June 21st OSU Mol. Spect. Symp.TB02 8:42 a.m Observation of Infrared Free Induction Decay and Optical Nutation Signals from Nitrous Oxide Using a Voltage Modulated Quantum Cascade Laser Geoffrey Duxbury, and Nigel Langford Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow, G4 0NG, UK James F. Kelly and Thomas A. Blake Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, PO Box 999, MS K-88. Richland, Washington 99352

June 21st 2011 Outline of Talk Design of pulse modulation QC system Examples of low pressure signals Examples of FID and rapid passage induced oscillations Power dependence of signals from higher pressure samples Saturation behaviour pinpoints different origins of oscillatory structure. 66 OSU Mol. Spect. Symp.TB02 8:42 a.m

June 21st 2011 Optical layout A schematic diagram of the optical layout for the free induction decay (FID)experiments 66 OSU Mol. Spect. Symp.TB02 8:42 a.m

June 21st 2011 Current pulse modulation 66 OSU Mol. Spect. Symp.TB02 8:42 a.m A schematic diagram showing the rectangular current pulse modulation and the resultant frequency tuning of the fast induced laser tuning. As the frequency tuning depends upon the Joule heating provided by the current pulse, the tuning reverses at the termination of the pulse. Its tuning rate is related to the heat loss as the laser approaches its equilibrium temperature.

June 21st 2011 Tuning range of slowly swept QC laser 66 OSU Mol. Spect. Symp.TB02 8:42 a.m The tuning range of the slowly swept QC laser through a representative part of the absorption spectrum of N 2 O used for these experiments. The heavy line is the spectrum recorded through the 10 cm reference cell, and the light line that through the Herriott cell with 100 m path length, and a nitrous oxide pressure of 203 mTorr. A, P(34), , 14 N 14 N 16 O, cm -1 ; B, P(41) f, , 14 N 14 N 16 O, cm -1 ; C, P(9), ), , 14 N 14 N 18 O, cm -1 ; D, P(29), , 14 N 15 N 16 O, cm -1.

June 21st 2011 Variation with probe pulse detuning of the difference between detector signal recorded with a pressure of 0.8 mTorr of nitrous oxide in the Herriott cell and the empty cell spectrum 66 OSU Mol. Spect. Symp.TB02 8:42 a.m In (i) and (ii), FID structure occurs on the turn on and turn off of the current pulse. Starting position of the FID oscillations is almost independent of the detuning. Oscillations vanish when the pulse start is blue detuned, (v). The beat frequency, (i) & (ii) from the almost free precession of the pseudo-spin vectors of the aligned molecules and the QC laser frequency is 145 MHz. The decay rate seems to be set by the Doppler dephasing of the pseudo-spin vectors, since the beat frequency remains constant as the amplitude of the emitted signal decays.. The duration of the current pulse is 50 ns (175 to 225 ns). Slow channel spectrum, Showing start of fast scan Rapid scan spectrum, origin varied from red(I) to blue (v)

June 21st 2011 Variation with probe pulse detuning of the difference between detector signal recorded with a pressure of 0.8 mTorr of nitrous oxide in the Herriott cell and the empty cell spectrum 66 OSU Mol. Spect. Symp.TB02 8:42 a.m As the scan range of the freq. up-chirped pulse is greater than the Doppler width of the line, FID signals occur either on turn-on or turn off of the current pulse. The beat frequency from the almost free precession of the pseudo-spin vectors of the aligned molecules and the QC laser frequency is reduced from 145 MHz. to 131 MHz, suggesting that the rate of precession is being altered by the intermolecular collision rate. The duration of the current pulse is 100 ns (175 to 275 ns). Slow channel spectrum, Showing start of fast scan Rapid scan spectrum, origin varied from red(I) to far red (v)

June 21st 2011 Variation with probe pulse detuning of the difference between detector signal recorded with a pressure of 20 mTorr of nitrous oxide in the Herriott cell and the empty cell spectrum 66 OSU Mol. Spect. Symp.TB02 8:42a.m In the slow and fast scans the laser radiation is never completely attenuated by the nitrous oxide.The contrast of the FID signals is much greater, providing the pulse does not start or terminate in the optically thick region, and the turn on FID oscillation has a similar number of oscillations to the turn off. When either are excited in the optically thick part of the line the FID oscillations are strongly damped The duration of the current pulse is 100 ns (175 to 275 ns). Slow channel spectrum, Showing start of fast scan Rapid scan spectrum, origin varied from red(I) to far red (iv) Under slow sweep conditions there should be no transmission at line centre

June 21st 2011 Location of the origin and end of fast scans relative to line centre using a nitrous oxide pressure of 203 mTorr 66 OSU Mol. Spect. Symp.TB02 8:42 a.m A pulse duration of 200 ns corresponds to a frequency up-chirp tuning range of ca, 830 MHz. (i) 100 m path length astigmatic Herriott cell, (ii) 10 cm path length.

June 21st 2011 Fast scan spectra corresponding to the scan ranges shown on the previous slide, using a nitrous oxide pressure of 203 mTorr 66 OSU Mol. Spect. Symp.TB02 8:42 a.m

June 21st 2011 Fast scan spectra using a nitrous oxide pressure of 203 mTorr 66 OSU Mol. Spect. Symp.TB02 8:42 a.m

June 21st 2011 Influence of laser power and gas pressure on FID and Nutation Signals 66 OSU Mol. Spect. Symp.TB02 8:47 a.m 20 mTorr, (i) maximum power, (ii)Laser power through the cell reduced by a factor of 38, FID greatly reduced 200 mTorr, (i) maximum power, (ii)Laser power through the cell reduced by a factor of 50, FID almost unchanged, nutation greatly reduced

June 21st 2011 Conclusions Normal FID signals occur at the lowest pressure used, 0.8 mTorr At higher pressures a rapid passage scan occurs. Leading to a buildup of a macroscopic polarization At high pressures, the non-linearity is shown by the changes in the signal at reduced power 200 mTorr, FID signals almost unchanged, Rapid Passage induced signal heavily damped

June 21st 2011 Acknowledgements This research was performed at the W.R. Wiley Environmental Molecular Sciences Laboratory, a national user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at the Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the United States Department of Energy by Battelle under Contract DE-AC06-76RLO G.Duxbury would like to thank The Leverhulme trust for the award of an Emeritus Fellowship, and the Royal Society of Edinburgh for the award of a travel grant.