An investigation of collisional processes in a Dicke narrowed transition of water vapor in the 7.8 μm spectral region by frequency down-chirped quantum cascade laser spectroscopy

Information about intermolecular potentials is usually obtained through the analysis of the absorption line shapes recorded in the frequency domain. This approach is also adopted to study the effects of motional narrowing and speed dependence of the pressure broadening coefficients. On the other hand, time domain measurements are directly related to molecular collisions and are therefore frequently employed to study molecular relaxation rates, as well as the effects of velocity changing collisions and the speed dependence of the absorption cross sections. Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. In the present work the frequency down-chirped radiation emitted by an intrapulsed quantum cascade laser operating near 7.8 mu m is employed to investigate the collisional relaxation processes, and the collisional narrowing, in the 15(0,15)<- 16(1,16) and 15(1,15)<- 16(0,16) doublet in the water vapor nu(2) band. The effects of He, Ne, Ar, N-2, and CO2 as collisional partners are investigated. The experimental results clearly indicate the dependence of the collisional cross sections upon the chirp rate. They also demonstrate that by using different chirp rates it is possible to gain information about the intermolecular processes driving the molecular collisions and the related energy transfer.