A pulsed distributed feedback quantum cascade laser operating near 970 cm<sup>-1</sup> (10.3 μm) was coupled with the technique of cavity ring-down spectroscopy, as described here for the first time. The newly constructed set-up was tested by recording three relatively weak rotational lines of the 10<sup>0</sup>0→00<sup>0</sup>1 vibrational band of CO<sub>2</sub> in the range from 966.75 cm<sup>-1</sup> to 971.5 cm<sup>-1</sup>. The CO<sub>2</sub> lines were recorded by measuring the decay time of a CO<sub>2</sub> - N<sub>2</sub> mixture flowing through an open sample tube placed between the cavity ring-down mirrors. The quantum cascade laser frequency was tuned at a rate of ~ 0.071 cm<sup>-1</sup>/K by changing the heat sink temperature in the range between -20 and 50 °C. The first results demonstrated the applicability and high sensitivity of the cavity ring-down spectroscopy - pulsed quantum cascade laser combination and encouraged us to extend our research to the study and detection of ammonia. We demonstrated that a detection limit of ammonia of ~ 25 ppbv can be attained with the current set-up. Basic instrument performance and optimization of the experimental parameters for sensitivity improvement are discussed.
Among other carbonaceous species, polycyclic aromatic hydrocarbons (PAHs) are of relevance for astrophysics. It is expected that they are present in the interstellar medium and they are considered as possible carriers of the diffuse interstellar bands. To prove this, absorption spectra of PAHs under conditions similar to those met in the interstellar medium should be obtained. We report here the application of cavity ring-down laser absorption spectroscopy to neutral PAHs, namely anthracene and pyrene, cooled in a supersonic jet. The absorption spectra corresponding to the S<sub>1</sub>(0) ← S<sub>0</sub>(0) transition of anthracene near 361 nm and to the S<sub>2</sub>(1,0) ← S<sub>0</sub>(0) transitions of pyrene around 321 nm and 316 nm have been studied. The results are similar to the excitation spectra previously reported in the literature.