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University of Illinois, Department of Chemistry June 24, 2009

INDIRECT TERAHERTZ SPECTROSCOPY OF MOLECULAR IONS USING HIGHLY ACCURATE AND PRECISE MID-IR SPECTROSCOPY. Andrew A. Mills, Kyle B. Ford, Holger Kreckel, Manori Perera, Kyle N. Crabtree, Benjamin J. McCall. University of Illinois, Department of Chemistry June 24, 2009.

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University of Illinois, Department of Chemistry June 24, 2009

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  1. INDIRECT TERAHERTZ SPECTROSCOPY OF MOLECULAR IONS USING HIGHLY ACCURATE AND PRECISE MID-IR SPECTROSCOPY Andrew A. Mills, Kyle B. Ford, Holger Kreckel, Manori Perera, Kyle N. Crabtree, Benjamin J. McCall University of Illinois, Department of Chemistry June 24, 2009

  2. Indirect THz Spectroscopy Outline • Motivations • How-to • Radiation source • Increasing accuracy and precision • Target ions • Sample production and detection • Future direction

  3. THz or IR Spectroscopy • THz • The THz regime opens new possibilities for interesting spectroscopy • Growing yet limited availability of THz sources • High sensitivity techniques infrequent • IR • Many available sources (OPO, DFG, QCL) • Historical accuracy ~30 MHz • Indirect THz via precise IR spectroscopy

  4. Indirect THz - HNN+ & Other Linear Molecules J’ 4 3 cm-1 IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions 2 1 0 6 5 cm-1 4 3 2 1 0 J”

  5. Test Molecule (HNN+) - Proof of Concept Dn THz ~ DnIR + DnMicrowave Dv (Line Accuracy)~30 MHz 30 K 300 K 1000 K • Increase IR transition accuracy Gudeman, C. S., Begemann, M. H. Pfaff, J.; Saykally, R. J. Velocity Modulated IR laser spectroscopy of Molecular Ions: The v1 band of HNN+ J. Chem. Phys 78(9) 1983 5837-8 P. Verhoeve, E. Zwart, M. Versluis, M. Drabbels, J.j. ter Meulen, W.Leo Meerts, A. Dymanus. Rev. Sci. Instrum. 61(6) 1990 1612-1625.

  6. DFG 532 nm pump laser Nd:YAG 1064 nm Ti:Sapph 700 – 1000 nm l/4 l/2 Reference Cavity AOM Beam Dump Polarising Prism InSb Optical Isolator l/2 PPLN LP filter Achromat dichroic Fabry-Perot Interferometer l-meter Fiber-Optic to Frequency Comb 20 MHz Accuracy

  7. l/2 l/2 PBS Menlo Systems Optical Frequency Comb • Stabilized mode-locked fs laser FT equally spaced lines in frequency • Frequency reference OCXO disciplined to GPS • 1x10-12 in 1s  200 Hz Accuracy • Unknown laser comb  Beat Frequency • Laser frequency ~ 1-100 kHz

  8. Target ions – Small Linear Polyatomics • IR transitions and J=10 Microwave transitions recorded • HNN+, HCS+, HOC+, HCO+, CO+, CH+, HCNH+ are known interstellar molecules. • Probe of chemical intermediates and conditions • Make in plasma discharge

  9. Ti:Sapph @ 925 nm Optical Isolator l-meter Etalon Finder FPI AOM Mode Matching Optics Ion Beam Spectrometer detector • Narrowing of line-widths • Physically separate ions from neutrals • Mass dependent Doppler splitting from cavity • Characterize w/ N2+ 1st bender lens Source chamber apertures beam modulator lens TOF detector 2nd bender drift region

  10. cw-CRDS

  11. cw-CRDS/Concentration Modulation CRDS • 99.99% RD Mirrors • 33 ms time constant • amin =4x10-9 cm-1 • Concentration Modulation • Beam on/ Beam off • amin =5x10-10 cm-1 • Reduces long term Drift

  12. Representative Scans 1-0 A2Pu-X2Sg+ amin=4.5e-10 x10-9 Absorbance (cm-1) Wavelength (nm)

  13. Ion Beam Velocity Modulation detector pzt • Use lower finesse cavity • Modulate the velocity of the ion beam by applying field to drift region • Use lock in detection to further reduce noise and increase signal Source chamber lens 1st bender steerer apertures PLL beam modulator lens TOF detector 2nd bender drift region

  14. J” Ion Beam + Frequency Comb = ? Low Accuracy Comb Accuracy • Ion beam  Narrower line widths • Narrower line widths  accurate line frequencies • Well calibrated lasers,  highly accurate spectra • Traditional accuracy 30 MHz, expect 2-3 orders of magnitude better than that

  15. Conclusions • The THz regime opens new possibilities for interesting spectroscopy • Indirect spectroscopy can be used to obtain the transitions for many interesting molecular ions of the interstellar medium • Indirect spectroscopy requires • Highly accurate and precise frequency references such as a frequency comb • Sub-Doppler line widths from fast ion beams • Sensitive, cavity enhanced spectroscopy

  16. the David Lucile & O O PackardFOUNDATION O RESEARCH CORPORATION for SCIENCE ADVANCEMENT A Foundation dedicated to science since 1855

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