Cavity Ring-down Absorption Research Articles

Cavity ring-down transient absorption spectroscopy (CRD-TAS): A proof of concept via detection of coherent artifacts of thin substrates

Cavity ring-down transient absorption spectroscopy (CRD-TAS): A proof of concept via detection of coherent artifacts of thin substrates

Nitrogen fixation in the stag beetle, Ceruchus piceus (Coleoptera: Lucanidae): could insects contribute more to ecosystem nitrogen budgets than previously thought?

Nitrogen (N) is a key nutrient required by all living organisms for growth and development, but is a limiting resource for many organisms. Organisms that feed on material with low N content, such as wood, might be particularly prone to N limitation. In this study, we investigated the degree to which the xylophagous larvae of the stag beetle Ceruchus piceus (Weber) use associations with N-fixing bacteria to acquire N. We paired acetylene reduction assays by cavity ring-down absorption spectroscopy (ARACAS) with 15N2 incubations to characterize rates of N fixation within C. piceus. Not only did we detect significant N fixation activity within C. piceus larvae, but we calculated a rate that was substantially higher than most previous reports for N fixation in insects. While taking these measurements, we discovered that N fixation within C. piceus can decline rapidly in a lab setting. Consequently, our results demonstrate that previous studies, which commonly keep insects in the lab for long periods of time prior to and during measurement, may have systematically under-reported rates of N fixation in insects. This suggests that within-insect N fixation may contribute more to insect nutrition and ecosystem-scale N budgets than previously thought.

Accurate measurement of trace H<sub>2</sub>S concentration based on cavity ring-down absorption spectroscopy under CO<sub>2</sub>/CO disturbance

Since H<sub>2</sub>S is a corrosive and toxic gas pollutant, the accurate measurement of its concentration is significant. However, in the practical industrial processes, it is difficult to implement because of the disturbance caused by other emissions such as CO<sub>2</sub> and CO. Therefore, in this work, the concentration of H<sub>2</sub>S, CO<sub>2</sub> and CO are measured simultaneously based on cavity ring-down spectroscopy (CRDS) as a viable alternative to measure the concentration of H<sub>2</sub>S accurately when CO<sub>2</sub> and CO exist. First, the wavelength of mixed gas within a range of 6336–6339 cm<sup>–1</sup> is selected as the target region where the spectral line intensity of H<sub>2</sub>S is stronger than 10 times that of CO<sub>2</sub> or CO and the water absorption is extremely weak. Second, the influence of the sampling length (Tm) on the accuracy of the ring-down time is analyzed by evaluating average (accuracy), standard deviation (precision) and consumption time (speed). Third, the experiments are carried out at different pressures in order to obtain the optimal pressure condition. Fourth, the concentration of trace H<sub>2</sub>S is measured when the disturbances caused by CO<sub>2</sub> or CO are added, and the error of the measured concentration is analyzed. Finally, the detection limit of CRDS-based system is calculated to be 6.9 ppb by analyzing the SNR of four groups of low concentration H<sub>2</sub>S spectra, while the lower limit of detection of CRDS-based system is calculated to be 2 ppb by analyzing the Allan variance of long-term data. The measured concentration and its desired value show a good linearity at different dilution ratios. Both the high linearity and the low detection limit of H<sub>2</sub>S indicate the effectiveness of the CRDS-based measurement system to measure H<sub>2</sub>S when CO<sub>2</sub> and CO exist. The successful application of the CRDS-based system to the measurement of H<sub>2</sub>S shows its promising prospect in gas concentration measurement for practical industrial processes.

In Vivo Temperature Dependency of Molybdenum and Vanadium Nitrogenase Activity in the Heterocystous Cyanobacteria Anabaena variabilis.

The reduction of atmospheric dinitrogen by nitrogenase is a key component of terrestrial nitrogen cycling. Nitrogenases exist in several isoforms named after the metal present within their active center: the molybdenum (Mo), the vanadium (V), and the iron (Fe)-only nitrogenase. While earlier in vitro studies hint that the relative contribution of V nitrogenase to total BNF could be temperature-dependent, the effect of temperature on in vivo activity remains to be investigated. In this study, we characterize the in vivo effect of temperature (3-42 °C) on the activities of Mo nitrogenase and V nitrogenase in the heterocystous cyanobacteria Anabaena variabilis ATTC 29413 using the acetylene reduction assay by cavity ring-down absorption spectroscopy. We demonstrate that V nitrogenase becomes as efficient as Mo nitrogenase at temperatures below 10-15 °C. At temperatures above 22 °C, BNF seems to be limited by O2 availability to respiration in both enzymes. Furthermore, Anabaena variabilis cultures grown in Mo or V media achieved similar growth rates at temperatures below 20 °C. Considering the average temperature on earth is 15 °C, our findings further support the role of V nitrogenase as a viable backup enzymatic system for BNF in natural ecosystems.

Mid-infrared interference coatings with excess optical loss below 10 ppm.

We present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 μm with record-low excess optical loss below 10 parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption. We report on a suite of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests to reveal the optical losses for a set of prototype mirrors. In the course of these measurements, we observe a unique polarization-orientation-dependent loss mechanism which we attribute to elastic anisotropy of these strained epitaxial multilayers. A future increase in layer count and a corresponding reduction of transmittance will enable optical resonators with a finesse in excess of 100 000 in the mid-infrared spectral region, allowing for advances in high resolution spectroscopy, narrow-linewidth laser stabilization, and ultrasensitive measurements of various light-matter interactions.

Doppler-limited precision spectroscopy of HD at 1.4 μm : An improved determination of the R (1) center frequency

We report on the improved determination of the center frequency of a hydrogen deuteride vibration-rotation transition. As a result of a refined analysis of cavity ring-down absorption spectra, it was possible to quantify the influence of the hyperfine structure on the line center frequency of the weak 2-0 $R(1)$ line, as retrieved in the Doppler-limited regime. It turns out that ignoring the hyperfine structure leads to an underestimation of the center frequency of about 350 kHz. The updated determination gives an absolute frequency of the $R(1)$ centroid of 217 105 181.936 MHz with a global uncertainty of 76 kHz.

Probing BrCl from photodissociation of CH2BrCl and CHBr2Cl at 248 nm using cavity ring-down spectroscopy.

Photodissociation of di- and tri-halogenated methanes including CH2BrCl and CHBr2Cl at 248 nm was investigated using cavity ringdown absorption spectroscopy (CRDS). The spectra of the BrCl(v'' = 2, 3) and Br2(v'' = 1, 2) fragments were probed over the wavelength range of 594.5-596 nm in the B3Π+0u ← X1Σ+g and B3Π (0+) ← X1Σ+ transitions, respectively. Their corresponding spectra were simulated for assignment of rotational lines at a given vibrational level. The quantum yields for Br2 eliminated from CHBr2Cl and BrCl from CH2BrCl were determined to be 0.048 ± 0.018 and 0.037 ± 0.014, respectively. The photodissociation of CHBr2Cl yielded only the Br2 fragment, but not the BrCl fragment in the experiments. An ab initio theoretical method based on the CCSD(T)//B3LYP/6-311g(d,p) level was employed to evaluate the potential energy surface for the dissociation pathways to produce Br2 and BrCl from CHBr2Cl, which encountered a transition state barrier of 445 and 484 kJ mol-1, respectively. The corresponding RRKM rate constants were calculated to show that the branching ratio of (Br2/BrCl) is ∼20. The BrCl spectrum is expected to be obscured by the much larger Br2 spectrum, explaining why BrCl fragments cannot be detected in the photolysis of CHBr2Cl.

Photodissociation of CH2BrCHBrC(O)Cl at 248 nm: probing Br2 as the primary fragment using cavity ring-down spectroscopy.

The photodissociation of 2,3-dibromopropionyl chloride (CH2BrCHBrC(O)Cl, 2,3-DBPC) at 248 nm was carried out to study Br2 as the primary molecular product in the B3Π+0u ← X1Σ+g transition using cavity ring-down absorption spectroscopy. The rotational spectra (v'' = 0-2) were acquired and assigned with the aid of spectral simulation. It is verified that the obtained Br2 fragment is attributed to the one-photon dissociation of 2,3-DBPC and is free from contributions of secondary reactions. The vibrational ratio of the Br2 population of v(0):v(1):v(2) is equal to 1:(0.58 ± 0.12):(0.23 ± 0.09), corresponding to the Boltzmann vibrational temperature of 623 ± 38 K. The quantum yield of Br2 eliminated from 2,3-DBPC is estimated to be 0.09 ± 0.04. The dissociation pathways of 2,3-DBPC and its potential energy surfaces were calculated using density functional theory. By employing the CCSD(T)//M062X/6-31+g(d,p) level of theory, transition state barriers and corresponding reaction energies were calculated for the Br, Cl, Br2, BrCl, HBr and HCl elimination channels. The unimolecular rate constant for Br2 elimination was determined to be 2.09 × 105 s-1 using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, thus explaining the small quantum yield of the Br2 channel.

Optical cavity with intracavity two-photon absorption

This paper presents a theoretical analysis of the optical properties of an optical cavity with both one- and two-photon intracavity absorption loss, without saturation. The cavity two-photon absorption loss per pass is assumed to be small, but otherwise the analysis is general. Analytical results for the cavity transmission as a function of cavity parameters, linear and two-photon intracavity loss, and detuning from resonance are derived and limiting cases discussed. Extraction of the two-photon loss from both cavity ring-down and cavity-enhanced absorption spectroscopy methods is analyzed and the predicted shot-noise limited sensitivities derived and compared. Numerical results are computed using the parameters of a recent experimental intracavity measurement of two-photon absorption of nitrous oxide. The results will be useful in the design and interpretation of cavity-enhanced, two-photon absorption spectroscopy.

Triplet and Singlet (n,π*) Excited States of 4H-Pyran-4-one Characterized by Cavity Ringdown Spectroscopy and Quantum-Chemical Calculations.

The 4H-pyran-4-one (4PN) molecule serves as a model for investigating structural changes following π* ← n electronic excitation. We have recorded the cavity ringdown (CRD) absorption spectrum of 4PN vapor at room temperature, over the wavelength region from 350 to 370 nm. This spectral region includes the T1(n,π*) ← S0 band system as well as the low-energy portion of the S1(n,π*) ← S0 system. Aided by predictions from ab initio (equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles, EOM-EE-CCSD) and density functional theory (time-dependent density functional theory with PBE0 functional, TDPBE0) calculations, we have made vibronic assignments for about 30 features in the CRD spectrum, mostly T1(n,π*) ← S0 transitions. We have used these results to correct certain vibronic assignments appearing in the previous literature for both T1(n,π*) ← S0 and S1(n,π*) ← S0 band systems. We conclude that the lowest-energy carbonyl wagging fundamentals (ν27, in-plane and ν17, out-of-plane) undergo significant frequency drops (28 and 50%, respectively) upon T1(n,π*) ← S0 excitation and similar drops (29 and 39%, respectively) for S1(n,π*) ← S0 excitation. We find that vibrational modes involving the conjugated ring atoms undergo relatively small frequency changes upon π* ← n excitation, for both T1 and S1 states. We have used the present spectroscopic results and vibronic assignments to test the accuracy of computed excited-state frequencies for 4PN. This benchmarking process shows that the economical time-dependent density functional theory method is impressively accurate for certain (but not all) vibrational modes. The highly correlated EOM-EE-CCSD ab initio method is capable of making accurate frequency predictions, but the results, unexpectedly, depend sensitively on basis set family. This anomaly is traceable to a computed conical intersection between the T1(n,π*) and T2(π,π*) surfaces near the T1(n,π*) potential minimum. Relatively small errors in the location of the conical intersection lead to enhanced mixing of the two electronic states and incorrect T1(n,π*) vibrational frequencies when certain triple-ζ quality basis sets are used.

Photodissociation of CH2BrI using cavity ring-down spectroscopy: in search of a BrI elimination channel.

Photodissociation of CH2BrI was investigated in search of unimolecular elimination of BrI via a primary channel using cavity ring-down absorption spectroscopy (CRDS) at 248 nm. The BrI spectra were acquired involving the first three ground vibrational levels corresponding to A3Π1 ← X1Σ+ transition. With the aid of spectral simulation, the BrI rotational lines were assigned. The nascent vibrational populations for v'' = 0, 1, and 2 levels are obtained with a population ratio of 1 : (0.58 ± 0.10) : (0.34 ± 0.05), corresponding to a Boltzmann-like vibrational temperature of 713 ± 49 K. The quantum yield of the ground state BrI elimination reaction is determined to be 0.044 ± 0.014. The CCSD(T)//B3LYP/MIDI! method was employed to explore the potential energy surface for the unimolecular elimination of BrI from CH2BrI.

Feed-forward comb-assisted coherence transfer to a widely tunable DFB diode laser.

The transfer of phase coherence from an ultrastable master laser to a distributed feedback diode laser, using an optical comb as a transfer oscillator, is obtained via a new scheme allowing continuous scanning across the whole tuning range of the slave laser together with absolute frequency determination. This is accomplished without phase lock loops, through a robust high-bandwidth feed-forward control acting directly on the slave laser output radiation. The correction is obtained by means of a dual-parallel Mach-Zehnder interferometer used as an optical single-sideband modulator. Coherence transfer across a master-slave frequency gap of 14 THz yields an ∼10 kHz linewidth providing high injection efficiency of an optical cavity with finesse 250 000. This allows demonstrating a cavity ring-down absorption spectrum of low-pressure ambient air over a 300 GHz spectral window.

Cavity Ring-Down Absorption Spectroscopy: Optical Characterization of ICl Product in Photodissociation of CH2ICl at 248 nm.

Iodine monochloride (ICl) elimination from one-photon dissociation of CH2ICl at 248 nm is monitored by cavity ring-down absorption spectroscopy (CRDS). The spectrum of ICl is acquired in the transition of B3Π0 ← X1Σ+ and is confirmed to result from a primary photodissociation, that is, CH2ICl + hν → CH2 + ICl. The vibrational population ratio is determined with the aid of spectral simulation to be 1:(0.36 ± 0.10):(0.11 ± 0.05) for the vibrational levels ν = 0, 1, and 2 in the ground electronic state, corresponding to a Boltzmann-like vibrational temperature of 535 ± 69 K. The quantum yield of the ICl molecular channel for the reaction is obtained to be 0.052 ± 0.026 using a relative method in which the scheme CH2Br2 → CH2 + Br2 is adopted as the reference reaction. The ICl product contributed by the secondary collisions is minimized such that its quantum yield obtained is not overestimated. With the aid of the CCSD(T)//B3LYP/MIDI! level of theory, the ICl elimination from CH2ICl is evaluated to follow three pathways via either (1) a three-center transition state or (2) two isomerization transition states. However, the three-center concerted mechanism is verified to be unfavorable.

HO2 reaction kinetics in an atmospheric pressure plasma jet determined by cavity ring-down spectroscopy

Hydroperoxyl () is detected in the effluent of an atmospheric pressure plasma jet (PJ) by cavity ring-down absorption spectroscopy. Laser radiation at is used to determine concentrations directly in the plasma effluent at atmospheric pressure as well as indirectly after sampling through a pinhole and expanding to . To unravel the reaction kinetics, the argon feed gas is humidified and the PJ is equipped with a gas curtain device that controls the surrounding atmosphere. Varying the curtain gas composition from pure nitrogen to pure oxygen leads to a five to six-fold increase in concentrations towards higher oxygen concentrations. A reaction kinetics model considering near-field and far-field reactions describes the reaction pathways that determine the hydroperoxyl concentration and is in good agreement with the measurements at both low and atmospheric pressure.

Method for High Frequency Underway N2 Fixation Measurements: Flow-Through Incubation Acetylene Reduction Assays by Cavity Ring Down Laser Absorption Spectroscopy (FARACAS).

Because of the difficulty in resolving the large variability of N2 fixation with current methods which rely on discrete sampling, the development of new methods for high-resolution measurements is highly desirable. We present a new method for high-frequency measurements of aquatic N2 fixation by continuous flow-through incubations and spectral monitoring of the acetylene (C2H2, a substrate analog for N2) reduction to ethylene (C2H4). In this method, named Flow-through Incubation Acetylene Reduction Assays by Cavity Ring-Down Laser Absorption Spectroscopy (FARACAS), dissolved C2H2 is continuously admixed with seawater upstream of a continuous-flow stirred-tank reactor (CFSR) in which C2H2 reduction takes place. Downstream of the flow-through incubator, the C2H4 gas is stripped using a bubble column contactor and circulated with a diaphragm pump into a wavelength-scanned cavity ring down laser absorption spectrometer (CRDS). Our method provides high-resolution and precise mapping of aquatic N2 fixation, its diel cycle, and its response to environmental gradients, and can be adapted to measure other biological processes. The short-duration of the flow-through incubations without preconcentration of cells minimizes potential artifacts such as bottle containment effects while providing near real-time estimates for adaptive sampling. We expect that our new method will improve the characterization of the biogeography and kinetics of aquatic N2 fixation rates.

Cl2 Elimination in 248 nm Photolysis of (COCl)2 Probed with Cavity Ring-Down Absorption Spectroscopy.

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time.

Cavity Ringdown Spectrum of 2-Cyclohexen-1-one in the CO/Alkenyl CC Stretch Region of the S1(n, π*)-S0 Vibronic Band System.

The 2-cyclohexen-1-one (2CHO) molecule serves as a prototype for understanding the photochemical properties of conjugated enones. We have recorded the cavity ringdown (CRD) absorption spectrum of 2CHO vapor at room temperature over the 360-380 nm range. This portion of the spectrum encompasses the S1(n,π*) ← S0 vibronic band system in the region of the C═C and C═O stretch fundamentals. We have assigned about 40 vibronically resolved features in the spectrum, affording fundamental frequencies for 7 different vibrational modes in the S1(n,π*) state, including the C═C (1554 cm-1) and OC-CH (1449 cm-1) stretch modes. The C═O stretch character is spread over at least four different vibrational modes in the S1(n,π*) state, with fundamentals spanning the 1340-1430 cm-1 interval. This finding stems from a significant reduction in C═O bond order upon excitation, which leads to near-coincidence of the C═O stretch and several CH2 wag frequencies. Such complexities make 2CHO an ideal candidate for testing excited-state computational methods. We have used the present spectroscopic results to test EOM-EE-CCSD harmonic-frequency predictions for the S1(n,π*) state. We have also benchmarked the performance of less costly computational methods, including CIS(D) and TDDFT. For certain density functionals (e.g., B3LYP and PBE0), we find that the accuracy of TDDFT frequency predictions can approach but not meet that of EOM-EE-CCSD.

Stationary afterglow measurements of the temperature dependence of the electron–ion recombination rate coefficients of and in He/Ar/H2/D2 gas mixtures at T = 80–145 K

We report measurements of the binary and ternary recombination rate coefficients of deuterated isotopologues of . A cavity ring-down absorption spectrometer was used to monitor the fractional abundances of , , and during the decay of a plasma in He/Ar// mixtures. A dependence of the measured effective recombination rate coefficients on the helium buffer gas density was observed and hence both the binary and the ternary recombination rate coefficients for and were obtained in the temperature range 80–145 K.

Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH3)2COO.

The Criegee intermediate acetone oxide, (CH3)2COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O2 and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO2 was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10-11 cm3 s-1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10-10 cm3 s-1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti-CH3CHOO with SO2, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N2 from cavity ring-down decay of the ultraviolet absorption of (CH3)2COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10-10 to (2.29 ± 0.08) × 10-10 cm3 s-1. Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, kH/kD = (0.53 ± 0.06), for reactions with SO2, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD3)2COO with NO2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10-12 cm3 s-1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of anti-CH3CHOO with NO2. Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH3)2COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s-1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH3)2COO with SO2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH3)2COO with NO2 and with SO2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.

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.