23 June 4:07 OSU-2009 TG10 1 Time dependent measurements of Dicke narrowing of a water line at 7.84 mm using a frequency down-chirped QC laser spectrometer Kenneth G. Hay Geoffrey Duxbury and Nigel Langford, a Department of Physics University of Strathclyde Glasgow, Scotland Nicola Tasinato Dipartimento di Chimica Fisica, Universita Ca Foscari di Venezia, Venezia Italy

23 June 4:07 OSU-2009 TG10 2 Outline of Talk Fundamental steps in understanding collision processes Intra-pulse spectrometers: rapid passage and chirp rate selection Chirp rate dependent scattering processes Comparison with other methods

23 June 4:07 OSU-2009 TG10 3 Fundamental steps in understanding collision processes Time dependent collision processes using electric field switching, with selected velocity groups. e.g RG Brewer and R.L. Shoemaker Phys.Rev A6, 2001 (1972) Energy transfer and propensity rules for molecular collisions Takeshi Oka. Adv. At Mol. Phys. 9,127 (1973), NH 3 velocity changing collisions, S.M. Freund, J.W.C. Johns, A.R.W. McKellar and T. Oka, JCP 59, 3445 (1973) Velocity dependence of collision broadening cross sections in NH 3. A.T. Mattick,(A. Sanchez) N.A. Kurnit and A. Javan. App.Phys.Lett 23, 675(1973) Chem. Phys. Lett. (1976)

23 June 4:07 OSU-2009 TG10 4 Studying time dependent processes: mainly microwave Molecular beam Fabry-Perot Fourier transform microwave spectrometer, Balle and Flygare JCP, 71, 2723 (1979), 72,922 (1980), Rev. Sci.Inst. 52,33 (1981) Chirped pulse Microwave,Brown, Dian, Douglass,Geyer, Shipman and Pate,Rev. Sci.Inst. 79, (2008)

23 June 4:07 OSU-2009 TG10 5 Studying time dependent processes: infrared Normal: collision studied via swept frequency or FTIR measurements of line shapes. No direct time information Chirped pulse infrared,QC, resultant spectra can be compared with both time dependent and time independent methods. QC time selection due to time dependent chirp rate within the output pulse.

23 June 4:07 OSU-2009 TG10 6 Experimental conditions adopted to record the water absorption line at slow, medium and fast chirp rates. Chirp Rate, MHz/nsLaser Temperature [°C]Drive Voltage [V] Slow chirp Medium chirp Fast chirp Pulse duration: 1.5  s;Repetition Rate: 2.5 kHz

23 June 4:07 OSU-2009 TG10 7 Selecting chirp rate dependence by temperature tuning the centre frequency of the chirp range Dicke narrowed water line recorded at different chirp rates: (―) 20 MHz/ns; (- -) 35 MHz/ns; (-  -) 77 MHz/ns. In order to illustrates the frequency down – chirp during the pulse the etalon fringe pattern is also shown.

23 June 4:07 OSU-2009 TG10 8 Variation of the water line half width with increasing Ar pressure Notice the strong narrowing affecting the line for pressures smaller than 250 Torr. (  ) Slow chirp rate; (  ) Fast chirp rate. The dashed second order polynomial fits are displayed to help the recognition of the data trend. The medium chirp results are not displayed for clarity.

23 June 4:07 OSU-2009 TG10 9 Change in the self-heterodyne emission signal with Ne Pressure Increase of the emission signal up to a Ne pressure of 47.7 Torr. The emission signal then starts to decrease as the Ne pressure is increased further. The magnitude of the emission peak is strictly related to the macroscopic polarization of the gas sample.

23 June 4:07 OSU-2009 TG10 10 Linear increase of line width in the carbon dioxide broadening experiments Line width observed in the CO 2 broadening experiments. (  ) Slow (  ) medium, (  ) fast chirp rate.

23 June 4:07 OSU-2009 TG10 11 Symmetric line shapes recovered with CO 2 as a buffer gas. ( ◦ ) Experimental spectrum: = 2.54 Torr, = 32.7 Torr, = 35 MHz/ns, path length = 37 m; (―) Voigt fit; (―) residuals. The residuals have been displaced for clarity. The water line has been marked, the remaining absorption are due to 18 O= 12 C= 16 O and the CH 4 impurities of the CO 2 sample.

23 June 4:07 OSU-2009 TG10 12 Effects of different perturbers on the Dicke line shape Effects of adding ca. 23 Torr of broadening gas on the Dicke narrowed line shape of the water absorption line. The effects of helium, neon, argon and molecular nitrogen are similar. Only carbon dioxide is efficient at quenching the rapid passage effects and recovering a symmetrical line shape.

23 June 4:07 OSU-2009 TG10 13 Chirp Rate/ [MHz ns -1 ] Broadening Coefficient [MHz Torr -1 ] Ne Ar He N 2 CO (11) 0.17(2)0.250(7) 0.217(8) 1.01(3) (5) 0.090(2)0.143(4) 0.205(8) 1.14(3) (5) 0.083(4)0.176(5) 0.316(12) 1.15(4) Chirp rate dependence of the Foreign gas pressure broadening coefficients of the Dicke narrowed water line.

23 June 4:07 OSU-2009 TG10 14 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 4:07 OSU-2009 TG10 15 Weak rapid passage induced gain in a Dicke Narrowed Water line Takeshi Oka JCP 59 (1973) pages ,re Blum et al. Science 177, 694 (1972), who observed Dicke narrowing. ”For the observed water transition, 16 1, ,15 the energy levels spacings are the order of 2000 cm-1, therefore molecules change their velocities many times before they change their rotational states” Rapid Passage, N 2 O, and Dicke narrowing, H 2 O Our unusual 7.84 micron 15 0, , 16. water transition is the mirror image of the earlier example of Blum et al.

23 June 4:07 OSU-2009 TG10 16 Conclusions Rapid chirp QC laser spectroscopy is a powerful tool for studying fundamental processes in molecular physics Some of these experiments may be considered as a modern extension of the pioneering experiments carried out from Many experiments exploit the fact that the rapid chirp of laser is faster than conventional relaxation processes at low pressure Absorption: study different collisional effects on the absorptive part of the signal. Delayed rapid passage (and emission) are directly related to the macroscopic polarisation of the gas. Gain information about processes occurring during molecular collisions and related enery transfer.

23 June 4:07 OSU-2009 TG10 17 Acknowledgements We are indebted to the the EPSRC for an instrumentation grant and a studentship for Kenneth 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.