Computational and infrared spectroscopic investigations of N-substituted carbazoles.

The carbazole moiety is a commonly identified structural motif in the high-molecular-weight components of petroleum, known as asphaltenes. Detailed characterization of carbazoles is important for understanding the structure of asphaltenes and addressing challenges in the areas of heavy oil recovery, transportation, upgrading, and oil spills, arising from asphaltene properties and composition. In this work we study carbazole and the four N-substituted carbazoles 9-methylcarbazole, 9-ethylcarbazole, 9-vinylcarbazole and 9-phenylcarbazole.

Photoacoustic Detection of Weak Absorption Bands in Infrared Spectra of Calcite.

Photoacoustic spectroscopic detection of infrared absorption often produces spectra with enhanced intensities for weaker peaks, enabling the detection of features due to overtones and combinations, as well as less-abundant isotopic species. To illustrate this phenomenon, we present and discuss photoacoustic infrared spectra of calcite. We use linearization of rapid-scan spectra, as well as comparing step-scan and rapid-scan spectra, to demonstrate that saturation is not the driving force behind these enhanced intensities.

Infrared spectra of micro-structured samples with microPhotoacoustic spectroscopy and synchrotron radiation.

Photoacoustic spectroscopy (PAS) measures the photon absorption spectrum of a sample through detection of the acoustic wave generated by the photothermal effect as one modulates the intensity of the incident radiation at each wavelength. We have recently demonstrated the implementation of PAS in a microscopy configuration with mid-infrared radiation (microPAS). In the present work, we describe the performance of microPAS using synchrotron radiation (SR) in diffraction-limited spectromicroscopy and imaging experiments.

Micro-solvation of CO in water: infrared spectra and structural calculations for (DO)-CO and (DO)-CO.

The weakly-bound molecular clusters (D2O)2-CO and (D2O)3-CO are observed in the C-O stretch fundamental region (≈2150 cm-1), and their rotationally-resolved infrared spectra yield precise rotational parameters. The corresponding H2O clusters are also observed, but their bands are broadened by predissociation, preventing detailed analysis. The rotational parameters are insufficient in themselves to determine cluster structures, so ab initio calculations are employed, and good agreement between the experiment and theory is found for the most stable cluster isomers, yielding the basic cluster geometries as well as confirming the assignments to (D2O)2-CO and (D2O)3-CO.

Infrared bands of CS dimer and trimer at 4.5 μm.

We report observation of new infrared bands of (CS) and (CS) in the region of the CS ν + ν combination band (at 4.5 µm) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium.

Thermal-wave resonant cavity signal processing.

The thermal-wave resonant cavity (TWRC) technique has been used for thermal diffusivity measurements by many researchers. This study aims to reduce the uncertainty associated with TWRC signal processing (curve fitting) by means of numerical simulation and experimental verification. Simulations show that the plot of signal amplitude versus cavity length can be fitted to a simplified model reported previously when the initial fitting position is at least twice the thermal-wave diffusion length (2 μ), and that the uncertainty caused by different end positions is negligible in the range of 6-10 μ.

Characterization of OCS-HCCCCH and NO-HCCCCH Dimers: Theory and Experiment.

The infrared spectra of the weakly bound dimers OCS-HCCCCH, in the region of the ν fundamental band of OCS (2050 cm), and NO-HCCCCH, in the region of the ν fundamental band of NO (2200 cm), were observed in a pulsed supersonic slit jet expansion probed with tunable diode/QCL lasers. Both OCS-HCCCCH and NO-HCCCCH were found to have planar structure with side-by-side monomer units having nearly parallel axes. These bands have hybrid rotational structure which allows for estimates of the orientation of OCS and NO in the plane of their respective dimers.

Discerning Inter- and Intramolecular Vibrations of Sulfur Polyaromatic Compounds.

Thiophenes are an important class of molecules in fields as diverse as petrochemistry, molecular electronics, and optoelectronics. Thiophenic submolecular motifs are thought to play a role in molecular association and nanoaggregation phenomena in both pure materials and natural and synthetic mixtures. Vibrational (infrared and Raman) spectroscopy provides the means to characterize these species.

New Infrared Bands of the C-Bonded Isomer of OC-NO and Determination of Two Intermolecular Frequencies.

Three combination bands involving intermolecular modes of the most stable isomer of the OC-NO van der Waals complex have been observed, two in the carbon monoxide CO stretch region (∼2150 cm) and one in the ν asymmetric stretch region of NO (∼2223 cm). Vibrational assignment is achieved by comparison with data recently published ( Barclay , A. ; et al.

Step-scan differential Fourier transform infrared photoacoustic spectroscopy (DFTIR-PAS): a spectral deconvolution method for weak absorber detection in the presence of strongly overlapping background absorptions.

The determination of small absorption coefficients of trace gases in the atmosphere constitutes a challenge for analytical air contaminant measurements, especially in the presence of strongly absorbing backgrounds. A step-scan differential Fourier transform infrared photoacoustic spectroscopy (DFTIR-PAS) method was developed to suppress the coherent external noise and spurious photoacoustic (PA) signals caused by strongly absorbing backgrounds. The infrared absorption spectra of acetylene (CH) and local air were used to verify the performance of the step-scan DFTIR-PAS method.

Detection of a higher energy isomer of the CO-NO van der Waals complex and determination of two of its intermolecular frequencies.

Infrared spectra in the carbon monoxide CO stretch region (∼2150 cm) and in the ν asymmetric stretch region of NO (∼2223 cm) are assigned to the previously unobserved O-bonded form of the CO-NO dimer ("isomer 2"). This van der Waals complex has a planar skewed T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective intermolecular distance between the centers of mass is 3.

Communication: Spectroscopic observation of the O-bonded T-shaped isomer of the CO-CO2 dimer and two of its intermolecular frequencies.

Infrared spectra in the carbon monoxide CO stretch region (≈2150 cm(-1)) are assigned to the previously unobserved O-bonded form of the CO2-CO dimer ("isomer 2"), which has a planar T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective center of mass intermolecular distances are 3.58 Å for isomer 2 as compared to 3.

New combination bands of N2O-CO2, N2O-OCS, and N2O-N2 complexes in the N2O ν(1) region.

Spectra of the weakly bound complexes N2O-CO2, N2O-OCS, and N2O-N2 were studied in the region of the ν1 fundamental of N2O (∼2224 cm(-1)) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion with an effective rotational temperature of about 2.5 K. One new combination band was observed for each complex: a band involving an intermolecular in-plane bending mode for N2O-N2, a band involving the disrotation (in-plane geared bend) for of N2O-CO2, and a band involving the out-of-plane torsional vibration for isomer b of N2O-OCS.

A simplified model for thermal-wave cavity self-consistent measurement of thermal diffusivity.

A simplified theoretical model was developed for the thermal-wave cavity (TWC) technique in this study. This model takes thermal radiation into account and can be employed for absolute measurements of the thermal diffusivity of gas and liquid samples without any knowledge of geometrical and thermal parameters of the components of the TWC. Using this model and cavity-length scans, thermal diffusivities of air and distilled water were accurately and precisely measured as (2.

A new look at the infrared spectrum of the weakly bound CO-N2 complex.

A broad-band (2135-2165 cm(-1)) infrared spectrum of the CO-N2 van der Waals complex is obtained, using a tunable quantum cascade laser to probe a pulsed supersonic expansion from a slit jet source. Analysis of the spectrum results in the characterization of four new 'stacks' of rotational levels for CO-orthoN2 (all in the v(CO) = 1 upper state) and five new stacks for CO-paraN2 (three in the upper state and two in the vCO = 0 lower state). This considerably expands our knowledge of a rather fundamental weakly bound complex and should lead to improved determinations of the intermolecular forces governing interactions between the carbon monoxide and nitrogen molecules.

Communication: Spectroscopic evidence for a planar cyclic CO trimer.

A high-resolution spectrum in the region of 2144 cm(-1) is assigned to the previously elusive CO trimer. In spite of interference from the CO dimer and some remaining unexplained details, there is strong evidence for a planar, cyclic, C-bonded trimer structure, with C(3h) symmetry and 4.42 Å intermolecular separation, in agreement with theoretical calculations.

CO dimer: the infrared spectrum revisited.

A broad-band (2135-2200 cm(-1)) infrared spectrum of the CO dimer is recorded using a tunable quantum cascade laser to probe a supersonic jet expansion with an effective rotational temperature of about 2.5 K. Analysis of the spectrum reveals the first known levels of the excited state (vCO = 1) with A(+) symmetry and establishes that resonant vibrational splittings are small (<0.

Dual beam photoacoustic infrared spectroscopy of solids using an external cavity quantum cascade laser.

Quantum cascade laser-based instrumentation for dual beam photoacoustic (PA) spectroscopy is described in this article. Experimental equipment includes a 4.55 μm (2141-2265 cm(-1)) continuous wave external cavity quantum cascade laser (EC-QCL), two gas-microphone PA cells, and two lock-in amplifiers.

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3.

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry.

On the sensitivity of thermophotonic lock-in imaging and polarized Raman spectroscopy to early dental caries diagnosis.

Dental caries is the leading cause of tooth loss, which can promptly be prevented if detected in early stages of progression. Unfortunately, conventional diagnostic modalities currently used in dentistry lack the sensitivity to detect early caries. The authors' intention is to compare the ability of polarized Raman spectroscopy and thermophotonic imaging to make early caries diagnosis.

Nonpolar nitrous oxide dimer: observation of combination bands of (14N2O)2 and (15N2O)2 involving the torsion and antigeared bending modes.

Spectra of the nonpolar nitrous oxide dimer in the region of the N(2)O ν(1) fundamental band were observed in a supersonic slit-jet apparatus. The expansion gas was probed using radiation from a quantum cascade or a tunable diode laser, with both lasers employed in a rapid-scan signal averaging mode. Four bands were observed and analyzed: new combination bands involving the intermolecular conrotation of the monomers (A(g) antigeared bend) for ((14)N(2)O)(2) and ((15)N(2)O)(2), the previously reported torsional combination band for ((14)N(2)O)(2) with improved signal-to-noise ratio, and the same torsional combination band for ((15)N(2)O)(2).

Soret effect and photochemical reaction in liquids with laser-induced local heating.

We report a theoretical model and experimental results for laser-induced local heating in liquids, and propose a method to detect and quantify the contributions of photochemical and Soret effects in several different situations. The time-dependent thermal and mass diffusion equations in the presence and absence of laser excitation are solved. The two effects can produce similar transients for the laser-on refractive index gradient, but very different laser-off behavior.

Photoacoustic spectroscopy using coherent synchrotron radiation: application to α-lactose monohydrate.

We produced coherent synchrotron radiation at the Canadian Light Source between about 5 and 30 cm(-1) in bursting and continuous emission modes and used it to acquire photoacoustic spectra of solids. A band was observed in the spectrum of α-lactose monohydrate at 18 cm(-1) and attributed to a rotational mode, in agreement with published data obtained using other numerical and experimental techniques.

Thermal-lens study of photochemical reaction kinetics.

We consider the time dependence of the absorption coefficient due to the photoinduced chemical reaction (PCR) and species diffusion to calculate the temperature rise in the thermal-lens (TL) effect. The TL signal at the detector plane is also calculated. This theoretical approach removes the restriction that the PCR time constant is much greater than the characteristic TL time constant, which was assumed in a previously published model.

Photoacoustic infrared spectroscopy of polymer beads.

Photoacoustic (PA) spectra of four types of polymer resin beads, ranging in size from 35 to 150 microm, were acquired using a Fourier transform infrared spectrometer capable of both rapid- and step-scan mirror movement. Thermal diffusion lengths were on the order of the particle sizes of the beads. The PA magnitude spectra were similar to absorption spectra; both positive- and negative-going features occurred in the phase spectra.