Variable Path-length Cell Research Articles

Infrared Spectra of Dimethylquinolines in the Gas Phase: Experiment and Theory

Infrared spectra of atmospherically important dimethylquinolines (DMQs), namely 2,4-DMQ, 2,6-DMQ, 2,7-DMQ, and 2,8-DMQ in the gas phase at 80 degrees C were recorded using a long variable path-length cell. DFT calculations were carried out to assign the bands in the experimentally observed spectra at the B3LYP/6-31G* level of theory. The spectral assignments particularly for the C-H stretching modes could not be made unambiguously using calculated anharmonic or scaled harmonic frequencies. To resolve this problem, a scaled force field method of assignment was used. Assignment of fundamental modes was confirmed by potential energy distributions (PEDs) of the normal modes derived by the scaled force fields using a modified version of the UMAT program in the QCPE package. We demonstrate that for large molecules such as the DMQs, the scaling of the force field is more effective in arriving at the correct assignment of the fundamentals for a quantitative vibrational analysis. An error analysis of the mean deviation of the calculated harmonic, anharmonic, and force field fitted frequencies from the observed frequency provides strong evidence for the correctness of the assignment.

Infrared spectra of dimethylnaphthalenes in the gas phase

We report the infrared spectra at 0.5 cm −1 resolution of three dimethylnaphthalenes (DMNs), namely 1,5-DMN, 1,6-DMN and 2,6-DMN in the gas phase at an elevated temperature recorded with the help of a variable path-length cell. DFT calculations have been carried out at B3LYP levels of theory with 6-31G* as basis set to determine the harmonic frequencies and intensities of the DMNs to assign the experimentally observed spectra. We have compared the experimentally observed and theoretically calculated spectra of the dimethylnaphthalenes and assignments have been made. The observed and predicted frequencies and relative intensities are generally in good agreement. The intense aromatic C–H out-of-plane bending vibration observed around 800 cm −1 and three methyl C–H symmetric and asymmetric stretching vibrations around 2900 cm −1 have been recognized as unique bands to identify various DMNs in a mixture. The high-resolution IR spectroscopy of these three important polycyclic aromatic hydrocarbons which are present in the atmosphere have been discussed.

Inexpensive Variable Pathlength Cells for Discovery-Based Investigation of the Beer-Lambert Law

Inexpensive Variable Pathlength Cells for Discovery-Based Investigation of the Beer-Lambert Law

Variable Path-Length Cells for Discovery-Based Investigation of the Beer-Lambert Law

An economical approach to the variation of path length for visible absorption measurements is demonstrated. The method is particularly useful for discovery-based learning of the Beer-Lambert law.

Variable pathlength cells for precise hypersonic spectrometry of liquids up to 5 GHz

A method is described to measure precisely the ultrasonic absorption coefficient α and sound velocity v 3 of liquids between about 300 MHz and 5 GHz. An easy-to-handle automatic measuring procedure enables relatively fast acquisition of data. The construction of two different specimen cells is described. One cell, using surface excitation of small rod-shaped piezoelectric lithium niobate crystals allows for broadband measurements up to about 3 GHz. The other cell utilizes thickness vibrations of thin ZnO films. It is appropriate for measurements above 2 GHz.

Diode-pumped thulium-doped zirconium fluoride fibreas afluorescent source for water sensing

Tm3+-doped zirconium fluoride fibre pumped with a low-cost diode laser at 785 nm is shown to function as a useful source for water or humidity sensing. The near-IR fluorescent emission band centred at 1.47 µm exhibits considerable overlap with an absorption band of H2O. Using a variable path-length cell and the residual pump signal as a reference, the fluorescent fibre source is shown to enable measurement of path lengths as low as 1 µm in liquid water. In addition, using a declad fibre as an evanescent-wave absorption probe, the feasibility of measuring water concentrations in other liquids is demonstrated.

The optical constants of di-iso-octyl phthalate and di-n-butyl phthalate

The optical constants-refractive index ( n) and absorptive index ( k) have been found for di-iso-octyl phthalate [C 6H 4(COO · C 8H 17) 2] and di- n-butyl phthalate [C 6H 4(COO · C 4H 9) 2] for a wave-length range extending from 2 to 16 μm. The absorptive index has been found by transmission measurements using a variable path length cell. Relative specular reflectances, at a near normal angle of incidence, were measured with distilled water as the reflectance standard. The reflectances and the measured k values were substituted into a form of the Fresnel equation to give the corresponding refractive index values. In the wavelength regions where the k value proved too great to be measured directly by the transmission technique the n and k values were found using a Kramers-Kronig dispersion analysis.

Ultrasonic absorption and velocity measurements at mW peak power level in the range 50-500 MHz with variable pathlength cell

Excitation of LiNbO3-transducers at odd harmonics up to 500 MHz with peak power below 5 mW, a new 0.01-7.5 mm pathlength cell with linear positioner driven by a stepping-motor, sensitive superhet-detection, and signal averaging are used to determine absolute ultrasonic absorption alpha /f2 and velocity nu in liquids to better than 1% accuracy. The cell can also be operated as ultrasonic resonator cavity near 24 MHz for distance measurements with 10 nm resolution and reproducibility.

A Variable Pathlength Cell for Spectrophotometry of Trace Gases

In the application of optical methods to remote sensing of air pollution, some difficulties have been experienced in calibrating instruments, based on resonance absorption, that are suitable for air quality measurements over a long pathlength. Instruments for this application are correlation spectrometers (i.e., Barringer Cospec, Barringer Research Ltd., Rexdale, Ontario, Canada) which use a modulated Xe lamp for average concentration measurements of SO2 at around 310 nm and of NO2 at around 440 nm, and tunable laser analyzers which use specific absorption of infrared light emitted by a diode laser or by other tunable laser devices.

Variable Path-Length Cell for the Measurement of the Absorption of Liquids in the Infra-Red Region of the Spectrum

A variable path-length cell is described for the quantitative examination of light absorption by liquids in the infrared region of the spectrum. The cell has a small capacity and shows excellent repeatability in setting at any given thickness. It is practically leak-proof when used with hydrocarbons and can be adjusted to a thickness of 0.004 in, with an error of about 1 per cent.