The enormous sensitivity of intracavity absorption spectroscopy (ICAS), as well as its unique ability to tolerate high broadband losses caused by, e.g., optical windows and light scattering, is being ...exploited by only a few research groups worldwide. The reason seems to be the lack of comprehensive literature, such that the field remains difficult to access for non-experts, in particular for engineers and chemists, who might derive the most benefits from applying ICAS. In particular, the missing connection to this target audience appears to be two-fold: (i) the seeming complexity of the theory, and (ii) the necessity to setup homemade laser systems. However, once some basic understanding and knowledge is obtained, both aspects appear to be of similar complexity as with other spectroscopic techniques. Therefore, the current work is aiming at (i) providing a comprehensive review of the theoretical basics of ICAS, and (ii) describing the most important practical aspects that need to be considered for a successful realization of ICAS measurements. To ensure maximum clarity, illustrative practical examples of recent work are used throughout the paper.
•Demand for clean energy requires multiplexed ultrasensitive diagnostics.•ICAS -Intracavity laser absorption spectroscopy.•ICAS for ultrasensitive detection of key intermediates in harsh ...environments.•Energy, combustion and material synthesis - relevant species are detected.•ICAS is currently expanded into mid-infrared and ultraviolet spectral ranges.
Meeting the demands of sustainable energy economy requires diagnostics of the chemical processes surrounding future fuels and contemporary combustion applications. Pioneered in 1970, Intracavity Laser Absorption Spectroscopy (ICAS) has evolved to be a powerful instrument in the toolbox of combustion diagnostics. It owes its ultra-high sensitivity to the enhancement of the effective absorption pathlength by placing the absorber inside the cavity of a broadband laser. In this review we introduce the complementary strengths of ICAS to other methods: ultra-high sensitivity to narrowband absorption alongside the immunity to broadband losses, multiplexed detection and (µs-scale)-temporal resolution. We outline the basic concepts and features of ICAS, focusing on the laser dynamics regime where an absorbing sample in the laser resonator yields the well-known Lambert-Beer law. We chart the progress made over the years in visible (dye-jet laser) and near infrared (fiber laser) ICAS speciation in flames, by highlighting case studies where species like long considered "hard-to detect" 1CH2 and HCO radicals, along with O-atoms, C2, NH2, HNO, CN, and HCN were measured, as well as thermometry and speciation applications demonstrated in shock tubes, flow-cells and flames based on (stationary or time-resolved) measurements of multicomponent spectral matrices containing lines of CH4, C2H2, CO2, CO, OH and H2O. We highlight the contributions of ICAS in gas-phase nanomaterial synthesis, exemplified in prototypical iron-doped flames and discuss prospective applications in spray-flame pyrolysis and metal-powder combustion. Finally, we present advances in the development of lasing media based on Cr2+ and Fe2+-doped chalcogenide crystals and fluoride crystals doped with trivalent lanthanides, that meet the (ICAS-specific) requirement associated with the necessity to have a gain media lasing directly in the desired wavelength range, and therefore to expand this technique into the important mid-infrared and ultraviolet spectral ranges.
This paper provides an overview of mid-infrared lasers based on rare-earth-ion-doped selenide glasses. Laser action was demonstrated at the transitions between the first excited and the ground levels ...of Ce3+, Pr3+, Nd3+ and Tb3+ ions. The highest output parameters for bulk glass lasers (over 40 mJ of output energy) and wavelength tuning in the range of 4.6–5.6 microns were obtained with Ce3+-doped glass. The highest output parameters for fiber lasers (150 mW at 5.1–5.3 μm under continuous pumping) were demonstrated with Tb3+ ions. The longest lasing wavelengths for any glass laser and tunability within the 5.56–6.01 µm spectral band were shown with Nd3+ ions in a Tb3+-Nd3+ co-doped system.
Proposed highlights for the manuscript titled “Revisiting iron oxidation chemistry in synthesis flames: insights from a shock-tube study with intracavity laser absorption spectroscopy tracking of ...FeO” submitted to the journal applications in energy and combustion science by M. Lalanne et al. are listed in the following:.•Reaction between fe and O2 was studied in the 1050–3400 K temperature range.•ICAS monitors fe oxidation directly by detecting FeO.•Oscillator strengths of 41 spectral transitions of FeO are reported.•Distinct temperature regimes of fe oxidation by O2 are demonstrated.
The reaction between Fe atoms and O2 in a mixture of iron-pentacarbonyl (IPC, 2 ppm) and oxygen (100 ppm), diluted in argon, has been studied in a shock tube behind reflected shock waves over the temperature and pressure ranges of 1050–3400 K and 0.7–2.0 bar. Time-resolved measurements of Fe and FeO have been performed simultaneously using a combination of atomic resonance absorption spectroscopy (ARAS) and intracavity laser absorption spectroscopy (ICAS) with a custom-made broadband dye laser, respectively. For ICAS, absorption features in the spectral range from 16,316 to 16,353 cm−1 have been evaluated, and the oscillator strengths for all 41 assigned ro-vibronic transitions have been expressed. For most of the experimental cases, the measured Fe and FeO traces agreed well with the mechanism reported in this work. The quantitative and highly-sensitive measurements revealed the presence of FeO at temperatures below 1400 K, leading to a reconsideration of rate coefficients for different Fe oxidation channels.
Gas-phase iron compounds strongly affect the flame structure already at very low concentrations, which implies the control of combustion efficiency, pollution formation, and materials synthesis in ...flames. The impact of iron pentacarbonyl on low-pressure premixed flames was investigated experimentally and numerically for a broad range of equivalence ratios. The burner was operated in top-to-bottom orientation, causing a strong effect of buoyancy on the flow field, a configuration, also known as buoyancy-opposed flame. The application of ultra-sensitive broadband intracavity laser absorption diagnostics enabled path-integrated measurements of gas-phase FeO in the particle-laden flow. Spatially-resolved temperature distributions were measured via OH laser-induced fluorescence. The measurements were complemented by detailed simulations of the down-firing flame to determine the (one-dimensional) flow field on the centerline of the burner. The experimental findings were the basis for extension of existing reaction schemes for iron-doped flames and a new skeletal scheme was proposed. Measured temperatures and normalized FeO concentrations were used to validate both the detailed and the skeletal scheme. The results of the optimization and reduction procedure helped to improve the understanding of the structure of the iron-doped flame and the role of iron-cluster formation in the interaction mechanisms which cause the flame inhibition or promotion by iron-compounds.
We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around
λ
= 3.1–3.4 µm. A detection scheme based on an integrated ...recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of
L
eff
≈ 850 m. Exemplary measurements of atmospheric H
2
O and CH
4
, and additionally introduced gas-phase HCl, C
2
H
4
, or C
2
H
6
are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 10
4
are discussed.
A broadband tunable Tm/Ho-doped fiber laser is developed for sensitive in situ measurements of intracavity absorption spectra in the spectral range of 4780–5560 cm
−1
. This spectral range includes ...an atmospheric transmission window enabling sensitive measurements of various species. The spectral bandwidth of laser emission varies from 20 to 60 cm
−1
and is well suitable for multicomponent spectroscopy. The sensitivity achieved in cw operation corresponds to an effective absorption path length of
L
eff
= 20 km, with a spectral noise of less than 1%. The spectroscopic system is applied for measurements of absorption spectra of H
2
O, NH
3
and for simultaneous in situ detection of three isotopes of CO
2
in human breath, which is important for medical diagnostics procedures.
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