Cavity Ring-down Spectroscopy Measurements Research Articles

Phase-shift cavity ring-down spectroscopy using mid-IR light from a difference frequency generation PPLN waveguide

Approximately 200 microW of mid-infrared (mid-IR) light around 3081 cm(-1), produced by difference frequency generation (DFG) in a periodically poled lithium niobate crystal waveguide, has been used for phase-shift cavity ring-down spectroscopy measurements. The overlapping (12)C(2)H(4) (R)P(0)(14) and (P)R(6)(10) and (12)CH(2) (13)CH(2) (P)Q(3)(10) rotational lines of the nu(9) fundamental ethene vibrational band at 3081.0016 cm(-1) were probed in proof-of-principle experiments, and ethene detection was demonstrated with a minimum absorption coefficient of 1.4 x 10(-7) cm(-1) (approximately 4 min acquisition time). The compact DFG system, with a >35 cm(-1) tuning range, has a considerable potential for use in trace gas detection and in molecular spectroscopy.

Time-Resolved CRDS Measurements of the N2(A3Σu+) Density Produced by Nanosecond Discharges in Atmospheric Pressure Nitrogen and Air

Cavity ring-down spectroscopy (CRDS) is used to measure the number density of N2(A3Sigmau+) metastables produced by nanosecond repetitively pulsed discharges in nitrogen and air preheated at 1000 K and atmospheric pressure. The densities of N2(A) are inferred from the absorbance of the Q1(22) and Q3(16) lines of the (2 <-- 0) vibrational band of the first positive system (B3Pig - A3Sigmau+) of N2 at 769.945 nm. The procedure for determining the temporal evolution of the density of metastable from the measured ring down signals is presented. The maximum number densities are in the range of 10(14)-10(15) molecules cm-3 for air and nitrogen discharges, respectively. In nitrogen, the decay of the N2(A) density is shown to be a second-order process with a rate coefficient of 1.1 x 10(-9) cm3 s-1 at 1600 K with a factor of 2 uncertainty. In air, the decay is estimated to be 1 order of magnitude faster than that in nitrogen owing to quenching by atomic and molecular oxygen. Furthermore, the rotational temperature is determined by comparison of CRDS measurements and simulations of several rotational lines of the (2 <-- 0) band of the first positive system of N2 between 769.8 and 770.7 nm. The rotational and vibrational temperatures are also determined by comparison of optical emission measurements and simulations of the second positive system of N2 between 365 and 385 nm. In these CRDS measurements, we achieved a temporal resolution down to 50 ns.

Ultra-sensitive optical measurements of high- J transitions in the O 2 A-band

We report frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) measurements of high- J O 2 A-band magnetic dipole line parameters. Our goals were to measure intensities and line shape parameters for the primary isotopologue of O 2 in the A-band region and to understand how these quantities compare to, and build on those contained in the H itran database. These experiments quantify line parameters of some of the weakest transitions ever observed in the laboratory. We have measured line intensities as low as 1 × 10 −30 cm molecule −1. Our data extend available data in the P-branch up to J′ = 50. Additionally, we have characterized the self-broadened and air-broadened line widths and provide new J-dependent correlation functions.

Cavity ringdown spectroscopy of collinear dual-pulse laser plasmas in vacuum

The plasma plume induced by dual-pulse laser ablation of a titanium target in vacuum was analyzed by the technique of cavity ringdown spectroscopy (CRDS). Large Doppler-splitting of the absorption spectral lines was observed which is due to increase of the velocity components parallel to the optical axis and specific features of the CRDS measurements. Vertical velocity component, the particle number density and plasma volume also show increase compared to the single-pulse laser ablation. The forward convolution best fit of absorption lineshapes was used to extract parameters describing dual-pulse laser ablation plasma plume.

Deep-UV Rayleigh scattering of N2, CH4 and SF6

Rayleigh scattering room temperature cross-section values of N2, CH4 and SF6 have been obtained between 198 and 270 nm by combining cavity ring-down spectroscopy (CRDS) and pressure ramp measurements. The experimental data have been fitted to a functional representation, describing the ∼1/λ4 like behaviour of the Rayleigh scattering cross section over a wide wavelength range. The resulting values are compared with numerical predictions, based on refractive indices and molecular anisotropy data available in the literature. From this, values of molecular volume polarizability αvol and depolarization ratios are derived. It is found that the optical extinction for all three gases is governed by Rayleigh scattering for wavelengths down to 200 nm. No absorption onsets in the specified deep-UV region have been observed.

NCN quantitative measurement in a laminar low pressure flame

The present work has been performed to quantitatively determine the NCN radical concentration profile in a laminar low-pressure CH4/O2/N2 (ϕ=1.25) flame. Laser induced fluorescence (LIF) technique has been implemented in order to record a NCN spectrum in the 326.7–330.1nm range in flame conditions, and secondly to measure the spatially resolved NCN profile in the flame by exciting the main congested branch of NCN around 329.13nm. The LIF profile has been calibrated into absorbance using cavity ring-down spectroscopy (CRDS). The calibration into absolute concentration has been done by using the PGOPHER spectral simulation program. The experimental and the simulated NCN spectra were found to be in very good agreement. It has allowed reconsidering the exact assignment of the transitions involved in the strong bandhead at flame temperature, which actually originate from the contribution of both the origin (000) band and the (010Σ+) component of the (010) band contrary to ambient temperature where only the (000) band contribution is significant around 329.13nm.Thanks to the simulation and using electronic transition dipole moments extracted from available NCN fluorescence lifetimes, we provide a direct determination of the absorption cross-section: σ(λ=329.13nm)=(9.4±1.8)×10−17cm2/molecule at 1830K. This cross-section was found constant within ±15% in the 1500–1900K temperature range. Its determination is very important for research groups concerned with NCN chemistry at high temperature and prompt-NO formation. From that cross-section the peak mole fraction of the NCN radical has been determined to be equal to 170±90ppb in the low-pressure flame. The experimental uncertainty is mainly linked to the CRDS measurement conditions close to the detection limit and should be reduced in flame conditions yielding higher NCN concentration.

Deep-UV absorption and Rayleigh scattering of carbon dioxide

Cavity ring-down spectroscopy and pressure ramp measurements have been used to determine the extinction coefficients of CO 2 in the deep-ultraviolet wavelength region, between 198 and 270 nm. The observed optical extinction confirms that there is a clear absorption onset of CO 2 in the deep-UV wavelength region. This onset has been reported previously around 205 nm. The new set of measurements presented here shows that the onset actually starts at higher energy, around 202 nm. For longer wavelengths it is found that the optical extinction demonstrates a 1/ λ 4 like behavior, as typical for Rayleigh scattering.

Sensitive Absorption Methods for Quantitative Gas Phase Kinetic Measurements. Part 2: Cavity Ringdown Spectroscopy

Cavity ringdown spectroscopy (CRDS) and frequency modulation spectroscopy (FMS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 2 of this review, the theoretical foundations of CRDS are addressed with a special emphasis on quantitative time-resolved measurements of concentration profiles. A complementary review of FMS can be found in part 1 (Z. Phys. Chem. 222 (2008) 1–30). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Recent kinetic studies based on CRDS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with 136 entries is included to facilitate the access to the large body of original literature.

High-resolution cavity ring-down spectroscopy measurements of blended H2O transitions

We probed four closely spaced rovibrational water vapor absorption transitions near $\tilde{\nu}$ =7100 cm-1 using frequency-stabilized cavity ring-down spectroscopy. Room-temperature spectra were acquired for pure water vapor in the Doppler limit and for mixtures containing ≈6.6 μmol mol-1 of water vapor in N2 at total gas pressures ranging from 6.5 kPa to 53 kPa. By measuring Lamb dips for each transition, we demonstrated a resolution of 50 kHz and determined relative transition frequencies with an uncertainty <0.4 MHz over a 10 GHz range. Pressure-induced broadening, collisional narrowing coefficients of the component transitions and line areas were retrieved by fits of model line shapes to the measured spectra. Standard and advanced models were considered including those which incorporated the combined effects of collisional narrowing and speed-dependent line broadening and line shifting. By measuring water vapor concentration with a transfer standard hygrometer, we determined line intensities in terms of measured line areas with a combined relative uncertainty of 0.6%.

Cavity ring-down spectroscopy measurement of single aerosol particle extinction. II. Extinction of light by an aerosol particle in an optical cavity excited by a cw laser

The authors present an analytical derivation of the scattered power from a spherical, homogeneous, nonabsorbing particle in a plane standing wave. The scattered power changes significantly with the position of the particle with respect to the peaks and nodes of the standing wave, even for particles whose diameters are many times the wavelength of the light. The analysis is applicable to continuous-wave cavity ring-down spectroscopy on aerosol particles, and the structure of the standing wave is expected to affect both the measured ring-down time and the shape of the ring-down trace. The dependence of the extinction on the phase of the standing wave at the location of the particle is captured in a parameter zeta which connects the current treatment to standard Mie scattering theory. Methods for calculating zeta are presented.

Cavity ring-down spectroscopy measurements of single aerosol particle extinction. I. The effect of position of a particle within the laser beam on extinction

A continuous wave distributed feedback diode laser operating in the near infrared at wavelengths close to 1650 nm has been used to measure the extinction of light by single aerosol particles. The technique of optical feedback cavity ring-down spectroscopy (CRDS) was used for measurement of CRDS events at a repetition rate of 1.25 kHz. This very high repetition rate enabled multiple measurements of the extinction of light by single aerosol particles for the first time and demonstrated the dependence of light scattering on the position of a particle within the laser beam. A model is proposed to explain quantitatively this phenomenon. The minimum detectable dimensionless extinction coefficient epsilonmin was determined to be 3x10(-6). Extinction values obtained for single spherical polymer beads from a monodisperse sample of particles of diameter of 4 microm are in near-quantitative agreement with the values calculated by the Mie scattering theory. The deviations from the Mie theory expected for measurement of extinction by CRDS using a continuous wave laser are discussed in the companion paper.

Night-time radical chemistry during the NAMBLEX campaign

Abstract. Night-time chemistry in the Marine Boundary Layer has been modelled using a number of observationally constrained zero-dimensional box-models. The models were based upon the Master Chemical Mechanism (MCM) and the measurements were taken during the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head, Ireland in July–September 2002. The model could reproduce, within the combined uncertainties, the measured concentration of HO2 (within 30–40%) during the night 31 August–1 September and of HO2+RO2 (within 15–30%) during several nights of the campaign. The model always overestimated the NO3 measurements made by Differential Optical Absorption Spectroscopy (DOAS) by up to an order of magnitude or more, but agreed with the NO3 Cavity Ring-Down Spectroscopy (CRDS) measurements to within 30–50%. The most likely explanation of the discrepancy between the two instruments and the model is the reaction of the nitrate radical with inhomogeneously distributed NO, which was measured at concentrations of up to 10 ppt, even though this is not enough to fully explain the difference between the DOAS measurements and the model. A rate of production and destruction analysis showed that radicals were generated during the night mainly by the reaction of ozone with light alkenes. The cycling between HO2/RO2 and OH was maintained during the night by the low concentrations of NO and the overall radical concentration was limited by slow loss of peroxy radicals to form peroxides. A strong peak in [NO2] during the night 31 August–1 September allowed an insight into the radical fluxes and the connections between the HOx and the NO3 cycles.

Detection of sputtered metals with cavity ring-down spectroscopy

We report on use of cavity ring-down spectroscopy (CRDS) as a means to detect and quantify ion sputtering of refractory metal species. CRDS measurements are made with a neodymium:YAG-pumped optical parametric oscillator laser system in the 375-400 nm region. CRDS sputtering measurements are presented for argon ions incident on iron, aluminum, molybdenum, and titanium. The measurements are based on absorption from fine-structure levels of the electronic ground-state multiplets. For each species, characteristic spectra are provided, the dependence of sputtered particle number density on the beam current is examined, measured densities are compared with a sputter model, and detection limits are determined. For iron, aluminum, and titanium we probe multiple fine-structure levels within the ground-state multiplet and obtain information on their relative populations.

Gold Atoms and Dimers on Amorphous SiO2: Calculation of Optical Properties and Cavity Ringdown Spectroscopy Measurements

We report on the optical absorption spectra of gold atoms and dimers deposited on amorphous silica in size-selected fashion. Experimental spectra were obtained by cavity ringdown spectroscopy. Issues on soft-landing, fragmentation, and thermal diffusion are discussed on the basis of the experimental results. In parallel, cluster and periodic supercell density functional theory (DFT) calculations were performed to model atoms and dimers trapped on various defect sites of amorphous silica. Optically allowed electronic transitions were calculated, and comparisons with the experimental spectra show that silicon dangling bonds [[triple bond]Si(.-)], nonbridging oxygen [[triple bond]Si-O(.-)], and the silanolate group [[triple bond]Si-O(-)] act as trapping centers for the gold particles. The results are not only important for understanding the chemical bonding of atoms and clusters on oxide surfaces, but they will also be of fundamental interest for photochemical studies of size-selected clusters on surfaces.

Development of Alternative Plasma Sources for Cavity Ring-Down Measurements of Mercury

We have been exploring innovative technologies for elemental and hyperfine structure measurements using cavity ring-down spectroscopy (CRDS) combined with various plasma sources. A laboratory CRDS system utilizing a tunable dye laser is employed in this work to demonstrate the feasibility of the technology. An in-house fabricated sampling system is used to generate aerosols from solution samples and introduce the aerosols into the plasma source. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Several microwave plasma discharge devices are tested for mercury CRDS measurement. Various discharge tubes have been designed and tested to reduce background interference and increase the sample path length while still controlling turbulence generated from the plasma gas flow. Significant background reduction has been achieved with the implementation of the newly designed tube-shaped plasma devices, which has resulted in a detection limit of 0.4 ng/mL for mercury with the plasma source CRDS. The calibration curves obtained in this work readily show that linearity over 2 orders of magnitude can be obtained with plasma-CRDS for mercury detection. In this work, the hyperfine structure of mercury at the experimental plasma temperatures is clearly identified. We expect that plasma source cavity ring-down spectroscopy will provide enhanced capabilities for elemental and isotopic measurements.

Measurement and modeling of a diamond deposition reactor: Hydrogen atom and electron number densities in an Ar∕H2 arc jet discharge

A combination of experiment [optical emission and cavity ring-down spectroscopy (CRDS) of electronically excited H atoms] and two-dimensional (2D) modeling has enabled a uniquely detailed characterization of the key properties of the Ar∕H2 plasma within a ⩽10-kW, twin-nozzle dc arc jet reactor. The modeling provides a detailed description of the initial conditions in the primary torch head and of the subsequent expansion of the plasma into the lower pressure reactor chamber, where it forms a cylindrical plume of activated gas comprising mainly of Ar, Ar+, H, ArH+, and free electrons. Subsequent reactions lead to the formation of H2 and electronically excited atoms, including H(n=2) and H(n=3) that radiate photons, giving the plume its characteristic intense emission. The modeling successfully reproduces the measured spatial distributions of H(n&amp;gt;1) atoms, and their variation with H2 flow rate, FH20. Computed H(n=2) number densities show near-quantitative agreement with CRDS measurements of H(n=2) absorption via the Balmer-β transition, successfully capturing the observed decrease in H(n=2) density with increased FH20. Stark broadening of the Balmer-β transition depends upon the local electron density in close proximity to the H(n=2) atoms. The modeling reveals that, at low FH20, the maxima in the electron and H(n=2) atom distributions occur in different spatial regions of the plume; direct analysis of the Stark broadening of the Balmer-β line would thus lead to an underestimate of the peak electron density. The present study highlights the necessity of careful intercomparisons between quantitative experimental data and model predictions in the development of a numerical treatment of the arc jet plasma. The kinetic scheme used here succeeds in describing many disparate observations—e.g., electron and H(n=2) number densities, spatial distributions of optical emission from the plume, the variation of these quantities with added flow of H2 and, when CH4 is added, absolute number densities and temperatures of radicals such as C2 and CH. The remaining limitations of the model are discussed.

Measurements of absolute concentration profiles of C 2 in non-sooting and sooting diffusion flames by coupling cavity ring-down spectroscopy and laser induced fluorescence

Coupling between cavity ring down spectroscopy (CRDS) and laser induced fluorescence (LIF) methods has been implemented to measure nascent C 2 (a 3Π u) in both non-sooting and sooting flames stabilised on a Wolfhard-Parker burner at atmospheric pressure. In the non-sooting flame, relative concentration profiles have been recorded by LIF measurements and calibrated by CRDS in absolute concentration (maximum of 3.4 × 10 −9 in mole fraction). In the sooting flame, it is shown that LIF is unusable to measure nascent C 2 in the sooting regions due to the creation of additional C 2 from laser soot vaporisation, even by using extremely low laser energies. It is besides demonstrated that, although CRDS measurements were also affected by PAH/soot interference, relative profile determination and absolute quantification could be satisfactorily obtained. Complete C 2 profiles in the sooting flame are thus obtained by combining LIF and CRDS. Hence, LIF can be advantageously used to measure relative profiles of C 2 in the vicinity of the burner (non-sooting zone) and CRDS can then provide determination of C 2 profiles in the presence of soot. Absolute concentration has been determined by CRDS, providing a peak mole fraction of 6.8 × 10 −9. Based on a comparison with PAH and soot profiles issued from a previous work, the role of C 2 in soot formation mechanism is discussed. A comparison with earlier profiles of CH is also proposed, showing a good correlation between C 2 and CH radical profiles.

Continuous wave cavity ring-down spectroscopy measurement of NO2 mixing ratios in ambient air

The performance of a continuous wave diode laser based cavity ring-down (CRD) spectrometer is demonstrated for retrieval of NO2 mixing ratios in ambient air. Measurements are made at wavelengths around 410 nm, a region free from other spectroscopic interferences, and the instrument is shown to be capable of quantitative and precise NO2 mixing ratio determinations without any requirement for calibration. The minimum concentration of NO2 that can be measured by the spectrometer is estimated to be 0.1 ppbv (parts per billion by volume) in 50 s at atmospheric pressure. Using dynamic dilution of a standard NO2 sample, the performance of the spectrometer has been validated for NO2 measurements over the range of 0.2 to 80 ppbv. NO2 measurements performed directly on laboratory air are compared with data obtained with a calibrated chemiluminescence instrument and shown to be in excellent agreement.

Sputter erosion measurements of titanium and molybdenum by cavity ring-down spectroscopy

We report cavity ring-down spectroscopy measurements of the gas-phase number density of titanium and molybdenum sputtered by argon ions. A neodymium: yttrium–aluminum–garnet pumped optical parametric oscillator laser system is used to probe optical absorption features of titanium and molybdenum in the 375–400 nm region. For an 18 mA ion beam current, and 750 eV ions, we find that the spatially averaged number density of sputtered atoms is 6.4±0.6×108 and 5.1±0.5×108 cm−3, for titanium and molybdenum, respectively. The measured number densities exhibit the expected linear behavior versus beam current, and are in reasonable agreement with values found from a simple sputtering model. Our current configuration yields number density detection limits of ∼9×106 and ∼7×105 cm−3, for titanium and molybdenum, respectively. The technique may ultimately provide a means to measure sputter erosion rates, which are of particular importance for the electric propulsion field.

Cavity ring-down spectroscopy measurements of the concentrations of C 2(X 1Σ g+) radicals in a DC arc jet reactor used for chemical vapour deposition of diamond films

Absorption spectroscopy measurements are reported of the absolute number density (3.0 ± 0.9 × 10 12 cm −3) and vibrational temperature (3000 ± 500 K) of C 2(X 1Σ g +) radicals in a DC arc activated chemical vapour deposition reactor under the standard operating conditions (3.3% CH 4 in H 2 and 6.4 kW discharge) used to grow polycrystalline diamond films. The ratio of number densities of C 2 radicals in their ground (X 1Σ g +) and low-lying electronically excited (a 3Π u) states is close to that expected from a Boltzmann distribution at the measured temperature, despite the more rapid rates of reaction of C 2(X) with H 2 and hydrocarbon molecules.