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

Two species-one wavelength detection based on selective optical saturation spectroscopy

Cross-sensitivity limits accurate quantitative detection of species concentrations in all sensor technologies, including laser-based absorption techniques. Absorption sensors capture a signal that combines contributions from all interfering species at a given detection wavelength. Careful selection of the probed spectral line, broadband detection, or upstream separation can partially mitigate cross-sensitivity, however, weak or unidentified signal interference remains a challenge for accuracy. Here, we present a proof-of-principle study to overcome cross-sensitivity by taking advantage of the distinct optical saturation characteristics of different gas mixture components. By controlling the absorption contribution of a selected species by intentional optical saturation, simultaneous and quantitative detection of two interfering species becomes possible even without the need for spectral scanning, hence offering two species-one wavelength detection (2S1W) capability. Demonstrated with direct absorption and cavity-ringdown setups, the method offers a new, previously unexploited opportunity to further enhance laser-based analyzers for complex gas mixture analysis in environmental, medical, and technical applications.

Quantitative Mid-Infrared Cavity Ringdown Detection of Methyl Iodide for Monitoring Applications

Methyl iodide is a toxic halocarbon with diverse industrial and agricultural applications, and it is an important ocean-derived trace gas that contributes to the iodine burden of the atmosphere. Quantitative analysis of CH₃I is mostly based on gas chromatography coupled with mass spectrometry or electron capture detection (GC-MS/ECD) as of yet, which often limits the ability to conduct in situ high-frequency monitoring studies. This work presents an alternative detection scheme based on mid-infrared continuous wave cavity ringdown spectroscopy (mid-IR cw-CRDS). CH₃I was detected at the ^RR₂(15) rovibrational absorption transition at ṽ = 3090.4289 cm^-1; part of the corresponding v₄ vibration band has been measured with Doppler-limited resolution for the first time. A line strength of S(T = 295 K) = (545 ± 20) cm/mol, corresponding to a line center absorption cross-section σ_c(p = 0 bar) = (1.60 ± 0.06) × 10⁵ cm²/mol, and pressure-broadening coefficients γ_p(Ar) = (0.094 ± 0.002) cm^-1/bar and γ_p(N₂) = (0.112 ± 0.003) cm^-1/bar have been determined. The performance of the detection system has been demonstrated with a tank-purging experiment and has been directly compared with a conventional GC-MS/ECD detection system. Quantitative detection with high reproducibility and continuous sampling is possible with a current noise-equivalent limit of detection of 15 ppb at 20 mbar absorption-cell pressure and 70 s averaging time. This limit of detection is suitable for practical applications in the ppm mixing ratio level range such as workplace monitoring, leak detection, and process studies. Natural environmental abundances are much lower, therefore possibilities for future improvement of the detection limit are discussed.

Doppler-limited high-resolution spectrum and VPT2 assisted assignment of the C-H stretch of CH2Br2

The Doppler limited non-saturated rotationally resolved infrared spectra of the symmetric and asymmetric CH-stretch bands of CH₂Br₂ have been measured. A continuous wave cavity ringdown setup with a widely tunable Mid-IR-OPO laser light source yielded a single-shot minimum absorption of 4.9×10^-8cm^-1. In contrast to the heavily congested ν₁ band, the ν₆ band showed partially resolved rotational features that may serve as suitable absorption targets in future environmental detection schemes for CH₂Br₂. A straightforward, VPT2 (second-order vibrational perturbation theory) assisted quantum-chemical approach for assigning the rotational structure has been tested using different model chemistries. The molecular structures, anharmonic frequencies and the structural changes upon vibrational excitation of CH₂Br₂ have been investigated. The predicted changes of the anharmonic rotational constants have been used together with available spectroscopic ground state constants to simulate the rovibrational structures of the ν₁ and ν₆ bands of CH₂Br₂. A refined analysis of the ν₆ band is presented yielding accurate values for the band origin and the rotational constants. A fit of the line positions of 312 prominent transitions of the three isotopologues revealed a low standard error of 0.00056cm^-1, hence within the absolute 0.0009cm^-1 wavelength accuracy of the used spectrometer setup. A combined analysis of the predicted line strengths and positions of the strong Q sub-branches of the ν₆ band has been performed to test the ability of the different density functionals for VPT2 prediction of anharmonic molecular constants. The M06/6-311++G(d,p) model chemistry turned out to yield reliable state-dependent rotational constants that are accurate enough to reproduce the overall rotational structure even without fitting.

Saturation dynamics and working limits of saturated absorption cavity ringdown spectroscopy

The decay transient dynamics and the optimum experimental conditions for reliable gas absorption measurements have been investigated using saturated CRDS (Sat-CRDS, SCAR).

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.

Sensitive Absorption Methods for Quantitative Gas Phase Kinetic Measurements. Part 1: Frequency Modulation Spectroscopy

Frequency modulation spectroscopy (FMS) and cavity ringdown spectroscopy (CRDS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 1 of this review, the theoretical foundations of FMS are addressed with a special emphasis on time-resolved quantitative measurements of concentration profiles. A complementary review of CRDS can be found in part 2 (Z. Phys. Chem. 222 (2008) 31–61). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Kinetic studies based on FMS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with more than a hundred entries is included to facilitate the access to the large body of original literature.