Experiments

Diode laser molecular spectroscopy

This experiment concerns the Molecular Spectroscopy of molecules of atmospheric interest, like CH4, CH3Cl, CH3F, CH3I, C2H2, C2H4, CO2, H2O, NH3, N2O, O2.
By using monomodal semiconductor lasers as the sources, noise reduction techniques, such like the Wavelength Modulation Spectroscopy (WMS) along with the phase detection, and the aid of multipass measurement cells, remarkable results have been obtained in the characterization of overtone resonances of such molecules in VIS and NIR part of the e.m. spectrum.
By working with the laser diodes in “free-running” or in “extended cavity” mode and sweeping them in frequency, specific ro-vibrational molecular parameters can be measured with high precision for each sampled molecule.
The maximum resolution attainable by this spectroscope is on the order of 0.01 cm^-1 (3σ) and its sensitivity permits the detection of absorption lines with intensity (S) around 10^-26 cm/molecule, with extinction coefficients of the order of magnitude of tens of km·amagat.
At the same time, the “Gas Filter Correlation” (GFC ) technique was implemented, using infrared Mid IR LED sources and a multipass White type cell. A suitable band pass filter permits to select only the specific wavelengths of the gas to be measured and consequently reduce interference from other gases present in the measurement environment. The heart of this technique is the filter cell, made up of two separate parts: one containing an inert gas and the other with the gas being measured. The development of this technique has shown good sensitivity and the possibility of integration in a low-cost portable device.

A particular of the laser diode beam reflections on one of the two mirrors of the multipass Herriott type measurement cell as seen by an infrared camera.

GFC system schematic. Controller: Mid IR LED controller; IR Det: IR detector; NBPF: narrow band pass filter; Preampl.: preamplifier.