Diode- and Difference-Frequency Laser Studies of Atmospheric Molecules in the Near- and Mid-infrared: H2O, NO2, and NH3

The advantages of using external cavity diode-lasers (ECDL) for quantitative spectroscopy in the laboratory are in particular the absence of any significant instrumental line shape (ECDL have typical emission line widths of 1 MHz i.e. a few 0.00001 cm-1), the fast temporal response and the high signal-to-noise ratio that can be achieved in rather short experiments. We have used near-infrared ECDL to study line widths and intensities of H2O around 820 nm and of NH3 around 1.5ãm. The results are compared with HITRAN and with previous experimental studies. For NH3 we have also recorded new near-infrared absorption spectra using a Bruker IFS 120 HR Fourier transform spectrometer, to investigate significant discrepancies concerning the line positions and intensities from the available literature. Recently we have constructed a compact difference frequency (DFG) laser that is operating in the mid-infrared region between 1900 ? 3100 cm-1 by mixing a tuneable ECDL (820 ? 880 nm range) with a cw Nd:YAG laser (1064 nm) using a Periodically Poled Lithium Niobate ? PPLN ? crystal as non-linear medium. This DFG laser (also 1 MHz linewidth) was used to study line positions and intensities of NO2 in the 3.5ãm region and in particular to compare the line intensities with the absorption cross-sections of NO2 in the 400 nm region that are known to within better than 2%. Again the results are compared with the HITRAN database. We are now moving towards using ECDL for studying unstable species and radicals.