Research Group Prof. Dr. G. Friedrichs

Cavity Ringdown Spectroscopy

Cavity Rindown Spectroscopycwopo

CRDS: Basics

CRDS_BasicsCavity ringdown spectroscopy (CRDS) is an ultra-sensitive absorption spectroscopic technique, which has developed rapidly during the last two decades. It has been implemented for diverse applications and has become a mature spectroscopic technique for variety of prototype and commercial instruments. Historically, the first implementation of the technique was pulsed-CRDS, where a pulsed laser was used as an excitation light source.

In principle, the basic experimental realization of CRDS is fairly simple. It is is based on merely four main components:

  • a laser as a light source,
  • a Fabry-Perot cavity consisting of two mirrors of high reflectivity (R > 99.99 %), which also confines the detection volume,
  • a light detector (photomultiplier or fast photodiode),
  • and a data acquisition system that determines the ringdown time constant from the experimentally observed mono-exponential signal decay.
     

The high detection sensitivity of CRDS is primarily based on a long effective absorption length (>10 km) arising from a multiple reflection of the laser beam, which is trapped in the optical cavity. In addition, since the CRDS signal is related to the temporal decay instead of the total intensity of the transmitted light, CRDS is inherently immune to laser intensity fluctuations, which often limit the sensitivity of conventional absorption methods. Due to absorption of the sample gas in the cavity, the intensity of the transmitted laser light decreases every roundtrip and the absorption coefficient in turn is directly related to the observed ringdown decay. Compared to an “empty” cavity it becomes faster in case of an absorbing species is present in the cavity.

crds_principle2

Today, in most cases narrow linewidth continuous-wave (cw) laser light sources have become more common. Typically, in a cw-CRDS experiment the cavity is made resonant with the excitation light by modulating its length by a piezo-driven mirror mount. With a resonant cavity, light intensity starts to build up until a preset light level is reached. Then, a fast optical switch (e.g., an acousto-optic modulator, AOM) shuts-off the excitation laser light and the ringdown decay can be observed. In contrast to pulsed-CRDS, the cw approach avoids the problems associated with multi-mode excitation of the optical cavity resulting in further sensitivity and spectral resolution enhancement.

Our research group focuses on the development of new CRDS-based methods in the IR range for gas phase measurements and in the NIR range for both gas phase and interface measurements (so-called evanescent wave CRDS). Moreover, we are involved in the application of field-deployable commercial CRDS instruments (e.g., for isotope selective detection of CO2, see here).

The developed spectrometers are used for fundamental spectroscopic work needed for environmental monitoring of various trace gases. Many trace gases are emitted from the ocean, where their abundance is determined by interacting physical, chemical and biological transport and transformation processes. For example, more recent research is concerned with photochemically active substances such as reactive organohalogen compounds containing Cl, Br, and I that are known to considerably influence the oxidation capacity of the atmosphere. Interface measurements using  evanescent wave CRDS hold the potential to directly investigate and understand the heterogeneous chemistry taking place at surfaces, which is difficult to study by other methods.

Saturation-IR-CRDS

Acwopo singly-resonant continuous-wave optical parametric oscillator (SR-cw-OPO) based CRDS experiment for sensitive and high-resolution IR detection has been setup.

The new cw-OPO-IR laser system (bluish illuminated devices) provides a wide tunability (λ = 1.38 – 1.60 μm & 3.2 – 4.6 μm), a narrow spectral bandwidth (Δν = 60 kHz at Δt = 500 ms), and a high output power (P > 1.0 W).

chr2_br2The wide tunability of the laser system in combination with the  the narrow linewidth enables the measurement of high-quality Doppler-limited rovibrational spectra. As an example, the linear CRDS spectrum illustrates the complex structure of the asymmetric CH stretch band of CH2Br2. The assignment of the spectrum and the determination of the spectroscopic constants is based on a fit of the effective Hamiltonian to the experimental spectrum.

sat_crds_principleUtilizing the high-power of cw-OPO-IR laser system, a sensitivity enhancement can be accomplished by the implementation of the so-called saturated absorption cavity ringdown spectroscopy (Sat.-CRDS/SCAR) [Giusfredi et al., Phys Rev Lett 104 (2010)]. Sat.-CRDS allows one to extract both the gas absorption and the empty cavity loss constant from a single ringdown signal. This is possible because the degree of sample saturation changes during the ringdown event causing a distinct non-exponential ringdown behavior. In simple terms, the sample can be considered to be strongly saturated at the beginning of the ringdown event (hence the observed decay is dominated by the empty cavity losses), but with decreasing light intensity the sample becomes non-saturated (hence the observed decay is influenced by the absorbing species lateron). Actually, the observed ringdown decay needs to be simulated by a ringdown model that takes into account the dynamics of the absorption saturation, which is controlled by the pumping (light power coupled into the cavity) and the relaxation rates of the molecular excited state (dominated by molecular collisions and hence the cell pressure).

Contributing Researchers: Ibrahim Sadiek and Gernot Friedrichs

NIR-cw-CRDS

Quantitative High Resolution Spectroscopy


cwcrdsCompared to pulsed CRDS, a further increase in sensitivity can be achieved by using narrow line width continuous wave (cw) laser sources. This approach, next to increasing the spectral resolution, avoids the problem of multi-mode excitation of the optical cavity, which causes multi-exponential ringdown decays and ultimately limits the precision of the ringdown time constant measurement. A cw-NIR CRDS experiment based on an external cavity diode laser emitting light at wavelengths around 1650 nm has been setup. A modular detection cell design was implemented allowing us to measure both gas phase as well as interface spectra (by means of evanescent wave CRDS). The near infrared (NIR) spectral range was chosen because state-of-the-art single mode diode lasers, the required high quality mirrors and fiber technology is readily available and affordable.

n2oIn contrast to distributed feedback (DFB) diode lasers, ECDLs can be tuned over a broader wavelength range, in our case over the wavelength range 1625-1690 nm, and thus allow one to detect several different chemical species with only one laser diode. However, it turned out that long-term frequency drift and jitter of the ECDL caused by changes of ambient conditions led to bad performance of the spectrometer when applied for high resolution spectroscopy. As a way out, we finally implemented a conceptually new Fourier Transform based wavelength calibration scheme that enables the acquisition of high quality spectra without the need for stabilization of the probe laser. Here, the output of a second laser (DFB), which is continuously locked to a known transition of CO2, is combined with the actual probe laser beam in an optical fiber and its frequency is measured relative to the reference laser frequency using a Fourier transform wavelength meter. An excellent wavelength precision of 5 · 10-8 was obtained, which is comparable to the precision of the most sophisticated commercially available instruments [1].

The spectrometer has been applied to detect nitrous oxide isotopomers. A precise determination of the so-called site-preference SP in natural N2O samples, SP = ([14N15NO]/[15N14NO] - 1), would allow one to figure out sources and sinks of this important atmospheric trace gas. For example, N2O formed by microbial processes, namely denitrification and nitrification, shows characteristic site-preferences of SP = 0‰ and 33‰, respectively.

[1] C. Fehling, G. Friedrichs, "A Precise High-Resolution Near Infrared cw-Cavity Ringdown Spectrometer using a Fourier Transform based Wavelength Calibration", Rev. Sci. Instrum. 81 (2010) 053109/1-8; doi:10.1063/1.3422254.
 
Contributing researchers: G. Friedrichs and (formerly) C. Fehling

Field Application of a cw-CRDS analyzer determining surface ocean fCO2 and δ13C(CO2)

The increasing amount of anthropogenic carbon dioxide (CO2) in the atmosphere, a major component of climate change, is also reflected in the world oceans' carbon budget. The oceans act as a net CO2 sink, which is driven by the CO2 partial pressure difference across the air-sea interface. Seasonal time-series data of surface-water-dissolved inorganic carbon (DIC) and its isotopic composition allow the separation of relative contributions of several processes to the surface layer carbon budget. Processes altering both DIC and its isotopic composition include photosynthesis, mixing, and the oceanic Suess effect. Isotope ratio mass spectrometry is the conventional method for accurately measuring isotope ratios, but sample collection, shipping, and processing is labor-intensive and costly. In this highly interdisciplinary project, the long-term stability, response time, susceptibility to external interferences, and precision of a CRDS based instrument for δ13C(CO2) isotope analysis of gaseous CO2 was thoroughly analyzed. A significant gas matrix effect, which is caused by subtle absorption line shape effects, was found and a correction procedure for field measurements of surface-water-dissolved CO2 has been worked out. For underway pCO2 measurements aboard a research vessel, the instrument was successfully operated aboard RV Polarstern during the ANT-XXVI/1 and ANT-XXVI/4 cruises (Bremerhaven - Punta Arenas (Chile)). For the first time, on-line data of oceanic δ13C(CO2) and pCO2 could be collected. By comparing the CRDS data with reference measurements performed by established methods (NDIR for pCO2, IRMS for isotope ratios) it turned out that the CRDS data were in good agreement with the reference data. The next step will be the installation aboard a voluntary observing ship (VOS) in order to obtain a high resolution dataset of pCO2 and δ13C(CO2) of the North Atlantic surface ocean.

allanAllan plot statistics of CRDS CO2 analyzer.







 

transect_exp
Schematic setup of the water supply, equilibration system and gas pathways aboard R/V Polarstern.










 


transect
CO2 fugacity, fCO2, and its stable carbon isotope ratio, δ13C(CO2), during two Atlantic transects.





 

[1] G. Friedrichs, J. Bock. F. Temps, P. Fietzek, A. Körtzinger, D. Wallace, "Toward Continuous Monitoring of Seawater  13CO2/12CO2 Isotope Ratio and pCO2: Performance of a Cavity Ringdown Spectrometer and Gas Matrix Effects", Limnol. Oceanogr.: Methods 8 (2010), 539-551;doi:10.4319/lom.2010.8.539.

[2] M. Becker, A. Körtzinger, N. Andersen, B. Fiedler, P. Fietzek, T. Steinhoff, G. Friedrichs, "Using Cavity Ringdown Spectroscopy for Continuous Monitoring of δ13C(CO2) and fCO2 in the Surface Ocean", Limnol. Oceanogr.: Methods 10 (2012) 752-766; doi: 10.4319/lom.2012.10.752

CRDS & Kinetics

A requirement for a simple quantitative measurements of concentration-time profiles by means of CRDS is a (quasi) steady concentration of the detected species during the whole ringdown (typically several tens of μs). In this case, the concentration of the species can be easily extracted from the single-exponential ringdown
time constants by pump-probe schemes with variable time delays between photolysis (pump) and ringdown (probe) laser pulses. For "fast reactions" that occur on the same time scale as the ringdown, the ringdown becomes nonexponential due to the convolution with the concentration change caused by the reactions. In this case, a more sophisticated ringdown model has to be applied.

CRDS for Fast Reactions: The eSKaR Model

Typically, kinetic applications of cavity-ringdown spectroscopy (CRDS) are limited to the investigation of relatively slow reactions with lifetimes of the measured species longer than the ringdown time. In this case, the concentration of the reactive species remains almost constant during the ringdown period and complete concentration-time profiles can be obtained from the single-exponential ringdown time constants by pump-probe schemes with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses.

crds1For fast reactions that occur on the same time scale as the ringdown, the ringdown becomes nonexponential due to the convolution with the concentration change caused by the reactions. In this case, rate constants can be extracted from the nonexponential ringdown signals by the simultaneous kinetics and ringdown (SKaR) model developed by Brown et al. (J. Phys. Chem. A 2000, 104, 7044 and 8600). However, nonexponential signals are also obtained in the absence of a reaction when the probe laser line width is comparable or exceeds the absorpion line width of the detected species. In this case, different frequency components of the probe laser light in the ringdown cavity experience different absorptions and thus decay with different time constants. The resulting ringdown exhibits multiexponential character even without convoluted kinetics. This is called the bandwidth effect.

We have developed an extended version (eSKaR) of the SKaR model to extract rate constants of fast reactions from nonexponential ringdown profiles by deconvolution of the ensuing reaction and the bandwidth effect. Detailed measurements of the rate constants of the reactios NH2 + NO, SiH2 + O2, and SiH2 + alkenes have shown that the eSKaR model provides a reliable method for extracting rate constants from the ringdown profiles. Small inaccuracies in the assumed absorption and probe laser line shapes were shown to induce only minor errors in the finally determined rate constant. The eSKaR method thus extends the applicability of CRDS measurements of fast reactions of small radicals using conventional pulsed dye lasers as the probe laser source.

 [1] Y. Q. Guo, M. Fikri, G. Friedrichs, F. Temps, "An Extended Simultaneous Kinetics and Ringdown Model: Determination of the Rate Constant for the Reaction SiH2 + O2", Phys. Chem. Chem. Phys. 5 (2003) 4622-4630.

[2] G. Friedrichs, M. Colberg, M. Fikri, Z. Huang, J. Neumann, and F. Temps, "Validation of the extended Simultaneous Kinetics and Ringdown Model by Measurements of the Reaction NH2 + NO", J. Phys. Chem. A 109 (2005) 4785-4795.

[3] G. Friedrichs, "Review paper: Sensitive Absorption Methods for Quantitative Gas Phase Kinetic Measurements. Part2: Cavity Ringdown Spectroscopy", Z. Phys. Chem. 222 (2008) 31-61.

[4] G. Friedrichs, M. Fikri, Y. Q. Guo, F. Temps, "Time-resolved cavity ringdown measurements and kinetic modeling of the pressure dependences of the recombination reactions of SiH2 with the alkenes C2H4, C3H6, and t-C4H8", J. Phys. Chem. A 112 (2008) 5636-5646.

Contributing researchers: G. Friedrichs, F. Temps and (formerly) Y. Guo, M. Fikri, J. Neumann, Z. Huang