Aqueous foam drainage characterized by terahertz spectroscopy

Aqueous foam drainage has been studied using terahertz (THz) spectroscopy. Water is highly absorbing of THz radiation, allowing drainage to be determined based on water content at respective foam height. These drainage profiles were validated using a model constructed from published equations and tailored to this specific study. In addition, a slow-draining foam was scanned to produce a two-dimensional foam image.     

Structural characterization of a procyanidin tetramer and pentamer from the apple by low-temperature NMR analysis

The structures of a procyanidin tetramer and pentamer from unripe apple (Malus pumila) were elucidated by low-temperature NMR analysis at −34 °C. These structures were [epicatechin-(4β→6)-epicatechin-(4β→8)-epicatechin-(4β→8)-epicatechin (1)] and [epicatechin-(4β→8)-epicatechin-(4β→8)-epicatechin-(4β→8)-epicatechin-(4β→8)-epicatechin (2)].     

Far-infrared characteristics of ZnS nanoparticles measured by terahertz time-domain spectroscopy

The optical and dielectric properties of ZnS nanoparticles are studied by use of terahertz time-domain spectroscopy (THz-TDS) over the frequency range from 0.3 to 3.0 THz. The effective medium approach combined with the pseudo-harmonic model of the dielectric response, where nanoparticles are embedded in the host medium, provides a good fit on the experimental results. The extrapolation of the measured data indicates that the absorption is dominated by the transverse optical mode localized at 11.6+/-0.2 THz. Meanwhile, the low-frequency phonon resonance of ZnS nanoparticles is compared with the single-crystal ZnS. The THz-TDS clearly reveals the remarkable distinction in the low-frequency phonon resonances between ZnS nanoparticles and single-crystal ZnS. The results demonstrate that the acoustic phonons become confined in small-size nanoparticles.     

Cure monitoring of two-component epoxy adhesives by terahertz time-domain spectroscopy

We report nondestructive measurements of the properties of two-component epoxy adhesives at terahertz wavelengths using a transmissive time-domain spectroscopy system. The results show that the different epoxies have measurably different THz properties, that the changes which occur during the curing process can be monitored by measurements of their refractive index and absorption at terahertz wavelengths, and that this technique is sensitive enough to record changes in the optical material parameters during the postcuring process.     

Characterizing polymorphism and crystal transformation of polylactide by terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz-TDS), which has been proved to show promising application in complex polymer systems, was employed to investigate the polymorphism phenomenon and crystal transformation of polylactide (PLA) in this study. The THz-TDS shows sensitive response on the crystal structure. The α'-form, α-form and stereocomplex crystals exhibit absorption peaks of lattice vibration at 1.82, 2.01 and 2.09 THz, respectively. THz-TDS has no direct chirality identification on the difference between poly (d-lactide) (PDLA) and poly(l-lactide) (PLLA). However, the PLA stereocomplex shows an extra and distinctive absorption peak at 1.43 THz compared with homo-PLA, and the peak was proved to be stemmed from the collective vibration of L-lactic unit and D-lactic unit pairs connecting by hydrogen bonds. This is the first time that THz-TDS has been proved to be of great potential in identification of polymer stereocomplex crystal. Also, the α'→α crystal transformation of PLA were intuitively investigated at 120 °C using THz-TDS, while the transformation rate was quite slow.     

Nonresonant ionization of oxygen molecules by femtosecond pulses: plasma dynamics studied by time-resolved terahertz spectroscopy

We show that optical pump-terahertz probe spectroscopy is a direct experimental tool for exploring laser-induced ionization and plasma formation in gases. Plasma was produced in gaseous oxygen by focused amplified femtosecond pulses. The ionization mechanisms at 400- and 800-nm excitation wavelengths differ significantly being primarily of a multiphoton character in the former case and a strong-field process in the latter case. The generation of the plasma in the focal volume of the laser and its expansion on subnanosecond time scale is directly monitored through its density-dependent susceptibility. A Drude model used to evaluate the plasma densities and electron-scattering rates successfully captures the observations for a wide range of pump intensities. In addition, rotational fingerprints of molecular and ionic species were also observed in the spectra.     

Classical theory of two-dimensional time-domain terahertz spectroscopy

A general theoretical framework of two-dimensional time-domain second-order and third-order terahertz spectroscopy has been presented. The theoretical treatment is based on a classical and phenomenological model with weak nonlinearities. Three types of nonlinearity sources, anharmonicity, nonlinear coupling, and nonlinear damping, were considered. The second-order THz spectroscopy has an exact correspondence to fifth-order off-resonance Raman spectroscopy, and it has been shown that the present treatment gives exactly the same results as of the quantum mechanical theory under the weak nonlinearity condition. General expressions for the nonlinear signal have been obtained for a single-mode system, and numerical calculations for delta-function incident terahertz pulses were shown. For the third-order signal, two-level systems were also considered for comparison. Contributions of two types of incident pulse sequences have been studied separately in the third-order signals. Profiles of the two-dimensional signals were found to depend on the origin and order of the nonlinearity and also on the pulse sequence. The results of the present study show that the two-dimensional signal features of second- and third-order nonlinear terahertz spectroscopy can clarify the nature of the system which is not accessible using linear spectroscopy.     

Temperature dynamics of magnetoactive compounds under terahertz irradiation: characterization by an EPR study

A possibility of fast rapid temperature changing for the studied compound makes it possible to study related phenomena, such as thermally induced trapping of metastable states in magnetoactive compounds and thermally activated catalytic and biological processes. The Electron Paramagnetic Resonance (EPR) spectroscopy station at the Novosibirsk Free Electron Laser (NovoFEL) allows one to study effects of powerful terahertz (THz) radiation on the spin state of paramagnetic systems. A change in the sample temperature as a result of radiation absorption is an inevitable consequence of such an exposure. However, the sample heating at the NovoFEL EPR station itself is of interest due to the record power of THz radiation and small sizes of the samples used. A combination of these two factors can provide a significantly high heating rate. The magnetoactive complex [Cu(hfac)₂L^Et] was chosen as a model system for studying the heating process, since the complex has a magnetostructural transition at ～125 K with substantially different spin states above and below the transition temperature. The heating processes with the amplitudes above 60 K were studied, the heating and cooling rates of the sample in similar experiments were estimated, and prospects for using the method to study various thermally induced effects are discussed.

     

Intermolecular vibrational mode of the benzoic acid dimer in solution observed by terahertz time-domain spectroscopy

The low-frequency modes of the benzoic acid (BA) dimer and its analogues in carbon tetrachloride (CCl(4)) have been investigated by terahertz time-domain spectroscopy. The solute spectrum is obtained by subtracting the solvent contribution from that of the solution. The difference spectrum of BA in CCl(4) has a broad band with a peak at 68 cm(-1). To assign the observed band, the spectrum is compared with spectra of other aromatic molecules, such as benzene and phenol in addition to p-methyl BA and deuterated BA species (BA-d(OH) and BA-d(5)) in CCl(4). The band at 68 cm(-1) is assigned to the cogwheel mode of the BA dimer. Density functional theory calculations also support this assignment. Finally, spectral lineshape analysis based on the multimode Brownian oscillator model is applied to the THz spectra for all the samples.     

Synthesis and characterization of degradable electrically conducting copolymer of aniline pentamer and polyglycolide

A novel block copolymer of polyglycolide (PGA) and aniline pentamer that is electro-active and degradable was synthesized. The copolymer was prepared by low-temperature polycondensation between carboxyl-capped aniline pentamer and hydroxyl-capped PGA, with N,N′-dicyclohexylcarbodiimide (DCC) as dehydrating agent. The structures of monomers and the copolymer were characterized by IR, ¹H NMR. The chemical redox process of the as-prepared copolymer was studied by UV-vis spectra and cyclic voltammetry (CV). Thermal analysis and the study of degradation show that the copolymer has good thermal stability and degradability.     

Angle-dependent terahertz time-domain spectroscopy of amino acid single crystals

The measurement of absorption spectra using angle-dependent terahertz (THz) time-domain spectroscopy for amino acid single crystals of l-cysteine and l-histidine is reported for the first time. Linearly polarized THz radiation enables us to observe angle-dependent far-infrared absorption spectra of amino acid single crystals and determine the direction of the oscillating dipole of the molecules in the 20-100 cm(-1) range. By comparing the THz spectra of a single crystal and powder, we found that there was a clear hydrogen-bond peak in the crystal spectrum as a result of the larger hydrogen-bond network. The low-temperature THz spectra of amino acid microcrystals showed more intermolecular vibrational modes than those measured at room temperature. An ab initio frequency calculation of a single amino acid molecule was used to predict the intramolecular vibrational modes. The validity of the calculation models was confirmed by comparing the results with experimentally obtained data in the Raman spectral region.     

Photoinduced electron transfer and solvation dynamics in aqueous clusters: comparison of the photoexcited iodide-water pentamer and the water pentamer anion

Upon photoexcitation of iodide-water clusters, I(-)(H(2)O)(n), an electron is transferred from iodide to a diffuse cluster-supported, dipole-bound orbital. Recent femtosecond photoelectron spectroscopy experiments have shown that, for photoexcited I(-)(H(2)O)(n) (n≥ 5), complex excited-state dynamics ultimately result in the stabilization of the transferred electron. In this work, ab initio molecular dynamics simulations of excited-state I(-)(H(2)O)(5) and (H(2)O)(5)(-) are performed, and the simulated time evolution of their structural and electronic properties are compared to determine unambiguously the respective roles of the water molecules and the iodine atom in the electron stabilization dynamics. Results indicate that, driven by the iodine-hydrogen repulsive interactions, excited I(-)(H(2)O)(5) rearranges significantly from the initial ground-state minimum energy configuration to bind the excited electron more tightly. By contrast, (H(2)O)(5)(-) rearranges less dramatically from the corresponding configuration due to the lack of the same iodine-hydrogen interactions. Despite the critical role of iodine for driving reorganization in excited I(-)(H(2)O)(5), excited-electron vertical detachment energies appear to be determined mostly by the water cluster configuration, suggesting that femtosecond photoelectron spectroscopy primarily probes solvent reorganization in photoexcited I(-)(H(2)O)(5).     

Synthesis and characterization of three novel perfluoro-oligothiophenes ranging in length from the trimer to the pentamer

In this Article, we report on the synthesis and full characterization of three perfluorinated oligothiophenes, ranging in length from the trimer to the pentamer (PF-nT, with n = 3, 4, or 5). The differential pulse voltammetry (DPV) analysis of the compounds showed that they can be both oxidized and reduced (i.e., they display a dual or amphoteric electrochemical behavior), with the reduction peaks positively shifted relative to those of the corresponding unsubstituted oligothiophenes. The electrochemically determined energy gaps are in agreement with those measured from the UV-vis-NIR absorption spectra in solution. The conjugational properties have been investigated by means of FT-Raman spectroscopy, both as pure solids and as dilute solutes in CH(2)Cl(2), revealing that: (i) pi-conjugation does not still reach saturation with chain length for the longest oligomer, and (ii) conformational distortions from a nearly coplanar arrangement of the successive thiophene units upon solution are not too large. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information about the optimized molecular structure, equilibrium atomic charges distribution, energies and topologies of the frontier molecular orbitals (MO) around the gap, vibrational normal modes associated with the most outstanding Raman scatterings, and vertical one-electron excitations that give rise to the main optical absorptions.     

Probing structural transition and guest dynamics of hydroquinone clathrates by temperature-dependent terahertz time-domain spectroscopy

The structural transition from hydroquinone clathrates to crystalline α-form hydroquinone was observed up to the range of 3 THz frequency as a function of temperatures. We found that all three hydroquinone clathrates, CO(2)-, CH(4)-, and CO(2)/CH(4)-loaded hydroquinone clathrates, transform into the α-form hydroquinone at around 102 ± 7 °C. The resonance peak of the CO(2)-loaded hydroquinone clathrate at 2.15 THz decreases with increasing temperature, indicating that CO(2) guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the hydroquinone clathrates may transform into the stable α-form hydroquinone via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks. A strong resonance of the α-form hydroquinone at 1.18 THz gradually shifts to the low frequency with increasing temperature and shifts back to the high frequency with decreasing temperature.     

High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.     

Laser-induced breakdown spectroscopy in analysis of Al3+ liquid droplets: on-line preconcentration by use of flow-injection manifold

Laser-induced breakdown spectroscopy (LIBS), combined with a flow-injection system, is demonstrated to analyze liquid droplets of aluminum salt, as generated with an electrospray ionization device. The spray needle also serves as the anode, through which the analyte solution is spread toward the other metal base as the cathode. Along the passage of the FI manifold, the Al-sample loading speed is controlled at 0.15 mL min⁻¹, restricted to the small diameter of the spray needle, and the loading volume amounts to 0.1 mL. The metal ion is retained in a cation-exchange resin microcolumn immobilized with Chromotrope 2B chelating agent, followed by elution with a 0.5 M HCl solution into LIBS. Upon laser irradiation at the preconcentrated liquid droplets, the time-resolved laser-induced breakdown (LIB) emission and plasma-induced current signals are acquired concurrently on a single-shot basis. The area under the LIB/current distribution increases in linear proportion as the concentration of the sample solution increases. The detection limit thus obtained can reach 1.5 mg L⁻¹, about an order of magnitude lower than those achieved previously using single-laser ablation without involvement of preconcentration. The linear dynamic range is more than two orders of magnitude.     

Quantification of pharmaceutical polymorphs and prediction of dissolution rate using theophylline tablet by terahertz spectroscopy

Theophylline has an anhydrous form and a monohydrated form, and the dissolution rate of the anhydrous form is higher than that of the monohydrated form. Terahertz (THz) spectra of theophylline tablet containing the theophylline anhydrous form, monohydrated form, microcrystalline cellulose and magnesium stearate exhibited a specific absorption peak at 0.96 THz, where the theophylline anhydrous form demonstrated an absorption peak. Additionally, the intensity of the peak at 0.96 THz gradually decreased as the proportion of the anhydrous form decreased. The multivariate data analysis was performed to correlate the THz spectra of theophylline tablets with the ratio of the theophylline anhydrous form. The calibration model used to predict the mixing ratio of the theophylline anhydrous form from the THz spectra achieved root-mean-squared errors of cross-validation (RMSECV) of 2.89%, a slope of 0.9934 and an R(2) of 0.9927. In addition, there were intentions to develop a prediction model for the dissolution rate of theophylline from the drug product. The dissolution rate of theophylline tablet was gradually delayed as the proportion of the anhydrous form was decreased. The multivariate data analysis was performed to correlate the THz spectra of theophylline tablets with the dissolution rate. The calibration model used to predict the percentage of theophylline dissolved in 45 min from the THz spectra achieved an RMSECV of 3.29%, a slope of 0.9260 and an R(2) of 0.9423. Furthermore, there were no significant differences between the predicted and measured percentages of theophylline dissolved in 45 min in the theophylline tablets that were stored at 84% relative humidity (RH) and 25 °C for 12 h or 3 d.