THZ/MM-WAVE SPECTROSCOPIC SENSORS, CATALOGS, AND UNCATALOGUED LINES

IVAN MEDVEDEVDepartment of Physics, Wright State University, Dayton, OH, USA; CHRISTOPHER F. NEESE, FRANK C. DE LUCIADepartment of Physics, The Ohio State University, Columbus, OH, USA.

Objectives• We attempt to numerically discriminate uncatalogued lines in

Doppler limited THz rotational spectra of complex mixtures of gasses

Approaches• In our earlier research projects we accumulated library

spectra of more than 40 gases, as well as mixture spectra of selected subset of our library molecules. All spectra cover the 210-270 GHz spectral range

• We performed Least Squares Fit of library to mixture spectra. We varied the number of library spectra included in the fit and gauged the performance of our algorithm.

• In a separate set of experiments we attempted to use line shape analysis to predict molecular masses of the unknown analytes.

A 210-270 GHz SMM/THz gas sensor based on CW electronic and frequency multiplication technologies (MACS)

Optics Letters, 35(10), 1533-1535 (2010)

Molecular Identification Based on Snippet Spectral Libraries

1 HCN 22 C2H3Cl2 ClCN 23 Oxetane3 BrCN 24 CF3CH3

4 CH3CN 25 CH3CCH5 OCS 26 F2CO6 CH3F 27 Thietane7 CH3Cl 28 CH3SH8 C2H3CN 29 CH3NCO9 SO2 30 CH3OH

10 CH2Cl2 31 F2SO11 CH3I 32 Acetone12 CH3Br 33 C2H3Br13 CH2F2 34 N2O14 Ethylene Oxide 35 CH2ClCH2Cl15 CHF3 36 CH3OCH3

16 C2H3CHO 37 CH3COOH17 C2H5CN 38 PhCl18 H2S 39 Thiophene19 Pyridine 40 CHCl3

20 CH2CF2 41 NO21 C2H3F

Molecular Identification Based on Wideband Incomplete Molecular Library Spectra

Mixture

12. CH3Br

9. SO2

Line Shape Considerations

Due to experimental variations in line shapes between libraries and mixture, we resorted to the use of positive lobes of the 2f spectral lines.

Mixture

12. CH3Br

9. SO2

1 HCN2 ClCN3 BrCN4 CH3CN5 OCS6 CH3F7 CH3Cl8 C2H3CN9 SO2

10 CH2Cl211 CH3I12 CH3Br13 CH2F214 Ethylene Oxide15 CHF316 C2H3CHO17 C2H5CN18 H2S19 Pyridine20 CH2CF221 C2H3F22 C2H3Cl23 Oxetane24 CF3CH325 CH3CCH26 F2CO27 Thietane28 CH3SH29 CH3NCO30 CH3OH31 F2SO32 Acetone33 C2H3Br34 N2O35 CH2ClCH2Cl36 CH3OCH337 CH3COOH38 PhCl39 Thiophene40 CHCl341 NO

Least Squares Fitting of Libraries to Mixture Spectra

1 HCN2 ClCN3 BrCN4 CH3CN5 OCS6 CH3F7 CH3Cl8 C2H3CN9 SO2

10 CH2Cl211 CH3I12 CH3Br13 CH2F214 Ethylene Oxide15 CHF316 C2H3CHO17 C2H5CN18 H2S19 Pyridine20 CH2CF221 C2H3F22 C2H3Cl23 Oxetane24 CF3CH325 CH3CCH26 F2CO27 Thietane28 CH3SH29 CH3NCO30 CH3OH31 F2SO32 Acetone33 C2H3Br34 N2O35 CH2ClCH2Cl36 CH3OCH337 CH3COOH38 PhCl39 Thiophene40 CHCl341 NO

Effect of Spectral Overlaps

1 HCN2 ClCN3 BrCN4 CH3CN5 OCS6 CH3F7 CH3Cl8 C2H3CN9 SO2

10 CH2Cl211 CH3I12 CH3Br13 CH2F214 Ethylene Oxide15 CHF316 C2H3CHO17 C2H5CN18 H2S19 Pyridine20 CH2CF221 C2H3F22 C2H3Cl23 Oxetane24 CF3CH325 CH3CCH26 F2CO27 Thietane28 CH3SH29 CH3NCO30 CH3OH31 F2SO32 Acetone33 C2H3Br34 N2O35 CH2ClCH2Cl36 CH3OCH337 CH3COOH38 PhCl39 Thiophene40 CHCl341 NO

What if one gas is missing from the library set?

Obvious overlaps

Need another round of Least Squares Fitting with the residuals of previous Fit masked out.

Wideband Residuals

Identification of Uncatalogued Lines

Line # Frequency/MHz Line Peak IntensitySignal to

Noise Probability it is Noise1 267932.53 4.19E-04 4.2 2.67E-052 267934.00 4.37E-04 4.4 1.08E-053 267940.84 1.27E-03 12.7 5.91E-374 267945.63 2.63E-04 2.6 9.32E-035 267989.03 2.07E-04 2.1 3.57E-026 267992.47 8.04E-04 8.0 1.24E-157 268007.63 4.86E-04 4.9 9.58E-078 268009.38 6.06E-04 6.1 1.06E-099 268025.06 1.13E-03 11.3 1.31E-29

10 268046.50 3.66E-04 3.7 2.16E-0411 268048.90 3.47E-04 3.5 4.65E-0412 268076.88 7.91E-04 7.9 2.78E-1513 268079.50 2.07E-04 2.1 3.57E-0214 268082.44 3.94E-04 3.9 9.62E-0515 268095.28 1.21E-03 12.1 1.50E-3316 268106.13 3.93E-04 3.9 9.62E-05

Probability of Spectral Line Detection

Are the weaker species really in the mixture?

Another Example of Absolute Calibration: Automated listing of Weak Uncataloged Lines in Methanol Spectrum

(RA06 - JAMES McMILLAN)1. Fit Observed Spectrum (OS) to Library Spectrum (LS) to obtain Concentration (Con)2. Calculate Figure of Merit (FOM)3. FOM = 1 => uncataloged line4. FOM < 1 => overlapped uncataloged line

FOM OS LS Con

OS

Weak uncataloged lines are due to ethyl cyanide, vinyl cyanide, methyl cyanide, and acetaldehyde

Obs

erve

d S

pec

tru

mF

igur

e of

Mer

it

Methanol Library

Blended Line

Expansion of Previous Chart to Show Uncataloged Lines

Obs

erve

d S

pec

tru

mF

igur

e of

Mer

it

Because overlap of methanol with uncataloged lines is with weak lines, no iteration is required

Can We Measure Molecular Mass from the Line Width?

1. We begin with a true line shape spectra (RA06 - JAMES McMILLAN)

2. We fit the line shape to a Voigt profile and calculate molecular mass from the Doppler contribution to Voigt lineshape

FrequencyCalculated

amuAverage

amu251890.9 32.02 32.10251895.7 32.10251900.5 31.26251905.7 32.17251917.1 32.77251923.7 32.30251984.8 32.11241879 32.05

241887.7 31.54218440 31.83

236936.1 32.94

Methanol ( 32 amu) Methyl Cyanide ( 41 amu)

FrequencyCalculated

amuAverage

amu588129.5 40.89 41.55588185.6 41.33588219.3 41.25606475.9 43.62606533.8 41.98606568.4 42.82606580 42.61

624498.4 42.05624629.1 42.12624736.1 41.66624926.5 42.27

Acetaldehyde ( 44 amu)

FrequencyCalculated

amuAverage

amu580257.1 44.28 44.83580285.7 45.27580190.1 43.36597402.2 45.96597681.5 46.07597698.6 48.03621594.4 47.88621663.8 41.48621888.8 44.44637851.8 45.32637898.2 44.88

Conclusions• Least Squares Fitting of incomplete spectral

libraries to Mixture spectra yields unassigned line lists limited by experimental signal to noise ratio.

• This method works very well for Doppler limited spectra with limited spectral overlaps, typical for rotational spectra

• True line shape spectra offer good potential for mass identification of unknown analytes.