1. Cavity Based Medium Resolution Spectroscopy Satyakumar Nagarajan, Frank C. De Lucia, Christopher Neese The 70 th International Symposium on Molecular Spectroscopy The Ohio State University

2. Overview Motivation Background Theory Experimental setup Results so far Main advantages Summary Benzonitrile DMMP Pentafluorobenzonitrile Chloromethane

3. Motivation Challenge of dealing with large molecules Spectra rapidly become denser with increasing molecular size Lines merge into clumps of lines, then quasi-continua Traditional signal processing techniques do not separate broad spectral features from spectrometer baseline Very difficult to detect continua with traditional methods – we ordinarily signal process for sharp lines Can lay foundation for development of THz chemical sensors We want to advance the spectroscopic understanding of these systems Hope to extend THz based sensors to dense quasi-continuum spectra

4. Medium Resolution Cavity Spectroscopy

5. Spectral Overlap Fig. 1: Lineshapes in Pressure Broadened Lines Microwave Molecular Spectra (pp. 49), Gordy & Cook, 1984 Fig. 2: Example of two separate lines overlapping to demonstrate signal growth

6. Theory

7. Experimental Setup

8. Inside the Chamber Incoming Beam Isolator - 2 Polarizers, 1 Faraday Rotator Beamsplitter Cavity Mirror

9. Full Power Profile

10. Snippet Scan of Cavity Modes Free Spectral Range = 257 MHz Higher order transverse modes TEM 00q Mode TEM 00q+1 Mode

11. Analysis

12. Pressure Broadened Spectrum of OCS J = 19 J = 18 J = 17

13. Pressure Broadened Spectrum of OCS J = 19 J = 18 J = 17

14. 2 Benzonitrile Spectra – The Effects of Pressure Broadening J = 88J = 87 2C = 2.428 GHz

15. Main advantages of this technique Sensitivity (Effective path length ~100 m) Higher pressure eliminates molecular saturation Power required to saturate varies as the square of pressure Provides an absolute measurement of absorbed power Broad spectral features do not get lost in signal processing

16. Summary Results so far look promising Will analyze other molecules Will further map out limitations and capabilities of Cavity Based Medium Resolution Spectroscopy We would like to thank The Defense Threat Reduction Agency for their support of this work