INTRODUCTION:

A wide variety of laser-based spectroscopic techniques are used to probe chemical dynamics, combustion and plasma environments, the earth's atmosphere, and many other fascinating problems in modern Chemistry. Methods such as Laser Induced Fluorescence (LIF), Resonance Enhanced Multiphoton Ionization (REMPI), Degenerate Four-Wave Mixing (DFWM) and Photoacoustic Spectroscopy rely on measuring a side effect of the laser excitation of the sample, and have proved to be sensitive tools for spectroscopic studies. An important place remains, however, for laser absorption spectroscopy since it offers the possibility of quantitative measurements and can be used to study molecules with very short-lived excited states. The major drawback has been that the sensitivity of direct absorption methods can be limited and the detection of weak absorptions is made difficult by large background fluctuations in absolute light intensity. Cavity ring-down spectroscopy (CRDS), by virtue of its insensitivity to fluctuations in laser output and the enormous (many kilometers) path lengths through the sample that can be achieved, has become the method of choice for ultra-sensitive and quantitative absorption measurements.

In this article, we outline the working background, implementation, and applications of CRDS. First, however, we outline the basics of absorption spectroscopy and consider alternative, high sensitivity laser methods commonly employed for laser spectroscopy and diagnostics so that we can draw comparisons with the CRDS method. This overview of laser methods is only cursory because of the extensive existing literature^{1, 2}. We also address what we regard as the primary limitations of CRDS and discuss likely future developments in the experimental methods that will extend the versatility of the technique.

HISTORY:

The work reported by Herbelin et al.³ (1980) and Anderson et al.⁴ (1984) can be regarded as precursors to the CRD technique, although the transmission of a light pulse through an optical cavity has already been studied for a long time were the first to propose the use of an optical cavity for measuring the reflectance of mirror coatings (the reflection losses are, in a way, the ‘absorption’ of the optical cavity). By intensity modulating a continuous wave (CW) light beam and measuring the phase shift introduced by the optical cavity, they were able to determine accurately the high reflectance of their mirrors. In 1984, Anderson et al. demonstrated that the reflectance could be measured even better by abruptly switching off the CW light source when the intracavity field exceeded a certain threshold value, followed by a recording of the subsequent intensity decay of the light in the optical cavity. In both techniques, injection of light into the cavity occurred via accidental coincidences of the (narrow-bandwidth) laser frequency with the frequency of one of the narrow cavity modes. In 1988 O’Keefe and Deacon^5,
6 showed that problems associated with mode coincidences could be circumvented by using a pulsed laser. Additionally, owing to the pulsed character, no electronics were needed for monitoring the intracavity power or for switching off the laser, before observing the decay transient, thus providing a simple experimental design for measuring the cavity loss. O’Keefe and Deacon realized that this method was suitable for measuring the absorption spectrum of molecules present in the cavity.

An overview of the use of CRDS for the study of fast (sub nanosecond) predissociation of electronically excited states of small molecules and radicals has been given by Wheeler et al.⁷(1998). Cheskis (1999) discussed the application of the CRD technique to the measurements of radicals in flames.

PRINCIPLE:

CRDS is a spectroscopic technique for measuring the transmission or more accurately, the absorbance of light through a material. CRDS can provide extremely sensitive measurements, allowing to measure very small differences in the amount of absorbed light. CRDS uses an optical cavity, a pair of mirrors facing each other. A brief pulse of laser light is injected into the cavity, and it bounces back and forth between the mirrors. A small amount of the laser light (typically 0.1% or less) leaks out of the cavity on each bounce. A detector measures this leakage. Since some light is lost on each reflection, the amount of light inside the cavity is slightly less after each bounce. Since the light that leaks out is a percentage of the light inside the cavity, the amount of light measured also decreases with each reflection. If something that absorbs light is placed in the cavity, the amount of light decreases faster, it makes fewer bounces before it is all gone. A CRDS setup measures how long it takes for the light to drop to a certain percentage of its original amount, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity.

Optical spectroscopic techniques in general and laser-based techniques in particular, have a great potential for detection and monitoring of constituents in gas phase. They combine a number of important properties, e.g. a high sensitivity and a high selectivity with non-intrusive and remote sensing capabilities. Laser absorption spectrometry has become the foremost used technique for quantitative assessments of atoms and molecules in gas phase. It is also a widely used technique for a variety of other applications, e.g. within the field of optical frequency metrology or in studies of Light matter interactions.

WORKING:

The basic experimental requirements are a tunable laser source, two highly-reflective (> 99.9%) dielectric-coated concave mirrors, a photo-sensitive detector and data acquisition equipment. The mirrors form the windows of a vacuum cell inside which the species of interest may be placed. Although CRDS using CW lasers is becoming widespread, the discussion here will concentrate on the use of pulsed lasers. The laser beam is aligned along the axis of the ring-down cavity defined by the two mirrors. Clearly most of the laser light is reflected straight back off the input mirror, but a small percentage (< 0.1%) is transmitted through. The mirrors are mounted so that their positions can be minutely adjusted, and with careful alignment the laser pulse may be trapped inside the cavity, being reflected backwards and forwards. In this way a pulse can be stored for microseconds (thousands of round trips, giving effective kilometres-long path lengths between the mirrors) before decaying away due to cavity losses (such as mirror transmission or sample absorption). CRDS apparatus is bounded by two high reflectivity mirrors without intervening surfaces and the separation of the two fixed mirrors determines the structure and frequency spacing of the cavity modes for light trapped within the cavity (Fig.1 and Fig. 2).

Fig 1: Schematic diagram of pulsed laser cavity ring-down apparatus

Fig 2: Schematic diagram of Cavity ring-down Spectroscopy apparatus*

*The key components illustrated are as follows:

1. Diode laser: Emits light energy

2. Isolator: Prevents light energy feedback from interfering with the laser

3. Acousto-Optical Modulator: Shuttering device for the light source

4. Absorption Cell: With mirrors, creates measurement cavity

5. Photodiode: Monitors the light energy from the absorption cell

6. Trigger: Works in concert with the photodiode and sends signal to the Acousto-Optical Modulator to activate the ring-down cycle.

A photo-sensitive detector is positioned behind the output mirror to record the (tiny) intensity of light transmitted through it; because the light intensity is reduced by a given percentage on each round trip, the detector sees an exponential decay of light ringing down. (If short laser pulses are used, a very fast detector will see a train of pulses within an exponential decay envelope, but the time response of detection electronics usually means the pulses are smoothed into a single exponential curve) (Fig. 3)

Fig 3: Exponential decay of light ringing down inside the cavity

The signal from the detector is amplified and digitized, and fed to a computer, which fits the trace to a first-order exponential function to determine the decay time constant for each pulse. This constant is determined by two factors: the reflectivity of the mirrors and attenuation of the laser pulse by any absorbing medium inside the cavity (i.e. the pulse rings down faster if some intensity is lost to absorption on each round trip). If the wavelength of the light within the cavity matches absorption of a sample gas held between the mirrors, the additional mechanism for loss of light from the cavity by sample absorption speeds up the decay of the trapped light intensity. For absorption conditions corresponding to Beer Lambert law behavior, the decay of the light intensity will still be exponential, with time dependence

ADVANTAGES OF CRDS:

There are main advantages to CRDS over other absorption methods:

1. Stability: It isn't affected by fluctuations in the laser. In most absorption measurements, the light source must be assumed to remain steady between blank (no analyte), standard (known amount of analyte), and sample (unknown amount of analyte). Any drift (change in the light source) between measurements will introduce errors. In CRDS, the ringdown time does not depend on the brightness of the laser, so fluctuations aren't a problem.

2. Sensitivity: It is very sensitive due to its long pathlength. In absorption measurements, the smallest amount that can be detected depends on the length that the light travels through a sample. Since the light reflects many times between the mirrors, it ends up traveling long distances. For example, a laser pulse making 500 round trips through a 1 meter cavity will effectively have traveled through 1 kilometer of sample

3. The sensitivity of CRDS stems in part from the huge number of passes each laser pulse makes between the ultra-high reflectivity mirrors - for reflectivities of 0.9999, the number of round trips is about 5000, giving an effective pathlength of 10 kilometres, far higher than for conventional multi-pass arrangements

4. Specificity: By measuring the ring-down decay rate rather than absolute intensity of the laser pulse, shot-to-shot variations in laser output are removed from the final spectrum

5. Absolute Measurement: CRDS is an absolute measurement, unlike most other spectroscopic techniques; it requires no calibration or gas standard

6. Self Calibration: Every measurement sequence incorporates an intrinsic recalibration by tuning the laser first to the baseline and then through the absorption peak. No special calibration procedure or reference gases are required

7. Situation: Non optical techniques such as gas chromatography, mass spectrometry, and ion mobility spectrometry all force physical separation of the target analyte from the matrix and then count up the results. CRDS measures the sample as it is, in situ, and accepts a full sample directly into the measurement cavity

8. Simplicity and Economical: CRDS requires only a few basic optical components and no moving parts and the apparatus required for CRDS is relatively compact and inexpensive, consisting of bench-top apparatus and a single laser system. Recent developments with CW diode lasers have realized the potential for compact instruments to perform in-situ measurements of atmospheric trace gas constituents

9. CRDS is applicable at any wavelengths for which the requisite highly-reflective mirrors are available, and thus encompasses a large spectrum extending from the mid-infrared to deep into the ultraviolet. As such, ring-down spectroscopy has been widely used to study vibrational and electronic spectra of molecules present at very low concentrations or having very low transition probabilities, making their detection difficult by conventional methods

10. Immunity to shot variations in laser intensity due to the measurement of a rate constant

11. High throughput, individual ring down events occur on the millisecond time scale

12. This technique is gas-phase, there is no need to account for solvent shifts in the measurements, meaning that our measurements directly probe structure and intramolecular dynamics, and can be easily used to search for molecules in space, or as benchmark data for improving computational chemistry methods

13. CRDS is a non sensitive to laser pulse to pulse fluctuations and it is applicable on non fluorescent species with High sensitivity

14. The advantage over normal absorption spectroscopy results from, firstly, the intrinsic insensitivity to light source intensity fluctuations and, secondly, the extremely long effective path lengths (many kilometers) that can be realized in stable optical cavity.

15. Whereas other techniques such as LIF may rival the sensitivity of CRDS, the ability to record quantitative absorption spectra makes the ring-down technique preferable in situations where absolute intensities are required, or where fluorescence yields are poor (e.g. in short-lived predissociated systems)

DISADVANTAGES OF CRDS:

1. Spectra cannot be acquired quickly due to the monochromatic laser source which is used.

2. CRDS mirrors often work only over a narrow wavelength range (e.g. 415nm ± 5nm)

3. Analytes are limited both by the availability of tunable laser light at the appropriate wavelength and also the availability of high reflectance mirrors at those wavelengths

4. Expense: The requirement for laser systems and high reflectivity mirrors often makes CRDS orders of magnitude more expensive than some alternative spectroscopic techniques

5. CRDS is non-species selective: All absorbing molecules (at the wavelength of the laser) in the cavity will contribute to the ring-down decay rate, causing spectral contamination in some instances

6. The technique requires constant mirror reflectivities over long timescales, something that is hard to achieve in "dirty" environments causing deposition on mirror surfaces. This may be overcome in some cases by directing inert gas streams across the mirror faces.

FIELDS OF APPLICATION:

1. Diagnostics - It is inevitable that it will find use as a new laser-based diagnostic technique in a whole range of analytic applications

2. Dating-To date, the CRD technique has been successfully applied in various environments

3. Environmental survey- CRDS has been applied to detect molecular species in samples from many different scientific research fields, ranging from such hostile environments as combustion and flames. High resolution spectroscopy studies have been performed on molecules in cells and supersonic jets and on transient molecules generated in discharges, flow reactors and flames

4. Flame analysis-Optimization of the gas seeders in automotive engines and mapping of the different elements in a laminar flame, as function of the distance to the burner

5. Trace analysis-With the use of CRDS, it is possible to detect absorbing species at trace levels, or to observe extremely weak absorptions such as high vibrational overtones or spin-forbidden electronic transitions

6. Its applications also include assignment of diffuse interstellar bands and spectroscopic characterization of biological relevant systems

7. CRDS is a very sensitive diagnostic technique for absorption measurements. It is capable of measuring the absolute line-of-sight (LOS) integrated density of negative hydrogen ions (H⁻, D⁻) which induce a weak absorption along a LOS in plasmas containing negative hydrogen ions⁸.

8. CRDS, which is most commonly used in the detection of trace concentrations⁹ of absorbing molecules, offers a real-time response in a simple, compact, less expensive arrangement that requires negligible sample preparation

9. The implementation of CRDS in an optical fiber resonator extends the viability of this highly sensitive technique for label-free detection¹⁰ of biological species. From the observed detection limits, based on a minimum detectable scattering cross section on the order of 10 µm², suggest a broad range of new applications in a simple, inexpensive device for real-time cavity ring-down biosensing

10. The CRD technique has been widely adopted for molecular spectroscopy applications ranging from the monitoring of disperse atmospheric species to the spectral resolution of forbidden overtone transitions in small molecules¹¹.

11. Cavity ring-down UV absorption spectroscopy was used to study the kinetics of the recombination reaction of FCO radicals¹² and the reactions with O₂ and NO

12. CRDS can be used as a detector¹³ for high performance liquid chromatography (HPLC)

13. High-resolution, pulsed infrared cavity ringdown laser spectroscopy was developed and implemented for the study of carbon clusters and hydrocarbon ions. Several supersonic molecular beam sources of ions and hydrocarbons were constructed and tested by infrared cavity ringdown spectroscopy

14. CRDS technique used for sputter erosion¹⁴ measurements

15. A simple, economic diode laser based cavity ringdown system for trace-gas applications in the petrochemical industry¹⁵.

16. Use of CRDS for measurement of number densities of sputtered particles and the applications for thruster characterization in electric propulsion and process monitoring in plasma engineering

17. Recent measurements of carbon isotopes¹⁶ in carbon dioxide using near-infrared, diode-laser-based CRDS system achieved good precision

18. CRDS is a highly sensitive spectroscopic technique that has been successfully applied to problems such as trace gas detection and the observation of weak spectra.

19. Application of CRDS has been limited to gas phase samples because solids show large optical losses by reflection. Here, we show that CRDS is applicable to an optically flat solid material.

FUTURE PROSPECTS:

From the foregoing it is obvious that CRDS offers a whole range of new possibilities; it is safe to assume that there are more, as yet unthought-of, to come. Before concluding with a paragraph summarizing some real and imagined prospects for the technique it is perhaps worth commenting on its limitations, amongst the more obvious are the following.

1. The method samples along a column, rather than at a point, which may prove restrictive when attempting spatial profiling in very inhomogeneous environments.

2. CRDS is a one-dimensional technique in the sense that the only user-selectable degree of freedom is the excitation frequency. All absorbing species in the cavity will contribute to the ring down time. Thus sample purity may be more of an issue than for rival techniques such as LIF (where the frequency dependence of both the excitation and detection steps can be used to aid species selectivity) or REMPI, where the spectral carrier can be distinguished not just by the spectrum in frequency space but also by monitoring the mass spectrum of the resulting ions or the kinetic energy distribution of the accompanying photoelectrons.

3. The mirror reflectivity, and its constancy over a long timescale, is crucial aspects of the technique, the maintenance of which may necessitate particularly careful cavity design in cases where CRDS is to be applied to “dirty” environments.

CONCLUSION:

Though CRDS well established within the scientific community for over ten years, CRDS is revolutionizing the sensitivity, speed, ease of use, robustness, and portability of trace chemical species detection. CRDS is thus a powerful technique particularly suited to the spectroscopy of very weak transitions, or of molecules present at very low concentrations. Today, CRDS is being commercialized across a broad range of application areas, including medical diagnostics, industrial process control, environmental monitoring, and civilian and military security. Finally the good sensitivity of CRDS combined with a fairly simple experimental set-up, makes these direct absorption techniques very powerful in many areas of research.

REFERENCES:

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14. A. Yalin, V. Surla, J. Williams and P. Wilbur, “Application of cavity ring down spectroscopy to sputter erosion measurements”, in laser applications to chemical and environmental analysis, Technical digest (optical society of America), 2004, paper MC4

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Received on 08.11.2009 Modified on 29.12.2009

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