THz Absorption Spectra Research Articles

THz spectra of hypoxanthine and inosine

Far-infrared absorption spectra and refractive indices of hypoxanthine and inosine have been measured by terahertz time-domain spectroscopy (THz-TDS) in the frequency range of 03—16THz at room temperature. The experimental results show that there is a distinct difference between the THz absorption spectra of hypoxanthine and inosine，which suggests that THz-TDS is highly sensitive to molecular structures and the components. Hartree-Fork calculation was used to optimize and simulate the THz spectra obtained by measurement. The low frequency vibrational mode at 14THz of inosine is attributed to the swing and torsion between purine and pentose rings according to the theoretical calculation. There is a linear relationship between the absorption intensity and the concentration of sample，which is consistent with Lambert-Beer's Law. Quantitative analysis of the components of mixtures of hypoxanthine and inosine was carried out by linear regression with non-negative constraint and the relative error was less than 7%. The sources of error were also discussed.

Characterization of Anhydrous and Hydrated Pharmaceutical Materials with THz Time-Domain Spectroscopy

The identification of anhydrous and hydrated forms of pharmaceutical substances is of great importance in pharmaceutical science and industry. We report the use of THz time-domain spectroscopy (THz-TDS) for pseudopolymorph investigation. The anhydrous forms of pharmaceutical substances including caffeine, theophylline, D-glucose, and ampicillin exhibit different THz absorption spectra from their hydrated forms, presumably due to their different intermolecular vibrational modes mediated by hydrogen-bonding. The data from X-ray powder diffractometry (XRPD) confirm the crystallinity differences between the anhydrous and hydrated forms. The temperature-dependent THz spectra of caffeine hydrate were also recorded, and it demonstrates that THz-TDS can be used to monitor the dehydration process of drug hydrates. We conclude that THz-TDS is an advantageous technique for the pseudopolymorph identification and study, and has great potential to become a process analytical technology (PAT) in pharmaceutical production and quality control.

Terahertz absorption spectra of some saccharides and their metal complexes

In this work, THz absorption spectra of some saccharides and their metal complexes were measured. The main purpose of this work is to investigate the M–O vibrations, intermolecular and intramolecular hydrogen bonds and other vibrations in the FIR region using powerful spectroscopic techniques adopting the metal–sugar complexes prepared in our laboratory. The M–O vibrations in the FIR spectra of metal–sugar complexes indicate the formation of metal complexes. The THz spectrum of glucose below 100 cm −1 was measured at first to confirm the THz experimental method. Characteristic absorption bands in the spectra of various samples are observed. THz spectra of saccharides below 100 cm −1 often have several absorption bands, and different saccharides have various absorption peaks in the THz region, which may be used to distinguish different saccharides. The differences in the number of bands observed are related to different structures of the samples, and these absorption bands are related to the collective motion of molecules. But the THz spectra of their metal complexes are different from the ligands, and no band appears in the region below 50 cm −1 at the present experimental condition, which indicates that THz spectroscopy may also be helpful to identify the formation of metal–sugar complexes, and the changes after complexation in the THz spectra below 100 cm −1 may be related to different metal ions. The metal–sugar complexes with similar crystal structures resemble mid-IR spectra, but their THz spectra may have some differences.

Substituent Effects on the Low‐Frequency Vibrational Modes of Benzoic Acid and Related Compounds

AbstractWell‐resolved absorption spectra of benzoic acid and its derivatives with one hydrogen atom replaced by a substituent group CH3, OH, NH2 or NO2 were reported in the frequency region between 6 and 67 cm−1 at room temperature with terahertz time‐domain spectroscopy (THz‐TDS). These substances can be distinguished easily based on the terahertz absorption spectra. The measurements suggested that even minor changes in the molecular configuration and chemical composition lead to distinct differences in THz spectrum. Density functional theory (DFT) method was used to assist the analysis and assignment of the individual THz absorption spectra of benzoic acid and its methyl derivatives. Observed THz responses of samples can be assigned to the collective vibrations associated with intermolecular hydrogen bonds.

Terahertz Time-Domain Spectroscopy of Four Hydroxycinnamic Acid Derivatives

The well-resolved absorption spectra of the hydroxycinnamic acid (HCA) derivatives, caffeic acid, ferulic acid, sinapic acid and chlorogenic acid, were measured over the frequency region from 0.3 to 2.0 THz at 294 K with terahertz time-domain spectroscopy (THz-TDS). Theoretical calculation was applied to assist the analysis and assignment of the individual THz absorption spectra of the HCA derivatives with density functional theory (DFT). The distinctive spectral features were originated from the collective motion of molecules held together by hydrogen bonds. The real and imaginary parts of dielectric function of the four HCA derivatives were also obtained.

Wideband terahertz spectroscopy of explosives

Time-domain terahertz spectroscopy (TDTS) has been shown to be a promising tool in detection of explosives and explosive related compounds. To date, TDTS for many explosives has been limited to spectral regions <3 THz. With recent improvements in emitters and sensors of THz radiation, we report the THz absorption spectra over an extended frequency band from 0.5 to 6 THz for four explosives: RDX (1,3,5-trinitroperhydro-1,3,5-triazine), HMX (1,3,5,7-tetranitroperhydro-1,3,5,7-tetrazocine), PETN (pentaerythritol tetranitrate), and TNT (2,4,6-trinitrotoluene). New distinctive spectral features are shown in these materials between 3 and 6 THz. These data may aide in the spectral identification of these explosives.

Nonharmonic transverse vibration of the H-bonded molecules and the THz spectra in ice and water

A nonlinear equation of motion is found for the dimer comprising two charged H 2O molecules. The THz dielectric response to nonharmonic vibration of a nonrigid dipole, forming the hydrogen bond (HB), is found in the direction transverse to this bond. An explicit expression is derived for the autocorrelator that governs the spectrum generated by transverse vibration (TV) of such a dipole. This expression is obtained by analytical solution of the truncated set of recurrence equations. The far infrared (FIR) spectra of ice at the temperature − 7 °C are calculated. The wideband, in the wavenumber (frequency) ν range 0…1000 cm − 1 , spectra are obtained for liquid water at room temperature and for supercooled water at − 5.6 °C. All spectra are represented in terms of the complex permittivity ε( ν) and the absorption coefficient α( ν). The obtained analytical formula for ε comprises the term ε ⊥ pertinent to the studied TV mechanism with three additional terms Δ ε q , Δ ε μ , and ε or arising, respectively, from: elastic harmonic vibration of charged molecules along the H-bond; elastic reorientation of HB permanent dipoles; and rather free libration of permanent dipoles in ‘defects’ of water/ice structure. The suggested TV-dielectric relaxation mechanism allows us: (a) to remove the THz ‘deficit’ of loss ε″ inherent in previous theoretical studies; (b) to explain the THz loss and absorption spectra in supercooled (SC) water; and (c) to describe, in agreement with the experiment, the low- and high-frequency tails of the two bands of ice H 2O located in the range 10…300 cm − 1 . Specific THz dielectric properties of SC water are ascribed to association of water molecules, revealed in our study by transverse vibration of HB charged molecules.

Terahertz Resonances in the Dielectric Response Due to Second Order Phonons in a GaSe Crystal

ABSTRACTThe dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4 to 2.4 THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp = 6.1 ± 0.5 THz, the average momentum relaxation time &lt;τ&gt; = 51 +± 6 fs for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons. This paper has already been published, see B. L. Yu, F. Zeng, V. Kartazayev, and R. R. Alfano, “Terahertz studies of the dielectric response of and second order phonons in a GaSe crystal,” Appl. Phys. Lett. 86, 182104 (2005).

Terahertz studies of the dielectric response and second-order phonons in a GaSe crystal

The dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4to2.4THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp=2.6±0.2THz, the average momentum relaxation time ⟨τ⟩=56±2fs, and the mobility μ=89cm2∕Vs for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons.

Terahertz absorption spectra of L-, D-, and DL-alanine and their application to determination of enantiometric composition

Absorption spectra of polycrystalline L-, D-, and DL-alanine have been measured by the terahertz time domain spectroscopy (THz-TDS) in the frequency range from 10to90cm−1 at room temperature. We observed several absorption bands, which have the large difference between enantiomers (L- and D-alanine) and the racemic compound (DL-alanine) in their peak frequencies. This obvious difference shows that the THz absorption bands are strikingly sensitive to the crystal structures. This result indicates that the THz-TDS can be used for distinguishing between the enantiomers and the racemic compound. We propose and demonstrate a method to determine the enantiometric composition of amino acids from the THz absorption spectra.

A comparative study of THz spectra

We have constructed two types of THz spectrometers using the frequency-tunable THz wave generators based on difference-frequency generation (DFG) in GaP crystal. The OPO-source system is for high accuracy measurement to detect fine structures in THz absorption spectra. The Cr:F-source system has merit in simple structure, easy maintenance and low-cost, but still has sufficient resolution for room temperature observation of solids or liquids. Comparing the absorption spectra of saccharides measured with these spectrometers and those measured with FT-IR or TDS, our spectrometers have superiority in frequency range and resolution, enough to record sub peaks, which can be analyzed as structural defects in organic compounds. (Contributed by Jun-ichi NISHIZAWA, M.J.A.)

Tunable terahertz detection based on a grating-gated double-quantum-well FET

We model resonant terahertz photoconductance recently observed in field-effect transistors with a double-quantum-well (DQW) channel. Comparison of the measured THz resonant photoresponse to the calculated THz absorption spectrum establishes that the resonances are determined by standing plasma waves in the DQW channel under metallic portions of the grating gate. It is found theoretically that the DQW asymmetry mixes the acoustic and optical plasmons resulting in a rather intense ac electric field between the QWs.