Laser Raman Spectroscopic Analyses of Dissolved Gases

Laser Raman spectroscopy is an optical technique which is capable of in situ molecular identification of solids, liquids and gases, and is well suited to extreme environments. It is a type of vibrational spectroscopy which is non-invasive and does not require reagents or consumables. A laser excites a target, and the spectrum of the energy-shifted, back-scattered radiation serves as a "fingerprint" - providing compositional and structural information. Raman scattering is a weak effect - only 1 in 10
⁸
photons is Raman scattered - but it benefits in the deep ocean from the lack of ambient light. The development of a sea-going Raman system capable of deployment on an ocean observatory will allow in situ analyses of dissolved gases in vent fluids at hydrothermal vent and other seafloor seep sites. Gases of interest at hydrothermal and cold seep sites include carbon dioxide (CO
₂
), methane (CH
₄
) and higher hydrocarbons, and hydrogen sulfide (H
₂
S). These gases are all Raman active and have distinct Raman signatures. For example, CO
₂
is characterized by a Fermi diad at ~1285 & ~1388 Deltacm
^-1
. However, the aqueous band positions are broadened and shifted from the gas-phase band positions. In addition to identifying gases by band positions, concentrations of dissolved gases can be inferred by the relative height of their Raman bands. Extracting this type of quantitative information from Raman spectra requires the use of a ratio technique as described by Wopenka and Pasteris [1987]. In this work the gas bands are compared to the water stretching band (~3435 Deltacm
^-1
). Although the position and intensity of some of the water bands are affected by temperature, the 3435 band height is relatively stable.