Continuous-wave Cavity Ring-down Spectrometer Research Articles

Understanding the high-resolution spectral signature of the N2-H2O van der Waals complex in the 2OH stretch region.

We present the observation of the N2-H2O van der Waals complex in the 2OH stretch overtone region. The high-resolution jet cooled spectra were measured using a sensitive continuous wave cavity ringdown spectrometer. Several bands were observed and vibrationally assigned in terms of ν1, ν2, and ν3, the vibrational quantum numbers of the isolated H2O molecule, as (ν1'ν2'ν3')←(ν1″ν2″ν3″)=(200)←(000) and (101) ← (000). A combination band involving the excitation of the in-plane bending motion of N2 and the (101) vibration of water is also reported. The spectra were analyzed using a set of four asymmetric top rotors, each associated with a nuclear spin isomer. Several local perturbations of the (101) vibrational state were observed. These perturbations were assigned to the presence of the nearby (200) vibrational state and to the combination of (200) with intermolecular modes.

Cavity ring-down spectroscopy of 17O-enriched water vapor between 12,055 and 12,260 cm−1

The absorption spectrum of oxygen-17-enriched water vapor in the 12,055–12,260 cm−1 region was recorded with a continuous-wave cavity ring-down spectrometer (CW-CRDS) at room temperature. In total, 1342 lines were observed in this region, while more than 1400 absorption transitions were assigned based on previously published experimental energies and the variational calculations based on the results of Partridge and Schwenke. The typical frequency accuracy of 0.002 cm−1 was achieved in this work. Three hundred and fifty-eight ro-vibrational energy levels of H217O water isotopologue were retrieved, which belong to the ν1+12ν2+ν3=3.5 and 4 polyads: (0 1 3), (0 3 2), (1 1 2), (1 3 1), (2 1 1), (2 2 1), (2 3 0), (3 0 1), and (3 1 0) states. One hundred and ten of them were reported for the first time. Comparisons of experimental line positions with those in the HITRAN database were given for H216O and H217O isotopologues. The obtained vibration-rotation energy levels were also compared with MARVEL values.

Cavity ring-down spectroscopy of 15N enriched N2O near 1.56 µm

The absorption spectrum of nitrous oxide around 1.56 µm has been recorded with Doppler limited resolution by a continuous-wave cavity ring-down spectrometer at room temperature using 15N-enriched samples. The typical sensitivity was at the level of 2 × 10−10 cm−1. In total, 3389 transitions were observed and ro-vibrationally assigned on the basis of the global effective Hamiltonian model for six nitrous oxide isotopologues (14N15N16O, 15N14N16O, 14N15N18O, 15N14N18O, 14N15N17O and 15N14N17O). The band-by-band analysis led to the determination of ro-vibrational parameters of 47 bands, 36 of them were newly observed, and the rotational analysis of 11 others were significantly extended and improved. New sets of the effective Hamiltonian parameters were determined for the 14N15N16O and 15N14N16O isotopic species.

CRDS absorption spectrum of 17O enriched water vapor in the 12,277–12,894 cm−1 range

The absorption spectrum of water vapor enriched in oxygen-17 has been recorded by a continuous-wave cavity ring-down spectrometer (CW-CRDS) at room temperature in the 12,277–12,894 cm−1 region. The typical sensitivity of 2 × 10−10 cm−1 allowed the detection of water vapor transitions with intensities larger than 1 × 10−28 cm−1/(molecule cm−2). In total, more than 3400 lines were observed in the recorded spectra with a typical frequency accuracy of 0.002 cm−1. In which more than 3400 transitions were ro-vibrationally assigned to five water isotopic species (H217O, H216O, H218O, HD16O, and HD17O) using the known empirical energy levels combined with the calculated variational line lists based on the results of Partridge and Schwenke. About 1300 transitions of H217O were assigned, leading to the determination of new energy levels of 17 vibration states. Those assigned H217O transitions are involved with the upper vibration states of (013), (023), (032), (051), (061), (070), (080), (090), (112), (131), (150), (160), (211), (230), (301), (310), and (320). The maximum values of the rotational numbers J and Ka were 16 and 9, respectively. The 5ν1 band of HD17O was assigned for the first time as well. The line positions and energy levels determined in this work were compared with literature, and it shows a significant improvement of the line positions of H217O and HD17O in the 12,277–12,894 cm−1 region.

Near infrared cavity ring-down spectroscopy for isotopic analyses of CH4 on future Martian surface missions

A compact Near Infrared Continuous Wave Cavity Ring-down Spectrometer (near-IR-cw-CRDS) was developed as a candidate for future planetary surface missions. The optical cavity was made of titanium with rugged quartz windows to protect the delicate super cavity from the harsh environmental changes that it would experience during space flight and a Martian surface mission. This design assured the long-term stability of the system. The system applied three distributed feedback laser diodes (DFB-LD), two of which were tuned to the absorption line peaks of 12CH4 and 13CH4 at 6046.954cm−1 and 6049.121cm−1, respectively. The third laser was tuned to a spectral-lines-free region for measuring the baseline cavity loss. The multiple laser design compensated for typical baseline drift of a CRDS system and, thus, improved the overall precision. A semiconductor optical amplifier (SOA) was used instead of an Acousto-Optic Module (AOM) to initiate the cavity ring-down events. It maintained high acquisition rates such as AOM, but consumed less power. High data acquisition rates combined with improved long-term stability yielded precise isotopic measurements in this near-IR region even though the strongest CH4 absorption line in this region is 140 times weaker than that of the strongest mid-IR absorption band., The current system has a detection limit of 1.4×10–12cm−1 for 13CH4. This limit corresponds to ~7pptv of CH4 at 100Torr. With no further improvements the detection limit of our current near IR-cw-CRDS at an ambient Martian pressure of ~6Torr (8mbar) would be 0.25ppbv for one 3.3 minute long analysis.

Cavity ring-down spectroscopy of the electric quadrupole transitions of [formula omitted] in the 784–852 nm region

Precision spectroscopy of the hydrogen molecule can be used to verify the quantum electrodynamics theory (QED) in a molecular system. The electric quadrupole transitions of the second overtone of H2 have been recorded with a continuous-wave cavity ring-down spectrometer. In total, eight lines in the 784–852nm range have been observed, including the extremely weak S3(5) line with a calculated intensity of 9.2×10-31cm/molecule. The absolute line positions at the zero pressure limit have been determined with an accuracy of 3×10-4cm-1, agreeing well with the high-level quantum chemical calculations including relativistic and QED corrections. The deviations between the experimental and theoretical frequencies are less than 5×10-4cm-1, which is only 20% of the claimed theoretical uncertainty.

Continuous-wave cavity ringdown spectroscopy based on the control of cavity reflection

A new type of continuous-wave cavity ringdown spectrometer based on the control of cavity reflection for trace gas detection was designed and evaluated. The technique separated the acquisitions of the ringdown event and the trigger signal to optical switch by detecting the cavity reflection and transmission, respectively. A detailed description of the time sequence of the measurement process was presented. In order to avoid the wrong extraction of ringdown time encountered accidentally in fitting procedure, the laser frequency and cavity length were scanned synchronously. Based on the statistical analysis of measured ringdown times, the frequency normalized minimum detectable absorption in the reflection control mode was 1.7 × 10(-9)cm(-1)Hz(-1/2), which was 5.4 times smaller than that in the transmission control mode. However the signal-to-noise ratio of the absorption spectrum was only 3 times improved since the etalon effect existed. Finally, the peak absorption coefficients of the C(2)H(2) transition near 1530.9nm under different pressures showed a good agreement with the theoretical values.

Cavity ring down spectroscopy of 18O and 17O enriched carbon dioxide near 795 nm

The dyad bands of 17O and 18O enriched carbon dioxide near 795nm have been recorded with a continuous-wave cavity ring-down spectrometer. Seven cold bands of four isotopologues: 12C18O2, 16O12C18O, 16O12C17O and 12C17O2 have been observed for the first time. The spectroscopic parameters Gν, Bν, and centrifugal distortion constants, have been determined.

High sensitivity cavity ring down spectroscopy of 13C16O2 overtone bands near 806 nm

The absorption spectrum of 13C16O2 near 806nm has been recorded with a continuous-wave cavity ring-down spectrometer. Two cold bands and one associated hot band are observed in this region. The line positions of these bands are determined with an accuracy of 0.003cm−1. The absolute line intensities have also been retrieved with an estimated accuracy of 4% for majority of the unblended lines. The vibrational transition dipole moment squared values and the empirical Herman–Wallis coefficients are determined for all the three bands. The comparison of the retrieved line positions and intensities to those given in the Carbon Dioxide Spectroscopic Databank shows large deviations in the line intensities of the 10052–00001 band. The effective dipole moment parameters describing the line intensities of 13C16O2 near 806nm are fitted according to the observed line intensities.

Cavity ring-down spectroscopy of Doppler-broadened absorption line with sub-MHz absolute frequency accuracy

A continuous-wave cavity ring-down spectrometer has been built for precise determination of absolute frequencies of Doppler-broadened absorption lines. Using a thermo-stabilized Fabry-Pérot interferometer and Rb frequency references at the 780 nm and 795 nm, 0.1 - 0.6 MHz absolute frequency accuracy has been achieved in the 775-800 nm region. A water absorption line at 12579 cm(-1) is studied to test the performance of the spectrometer. The line position at zero-pressure limit is determined with an uncertainty of 0.3 MHz (relative accuracy of 0.8 × 10(-9)).

Electric-quadrupole transition of H2determined to10−9precision

The $S$(3) electric-quadrupole transition of the (3-0) overtone of molecular hydrogen has been recorded with a continuous-wave cavity ring-down spectrometer and calibrated with sub-megahertz accuracy using a temperature-stabilized Fabry-P\'erot interferometer. The collision-induced frequency shift and the frequency at zero pressure limit are derived from a fit of the spectral profiles. The absolute transition frequency of the $S$(3) line is determined to be 376 531 818.1 MHz with a 1.6-MHz accuracy ($\ensuremath{\delta}\ensuremath{\nu}/\ensuremath{\nu}=4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$) by measuring the shift to a Rb line at 795 nm. As the best determined transition frequency of neutral H${}_{2}$ in the ground electronic state, this value agrees well with recent high-level quantum chemistry calculations including relativistic and quantum electrodynamics corrections.

Laser-locked, continuously tunable high resolution cavity ring-down spectrometer

A continuous-wave cavity ring-down spectrometer with sub-MHz precision has been built using the sideband of a frequency stabilized laser as the tunable light source. The sideband is produced by passing the carrier laser beam through an electro-optic modulator (EOM) and then selected by a short etalon on resonance. The carrier laser frequency is locked to a longitude mode of a thermo-stabilized Fabry-Perot interferometer (FPI) with a long-term absolute frequency stability of 0.2 MHz (5 × 10(-10)). Broad and precise spectral scanning is accomplished, respectively, by selecting a different longitudinal mode of the FPI and by tuning the radio-frequency driving the EOM. The air broadened water absorption line at 12,321 cm(-1) was studied to test the performance of the spectrometer.

High-resolution spectroscopy of the ν8 band of methylene bromide using a quantum cascade laser

A continuous wave cavity ringdown spectrometer with a Fabry-Perot quantum cascade laser has been used to collect a rotationally-resolved infrared spectrum of the ν 8 vibrational band of methylene bromide in a slit nozzle expansion. In our laboratory, previous observations of the vibrational band were limited by spectral coverage to only the P and Q-branches and by the 24 MHz step-size of the laser [1]. The issue of limited spectral coverage has been resolved using a Fresnel rhomb and a wire grid polarizer to protect the laser from the destabilizing effects of back-reflection from the ringdown cavity. The frequency step-size of the spectrometer has been reduced from 24 MHz to 2 MHz. With both of these instrument enhancements, we have been able to record the R-branch of the vibrational band, and can resolve many lines that were previously blended in spectra acquired using a pinhole expansion nozzle. Significant hyperfine splitting was observed for the low- J transitions in the P and R-branches. It was possible to neglect the effects of hyperfine splitting for transitions involving J″ > 2 in the spectral assignment, and simulations using the constants obtained by fitting to Watson’s S-reduced Hamiltonian for CH 2 79Br 81Br, and the A-reduced form for CH 2 79Br 2 and CH 2 81Br 2, provide a good match to experimental spectra. A total of 297 transitions have been assigned for all three isotopologues, with a standard deviation of 0.00024 cm −1(∼7 MHz).

First Direct Detection of HONO in the Reaction of Methylnitrite (CH3ONO) with OH Radicals

We report on the development of a new environmental simulation chamber coupled with an in situ continuous wave cavity ring-down spectrometer operating in the near IR (∼1.5 μm). The first application reported in this paper dealt with the chemical mechanism of UV photolysis of methyl nitrite (CH(3)ONO) in air. HONO has been detected for the first time and shown to be formed in the OH + CH(3)ONO reaction. A dense spectrum of cis-HONO absorption lines has been observed near 1.5 μm, in agreement with a previous study (Guilmot et al.). CH(2)O has been measured as primary product with good sensitivity and time resolution. In contrast to Zhao et al., we did not detect any NO(2) absorption features in this wavelength range. Calibration experiments provided very low NO(2) absorption cross sections in this region (∼10(-25) cm(2)), leading to conclude that NO(2) cannot be observed in this wavelength range in the presence of equal amounts of CH(2)O.

High sensitivity cavity ring down spectroscopy of CO 2 overtone bands near 790 nm

The dyad bands of C 12 O 2 16 near 790 nm have been recorded with a continuous-wave cavity ring-down spectrometer. Two cold bands and the two associated hot bands are observed in this region. High sensitivity of the 1×10 −10/cm level allows one to detect weak transitions with satisfied accuracy. The absolute line intensities have also been retrieved with an estimated accuracy of 2% for majority of the unblended lines. The vibrational transition dipole moment squared values and the empirical Herman–Wallis coefficients are presented for two cold bands. The comparison of the retrieved line positions and intensities with those given in the Carbon Dioxide Spectroscopic Databank shows clear deviation and indicates the empirical calculation can be further improved using the present spectroscopy data.

Toward continuous monitoring of seawater 13CO2/12CO2 isotope ratio and pCO2: Performance of cavity ringdown spectroscopy and gas matrix effects

The potential of a continuous wave cavity ringdown spectrometer for monitoring the isotope ratio 13CO2/12CO2 and the partial pressure pCO2 of CO2 dissolved in water was thoroughly analyzed by quantitative measurements. Running calibration gas standards under typical operation conditions, a relative accuracy of Δ(δ13C[CO2]) = ±0.1‰ with 120 min averaging time has been demonstrated. Absolute uncertainties were determined to be Δ(δ13C[CO2]) = ±0.2‰ and Δ(xCO2) = ±0.5 ppmv. No principle problems were encountered when using the instrument in combination with a water‐air equilibration setup. By contrast, when performing measurements of CO2 in gas matrices with a composition different from that of ambient air, pressure broadening linewidth effects induced significant errors in both δ13C(CO2) and xCO2 values. These effects, which compromise the accessible accuracy in environmental studies, can be quantitatively taken into account by using a spectroscopically based correction procedure. Relying on linewidth analysis, the instrument was shown to be capable of continuous and simultaneous measurement of δ13C(CO2), pCO2, as well as water content and O2 supersaturation, and thus holds the potential for online monitoring of these quantities aboard research vessels.

Cavity ring-down spectroscopy of the [formula omitted] bands of 15N substituted N 2O

The 6 ν 3 and ν 2 + 6 ν 3 − ν 2 bands of 15N substituted nitrous oxide isotopologues have been recorded by a continuous-wave cavity ring-down spectrometer (CW-CRDS) operated near 0.8 μ m . The sensitivity limit was at the level of 1 × 10 − 10 / cm . In total, 213, 86 and 191 transitions were observed for the 14N 15N 16O, 15N 14N 16O and 15N 2 16O isotopologues, respectively. The ro-vibrational spectroscopic parameters of the upper states are determined from least square fitting of the transitions. The absolute line intensities of the 6 ν 3 cold bands have been retrieved by a multi-line fitting procedure from the spectra with an estimated accuracy of 4% for majority of the unblended lines. The vibrational transition dipole moment squared values and the empirical Herman–Wallis coefficients are also presented.

A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source

A new instrument has been constructed that couples a supersonic expansion source to a continuous wave cavity ringdown spectrometer using a Fabry-Perot quantum cascade laser (QCL). The purpose of the instrument is to enable the acquisition of a cold, rotationally resolved gas phase spectrum of buckminsterfullerene (C(60)). As a first test of the system, high resolution spectra of the nu(8) vibrational band of CH(2)Br(2) have been acquired at approximately 1197 cm(-1). To our knowledge, this is the first time that a vibrational band not previously recorded with rotational resolution has been acquired with a QCL-based ringdown spectrometer. 62 transitions of the three isotopologues of CH(2)Br(2) were assigned and fit to effective Hamiltonians with a standard deviation of 14 MHz, which is smaller than the laser frequency step size. The spectra have a noise equivalent absorption coefficient of 1.4 x 10(-8) cm(-1). Spectral simulations of the band indicate that the supersonic source produces rotationally cold (approximately 7 K) molecules.

A precise high-resolution near infrared continuous wave cavity ringdown spectrometer using a Fourier transform based wavelength calibration

A wavelength calibration technique is described, which is based on a combination of a Fourier transform wavelength meter and a distributed feedback laser locked to a molecular transition as a frequency marker in the spectrum. The technique provides a reliable wavelength scale to be used in high resolution continuous wave cavity ringdown spectroscopy without need for stabilization of the probe laser and accurately known molecular transitions in the scanned wavelength range. Due to a continuous reference measurement, ambient influences on the laser sources are effectively suppressed. As an example, we measured highly resolved cavity ringdown spectra of N(2)O isotopomers and determined the line strength of several absorption lines at a wavelength around 1687 nm. A near infrared wavelength precision of 6 x 10(-8) and an absolute accuracy on the order of 1 x 10(-7) was readily achieved. The general concept is easy to implement and can be further refined by using additional reference lasers, thus holding the potential of even higher wavelength accuracy.

Precision continuous-wave cavity ringdown spectroscopy of CO2 at 1064 nm

A high performance continuous-wave cavity ringdown spectrometer using a rapidly swept cavity has been applied to investigate weak absorption of CO2 line near 1064 nm. In the experimental setup, offset locking of the Nd:YAG laser to other iodine-stabilized Nd:YAG laser provides a frequency accuracy better than 20 kHz. The ringdown time was obtained by fitting the experimental ringdown data to a theoretically obtained ringdown curve. The absorption coefficient and absolute center wavelength are determined with accuracy better than two orders of magnitude with respect to the previous observations. Our experimental setup yields high performance in a relatively simple, low cost, and compact system that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.