Project Alma Mater

Absorption of Light, Macro algae and Atmosphere

 

Marie Curie Intra European Fellowship (IEF)

 

Project No: 3012109

 

Persons in Charge: Dr. Ranjini Raghunandan, Dr. Andy Ruth

 

 

 

Background Methodology Outreach Project Output

 

             IBBCEAS FT-IBBCEAS Off-Axis CEAS

 

 

 

Methodology

 

A very powerful and widely applicable principle to detect trace amounts of gaseous substances is by means of their optical absorption properties. For the detection of halocarbons from seaweed, a cavity-enhanced absorption instrument at the host institute will be developed and advanced - the basic considerations in this context will be outlined in the following. The key qualities of any in situ spectroscopic absorption instrument are its detection sensitivity and species selectivity.

 

The sensitivity of an absorption method depends on the length over which light can interact with the substance(s) of interest. The sensitivity increases linearly with the absorption path length. Over the past two decades approaches have been developed (mainly for gas detection) that use optically stable cavities consisting of two highly reflecting mirrors (opposing each other) to dramatically increase the absorption path length and hence the sensitivity. Light entering a cavity through one mirror is reflected back and forth many times before leaving the cavity through the other mirror and entering a light detector.

 

Although many different species may absorb light at one or more wavelengths, the total spectral profile of any particular species is unique - every species has a so-called "spectral fingerprint", which is generally more pronounced in the (near) infrared region than in the visible or UV. The selectivity of an instrument is its ability to distinguish between different species absorbing at similar wavelengths. In practice, selecting an appropriate wavelength range that is specific to a target species can pose a challenge, especially if "interfering" species are present (which will be the case for seaweed emissions). Therefore high spectral resolution methods (such as Fourier transform spectroscopy suggested here) generally have a high selectivity if applied in an appropriate wavelength range.

 

In this project a cavity-enhanced absorption spectroscopy (CEAS) instrument with improved sensitivity and selectivity for trace gas monitoring will be developed by combining "classical" Fourier transform spectroscopy (FTS) with 'incoherent broadband cavity enhanced absorption spectroscopy' (IBB-CEAS); the latter method was pioneered by the host group.

 

In addition to FTIBBCEAS, an off axis CEAS spectrometer has been set up to measure biogenic emissions using a more 'conventional' approach using tuneable external cavity diode lasers.