THZ/MM-WAVE SPECTROSCOPIC SENSORS, CATALOGS, AND UNCATALOGUED LINES IVAN MEDVEDEV Department of Physics, Wright State University, Dayton, OH, USA; CHRISTOPHER F. NEESE, FRANK C. DE LUCIA Department 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 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 GHz SMM/THz gas sensor based on CW electronic and frequency multiplication technologies (MACS) Optics Letters, 35(10), (2010) Molecular Identification Based on Snippet Spectral Libraries

1HCN22C 2 H 3 Cl 2ClCN23Oxetane 3BrCN24CF 3 CH 3 4CH 3 CN25CH 3 CCH 5OCS26F 2 CO 6CH 3 F27Thietane 7CH 3 Cl28CH 3 SH 8C 2 H 3 CN29CH 3 NCO 9SO 2 30CH 3 OH 10CH 2 Cl 2 31F 2 SO 11CH 3 I32Acetone 12CH 3 Br33C 2 H 3 Br 13CH 2 F 2 34N2ON2O 14Ethylene Oxide35CH 2 ClCH 2 Cl 15CHF 3 36CH 3 OCH 3 16C 2 H 3 CHO37CH 3 COOH 17C 2 H 5 CN38PhCl 18H2SH2S39Thiophene 19Pyridine40CHCl 3 20CH 2 CF 2 41NO 21C2H3FC2H3F Molecular Identification Based on Wideband Incomplete Molecular Library Spectra Mixture 12. CH 3 Br 9. SO 2

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. CH 3 Br 9. SO 2

1HCN 2ClCN 3BrCN 4CH3CN 5OCS 6CH3F 7CH3Cl 8C2H3CN 9SO2 10CH2Cl2 11CH3I 12CH3Br 13CH2F2 14Ethylene Oxide 15CHF3 16C2H3CHO 17C2H5CN 18H2S 19Pyridine 20CH2CF2 21C2H3F 22C2H3Cl 23Oxetane 24CF3CH3 25CH3CCH 26F2CO 27Thietane 28CH3SH 29CH3NCO 30CH3OH 31F2SO 32Acetone 33C2H3Br 34N2O 35CH2ClCH2Cl 36CH3OCH3 37CH3COOH 38PhCl 39Thiophene 40CHCl3 41NO Least Squares Fitting of Libraries to Mixture Spectra

1HCN 2ClCN 3BrCN 4CH3CN 5OCS 6CH3F 7CH3Cl 8C2H3CN 9SO2 10CH2Cl2 11CH3I 12CH3Br 13CH2F2 14Ethylene Oxide 15CHF3 16C2H3CHO 17C2H5CN 18H2S 19Pyridine 20CH2CF2 21C2H3F 22C2H3Cl 23Oxetane 24CF3CH3 25CH3CCH 26F2CO 27Thietane 28CH3SH 29CH3NCO 30CH3OH 31F2SO 32Acetone 33C2H3Br 34N2O 35CH2ClCH2Cl 36CH3OCH3 37CH3COOH 38PhCl 39Thiophene 40CHCl3 41NO Effect of Spectral Overlaps

1HCN 2ClCN 3BrCN 4CH3CN 5OCS 6CH3F 7CH3Cl 8C2H3CN 9SO2 10CH2Cl2 11CH3I 12CH3Br 13CH2F2 14Ethylene Oxide 15CHF3 16C2H3CHO 17C2H5CN 18H2S 19Pyridine 20CH2CF2 21C2H3F 22C2H3Cl 23Oxetane 24CF3CH3 25CH3CCH 26F2CO 27Thietane 28CH3SH 29CH3NCO 30CH3OH 31F2SO 32Acetone 33C2H3Br 34N2O 35CH2ClCH2Cl 36CH3OCH3 37CH3COOH 38PhCl 39Thiophene 40CHCl3 41NO

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/MHzLine Peak Intensity Signal to NoiseProbability it is Noise E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-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 line 4. FOM overlapped uncataloged line Weak uncataloged lines are due to ethyl cyanide, vinyl cyanide, methyl cyanide, and acetaldehyde Observed Spectrum Figure of Merit

Methanol Library Blended Line Expansion of Previous Chart to Show Uncataloged Lines Observed Spectrum Figure of Merit 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

Frequency Calculated amu Average amu Methanol ( 32 amu) Methyl Cyanide ( 41 amu) Frequency Calculated amu Average amu Acetaldehyde ( 44 amu) Frequency Calculated amu Average amu

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.