovered using laser-induced fluorescence, following the reaction of laser-ablated scandium metal with acetylene or methane under supersonic jet-cooled conditions. The new bands are weak, since the a4Πi lower state is known from previous work to lie 160 cm−1 above the X2Π ground state; several spectra needed to be co-added in order to improve the signal-to-noise ratio. The rotational structure of the new transition is dense and heavily blended, but analysis has been possible for the strongest sub-bands of the (1,0) and (2,0) bands, respectively at 13,301 and 13,899 cm−1; extensive use was made of a band contour program for 4Σ − 4Π bands which listed the details of each line that it plotted. The bond length in the new 4Σ state is r0 = 1.964 Å, which is 0.01 Å longer than in the ground state; its vibrational frequency is 607 cm−1. There is no information in the spectrum giving the Kronig symmetry of the new state, though its electron configuration is probably (C 2pσ)1 (C 2pπ)2 (Sc 4sσ)1 (Sc 3dδ)1. Unexpectedly, the spin-spin interaction parameter, λ, in the new 4Σ state is negative, which suggests complicated interactions with nearby unseen electronic states.

Modulations of νO-H and νC═O Stretching Frequencies of Difluoroacetic Acid with Internal Rotation of CHF2 Rotor: A Combined Vapor Phase and Matrix Isolation Infrared Spectroscopy Study.

Mid-infrared spectra of difluoroacetic acid (DFAA) have been measured by isolating the molecule in argon and nitrogen matrices at 8 K and also in the vapor phase at room temperature. In argon matrix, the O-H stretching fundamental (νO-H) of -COOH group appears as a doublet with band maxima at 3554 and 3558 cm-1, and a similar doublet for C═O stretching fundamental appears at 1800 and 1810 cm-1. In the vapor phase, the νO-H transition is featured with multiple peaks, and the observed band shape has been deconvoluted as superposition of two transitions both having A-type rotational band contours. We have attributed these transitions to the two internal rotational isomers corresponding to the two distinct minima along -CHF2 torsional coordinate of the molecule. Natural bond orbital (NBO) analysis reveals that these torsional minima are the manifestations of different second order interactions involving bonding and antibonding orbitals corresponding to the rotor -CHF2 and COOH groups of the molecule. By use of the theoretically predicted rotational constants of the rotamers, the band profile for νO-H has been simulated satisfactorily by means of the PGOPHER method, and this has allowed estimating accurately the energy difference between the two rotamers as 0.54 kcal/mol. The predicted energy barrier for interconversion between the rotamers is very small, ∼0.5 kcal/mol from rotamer II to rotamer I, which implies that the molecule could hop almost freely between the two rotameric forms at room temperature. As a result, the frequencies of the key stretching vibrational modes, like νO-H, νC═O, and νC-H, undergo modulation with internal rotation of the rotor -CHF2 group. Such modulation of high frequency modes could be an efficient mechanism for acceleration of rotor-induced IVR (intramolecular vibrational redistribution) well documented in the literature. Furthermore, the spectra measured in matrix isolated environment show signatures for an energetically higher third rotamer, where -OH and -C═O groups are in anti orientation. It has also been shown that DFAA can easily form weak hydrogen bonded dimeric complexes with molecular nitrogen (N2), which causes νO-H to undergo a red shift of ∼30 cm-1 in argon matrix for all three DFAA monomers.

CH Stretch Activation of CH3CHOO: Deep Tunneling to Hydroxyl Radical Products.

Alkene ozonolysis, an important source of hydroxyl (OH) radicals in the Earth's troposphere, proceeds through unimolecular decay of Criegee intermediates. In this work, infrared activation of the methyl-substituted Criegee intermediate, syn-CH3CHOO, in the CH stretch fundamental region (2850-3150 cm-1) is shown to result in unimolecular decay to OH radical products. These excitation energies correspond to only half of the transition state barrier height, and thus the resultant 1,4 H atom transfer that leads to OH products occurs exclusively by quantum mechanical tunneling. Infrared action spectra recorded with UV laser-induced fluorescence detection of the OH products reveal the four CH stretch fundamentals and CO stretch overtone predicted to have strong transition strength. The vibrational band origins, relative intensities, and transition types derived from rotational band contour analyses are in good accord with theory. Distinctly different Lorentzian line broadening of the observed features is attributed to mode-specific anharmonic couplings predicted theoretically between spectroscopically bright and nearby dark states. The measured OH product state distribution shows a strong λ-doublet preference arising from pπ orbital alignment, which is indicative of the vinyl hydroperoxide intermediate along the reaction pathway. The unimolecular decay of syn-CH3CHOO at ca. 3000 cm-1 is predicted to be quite slow (ca. 105 s-1) using statistical Rice-Ramsperger-Kassel-Marcus theory with tunneling and much slower than observed at higher energies.

Dynamic Void Growth and Localization Behaviors of Glassy Polymer Using Nonlocal Explicit Finite Element Analysis

This work introduces the nonlocal integral theory into the finite-deformation viscoplastic Gurson-type model for glassy polymer with plastic softening under strain rate loading. In order to improve computational efficiency and numerical robustness, nonlocal averaging on the void volume fraction is coupled with the update of stress and strain using ABAQUS dynamic subtoutines. From two numerical examples using nonlocal FEA on the unnotched square plate and the dumbbell-shaped specimen under tension respectively, the introduced length scale is demonstrated to regularize the dynamic initial-value problem well that remains hyperbolic by predicting the nonlocal void growth, the objective load responses, and the contours of localization bands accurately. In addition, the relationship between the width of localization band and the length scale is also studied.

The FELion cryogenic ion trap beam line at the FELIX free-electron laser laboratory: infrared signatures of primary alcohol cations.

The combination of a 4 K 22-pole ion trap instrument, FELion, with the widely tunable free electron lasers at the FELIX Laboratory is described in detail. It allows for wide-range infrared vibrational spectroscopy of molecular ions. In this study, the apparatus is used for infrared vibrational predissociation (IR-PD) measurements of the simple alcohol cations of methanol and ethanol as well as their protonated forms. Spectra are taken by tagging the cold molecular ions with He atoms. The infrared spectrum of protonated methanol is recorded for the first time, and the wavelength coverage for all other species is substantially extended. The bands of all spectra are analysed by comparison to ab initio calculation results at different levels of theory. Vibrational bands of different isomers and conformers (rotamers) are discussed and identified in the experimental spectra. Besides the measurement of IR-PD spectra, the method of infrared multiple photon dissociation IR-MPD is applied for some cases. Spectral narrowing due to the cold environment is observed and rotational band contours are simulated. This will help in identifying more complex species using the IR-MPD method in future measurements. Overall the IR-PD spectra reveal more bands than are observed for the IR-MPD spectra. In particular, many new bands are observed in the fingerprint region. Depletion saturation of the finite number of trapped ions is observed for the IR-PD spectra of the ethanol cation and the presence of only one isomeric species is concluded. This special feature of ion trapping spectroscopy may be used in future studies for addressing specific isomers or cleaning the ion cloud from specific isomers or conformers. In addition, the results of this study can be used as a basis to obtain high-resolution infrared vibrational and THz rotational spectra of alcohol ions in order to detect them in space.

Electrically controlled beam steering with wide deflection angles in liquid crystal photonic crystals

Electrically controlled beam steering behaviors in a 2D square photonic crystal (PC) composed of Si cylinders surrounded by nematic liquid crystal are studied. The results show that by applying an external saturated electric field, the structure becomes anisotropic and results in remarkable change of band structure to produce a single band from 0.3105 to 0.3245(c/a) accordingly. This special single band is high anisotropic and only allows one mode with negative refraction to be excited in the ГM direction. Further studies show that as the external saturated electric field rotates from 0° to 90°, the equifrequency contours of the single band deform continuously from rectangle-like to parallelogram-like shapes, and then recover to rectangle-like shapes but with the long-side rotated by 90°. Moreover, we demonstrate continuous tunable beam steering behaviors with wide deflection angles from −38° to +38° at normal incidence by finite-difference time-domain simulations. These results have the potential for designing various tunable PC devices.

High resolution rovibrational analysis of dimethyl sulfide spectrum in the 10 µm atmospheric window combining supersonic jet-quantum cascade laser and FTIR spectroscopies

The rovibrational spectrum of the v14 CH3-bending mode of dimethyl sulfide CH3SCH3 was recorded in the 945–1005 cm−1 spectral region combining data from a tunable infrared quantum cascade laser spectrometer coupled to a pulsed slit jet and from a high resolution Fourier transform spectrometer coupled to a room temperature gas absorption cell. The characteristic PQR band contour of a vibrational mode with a b1 symmetry species in the C2v group was observed with both infrared methods but the rovibrational cooling achieved in the pulsed supersonic expansion (20 K in rotation) proved to be essential for starting the rovibrational analysis of such asymmetric top molecule. Internal rotation splittings were recently observed and analyzed in the ground and first torsional excited states. However, they are expected to be very small and could not be resolved in the v14 state using our infrared spectroscopic probes. Therefore a classic reduced effective Hamiltonian of an asymmetric rigid rotor was used. Finally, 1119 transitions from both jet-cooled and room temperature experiments were assigned and included in a global fit with J ≤ 29. A total of 9 rovibrational parameters including pure rotational constants as well as quartic centrifugal distortion constants were determined with high accuracy. These parameters can be used to produce a reliable line-list for an atmospheric detection of DMS.

Quantification of distortion of the water OH-band using picosecond Raman spectroscopy

An anomalously large (~40 cm−1) blue-shift of the Raman OH stretching vibration band was observed when bulk water was probed by picosecond versus nanosecond pulses. The temperature distortion of the OH band contour (3200–3600 cm−1) was studied by both nanosecond and picosecond Raman techniques. It was observed that the OH band centroid shifted linearly with temperature (~0.87 cm−1 °C−1) for both nano- and picosecond pulses. This feature of the picosecond Raman spectra was explained by the dynamics of the non-equilibrium hydrogen bond structures in water.

Effect of resonance dipole-dipole interaction on the infrared spectra of adsorbed CF4. Experimental investigation and theoretical modeling

Adsorption of CF4 on ZnO has been studied by means of FTIR spectroscopy at 55–300K. The contour of the band of the ν3 vibration in the spectra obtained is shown to depend crucially on surface coverage, due to strong resonance dipole-dipole interaction in the layer of adsorbed molecules. At high surface coverage the band is split into two components at 1320 and 1245cm−1, about 75cm−1 apart. The shape of the low frequency component is complicated by an “Evans hole” at 1258cm−1. At lowest coverage only a weak band of ν3 vibration was seen at 1270–1272cm−1; about 10cm−1 below the corresponding wavenumber for gaseous CF4. Preliminary adsorption of CO does not affect significantly the spectrum of adsorbed CF4, leading only to slight decrease of the distance between the two components. Experimentally observed band contours were compared with those calculated theoretically using a model developed earlier for the spectra of interacting molecules in liquid phase, or dissolved in liquefied noble gases, and updated for the case of 2D systems. The calculated spectra reproduce well the splitting of the ν3 band for a monolayer, as well as the coverage and temperature dependence of the IR spectra.

Efficiency of the intermolecular interaction of salicylic acid neutral form and monoanion with Cd2 + ion studied by methods of absorption and fluorescence

The methods of absorption and fluorescence were used to study the efficiency of the interaction between salicylic acid derivatives SAD (neutral SA form and SA monoanion) and Cd2+ ions (in CdBr2 salt) within the range рН=1.5÷8. The efficiency was determined from the change in both the absorption band contour and the fluorescence intensity of various SAD forms. It has been established that depending on the SAD form, the addition of CdBr2 to a starting solution leads to the formation of additional absorption for both the shorter wave lengths in the absorption spectrum of the neutral form (at рН<3) and the longer wave lengths in the absorption spectrum for the НSal− monoanion (at рН>4). In the fluorescence spectra, the intensity was observed to increase for the neutral SAD form (at рН<3) and to decrease for the НSal− monoanion (at рН>4) after addition of CdBr2.The spectral changes were interpreted in the framework of common notions about the effect of three physicochemical factors that determine the interaction between the SAD and the Cd2+ ion and affect the parameters of absorption and fluorescence spectra. These factors are: (1) the decrease in рН after addition of CdBr2 to the SAD solution, (2) the decrease in the efficiency of the Н-bonding of SAD molecules to the water ones, and (3) the existence of electrostatic ion-ion interaction between the НSal− monoanion and the Cd2+ ion.The bimolecular fluorescence quenching constants Kq of НSal− monoanion fluorescence quenching by the Cd2+ ion appeared to be substantially less than those of the quenching which would follow either the dynamic (diffusion) or the concentration (static) mechanisms.

Polymorphic behavior of an organic compound

Objective: Polymorphic crystals were exhibited in many organic compounds. The frequency changes, relative intensities, band contours, and numberof bands were observed in the spectra of different polymorphism which may be due to molecule-molecule interactions in the crystal unit cells. Theshape of a molecule at its site in the unit cell is distorted by molecular interactions.Methods: The identification of a pure crystal form and to quantify a mixture of two forms infrared and Raman spectra of different crystalline formsof the same organic compound can be used. 2’-chloro-4-methoxy-3-nitro benzil (1) was synthesized and its two polymorphic forms were obtainedby recrystallization from the solvents acetone/chloroform and ethanol. The polymorphism present in the compound was confirmed by single crystalX-ray crystallography and differential scanning calorimetry.Results: The polymorph 1.1 crystallizes as triclinic P-1 space group in the solvent acetone – chloroform and the polymorph 1.2 crystallizes asmonoclinic P21/c space group in the solvent ethanol.Discussion: The intermolecular lattice energy and the interplay of molecular conformation in the crystallization and stability of polymorphs areidentified by X-ray crystal structures of conformational polymorphs.Keywords: Conformational polymorphism, Organic compounds, Single crystal growth, X-ray diffraction.

Near infrared overtone (vOH = 2 ← 0) spectroscopy of Ne-H2O clusters.

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H2O complexes (D0(para) = 31.67 cm-1, D0(ortho) = 34.66 cm-1) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-⟩←|00+⟩ and |02+⟩←|00+⟩) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ←Σ,Π←Σ, and Σ←Π infrared bands in the overtone spectra, where Σ(K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne-H2O internuclear axis. End-over-end tumbling of the ortho Ne-H2O cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02+⟩fΠ(111)←eΣ(000) para Ne-H2O spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (Trot ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-H2O clusters and provide support for similar cooling dynamics in rare gas-tagging studies.