Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: rapid passage signals and the time dependence of collisional processes

Nicola Tasinato, Kenneth G. Hay, Nigel Langford, Geoffrey Duxbury, David Wilson

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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. The intrapulse time domain spectra closely resemble those recorded in coherent optical nutation experiments. In the present paper, the frequency down-chirped technique is employed to investigate the nitrous oxide-foreign gas collisions. We have demonstrated that the measurements may be characterized by the induced polarization dominated and collision dominated measurement limits. The first of these is directly related to the time dependence of the long range collision cross sections. Among the collisional partners considered, carbon dioxide shows a very unusual behavior of rapid polarization damping, resulting in the production of symmetrical line shapes at very low gas buffer pressures. In the collision dominated regime, the pressure broadening parameters, which we have derived, are comparable at slow chirp rates, with those derived from other experimental methods. By comparing the pressure broadening coefficients of Ar, N-2, and CO2 with those of He, making use of the chirp rate independence of the pressure broadening by helium, we have shown that at higher chirp rates there is clear evidence of the chirp-rate dependence of the pressure broadening parameters of N2 and CO2. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3386385]

Original languageEnglish
Article number164301
Number of pages11
JournalJournal of Chemical Physics
Issue number16
Early online date22 Apr 2010
Publication statusPublished - 28 Apr 2010



  • broadened linewidths
  • velocity dependence
  • coherent transients
  • nonlinear optics
  • gas
  • spectroscopy
  • spectrum
  • pulse
  • N2O
  • excitation

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