Synchronous Fluorescence as a Sensor of Trace Amounts of Polycyclic Aromatic Hydrocarbons.

Synchronous fluorescence spectroscopy (SFS) is a technique that involves the simultaneous detection of fluorescence excitation and emission at a constant wavelength difference. The spectrum yields bands that are narrower and less complex than the original excitation and emission bands. The SFS bands correspond uniquely to the fluorescing molecule. Our investigation focuses on evaluating the sensitivity of the SFS technique for the detection and quantitation of PAHs relevant to astrochemistry. Results are presented for naphthalene, anthracene, and pyrene in three different solvents: n-hexane, water, and ethanol. SF bands are obtained with a constant wavelength difference between the peak excitation and emission wavelength (Δλ = λex - λem) at a concentration ranging from 10-4 to 10-10 M. Limit of detection (LOD) and limit of quantitation (LOQ) calculations are based on integrated SF band areas at different concentrations. Spectra of 23 pg/g of anthracene, 16 pg/g, and 2.6 pg/g of pyrene are recorded using ethanol as the solvent. The PAHs exhibit detection limits in the fractions of parts-per-billion (ng/g) range. Through comparison with similar prior studies employing fluorescence emission, our findings reveal a better detectability limit, demonstrating the effectiveness and applicability of the SFS technique.

Vibrational fundamental and overtone spectra of C2H4 in cryogenic liquid solutions.

Vibrational Fundamental and overtone transitions of C2H4 around the Δυ = 1-3 in liquid Ar, Kr, Xe and N2 solutions have been obtained using a low temperature cryostat and a Fourier transform infrared spectrophotometer for wavenumbers between 800 and 10,000 cm-1. The visible CH spectra for Δυ = 6 of ethylene in cryogenic solutions of the same solvents and liquid ethane have been measured with a low temperature cell and the thermal lens technique. Spectra in solutions show great simplification of the bands with respect to gas phase absorption bands. Assignments have been made based on gas phase transitions. Peak positions (ν), wavenumber shifts (Δν), and full widths at half maximum (Δω1/2) are reported. Changes of C2H4 frequencies in liquid Ar, Kr, and Xe seem to correlate with an increase in molar volume, dielectric constant, temperature, and polarizability of the solvents. Influence of the solvent on some fundamental vibrational frequencies are explored using the Onsager model and the polarizable continuum model (PCM). When used in conjunction with calculated anharmonic frequencies, the PCM model shows qualitative agreement with frequency shifts.

Linear and Nonlinear Thermal Lens Signal of the (Δν = 6) C-H Vibrational Overtone of Naphthalene in Liquid Solutions of n-Hexane.

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of naphthalene (C10H8) in liquid hexane. The C-H fifth vibrational (Δν = 6) overtone spectrum of C10H8 is detected at room temperature for mole fractions from 0.08 to 19 × 10-6 using n-C6H14 as solvent. By detecting the absorption band in a 19 ppm (parts per million) solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10-7 cm-1. A plot of normalized integrated intensity as a function of the mole fraction of naphthalene in solution reveals a dependence of the magnitude of the signal with the probe laser wavelength. If the wavelength of the probe laser is 568 nm, the thermal lens signal (TLS) is linear as a function of the mole fraction of the solution. When the wavelength of the probe laser is 488 nm, the TLS is nonlinear as a function of the concentration. Three different models of nonlinear absorption are discussed. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers could explain the enhanced magnitude and the nonlinear behavior of the TLS for solutions of mole fraction < 0.1%.

Description of an air temperature sensor based on O2 absorption spectroscopy.

In this paper, we describe in detail an assembled open-path optical cavity to act as a temperature sensor of air. A metal absorption cell in a temperature-regulated tube furnace is placed at the center of an optical cavity. The optical cavity consists of two mirrors, two nitrogen buffer sleeves, and the open cell. The air is injected through a fitting in one extreme of the metal tube and travels half the tube length through a channel in the wall of the tube. The channel directs the air towards the center of the cell. The air flowing is heated at the temperature of the metal tube in contact with the furnace. The heated air injected at the center of the tube, flows towards the open extremes of the tube. The nitrogen buffer sleeves protect the mirrors from the heated air. The temperature of the air flowing through the tube is determined by measuring the absorption of the A band of oxygen as a function of the wavenumber in the 769-755 nm wavelength range. The absorption technique is phase-shift cavity ring down spectroscopy. To obtain the temperature, the energy of the lower rotational state for eleven selected rotational transitions is linearly fitted to a logarithmic function that contains the relative intensity of the rotational transition, the initial and final rotational quantum numbers and the energy of the transition. Accuracy of the measurement is determined by comparing the calculated temperature from the spectra with the analog reading of the temperature-regulated tube furnace. This technique is proposed for exhaust air temperature measurements of combustion chambers and cooling air after passing through the blades of a turbine.

C-H Infrared Absorption and Solubility of Ethylene, Propyne, 2-methyl-2-butene, and 2-methyl-1, 3-butadiene (Isoprene) in Liquid Argon Solutions.

The solubility of ethylene (H2C=CH2), propyne (CH3-C≡C-H), 2-methyl-2-butene (CH3-CH=C(CH3)2), and isoprene or 2-methyl-1, 3-butadiene (H2C=C(CH3)-CH=CH2) in liquid argon has been measured using mid-infrared and near-infrared (NIR) absorption. Spectra were recorded in the C-H infrared (IR) region. Spectra were obtained at increasing solution composition until the magnitude of the integrated absorption band reached a maximum value, indicating a saturated solution. The approximate experimental solubilities are: (600 ± 100) ppm at 92 K for ethylene, (22 ± 9) ppm at 100 K for propyne, (9 ± 5) ppm at 100 K for 2-methyl-2-butene, and (12 ± 2) ppm at 86 K for isoprene. The experimental solubility values at the corresponding temperature were used with solubility parameters of two separate models: the perturbed-chain statistical associating fluid theory (PC-SAFT) and the regular solution theory. Solvent-solute interaction parameters k12 (PC-SAFT) and [Formula: see text] (RST) were obtained for each solute in the presence of argon as the solvent. Data from experimental measurements are important for more realistic simulations of solubility of solids in cryogenic liquids.

Cavity Ring-Down Absorption of O2 in Air as a Temperature Sensor for an Open and a Cryogenic Optical Cavity.

The A-band of oxygen has been measured at low resolution at temperatures between 90 K and 373 K using the phase shift cavity ring down (PS-CRD) technique. For temperatures between 90 K and 295 K, the PS-CRD technique presented here involves an optical cavity attached to a cryostat. The static cell and mirrors of the optical cavity are all inside a vacuum chamber at the same temperature of the cryostat. The temperature of the cell can be changed between 77 K and 295 K. For temperatures above 295 K, a hollow glass cylindrical tube without windows has been inserted inside an optical cavity to measure the temperature of air flowing through the tube. The cavity consists of two highly reflective mirrors which are mounted parallel to each other and separated by a distance of 93 cm. In this experiment, air is passed through a heated tube. The temperature of the air flowing through the tube is determined by measuring the intensity of the oxygen absorption as a function of the wavenumber. The A-band of oxygen is measured between 298 K and 373 K, with several air flow rates. To obtain the temperature, the energy of the lower rotational state for seven selected rotational transitions is linearly fitted to a logarithmic function that contains the relative intensity of the rotational transition, the initial and final rotational quantum numbers, and the energy of the transition. Accuracy of the temperature measurement is determined by comparing the calculated temperature from the spectra with the temperature obtained from a calibrated thermocouple inserted at the center of the tube. This flowing air temperature sensor will be used to measure the temperatures of cooling air at the input (cold air) and output (hot air) after cooling the blades of a laboratory gas turbine. The results could contribute to improvements in turbine blade cooling design.

Phase shift cavity ring down and Fourier transform infrared measurements of C-H vibrational transitions, energy levels, and intensities of (CH3)3Si-C≡C-H.

Phase shift cavity ring down and Fourier transform IR techniques have been used to observe the C-H stretch fundamental and overtone absorptions of the acetylenic (Δυ = 1-5) and methyl (Δυ = 1-6) C-H bonds of trimethyl-silyl-acetylene [(CH3)3CSi≡CH] at 295 K. Harmonic frequencies ω(ν1), ω(a), and ω(s) and anharmonicities x(ν1), ω(a)x(a), ω(s)x(s) were calculated for the acetylenic, methyl out-of-plane, and methyl in-plane C-H bonds, respectively. The harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. A hot band, assigned as υν1 + ν24 - ν24 is observed for transitions with Δυ = 1-5 in a region near the acetylenic stretch. The intensity of the hot band is reduced considerably at 240 K. The strength of a Fermi resonance between C-Ha transition (υν(a)) and the combination band ((υ-1)ν(a) + 2ν(bend)) with (υ = 3-6) was calculated using the experimental perturbed energies and relative intensities. The main bands are separated by computer deconvolution and are integrated at each level to get the experimental band strengths. For methyl absorptions, the dipole moment function is expanded as a function of two C-H stretching coordinates and the intensities are calculated in terms of the HCAO model where only the C-H modes are considered. Acetylenic intensities are derived with a one dimensional dipole moment function. The expansion coefficients are obtained from molecular orbital calculations. The intensities are calculated without using adjustable parameters and they are of the same order of magnitude of the experimental intensities for all C-H transitions.

Vibrational overtone spectroscopy, energy levels, and intensities of (CH3)3C-C≡C-H.

The vibrational overtone spectra of the acetylenic (Δυ = 4, 5) and methyl (Δυ = 5, 6) C-H stretch transitions of tert-butyl acetylene [(CH(3))(3)C-C≡C-H] were obtained using the phase shift cavity ring down (PS-CRD) technique at 295 K. The C-H stretch fundamental and overtone absorptions of the acetylenic (Δυ = 2 and 3) and methyl (Δυ = 2-4) C-H bonds have been obtained using a Fourier transform infrared and near-infrared spectrophotometer. Harmonic frequency ω(ν(1)) and anharmonicities x(ν(1)) and x(ν(1), ν(24)) are reported for the acetylenic C-H bond. Molecular orbital calculations of geometry and vibrational frequencies were performed. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. Band strength values were obtained experimentally and compared with intensities calculated in terms of the HCAO model where only the C-H modes are considered. No adjustable parameters were used to get order of magnitude agreement with experimental intensities for all pure local mode C-H transitions.

Cavity ring down and Fourier transform infrared spectroscopy at low temperatures (84-297 K): Fermi resonance and intensities of the C-H fundamental and overtone (Delta(upsilon) = 1-6) transitions of CHD(3).

The C-H stretch fundamental and overtone absorptions of CHD(3) have been obtained using Fourier transform infrared (FTIR), near-infrared, and phase shift and pulsed cavity ring down (CRD) techniques at temperatures between 84 and 297 K. The partially resolved rotational-vibrational spectra of CHD(3) that included the fundamental transition nu(1), the overtone transitions, and combination bands 5nu(1) and 4nu(1) + 2nu(5) and 6nu(1) and 5nu(1) + 2nu(5) were obtained and compared with the simulated spectra at the corresponding temperature. The strength of the Fermi resonance between levels upsilonnu(1) and (upsilon - 1)nu(1) + 2nu(5) with upsilon = 2-6 was calculated using the experimental perturbed energies and relative peak intensities. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the cross section and the band strength of the Deltaupsilon = 5 and 6 transitions. Ab initio molecular orbital calculations were done to obtain the dipole moment function (DMF) as a function of the C-H internuclear distance. Intensity calculations with the DMF correctly predict the order of magnitude of the experimental band strengths.

Vibrational overtone spectroscopy of saturated hydrocarbons dissolved in liquefied Ar, Kr, Xe, and N2.

This article presents a collection of vibrational overtone spectra of hydrocarbons in cryogenic solutions. Vibrational overtone spectra of ethane and propane dissolved in liquid argon and n-butane and isobutane dissolved in liquid krypton were recorded between 5000 and 14,000 cm(-1). Spectral regions for the first four overtones were measured using a Fourier transform spectrophotometer. The fifth overtone (Deltaupsilon = 6) spectra were recorded with a double beam (pump-probe) thermal lens technique using concentrations as low as 10-3 mole fraction. We obtained the C-H (Deltaupsilon = 6) spectra of (a) liquid ethane at 100 K and ethane in solutions in liquid Ar at 92 K and liquid N2 at 85 K, (b) liquid propane at 148 K and propane in liquid Ar at 93 K, (c) n-butane in liquid Kr at 129 K, (d) n-pentane in liquid Xe at 160 K, and (e) isobutane liquid at 135 K and isobutane in liquid Kr at 130 K. Local-mode parameters were calculated for primary, secondary, and tertiary C-H oscillators in solution and compared with gas-phase local-mode parameters. The peak frequency shift (Deltaomega) from gas phase to solution is explained by the change in harmonic frequency and anharmonicity in solution with respect to the gas-phase values. The bandwidth (Deltaomega1/2) of the (Deltaupsilon = 6) C-H absorption bands of ethane in solution can be explained in terms of collisions with the solvent molecules.

Thermal lens spectroscopy in cryogenic solutions: Analysis and comparison of intensities in CH4-N2 and CH4-Ar liquid solutions.

The C-H (Delta upsilon = 6) absorption spectrum of methane has been obtained in liquid nitrogen and argon solutions using thermal lens spectroscopy. Mathematical models for continuous and periodic excitation were used to describe the concentration dependence of thermal lens intensities. Better fitting of the experimental results was accomplished by taking into account the periodic nature of the excitation. Comparison of thermal lens intensities in nitrogen and argon allowed calculation of relative enhancement factors. In dilute solutions, the intensity in liquid nitrogen was 1.39 +/- 0.08 times higher than that in argon. The literature estimation of this ratio is 1.49 +/- 0.61. In contrast to the estimated value, our result confidently shows actual variation in enhancement factors. This article not only shows the applicability of thermal lensing to cryosamples but also demonstrates that accurate measurements at low temperature are possible.

Thermal lens spectroscopy in liquid argon solutions: (Deltav = 6) C-H vibrational overtone absorption of methane.

A dual-beam thermal lens technique has been used to obtain the absorption spectrum of the (Deltav = 6) C-H stretch of liquid methane and methane in liquid argon solutions. The lowest concentration detected was 1 x 10(-3) (mole fraction) of CH(4) in liquid Ar with a continuous wave laser power of 20 mW. The thermal lens signal is linear with the mole fraction of methane up to 1 x 10(-2) but not for higher concentrations. Considering the system CH(4)-Ar as an ideal solution, the factors that contribute to the thermal lens signal were calculated as a function of the concentration of methane. A mechanism of energy transfer based on the gas-phase results could explain qualitatively the dependence of the magnitude of the signal on the mole fraction of methane.