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

Multi-Component Gas Monitor GHGs, VOCs, or breath analysis Formaldehyde CO Methanol Acetone Ethanol CO 2 ( 18 O) N2ON2O NO THz L.S. Rothman et al, “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 96, (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, (1998).

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

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

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))

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

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

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

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 (2011),

Time (ns) Frequency (GHz) ns - 10 GHz Chirped THz pulse 550 – 560 GHz

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, (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, (1998).

Absorption Data Results Many improvements in the pipeline

Magnitude Spectrum of FID THz FID Signal (a.u.) 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, (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, (1998) GHz in 500 nsec – 80K averages in 60 sec 5 Component Gas Mix

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

Fit to Quadratic Phase Change

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

Magnitude vs. Im Component

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, (2005).

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 GHz  Need to implement automated algorithms developed for NMR to phase the broadband spectrum

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

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))