Cavity Enhanced Absorption Spectroscopy

The Cavity-Enhanced absorption spectroscopy (CEAS) belongs, as the cavity ring-down spectroscopy (CRDS) to the methods of spectroscopy by means of optical resonators . Usually only the English term is used, it can be translated as "resonator-enhanced absorption spectroscopy". The alternative designation Integrated Cavity Output Spectroscopy (ICOS) is less common .

Functional principle of resonator-enhanced spectroscopy

functionality

The cavity enhanced absorption spectroscopy is based on the virtual extension of a light path through an optical resonator . Similar to White or Herriott cells , the intensity transmitted through an absorbing medium decreases proportionally to the light path according to the Lambert-Beer law . A long light path can improve the signal-to-noise ratio of a transmission measurement through a weakly absorbing sample.

A typical structure consists of a light source, coupling optics, the optical resonator, coupling optics and a detector. The optical resonator consists of two highly reflective concave mirrors at a distance between concentric and confocal geometry.

A good understanding of how CEAS works is provided by considering an infinitesimally small packet of light: If it can be assumed that there is no absorption in the highly reflective layer and the mirror substrate, the sum of transmission and reflection is just one. The fraction of a light packet, which is also emitted by the light source, penetrates the mirror. If you consider a complete resonator with two mirrors and a medium with the absorption , the detector measures behind the resonator after a simple passage of the light packet through the resonator: ${\ displaystyle T}$ ${\ displaystyle R}$ ${\ displaystyle I_ {0}}$ ${\ displaystyle I = I_ {0} \ cdot (1-R)}$ ${\ displaystyle (1-A)}$

{\ displaystyle I_ {1} = I_ {0} \ cdot (1-R) ​​\ cdot (1-A) \ cdot (1-R)}

The significantly larger portion of the light package is reflected on the second mirror (output mirror) and passes through the resonator one more time, where the process is repeated on the first mirror (input mirror). If you consider an infinite number of light packages - i.e. a continuously working light source - the intensity measured at the output mirror is the sum of the power transmitted with each passage:

{\ displaystyle I_ {n} = I_ {0} \ cdot (1-R) ​​\ cdot (1-A) \ cdot (1-R)}

{\ displaystyle \ quad + I_ {0} \ cdot (1-R) ​​\ cdot (1-A) \ cdot R \ cdot (1-A) \ cdot R \ cdot (1-A) \ cdot (1-R )}

{\ displaystyle \ quad + \ dots}

{\ displaystyle \ quad + I_ {0} \ cdot (1-R) ​​^ {2} \ cdot R ^ {2n} \ cdot (1-A) ^ {2n + 1}}

{\ displaystyle I_ {n} = I_ {0} \ cdot (1-R) ​​^ {2} \ cdot (1-A) \ cdot \ sum _ {n} R ^ {2n} (1-A) ^ { 2n}}

For this series converges to: ${\ displaystyle R, A <1}$

{\ displaystyle I = I_ {0} {\ frac {(1-R) ​​^ {2} \ cdot (1-A)} {1-R ^ {2} \ cdot (1-A) ^ {2}} }}

For the empty resonator is and in general , and thus: ${\ displaystyle A = 0}$ ${\ displaystyle 1 + R \ approx 2}$

{\ displaystyle I _ {\ text {empty}} = I_ {0} {\ frac {(1-R) ​​^ {2}} {1-R ^ {2}}} \ approx I_ {0} {\ frac { 1-R} {2}}}

If the absorption losses with a filled resonator are strictly according to Lambert-Beer, the following applies:

{\ displaystyle (1-A) = e ^ {- \ alpha \ cdot L}}

With a known resonator length , absorption length and mirror reflectivity, the absorption coefficient of the absorbing medium in the resonator can be determined: ${\ displaystyle L_ {0}}$ ${\ displaystyle L}$ ${\ displaystyle R}$ ${\ displaystyle \ alpha}$

{\ displaystyle \ alpha \ approx {\ frac {1} {L_ {0}}} \ left ({\ frac {I _ {\ text {empty}}} {I}} - 1 \ right) \ cdot (1- R)}

As an alternative to this phenomenological derivation, the process can be described from a probability analysis of finding a photon within the optical resonator.

Sensitivity and Limit of Detection

A discussion about sensitivity and detection limit can be found e.g. B. in.

Methods

The various CEAS methods differ in the type of light source and in the different approaches used to determine the reflectivity of the mirrors. ${\ displaystyle R}$

Coherent CEAS

CEAS, OF-CEAS

Incoherent broadband CEAS

BB-CEAS, CE-DOAS

application

Trace gases, particles, liquids

literature

Giel Berden, Richard Engeln: Cavity Ring-Down Spectroscopy - Techniques and Applications. Wiley-VCH, Chichester 2009, ISBN 978-1-405-17688-0 .

credentials

↑ ^a ^b Fraunhofer IPM
↑ U. Platt, J. Meinen, D. Pöhler, T. Leisner: Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) - applicability and corrections. In: Atmospheric Chemistry and Physics. Volume 2, 2009, doi : 10.5194 / amt-2-713-2009 , pp. 713–723 (online)
^ SE Fiedler, A. Hese, AA Ruth: Incoherent broad-band cavity-enhanced absorption spectroscopy of liquids. In: Rev. Sci. Instrum. Volume 76, 2005, p. 23107
↑ J. Meinen, J. Thieser, U. Platt, T. Leisner: Technical Note: Using a high finesse optical resonator to provide a long light path for differential optical absorption spectroscopy: CE-DOAS. In: Atmospheric Chemistry and Physics. Volume 10, 2010, doi : 10.5194 / acp-10-3901-2010 , pp. 3901-3914. (on-line)

[IPM-1] Fraunhofer IPM

[2] U. Platt, J. Meinen, D. Pöhler, T. Leisner: Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) - applicability and corrections. In: Atmospheric Chemistry and Physics. Volume 2, 2009, doi : 10.5194 / amt-2-713-2009 , pp. 713–723 (online)

[3] SE Fiedler, A. Hese, AA Ruth: Incoherent broad-band cavity-enhanced absorption spectroscopy of liquids. In: Rev. Sci. Instrum. Volume 76, 2005, p. 23107

[Meinen-4] J. Meinen, J. Thieser, U. Platt, T. Leisner: Technical Note: Using a high finesse optical resonator to provide a long light path for differential optical absorption spectroscopy: CE-DOAS. In: Atmospheric Chemistry and Physics. Volume 10, 2010, doi : 10.5194 / acp-10-3901-2010 , pp. 3901-3914. (on-line)