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 0 1 2 3 4 5 6 0 1 2 3 4 J’ cm -1 J” IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions

5. Test Molecule (HNN + ) - Proof of Concept 30 K 300 K 1000 K  THz ~  IR +  Microwave  v (Line Accuracy)~30 MHz 1)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. dichroic Polarising Prism Ti:Sapph 700 – 1000 nm 532 nm pump laser Reference Cavity /2 Nd:YAG 1064 nm AOM PPLN /2 /4 Fabry-Perot Interferometer Achromat InSb Optical Isolator -meter DFG Fiber-Optic to Frequency Comb Beam Dump LP filter 20 MHz Accuracy

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

10. cw-CRDS

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

12. Representative Scans 1-0 A 2  u -X 2  g +  min =4.5e-10 Wavelength (nm) Absorbance (cm -1 ) x10 -9

13. drift region Source chamber 1 st bender 2 nd bender TOF detector lens steerer apertures beam modulator lens 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 Ion Beam Velocity Modulation PLL pzt detector

14. Ion Beam + Frequency Comb = ? 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 J” Low Accuracy Comb Accuracy

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

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