Incoherent broad-band cavity-enhanced absorption spectroscopy
The technique is realized using incoherent sources of radiation e.g. Xenon arc lamps, LEDs or supercontinuum (SC) lasers, hence the name.
Typically in IBBCEAS, the wavelength selection of the transmitted light takes place after the cavity by either dispersive or interferometric means.
[1] Consider a cavity of length d formed by two identical high reflectivity mirrors (R1 = R2 = R > 99.9%) with losses L, which is continuously excited with incoherent light of intensity Iin.
For an empty resonator with L = 0, the time integrated transmitted intensity I0 is given by The intensity of light transmitted by the cavity, I( = I0 + I1 + I2 + ⋯ ), can be described by the superposition of the light after an odd number of passes, leading to a geometric series: Since R < 1 and L < 1 the series converges to: Assuming the losses per pass to be solely due to Lambert-Beer attenuation, i.e. (1 − L) = e(-αd), the extinction coefficient, α can be written as In case of small losses per pass, L → 0, and high reflectivities of the mirrors, R → 1, α can then be approximated as Approximating Δ I / I 0 ≈ (I 0 - I) / I 0 ≈ (I 0 - I) / I, the minimum absorption coefficient, αmin, can be expressed by the following equation: where Δ I min is the minimum detectable change in intensity smaller than I_{0}.
[6] A basic IBBCEAS setup consists of an incoherent light source, collimation optics, the absorber of interest and a detector.
Light transmitted through the cavity is detected using a suitable detector, for example, a monochromator / charge-coupled device (CCD) combination interfaced with a computer.
By knowing the number density n (molecules/cm3) and wavelength-dependent absorption cross-section of the calibration sample, the effective reflectivity Reff(λ) can be determined by:
In the single coupler configuration, the ASE gain medium placed inside the cavity is used as a broadband incoherent source.
On the other hand, the approach provides an improvement to conventional Fourier Transform spectroscopy for gas applications where small sample volumes are required (e.g. for discharges, combustion plasmas, flames or chemical flow reactors).
The figure above shows the spin forbidden O2 b-band at ~ 14500 cm−1 (688 nm) measured[7] in ambient air at atmospheric pressure using a xenon arc lamp compared against a calculated HITRAN spectrum.
In order to fully exploit the selectivity feature of Fourier transform spectroscopy, the near infrared region is of interest because many overtone spectra of atmospherically relevant gases are located in this part of the spectrum.
Recent studies have demonstrated their application in Evanescent Wave-IBBCEAS using a mirror-prism-mirror cavity configuration to measure absorption spectra of metallo-porphyrins in thin solution layers.
More recently, LED-IBBCEAS has been applied to simultaneous open path measurements of HONO and NO2 in the UV region with detection limits of 430 pptv and 1 ppbv respectively and acquisition times in the order of a few seconds.
[26] SC sources are attractive for spectroscopic applications owing to their broad wavelength coverage, which enables spectral signatures of multiple species to be detected simultaneously.
Detection sensitivities at picomolar concentration levels in solution have been reported for BBCEAS measurements with SC sources with signal acquisition times in the lower millisecond range.
