Observation of saturation and rapid passage signals in the 10.25 micron spectrum of ethylene using a frequency chirped quantum cascade laser

Abstract

Rapid passage signals exhibiting saturation effects have been observed when a low-pressure sample of ethylene, within a multiple pass absorption cell, is subjected to radiation from a repetitively pulsed 10.25 micron quantum-cascade laser. Within each pulse the laser frequency sweeps 36 GHz from high to low frequency in a time of 140 ns. At the low gas pressures, less than 20 mTorr, in the absorption cell the sweep rate through a Doppler-broadened absorption line (ca. 0.5 ns), is much faster than the collisional relaxation time of the ethylene and this leads to rapid passage effects. Examples are given of the complex rapid passage signals observed in pure and nitrogen broadened spectra. The rapid passage effects, which lead to the variety of the observed signals, have been modelled by numerical solution of the coupled Maxwell–Bloch equations for four sets of two-level systems.

Acknowledgements

The authors would like to thank the United Kingdom Engineering and Physical Sciences Research Council (EPSRC) for funding the initial part of this work through the research grant GST/M69111 1999, and National Environmental Research Council (NERC) for funding the main part through the research grant NER/T/S/2002/000052, and for the use of the NERC molecular Spectroscopy Facility at Rutherford Appleton Laboratory. One of us (MTM) is grateful for the award of an EPSRC Research Studentship, and would like to thank K.M. Smith and R.A. McPheat for their assistance with the experiments at the NERC MSF. We would also like to thank our colleague N.R. Badnell for access to the new Atomic Physics computer network at Strathclyde University. Finally we would like to thank the following staff of the Environmental Molecular Science Laboratory at Pacific Northwest National Laboratory, T.A. Blake, S. W. Sharpe and R.A. Sams for supplying us with high resolution Fourier transform spectra of ethylene, and J.F. Kelly for his stimulating discussions about nonlinearity and rapid passage.