June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 1 Time Dependent Measurements of Nitrous Oxide- Foreign Gas Collisional Relaxation Processes Using a Frequency Down-Chirped 7.84 Microns Quantum Cascade Laser Kenneth Hay, Geoffrey Duxbury Nigel Langford and David Wilson Department of Physics University of Strathclyde Glasgow, Scotland and Nicola Tasinato, Universita Ca Foscari di Venezia,Venice, Italy

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 2 Outline of Talk Intra-pulse spectrometers: rapid passage and molecular alignment Chirp induced “rapid passage” effects in the spectrum of nitrous oxide Signatures of molecular alignment Collisional effects on gain and absorption Chirp rate dependent scattering processes Analogy with chirped frequency Microwave spectrometers Conclusions

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 3 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

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 4 Transmission spectra of a section of the band of nitrous oxide green slow, red fast chirp rate a)Spectrum as a function of time, b) total spectrum as a function of wavenumber. Conversion from time to relative wavenumber via Ge etalon fringes

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 5 Rapid Passage and Molecular Alignment Rapidly frequency 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 de-phasing time, ~100 ns at 1 Torr.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 6 R branch structure of transition in the band of 16 O 12 C 18 O Path length 37 m (74 passes) Laser T -20 C, Drive voltage 15 Pulse duration 1500 ns Rep. rate 2.5 kHz Cumulative average of 8000 spectra Total pressures, Torr

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 7 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 (see references) is that a 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.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 8 Pressure dependence of the rapid passage structure part of the P branch of nitrous oxide High pressure, very large gain signals

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 9 Number density equivalence of the rapid passage structure of absorption lines of nitrous oxide Note similarity of the rapid passage structures, of P9 and the R8 and P4 lines of the isotopologues when their number densities are equivalent, and the variation of the rapid passage structure woth chirp rate.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 10 Quenching the induced polarization of the P(9) line of N 2 O (a)He (b) Ar (c) N 2 (d)CO 2

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 11 Broadening of the absorptive part of the P(9) line of N 2 O (a)He (b) Ar (c) N 2 (d)CO 2

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 12 Normalised sweep rate and rapid passage Buffer gas pressure > 20 Torr, a>>1 Gain due to self induced heterodyne depends on sweep rate Collision dominated Torr, a< 10

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 13 Linear dependence of the Lorentzian half widths on the gas buffer pressure for the P(9) (upper panel) and P(10) (lower panel) transitions ( , green) helium broadening, ( , red) argon broadening, (, black) nitrogen broadening, ( , blue) carbon dioxide broadening. The error weighted linear fits used to retrieve the effective broadening parameters are also displayed.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 14 Retrieved effective nitrous oxide – foreign gas broadening parameters,  0, and their dependence on the chirp rate ( , green ), ( , red ), (, black ), ( , blue ). Open, half- open and solid symbols refer to P(9), P(10) and P(11) transitions respectively.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 15 Conclusions:Propagation with slow chirp rate in an optically thin or optically thick medium Rapid chirp QC laser spectroscopy is a powerful tool for studying fundamental processes in non-linear optics and molecular physics More complete model, need to consider a set of open two level systems to aid the interpretation of the experimental data. At low pressure, collisions causing velocity and speed changing, and rotational energy transfer, become important Advantage is the ability to probe time dependent collisional effects, overlap with molecular beam scattering experiments 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). Many papers using this technique have been presented here recently.

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 16 Acknowledgements We are indebted to the the ERPSRC for an instrumentation grant and for the awards to K.G. Hay and D.Wilson of studentships through the Doctoral Training Fund, and to NERC for the award of a COSMAS grant for the development of the intra-pulse spectrometer. Nicola Tasinato is very grateful to the Universita Ca’ Foscari di Venezia for a studentship, and for support of his collaboration with the Strathclyde University group. GD has greatly benefited from the award of an Emeritus Followship by the Leverhulme Trust. We would also like to thank Cascade Technologies for their technical support..

June 22nd OSU Mol. Spect. Symp.TA01 8:30 a.m 17 References 1] G. Duxbury, N. Langford, M.T. McCulloch and S. Wright, “Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide”, Mol. Phys.105, 741 (2007) 2] G. Duxbury N. Langford and K. Hay, “Delayed rapid passage and transient gain signals generated using a chirped 8 micron quantum cascade laser”, J. Mod. Opt. 55, 3293 (2008) [3] N.Tasinato, N.Langford, G.Duxbury and K.G. Hay, “ An investigation of collisional processes in a Dicke narrowed transition of water vapor in the 7.8 micron spectral region by frequency down-chirped quantum cascade laser spectroscopy “ J.Chem. Phys. 132, (2010) [4] N.Tasinato, K.G. Hay, N.Langford, G.Duxbury and D. Wilson,“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”,J.Chem. Phys. 132, (2010