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
𝝎𝑹 ∝ 𝑬𝝁𝒊𝒋
Peak polarization
Dependent on:
- Source power curve
- Transition dipole
Mass Determination: Measuring the
Collisional Relaxation Rate by Pulse Echoes
𝑭𝒓𝒆𝒆 𝑰𝒏𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝑫𝒆𝒄𝒂𝒚:
𝑬 𝒕 ∝ 𝐜𝐨𝐬(𝝎𝒕)
−𝒕
𝒆𝑻𝟏
𝒆
−𝒕 𝟐
𝟐𝒔
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
𝑭𝒓𝒆𝒆 𝑰𝒏𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝑫𝒆𝒄𝒂𝒚:
𝑬 𝒕 ∝
−𝒕
𝐜𝐨𝐬(𝝎𝒕) 𝒆𝑻𝟏
𝒕 𝟐
−
𝒆 𝟐𝒔
𝒄
𝒎
𝒔=
𝝎 𝟐𝒌𝑻
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).