23 June, 3:19OSU-2009 TG081 Molecular alignment effects in ammonia at 6.14  m using a down-chirped quantum cascade laser spectrometer Kenneth G. Hay, Geoffrey Duxbury and Nigel Langford, Department of Physics University of Strathclyde Glasgow, Scotland

23 June, 3:19OSU-2009 TG082 Outline of Talk Intra-pulse spectrometers: rapid passage and molecular alignment Chirp rate dependent scattering processes Analogies with chirped frequency Microwave spectrometers Conclusions

23 June, 3:19OSU-2009 TG083 Doppler limited Experiments: The Intra-Pulse Method Apply a ns top hat current pulse to a DFB QC laser. Obtain a light pulse in time domain with a frequency down chirp. Pass pulse through absorbing species and monitor pulse absorption in time domain. Laser and Vigo MCT detector both Peltier cooled, no liquid nitrogen needed

23 June, 3:19OSU-2009 TG084 The Intra-Pulse Method : Chirp rate and pattern recognition Apply a 2  s (2000 ns) top hat current pulse to a new design 6.1  m DFB QC laser. Drive voltage 8.5 V. Total tuning range is much shorter than for 7.84  m DFB QC Rate of change of frequency down-chirp is much greater. 14 NH 3 spectra look very different when using the time or the wavenumber scales for the tuning range. Transit through Doppler line, lhs. 80 MHz/ns, 2 ns: rhs. 6 MHz/ns, 27 ns.

23 June, 3:19OSU-2009 TG085 Rapid Passage and Molecular Alignment Rapidly down-chirped radiation interacts with absorbing gas molecules. Interaction time determined by chirp rate and linewidth of transition. |d /dt| = 40 MHz/ns, ∆ D = 80 MHz (typical Doppler linewidth). Interaction time 2 ns. Interaction time less than Doppler de-phasing time (~1/∆ D ), ~12.5ns, and much less than rotational dephasing time, ~100 ns at 1 Torr. Resolution set by time-frequency uncertainty, not “laser linewidth” ∆ =  (C d /dt), Full width at half height. Rectangular inst. fn. C= Upper limit, Chirp 200 MHz/ns, res cm -1, 20 MHz/ns cm -1.

23 June, 3:19OSU-2009 TG086 Rapid passage processes Maxwell-Bloch equations Treat the system as a 2 level system with number density N. Transition has dipole moment µ 12 driven by a field E. For a frequency chirp , evolution of polarization P and population inversion w for a velocity component v z given by G. Duxbury, N. Langford, M.T. McCulloch & S. Wright, Chem. Soc. Rev, 34, 1 (2005) M. T. McCulloch, G. Duxbury & N. Langford, Mol. Phys. 104, (2006) G. Duxbury N. Langford, M.T. McCulloch & S. Wright, Mol. Phys. 105, (2007 ) G. Duxbury N. Langford and K. Hay, J. Mod. Opt. 55, 3293 (2008)

23 June, 3:19OSU-2009 TG087 Rapid passage processes Maxwell-Bloch equations The polarization P is a complex polarization, where the real part relates to the refractive index, and the imaginary part to the absorption coefficient. Molecular alignment effects are included in the calculation, see G. Duxbury N. Langford, M. T. McCulloch and S. Wright, Mol. Phys. 105, (2007) For descriptions of similar rapid passage effects in microwave spectra see also J.C. McGurk, T.G.Schmaltz and W.H. Flygare, J. Chem. Phys. 60, 4181 (1974)

23 June, 3:19OSU-2009 TG088 The 4 band of 14 NH 3 and 15 NH 3 Perpendicular band, slightly weaker than the 2 band in the 10  m region. Transition dipole moment D, D. Doppler fwhm 10  m 88 MHz, 6.14  m 148 MHz Inversion splitting much smaller than that in the 2 band QC laser emission is resonant with a range of very strong Q branches near the band centre. Tuning range by tuning substrate temperature, ~ cm -1 (-30 C) to ~1624 (20 C) cm -1. Max drive voltage 8.5 V, minimum 7.4 V Central part of laser tuning range is coincident with atmospheric water line

23 June, 3:19OSU-2009 TG089 The 4 band of 14 NH 3 and 15 NH 3 narrow window in FT spectra of laser region Central part of laser tuning range is coincident with atmospheric water line FT spectrum of 15NH3, resolution 0.02 cm-1 QC spectra of 14NH3 bracketing the water line

23 June, 3:19OSU-2009 TG0810 Rapid passage structure of the 6.1  m Q branch region of the 4 band 14 NH 3 : 2 microsecond scans: chirp rate of laser through NH 3 lines shown Expanded overlap region: Q branch assignments given. Rapid passage emission in fast chirp rate, almost absent slow chirp.  self /MHz/Torr (fwhm),39 Q7, Q6,0, 34 Q9,2 (HITRAN) The laser base temperatures used were -30 C and -20 C, the drive voltage 8.5 V

23 June, 3:19OSU-2009 TG0811 Pressure broadened structure 6.1  m Q branch region of the 4 band 14 NH 3 : Pressure broadened by adding nitrogen up to a total cell pressure of Torr.  air ~12 MHz/Torr (fwhm) (HITRAN). The pressure broadened line shapes in the overlap region are now identical. The effects of coherence destroying molecular collisions dominates the effects of the rapid laser downchirp. Full scan range, laser temp. -30 C to -20 CExpanded overlap region

23 June, 3:19OSU-2009 TG0812 Rapid passage effects at ammonia pressures below 1 mTorr Gas pressures/mTorr Heavy line 0.03 Light line0.1 Dashed light0.2 Dashed heavy0.33 At very low pressure rapid passage is still seen Main scattering events at these low pressures are due to velocity changing collisions. (see J.L.Hall 1973, Mattick et al., 1976) These are not included in the Maxwell Bloch model.

23 June, 3:19OSU-2009 TG0813 Rapid passage effects at ammonia pressures above 20 mTorr Very large delayed rapid passage emission signals seen at rapid chirp rate ~ 80 MHz/ns (red). Constructive interference between the incident laser field and the field generated within the ammonia. Chirp rate of the laser is faster than the collisional reorientation time of the ammonia molecules. Explained using MB equations. J.M.O. 55, 3293 (2008). (a) overlap region, pressures 51,113, 237, 407, 550,772 mTorr (b) expanded to display unusual nonlinearities in slow chirp. Press as (a) plus 26 mTorr start.  self /MHz/Torr (fwhm), 39 Q7, Q6,0, 34 Q9,2 (HITRAN)

23 June, 3:19OSU-2009 TG0814 Effects of N 2 collisions on rapid passage in low concentration 14 NH 3 at slow and fast chirp rates Comparison of the effects of nitrogen collision frequency on rapid passage effects. (a) Ammonia pressure 4.5mTorr Nitrogen pressures/Torr (b) 4.2, (c) 12, (d) 24 At low ammonia pressure the asymmetry of the slow and the fast chirp absorptive parts of the two strong lines, and their pressure broadening, are very similar. The nitrogen collisions mainly damp the rapid passage emission spike.

23 June, 3:19OSU-2009 TG0815 Calculated effects of N 2 collisions on rapid passage in low concentration 14 NH 3 at slow and fast chirp rates (a)Ammonia pressure (b)4.5mTorr Nitrogen pressures/Torr (b) 4.2, (c) 12, (d) 24

23 June, 3:19OSU-2009 TG0816 Propagation in an optically dense medium with rapid chirp rate Delayed signals occur when a chirped pulse propagates through an optically dense medium with minimal collisional damping. The inference from our Maxwell -Bloch calculations is that resultant nutation results from a rapid driving between the upper and lower molecular vibration-rotation levels at the Rabi frequency. The transient gain is due to constructive interference between the incident laser field and the field generated by the molecular response. Constructive interference occurs when the chirp rate of the laser is faster than the molecular collisional reorientation time.

23 June, 3:19OSU-2009 TG0817 Conclusions Rapid chirp QC laser spectroscopy is a powerful tool for studying fundamental processes in non-linear optics and molecular physics Many experiments exploit the fact that the rapid chirp of laser is faster than conventional relaxation processes at low pressure Large optical depth due to long absorption path in Herriott cell but low gas pressure. A modern microwave “broadband Fourier Transform” version of the long chirp excitation of molecular coherence was described by Pate and his group in Brown et al. Rev Sci Inst. 79, (2008)

23 June, 3:19OSU-2009 TG0818 Acknowledgements We are indebted to the the ERPSRC for an instrumentation grant and support for KG Hay, and to NERC for the award of a COSMAS grant. GD is grateful to the Leverhulme Trust for the award of an Emeritus Fellowship We would also like to thank A.W.E Aldermaston, for their support in the development of the QC laser spectrometer.