Laser Cavity Ring-down Spectroscopy Research Articles

Trace detection of volatile organic compounds by diode laser cavity ring-down spectroscopy

The use of continuous wave cavity ring-down spectroscopy (cw CRDS) with near infra-red diode lasers is demonstrated for quantitative detection of trace levels of unsaturated volatile organic compounds (VOCs) at wavelengths that avoid overlapping absorptions by more abundant atmospheric constituents such as H2O and CO2. The current detection limit, with due allowance for pressure broadening by 1 atmosphere of air, is 6 parts per billion by volume (ppbv) for ethyne at an air wavelength of 1519.670 nm, and is sufficient for direct atmospheric detection of this molecule in many urban environments. Detection limits for alkenes are inferior, and, without incorporating the consequences of pressure broadening, include 78 ppbv for ethene and 900 ppbv for 1,3-butadiene. While the CRDS detection method offers several advantages over established gas chromatographic techniques for monitoring of small VOCs such as ethyne, it appears to be less well suited to study of larger organic compounds. Methods are discussed for improving the instrument to reach the sensitivities required to monitor the various alkenes and other C–H containing molecules in the troposphere.

High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm−1 in slit jet expansions and at room temperature

The ν2+2ν3 combination band of CH412 near 7510 cm−1 was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. ν2+2ν3 is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of ν2+2ν3 at 7510.3378±0.0010 cm−1, the integrated band strength G=(1.3±0.2)×10−4 pm2 and the vibrational transition moment |μν|=(1.0±0.1)×10−3 D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F1-component of ν2+2ν3 interacting by a strong Coriolis resonance with the IR-active F2-component appears to be a dominant perturber.

Laser-induced dispersed vibration–rotation fluorescence: a new technique to study molecular states

Two sensitive high-resolution laser absorption techniques, intra-cavity photo-acoustic and cavity ring-down spectroscopy (CRDS), are discussed in the context of laser-induced dispersed fluorescence studies of molecular overtone states. These absorption methods provide spectral data of comparable quality. The laser-induced dispersed fluorescence experiment offers new possibilities when applied to vibration–rotation states within the ground electronic states. Particularly, some new states in symmetrical molecules not accessible by one-photon absorption spectroscopy can be reached. Interesting collisional effects including nuclear spin conversion can also be investigated.

Laser absorption spectroscopy diagnostics of nitrogen-containing radicals in low-pressure hydrocarbon flames doped with nitrogen oxides.

Absolute concentration profiles of NH2 and HNO have been measured in low-pressure methane/air flat flames doped with small amounts of NO and N2O. Addition of a small amount of nitrogen oxides does not alter significantly the flame speeds, temperature profiles and other parameters of the relatively well-understood methane/air flames. Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) are high-sensitivity techniques used to measure absolute concentrations of minor species in flames. In this work ICLAS is used to monitor NH2 and HNO, whereas CRDS is used for temperature measurements using OH spectra in the UV range. The (090)-(000) and (080)-(000) bands of the A2A1-X2B1 electronic transition of NH2 and (100)-(000) and (011)-(000) bands of the A1A"-X1A' transition of HNO are used. Methane flames of different equivalence ratios are used. NH2 and HNO are observed in the flame as well as in the zone surrounding the flame, closer to the walls of the low-pressure chamber where the burner is located. An absorption originating from the species in this zone can affect substantially the results of line-of-sight experiments. A slow flow of nitrogen through the optical window holders was added in order to separate the spectra of HNO originating from the central flame zone. Calculations based on the commonly used GRI-Mech chemical mechanism predict two maxima in the HNO concentration profile in the NO doped flames. The first is located in the vicinity of the burner, and the second is closer to the luminescence flame zone. We were able to observe the first maximum, and its measured location agrees well with prediction. On the other hand, GRI-Mech strongly underpredicts the observed absolute concentration of HNO in this maximum. The measured absolute concentrations of NH2 are in reasonable agreement with the GRI-Mech predictions.

Carbon chains and rings in the laboratory and in space

Seventy-seven reactive organic molecules of astrophysical interest have been identified in a supersonic molecular beam, 73 in the radio band by Fourier-transform microwave spectroscopy, four in the optical by laser cavity ringdown spectroscopy. Most are linear carbon chains, but six consist of carbon chains attached to the compact, highly polar C 3 ring, and two are rhomboidal cyclic configurations of SiC 3. The laboratory astrophysics of the radio molecules is complete for the time being, in the sense that essentially all the rotational transitions of current interest to radio astronomy (including hyperfine structure when present) can now be calculated to a small fraction of 1 km s -1 in equivalent radial velocity; six of the radio molecules have already been detected in space on the basis of the present data. The FTM spectrometer employed in this work is far from fundamental limits of sensitivity, so many more molecules can probably be found by refinements of present techniques. The density of reactive molecules in our supersonic beam is generally high by the standards of laser spectroscopy, and many of the radio molecules probably have detectable optical transitions which we are attempting to find, largely motivated by the long-standing problem of the diffuse interstellar bands. Our most interesting result to date is the detection of a fairly strong molecular band at 443 nm in a benzene discharge, in exact coincidence with the strongest and best known interstellar band. Isotopic shifts measured with partially and totally deuterated benzene suggest that the carrier of the laboratory band is a hydrocarbon molecule with the elemental formula C n H 5, with n most likely in the range 3–6.

Ultraviolet cavity ring-down spectroscopy of free radicals in etching plasmas

Many reactive species of interest in technological plasmas absorb light in the UV spectral region (200–300 nm). Measurement of these weak absorbances (typically 10 −2–10 −4 for a single pass) allows us to determine their absolute concentration. Low-resolution absorption spectra of these systems have previously been obtained by broad-band absorption spectroscopy. Here we present spectra obtained using laser cavity ring-down spectroscopy, which has much higher spectral resolution, and potentially higher sensitivity. Spectra were obtained for CF, CF 2, AlF and SiF 2 radicals in capacitively-coupled radio-frequency plasmas in fluorocarbon gases. This technique offers the possibility of real-time (1 s) absolute concentration measurements during wafer processing.

Absorption spectroscopy measurements of NH and NH 2 absolute concentrations in methane/air flames doped with N 2O

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) were used to measure concentration profiles of NH 2 and NH in low-pressure (30 torr) methane/oxygen/nitrogen flames doped with a small amount of N 2 O. High sensitivity of these absorption spectroscopy methods was demonstrated (4×10 10 cm −3 for NH and 8×10 10 cm −3 for NH 2 ). The absolute NH concentrations agree well with those predicted by the GRI-Mech 2.11 mechanism and one-dimensional PREMIX code. The absolute NH 2 concentrations in the lean flame are fitted well by the calculations, but in the stoichiometric and the rich flames, the observed concentrations are about twice that predicted. In the rich flame ( ϕ =1.2), the calculations underpredict the NH concentrations at large distances from the burner. The strongest difference was found for the NH 2 radical at large distances above the burner. The strong spectra of NH 2 were observed even at 40 mm above the burner where the model calculations predict very insignificant concentration of NH 2 . The observed discrepancy can be caused by limitations of both the chemical mechanism and one-dimensional PREMIX code used for calculations.

Rate Coefficients for the Propargyl Radical Self-Reaction and Oxygen Addition Reaction Measured Using Ultraviolet Cavity Ring-down Spectroscopy

By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH2CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the self-reaction was determined, k∞(C3H3+C3H3) = (4.3 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 295 K. This rate coefficient, which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C3H3+H 665 Pa He) = (2.5 ± 1.1) × 10-10 cm3 molecule-1 s-1...

Radical concentration profiles in a low-pressure methane-air flame measured by intracavity laser absorption and cavity ring-down spectroscopy

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) were used to measure temperature and concentration profiles in low-pressure (30 torr) methane/oxygen/nitrogen flames. Concentration profiles of the HCO and 1CH2 radicals were measured by ICLAS, whereas the OH radical profiles and temperature profiles were measured by CRDS. Flames with equivalence ratio =0.8, 1.0, and 1.2 were studied. HCO profiles and peak amounts agree well with model predictions based on the GRI-Mech 2.11 mechanism for the stoichiometric flame (=1.0). The absolute concentration of singlet CH2 radical could not be determined accurately, because of large uncertainty in the absorption cross section of this radical. Nevertheless, the experimental singlet CH2 concentration seems to be higher than predicted by model (by more than 20 times, based on the estimated cross-section data). The relative 1CH2 profile is very close to the prediction. The OH profiles agree well with model calculation: however, absolute experimental OH peak concentration is about two times lower than the predicted value.

Flame structure studies by high resolution quadrupole mass spectrometry

Absolute concentration measurements of atoms and radicals in flames are very important for a better understanding of combustion mechanisms. Recently, several laser based methods were successfully used for such measurements. Among them are laser induced fluorescence (LIF), intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS). This article attempts to review and discuss the current status of the application of these techniques to the absolute concentration measurements in flames. The principles of two absorption spectroscopy methods, ICLAS and CRDS, are discussed in more detail as these methods are only at the beginning stage of their application to combustion diagnostics. The final portion of the review is devoted to discussing some recent measurements of absolute concentrations of radicals in hydrocarbon flat premixed flames and comparing these measurements with computer simulations.