Water vapor spectroscopy in the blue-near UV range

For several years now, the LAME team of LIPhy has been at the forefront of the development of new laser techniques that considerably increase the sensitivity of physical measurements, thanks to the use of very high-finesse optical cavities (resonators). The major advantage of these cavities is that they enable equivalent optical paths of several kilometers to be obtained for an actual length of one meter!

The measurement technique (mode-locked cavity enhanced absorption spectroscopy (MLCEAS) [1]) is based on a method developed in the laboratory in 2002. This specific experiment applies this technique to a femtosecond pulsed titanium-sapphire laser (frequency doubled to reach wavelengths from ultra-violet to visible (340 to 540 nm)), combined with a very high-resolution spectrometer (resolving power of 800000). This frequency conversion complements the wavelengths currently available at LIPhy, with the aim of providing a laboratory instrument for the spectroscopy of various molecules of atmospheric interest. A first application will be the absorption spectroscopy of the water molecule in the blue and near-UV wavelengths. There is currently some debate as to the value of the absorption due to water vapor in this range [2].

The internship will involve recording and analyzing spectra of pure water vapor in the 380-440 nm range accessible with the mirrors currently in place on the high-finesse cavity. Each spectrum will be individually frequency-calibrated using the emission lines of a uranium lamp and the absorption spectrum of an iodine cell, as illustrated in the schematic diagram of the experiment above. The resulting water vapor spectrum will be compared with existing literature. The internship will be predominantly experimental, involving laser alignment and spectrum acquisition. It will be complemented by an analysis of the recorded data.

References :
[1] T. Gherman et D. Romanini, Optics Express (2002), doi.org/10.1364/OE.10.001033
[2] Q.-Y. Yang et al., Atmos. Meas. Tech. (2022), doi.org/10.5194/amt-15-4463-2022