Gas Analysis by Fourier Transform Millimeter Wave Spectroscopy Brent J. Harris, Amanda L. Steber, Kevin K. Lehmann, and Brooks H. Pate Department of Chemistry University of Virginia Chemical Analysis by Rotational Spectroscopy Spectroscopy and its Applications Frequency Molecular Energy Levels Commercial Techniques RF (<1 GHz) Nuclear Spin in Magnetic Field NMR, MRI MW (9-90 GHz) Electron Spin in Magnetic Field EPR/ESR MW-THz (2-2000 GHz) Overall Molecular Rotation NONE IR (100-4000 cm-1) Molecular Vibration FTIR, NDIR diode laser UV-VIS (100-800 nm) Electronic Excitation Fluorimeters, Imaging, LIBS, Raman microscopy X-rays (< 100 nm) Inner Core Electron Excitation X-ray emission for chemical analysis Analytical Chemistry Applications: Gas Analysis using mm-wave Rotational Spectroscopy Analysis of Complex Gas Mixtures 1) Gas Monitoring or Sensing High sensitivity monitoring of a known gas species Often there are requirements for low false positive detection rates Double-resonance verification Two-color FTMW Spectrometer 2) Gas Analysis Gas sample composition analysis Identification of molecules without the need for experimental spectrum libraries How do we automate rotational spectroscopy analysis? Segmented Chirped-Pulse Fourier Transform Spectrometer Unique Features of Fourier Transform mm-wave Spectroscopy Dynamic Range in Gas Analysis Noise level of 0.0006 mV suggests sensitivity to 12 OCS isotopes in natural abundance Ex: O13C36S (1:666,000) OC34S (4%) Vibrationally excited states are present for each isotope The sample purity is quoted as >97.5% 65 cm path length Spectral Transition Density and Dynamic Range Also present: - Manufacturer impurities - Sample cell impurities Already a predominantly unassigned spectrum OCS Normal Species: 1500 mV Greater than 1,000,000:1 dynamic range Double Resonance Verification K. Kubo, T. Fuyura, S. Saito, J. Mol. Spec, 222 (2003) 255 Match to Calculated Spectra from Analyzed Spectra Unique Strengths of Molecular Rotational Spectroscopy Quantitative Spectrum Analysis (Ground Vibrational State) Frequency accuracy of mm-wave spectroscopy OCS Normal Species: 1500 mV Chemical Analysis Database of Experimental Spectra The power of rotational spectroscopy for molecule identification would increase greatly if standards for archiving experimental spectra could be developed. Identification of Unknown Species Information in the rotational spectrum 3D Structure: A, B, C (moments-of-inertia) Double Resonance Spectroscopy (next talk) Atom Positions (isotopic assignment) Isotope fingerprint for verification Absolute Stereochemistry Stark Effect FTMW Spectroscopy Electronic Properties: Dipole Moment Direction in Principal Axis System Relative intensities of a-, b-, and c-type transitions Magnitude of the Dipole Moment Rabi cycle excitation Molecular Mass: Determination of Doppler Contribution (M/T) Line Shape Analysis Magnitude of the Dipole Moment: Rabi Cycle ∝ Peak polarization Dependent on: - Source power curve - Transition dipole Mass Determination: Measuring the Collisional Relaxation Rate by Pulse Echoes : ∝ () − − T1 = Collisional Decay s = Doppler Decay Nutation experiment determines the pi-pulse for repolarization of the Doppler dephased FID (echo). Mass Estimation from Doppler Contribution to FID Relaxation : ∝ − () − = 10 FIDs per data trace 20 different gates Results is an average of 200 fits Determined to < 1amu typical Mass Determination from FID Analysis OCS measurements across multiple transitions and isotopes ~3% or less error (OMC) For unassigned lines, the mass estimate refines the search for a molecular carrier M o lecule M ethano l P ro pyne A cro lein A cro lein Carbo nyl Sulfide-main Carbo nyl Sulfide-13C Carbo nyl Sulfide-33S Carbo nyl Sulfide-34S Carbo nyl Sulfide-18O Carbo nyl Sulfide-13C 34S Furan * mass values in units o f amu Literature M ass * M easured M ass * Uncertainty * Fractio nal Erro r 32 40 56 56 60 61 61 62 62 63 68 33.3 39.2 57.9 55.0 59.1 61.7 61.6 62.7 63.0 64.1 68.7 0.3 0.1 0.5 0.2 2.2 1.5 1.5 2.3 2.2 1.2 1.7 0.04 0.02 0.03 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.01 Conclusions 1) Rotational spectroscopy has unique strengths for chemical analysis of complex gas mixtures 2) Instrumentation for room-temperature rotational spectroscopy is advancing rapidly 3) Does the value of rotational spectroscopy data merit a community wide effort to improve archiving and data analysis methods? Acknowledgments Brent Harris is supported by an NSF Graduate Fellowship NSF I-Corps Program BrightSpec D. Patterson, M. Schnell, and J.M Doyle, Nature 497, 475- 478 (2013).