Cavity Ring-down Absorption Research Articles

Automated high-resolution frequency-stabilized cavity ring-down absorption spectrometer

We describe a frequency-stabilized cavity ring-down absorption spectrometer with automated scanning capabilities. The system utilizes the comb of ring-down cavity resonances as frequency markers for spectral scans. The comb is actively stabilized with respect to a frequency-stabilized reference laser, and the continuous wave, single-mode probe laser frequency is in turn actively stabilized with respect to resonances of the ring-down cavity frequency comb. Ring-down spectra are obtained by locking the external-cavity diode laser probe to sequential cavity resonances (separated by the ring-down cavity free spectral range), and the ring-down cavity frequency comb is translated by controlled amounts using acousto-optic frequency shifting methods for fine frequency steps less than the free-spectral range. A computer-controlled method for automating frequency tuning and probe laser locking to the cavity resonances is described, and high-resolution near-infrared (vicinity of 10712cm−1) absorption spectra of water vapor are presented. A frequency resolution of ≈1MHz and minimum measurable line intensity of 10−27cm2cm−1molecule−1 are demonstrated.

Development of High Dose Rate Sensing Method Based on Cavity Ring-Down Laser Spectroscopy

We have developed a new method for high radiation dose rate monitoring based on cavity ring-down laser absorption spectroscopy and have measured 60Co gamma-ray induced chemical species in stationary and flowing air at the wavelength of 532 nm to check the basic performance of the new method. In the stationary air measurement, we confirmed linearity between the absorbed dose and the measured light absorbance by the radiation induced chemical species. Furthermore, in the flowing air measurement, we found the response time and the magnitude of the light absorbance largely depend on the air flow rate. In addition, a preliminary experiment was done to demonstrate the applicability to an intensity monitor for the intense undulator X-rays of BL47XU beamline at SPring-8. Results showed that the measured ring-down rate is proportional to the photon intensity and the detectable range is from 2×1011 to 3×1015 photons/s with time resolution around 3×102 s.

Recombination studies in a He-Ar-H2 plasma

The recombination of H+3 ions with electrons has been studied in afterglow plasma in three different experiments. In two experiments, using the Variable Temperature Stationary Afterglow (VT-AISA) and the Variable Temperature Flowing Afterglow (VT-FALP) techniques, a decay of the electron number density was measured by an electrostatic Langmuir probe to determine the recombination rate coefficient. In the third experiment a near infrared Cavity Ring-Down Absorption Spectrometer (CRDS) was used to monitor the decay of the H+3 (v = 0) ion density during the afterglow. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen at total pressures ranging from 150 up to 1200 Pa and at buffer gas temperatures ranging from 100 up to 330 K. In the experiments the partial number density of hydrogen was varied from 5 × 1010 up to 1 × 1016 cm−3 and for this broad range of hydrogen number densities effective recombination rate coefficients were obtained, which varied over three orders of magnitude from 2 × 10−9 cm3s−1 at [H2] = 5 × 1010 cm−3 up to 3 × 10−6 cm3s−1 at [H2] = 1 × 1016 cm−3. Using our experimental results we discuss possible mechanisms of recombination in hydrogen plasma in a very broad range of several parameters: buffer gas pressure, temperature, electron number density, hydrogen number density and internal excitation of recombining ions.

The recombination of spectroscopically identified H3+ (v=0) ions with electrons

We report a study of the recombination of H3+ (v = 0) ions with thermal electrons at 330 and 100 K. A near infrared cavity ring-down absorption spectrometer (CRDS) working on the v2 = 3 ← 0 transition of H3+ (λ = 1382 nm) has been used to monitor the H3+ (v = 0) ion number density in decaying afterglow plasma. The plasma was created in helium gas with small admixtures of argon and hydrogen by pulses of microwaves. The measurements were carried out for hydrogen number densities ranging from 1013 up to 1016 cm−3. The total pressure in the discharge tube was 4–10 mbar. The temperature of the recombining ions was determined from the Doppler broadening of absorption lines. The obtained effective recombination rate coefficients are α (H3+ (v = 0)) = (0.8 ± 0.3) × 10−7 cm3s−1 and α(H3+ (v = 0)) = (2.3 ± 1.1) × 10−7 cm3s−1 at 330 and 100 K, respectively. The influence of the formation of H5+ at higher pressures and lower temperature is also discussed.

Rapid measurement of cavity ringdown absorption spectra with a swept-frequency laser

A novel cavity ringdown spectrometer, incorporating a miniature continuous-wave swept-frequency laser that is widely tunable, requires less than 1 s to record wide-ranging absorption spectra with high sensitivity in a single rapid sweep of the laser frequency. The free spectral range of the ringdown cavity defines a sampling grid to measure absorbance-dependent ringdown times at successive cavity-resonance frequencies. The spectrometer has a single-ended transmitter-receiver configuration based on retro-reflected optical-heterodyne detection and exploiting fibre-optical telecommunications components. This swept-frequency approach to cavity ringdown spectroscopy yields a simple, low-cost, compact, rugged, versatile instrument for efficient sensing of gases. The performance of the spectrometer is demonstrated by measuring weak near-infrared rovibrational spectra of carbon dioxide gas within the 1.5–1.6 μm wavelength range.

Molecular elimination of Br2 in 248 nm photolysis of bromoform probed by using cavity ring-down absorption spectroscopy.

By using cavity ring-down spectroscopy technique, we have observed the channel leading to Br(2) molecular elimination following photodissociation of bromoform at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolysis laser beam in a ring-down cell, is used to probe the Br(2) fragment in the B(3)Pi(ou)(+)-X(1)Sigma(g)(+) transition using the range 515-524 nm. The ring-down time lasts 500 ns, so the rotational population of the Br(2) fragment may not be nascent nature, but its vibrational population should be. The vibrational population ratio of Br(2)(upsilon=1)/Br(2)(upsilon=0)=0.8+/-0.2 implies that the fragmented Br(2) is vibrationally hot. The quantum yield of the molecular elimination reaction is 0.23+/-0.05, consistent with the values of 0.26 and 0.16 reported in 234 and 267 nm photolysis of bromoform, respectively, using velocity ion imaging. A plausible photodissociation pathway is proposed, based upon this work and ab initio calculations. The A(1)A(2), B(1)E, and C(1)A(1) singlet states of bromoform are probably excited at 248 nm. These excited states may couple to the high vibrational levels of the ground state X(1)A(1) via internal conversion. This vibrationally excited bromoform readily surpasses a reaction barrier 389.6 kJ/mol prior to decomposition. The transition state structure tends to correlate with vibrationally hot Br(2). Dissociation after internal conversion of the excited states to vibrationally excited ground state should result in a large fraction of the available energy to be partitioned in vibrational states of the fragments. The observed vibrationally hot Br(2) fragment seems to favor the dissociation pathway from high vibrational levels of the ground state. Nevertheless, the other reaction channel leading to a direct impulsive dissociation from the excited states cannot be excluded.

Cavity ring-down laser absorption spectroscopy of jet-cooled anthracene

Polycyclic aromatic hydrocarbons (PAHs) have been suggested as possible carriers of diffuse interstellar bands. To verify this, absorption spectra of PAHs under conditions similar to those encountered in the interstellar medium should be obtained. We report here the application of cavity ring-down laser absorption spectroscopy to the study of neutral anthracene expanded in supersonic jets. Absorption spectra of the S1←S0 transition of anthracene near 361 nm have been obtained with significantly higher resolution than reported previously. Assignments of the vibrational modes involved in the sequence bands observed on the red side of the origin band are proposed.

Cavity Ring-Down Laser Absorption Spectroscopy of the E3Δ−X3Δ Transition of VN

The (0,0) band of the electronic transition of VN near 450.5 nm has been investigated using the technique of laser vaporization/reaction with free jet expansion and cavity ring-down laser absorption spectroscopy. A new transition system was observed, which has been assigned as the E3Δ−X3Δ system. All three ΔΩ = 0 subband transitions were recorded and rotationally analyzed. A least-squares fit of the measured line positions yielded molecular constants for the new E3Δ state. The bond length, ro, of the E3Δ state was determined to be 1.6937 A, which is the longest among the known states of VN. The E3Δ state is expected to arise from the electronic configuration 1δ110σ1, where the 10σ orbital is an antibonding orbital. A comparison of the observed electronic states of VN to those of the isoelectronic TiO molecule supports the assignment.

Analysis of a-Si:H subgap absorption spectra obtained from absolute cavity ringdown absorption spectroscopy using an empirical DOS model

The novel thin film cavity ringdown absorption technique in combination with transmission reflection spectroscopy has been employed to obtain absolute absorption spectra of a-Si:H films of 4 nm and 1031 nm thickness between photon energies of 0.7 and 4 eV. Using an empirical density-of-states (DOS) model the absorption spectra have been deconvoluted and the DOS for both films could be determined within conceivable limits. The DOS for the bulk and surface dominated films are clearly different and the dipole matrix elements for the different transitions as well as the resulting dipole matrix function is discussed.

D 2 ← D 0 transition of the anthracene cation observed by cavity ring-down absorption spectroscopy in a supersonic jet

In an attempt to contribute to the explanation of the diffuse interstellar bands (DIBs), we have studied the absorption spectrum of the anthracene cation (An +) in a supersonic jet by cavity ring-down spectroscopy. The D 2 ← D 0 transition of An + has been located at 708.76 ± 0.13 nm. This means that the same transition observed in an argon matrix is redshifted by 13.65 nm. Although the band position determined in the laboratory coincides with the position of a weak feature in the DIB spectrum, the measured width is too broad to comply with the astronomical observation.

Cavity ring-down laser absorption spectroscopy of IrC

The absorption spectrum of IrC at wavelength between 445 and 500 nm has been investigated using the technique of laser vaporization/reaction with free-jet expansion and cavity ring-down laser absorption spectroscopy. This wavelength region covers the (0,0), (1,0) and (2,0) bands of the L 2 Φ 7/2– X 2 Δ 5/2 transition. IrC molecules were produced by reacting laser vaporized iridium atoms and methane. Analysis of the spectra gives refined band origins, vibrational and rotational constants for the L 2 Φ 7/2 level.

Cavity ring-down absorption and laser-induced fluorescence for quantitative measurements of CH radicals in low-pressure flames

The absolute, quantitative spatially resolved distribution of CH radicals was measured in the reaction zone of a low-pressure methane/air flame (25 Torr) using a combination of laser-induced fluorescence (LIF) and cavity ring-down (CRD) absorption spectroscopy operating on the A 2Δ–X 2Π(0,0) transition. The spatially resolved 1-D image of LIF provides a direct measure of the CH distribution along the path of the laser beam in the CRD cavity. The temperature distribution was determined from measurements on a pair of rotational transitions. A series of LIF line images and CRD absorption measurements taken at various burner heights are combined to form a quantitative 2-D image of the CH distribution. This is used to interpret the CRD measurements along this inhomogeneous path. The 10 ppm peak CH concentration measured here on the centerline of the flame is in good agreement (within 15%) with earlier CH A–X LIF measurements calibrated by Rayleigh and Raman scattering. A 1-D LIF image collected simultaneously with CRD absorption was also used to quantify and optimize the spatial resolution of the CRD measurement.

Lowest n,π* Triplet State of 2-Cyclopenten-1-one: Cavity Ringdown Absorption Spectrum and Ring-Bending Potential-Energy Function

Author Institution: Department of Chemistry, University of Wisconsin-Eau Claire; Department of Chemistry, Hanyang University; Department of Chemistry, Texas A \& M University

Cavity ring-down measurements of seeded NO in premixed atmospheric-pressure H2/air and CH4/air flames

Quantitative aspects of using cavity ring-down absorption spectroscopy near 226 nm for measurements of NO mole fractions in premixed atmospheric-pressure flames are discussed. Measurements in methane–air flames showed strong broadband absorption near 226 nm by hot CO2 molecules, precluding using the cavity ring-down method in these flames at atmospheric pressure. In hydrogen–air flames, the broadband absorption at this wavelength was substantially lower. Absorption cross sections derived from non-seeded cavity ring-down spectra suggest that absorption by water is the major contribution to the background in these flames. The detectability limit for NO by cavity ring-down measurements in hydrogen–air flames using the current setup is estimated to be ∼10 ppm. Effects of the cold boundary layer on the measured NO mole fraction were accounted for by measuring the radial distributions of temperature and NO mole fraction using coherent anti-Stokes Raman scattering and laser-induced fluorescence (LIF), respectively. Measurements performed in seeded stoichiometric and lean hydrogen–air flames showed no reburning at temperatures above 1750 K, demonstrating the adequacy of using these flames for calibration of LIF measurements. At lower temperatures, the mole fraction of NO in the hot gases was up to 30% lower than that expected from the degree of seeding in the cold gases.

Number density and temperature of acetylene in hot-filament and arc-jet activated CH 4/H 2 gas mixtures measured using diode laser cavity ring-down absorption spectroscopy

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) with an external cavity diode laser has been used to make diagnostic measurements of acetylene absorption in hot-filament and DC-arc plasma jet reactors operating with 1% CH 4 in H 2 and ≤0.8% CH 4/H 2 in excess Ar, respectively, and used for diamond chemical vapour deposition. The acetylene was widely distributed throughout the reactor, extending well beyond the regions of highest thermal activation as a result of diffusion processes. The average acetylene concentration was determined as (2.9±0.6)×10 13 molecules cm −3 in the hot filament reactor and (1.2±0.2)×10 14 molecules cm −3 in the DC arc jet reactor, accounting for ∼1.7% and 40% of the total C budget in the respective cases. The average acetylene gas temperature along the viewing column was 550±150 K in both environments. Variations in the concentration and temperature of acetylene with the gas feed composition and input power processing conditions for the two reactors are presented. The experiments demonstrate the effectiveness of near-IR diode laser CRDS as a probe of highly luminous environments.

Study of the morphology of a laser-produced aerosol plume by cavity ringdown laser absorption spectroscopy.

Cavity ring-down laser absorption spectroscopy (CRLAS) was applied for the first time to detection and characterization of laser breakdown generated aerosols. The method provided time-resolved morphological information on the aerosol plume, which is of importance in laser ablation (LA) and deposition, in laser-induced breakdown spectroscopy (LIBS) analysis, and in laser ablation inductively coupled plasma (LA-ICP) methods. This method provides sensitive detection of a variety of aerosols produced under ambient conditions. The morphological investigation revealed that the aerosol density has a reproducible pattern as a function of distance from the surface, although its details depend on time, on geometrical parameters and on the surface characteristics.

Swept-Cavity Ringdown Absorption Spectroscopy: Put Your Laser Light In and Shake It All About

Cavity ringdown (CRD) absorption spectroscopy with continuous-wave (cw) tunable coherent light sources provides a convenient, sensitive, and precise way to measure very weak optical absorption spectra and to analyze trace gas concentrations. Several recently introduced innovations in cw-CRD spectroscopy are described. Our approach involves one or more single-mode tunable diode laser sources, a rapidly swept ringdown cavity, and a simple optical heterodyne detected approach (based on the dynamics of coherent light reflected from such an active optical cavity). This offers high performance in a relatively simple, low-cost, compact instrument that is facilitated by fibre optic and other standard photonics or telecommunications components. New experimental results are reported, comprising CRD spectra of water vapour in the 0.815 µm wavelength region and of mixtures of CO2 and CO in the range of 1.56–1.59 µm. Promising applications of such techniques include chemical analysis of trace gases in medicine, agriculture, industry, and the environment.

Fiber-loop ring-down spectroscopy

Pulsed, visible and near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. Measurements of the loss-per-pass and of the ring-down time allow for characterization of the different loss mechanisms of the light pulses in the fiber and splice connector. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that by measuring the ring-down times one can accurately determine the absolute transmission of an optical fiber and of the fiber connector. In addition it is demonstrated that the technique is useful as an absorption spectroscopic technique of very small sample volumes. A solution of an organic dye was placed between the fiber ends instead of the usual index matching fluid, and an absorption spectrum of 7×10−15 mol of the dye 1,1′-diethyl-4,4′-dicarbocyanine iodide in 7×10−12 L of dimethylsulfoxide was recorded.

Rapidly swept, continuous-wave cavity ringdown spectroscopy with optical heterodyne detection: single- and multi-wavelength sensing of gases

Spectroscopic sensing of gases can be performed with high sensitivity and photometric precision by cavity ringdown (CRD) absorption spectroscopy. Our cavity ringdown spectrometer incorporates continuous-wave (cw) tunable diode lasers, fibre-optic coupling and standard photonics and optical telecommunications components. It comprises a rapidly swept optical cavity in a single-ended optical heterodyne transmitter–receiver configuration, enabling optical absorption of gases to be recorded either as single-frequency scanned spectra or as simultaneous, multi-wavelength tailored spectra. By measuring weak near-infrared rovibrational spectra of carbon dioxide gas (CO2), with high resolution in the vicinity of 1.53 μm, we have realised a noise-limited absorption sensitivity of 2.5×10-9 cm-1 Hz-1/2. Analytical sensitivity limits (both actual and projected) and prospective gas-diagnostic applications are discussed. Our approach to cw-CRD spectroscopy offers high performance in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medical, agricultural, industrial and environmental situations.

The T 1←S 0 Absorption Spectrum of Gaseous 4H-Pyran-4-thione

The T 1←S 0 absorption spectrum of 4H-pyran-4-thione (PT) was measured in a static cell at room temperature (550–620 nm) and in a seeded cold supersonic jet (580–600 nm) using the cavity ring-down (CRD) method. In the static cell absolute extinction coefficients were determined between 573 and 610 nm with an accuracy of ∼±5%. In this region 22 harmonic sequences and 18 hot bands were observed. The energetically lowest ground state vibration at 167.5 cm −1 was identified as the promoting mode in the static PT gas. The mode in the triplet state was found at 152.3 cm −1. The CRD absorption spectra of static PT gas and jet-cooled PT are compared with the phosphorescence excitation spectrum of isolated PT. The weak S 1, 0←S 0, 0 absorption was tentatively assigned to a transition at ∼17433 cm −1.