1. Progress Towards Obtaining Lineshape Parameters Using Chirped Pulse THz Spectroscopy Eyal Gerecht, Kevin O. Douglass, David F. Plusquellic National Institute of Standards and Technology Optical Technology Division, Gaithersburg, MD

2. Multi-Component Gas Monitor GHGs, VOCs, or breath analysis Formaldehyde CO Methanol Acetone Ethanol CO 2 ( 18 O) N2ON2O NO 0.8050.875 THz L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005). H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, 883-890 (1998).

3. Spectral Line Shapes + = ReIm Mag Resulting from FFT of a damped oscillator DispersionAbsorption FFT 

4. Spectral Line Shapes Resulting from FFT of a damped oscillator ● Higher resolution  Improved spectral discrimination ● measurement of lineshape parameters

5. Spectral Line Shapes: Issues Recorded spectra typically appear as a linear combination Re and Im components Re(F(ω)) = cos ϕ A(ω) + sin ϕ D(ω) Im(F(ω)) = -sin ϕ A(ω) + cos ϕ D(ω) The pure Absorption and Dispersion spectra can be determined with the correct phase angle A(ω) = cos ϕ Re(F(ω)) - sin ϕ Im(F(ω)) = Im(F(ω) exp(-i ϕ)) D(ω) = -sin ϕ Re(F(ω)) + cos ϕ Im(F(ω)) = Re (F(ω) exp(-i ϕ))

6. Determining the phase angle over the Full Spectrum  Time delay in acquisition leads to a frequency dependent phase shift (Shift Theorem) ϕ( ω ) = ω t delay  In NMR zero order term sets initial phase and the linear term accounts for acquisition delay ϕ( ω ) = ϕ 0 + t delay ( ω – ω a )  Need to account for quadratic phase accumulation due to chirped pulse excitation

7. Broad bandwidth Phase Correction Approaches in the Past: FT-ICR 1.Xian, F. et al Anal. Chem. 2010, 82, 8807 – 8812 2.Beu,S. C., Anal. Chem. 2004, 76, 5756 – 5761 3.Qi, Y, J. Am. Soc. Mass Spectrom. 2011, 158 164 4.Brouwer, H. de, JMR 201 (2009) 230–238

8. Broad bandwidth Phase Correction Current Approach  Challenges  rapidly accumulate phase  ϕ(ω) = ϕ 0 + t delay (ω – ω a ) + ϕ chirp Obtained by fitting transmitted chirped pulse phase angle Estimate and vary t delay and ϕ 0 until phase is aligned simplify to NMR approach

9. x48 White Cell 9 GHz Source Mix AMC x48 YIG AWG 12GS/s Chirped-Pulse THz Spectrometer E. Gerecht, K.O. Douglass, D.F. Plusquellic, Optics Express, April 22, 2011, Vol. 19, Issue 9, pp. 8973-8984 (2011),

10. Time (ns) 0 100 Frequency (GHz) 0 12 25 ns - 10 GHz Chirped THz pulse 550 – 560 GHz

11. Direct Absorption of a 5 Component Gas Mix L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005). H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, 883-890 (1998).

12. Absorption Data Results Many improvements in the pipeline

13. Magnitude Spectrum of FID 0.54600 0.55666 THz FID Signal (a.u.) 0.55133 0.55399 0.54866 N2ON2O OCS EtOH MeOH Acetone H2OH2O 100,000:1 x500 L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005). H. M. Pickett, R. L. Poynter, E. A. Cohen, M. L. Delitsky, J. C. Pearson, and H. S. P. Muller, "Submillimeter, Millimeter, and Microwave Spectral Line Catalog," J. Quant. Spectrosc. Radiat. Transf. 60, 883-890 (1998). 10.6 GHz in 500 nsec – 80K averages in 60 sec 5 Component Gas Mix

14. Phase Correcting a Single Peak ϕ 0 = 147⁰ A(ω)=Im(F(ω) exp(-i ϕ 0 )) Fit Results (MHz) wG1.05626+/- 0.00149 wL0.06852 +/- 0.00208 HITRAN (MHz) wG 0.87 wL 0.8 10 mTorr 1% OCS in Ne L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005).

15. Fit to Quadratic Phase Change

16. Magnitude vs. Im Component ϕ(ω) = ϕ 0 + t delay (ω – ω a ) + ϕ chirp

17. Magnitude vs. Im Component

18. 779.760869.760 ν / GHz Extending to Higher Bandwidths Justin Neil RC06 90 GHz FID near 850 GHz MeOH -1.2 mTorr Pure 2 ms acquisition time L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, 139-204 (2005).

19. Conclusions  Obtain lineshapes that are in good agreement with HITRAN from direct absorption measurements  Lineshapes measurements from FID are possible and demonstrated for a single transition of OCS at 546.859 GHz  Need to implement automated algorithms developed for NMR to phase the broadband spectrum

20. Acknowledgements  Virginia L. Perkey – SURF student  Eric M. Vess - SURF student  Tektronix – equipment loan  Upper Atmospheric Research Program of the National Aeronautics and Space Administration (NNH09AK47I) NIST National Research Council Program Post Doctoral Research Opportunities http://www.nist.gov/pml/div685/grp08/biophysics-group-research-opportunities.cfm

21. Spectral Line Shapes: Issues Recorded spectra typically appear as a linear combination Re and Im components Re(F(ω)) = cos ϕ A(ω) + sin ϕ D(ω) Im(F(ω)) = -sin ϕ A(ω) + cos ϕ D(ω) The pure Absorption and Dispersion spectra can be determined with the correct phase angle A(ω) = cos ϕ Re(F(ω)) - sin ϕ Im(F(ω)) D(ω)= -sin ϕ Re(F(ω)) + cos ϕ Im(F(ω))