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 FrequencyMolecular Energy LevelsCommercial Techniques RF (<1 GHz)Nuclear Spin in Magnetic FieldNMR, MRI MW (9-90 GHz)Electron Spin in Magnetic FieldEPR/ESR MW-THz ( GHz)Overall Molecular RotationNONE IR ( cm -1 )Molecular VibrationFTIR, NDIR diode laser UV-VIS ( nm)Electronic ExcitationFluorimeters, Imaging, LIBS, Raman microscopy X-rays (< 100 nm)Inner Core Electron ExcitationX-ray emission for chemical analysis Spectroscopy and its Applications

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 mV suggests sensitivity to 12 OCS isotopes in natural abundance Ex: O 13 C 36 S (1:666,000) Vibrationally excited states are present for each isotope The sample purity is quoted as >97.5% OC 34 S (4%) 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 mVGreater 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 T 1 = 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

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? D. Patterson, M. Schnell, and J.M Doyle, Nature 497, (2013). Acknowledgments Brent Harris is supported by an NSF Graduate Fellowship NSF I-Corps Program BrightSpec