1. Initial Development of High Precision, High Resolution Ion Beam Spectrometer in the Near- Infrared Michael Porambo, Brian Siller, Andrew Mills, Manori Perera, Holger Kreckel, Benjamin J. McCall International Symposium on Molecular Spectroscopy The Ohio State University 18 June 2012

2. Outline Introduction: Why a Fast Ion Beam? Ion Beam Description NIR Spectra Summary and Future Work

3. Molecular Ions Important in many areas of nature and science H2+H2+ H3+H3+ CH + CH 2 + CH 3 + CH 5 + CH 4 C2H3+C2H3+ C2H2C2H2 C3H+C3H+ C3H3+C3H3+ H2H2 H2H2 H2H2 H2H2 H2H2 C e C+C+ e C+C+ OH + H2O+H2O+ H3O+H3O+ H2OH2O OH e O H2H2 H2H2 HCO + CO HCN CH 3 NH 2 CH 3 CN C 2 H 5 CN N, e NH 3, e HCN, e CH 3 CN, e e CO, e H 2 O, e CH 3 OH, e CH CH 2 CO CH 3 OH CH 3 OCH 3 CH 3 + C2H5+C2H5+ e C2H4C2H4 e C3H2C3H2 e C3HC3H e C2HC2H Astrochemistry Atmospheric science Fundamental physics and chemistry CH 5 + From White et al. Science, 1999, 284, 135–137. From B. J. McCall, Ph.D. Thesis, Univ. of Chicago, 2001. NASA Picture of the Day, Expedition 13 Crew, International Space Station, NASA Challenge: How to produce ions in the laboratory effectively to study them?

4. Ion Production Methods Hollow Cathode Supersonic Expansion Positive Column Way to bring low rotational temperature and ion-neutral discrimination together? No ion-neutral discrimination Low rotational temperature No ion-neutral discrimination Ion-neutral discrimination with velocity modulation No low rotational temperature Ion Beam Spectroscopy -last attempted in 1980s–1990s 1 -advances in technology open new opportunities 1 Coe et al. J. Chem. Phys. 1989, 90, 3893.

5. Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES TOF mass spectrometer Source chamber Overlap region Laser in cavity Electrostatic Bender 2 Rigorous ion- neutral discrimination Can perform low temperature spectroscopy with a supersonic discharge source Low ion density Make up for this with cavity- enhanced spectroscopy 2 Kreckel et al. Rev. Sci. Instrum. 2010, 81, 063304.

6. Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES

8. Spectroscopic Detection Noise Immune Cavity Enhanced - Optical Heterodyne Molecular Spectroscopy Cavity enhancement for longer pathlength (× Finesse/π)

9. Spectroscopic Detection Noise Immune Cavity Enhanced - Optical Heterodyne Molecular Spectroscopy Heterodyne/Frequency Modulation Detection for Lower Noise EOM NICE- OHMS Signal

10. Spectroscopic Detection EOM Lock-In Amplifier NICE-OHMS Signal Noise Immune Cavity Enhanced - Optical Heterodyne Molecular Spectroscopy Also velocity modulate the ion beam and demodulate at this signal.

11. Ion Beam Doppler Splitting Ion Beam red blue Mass information encoded in the optical spectrum!

12. First Spectroscopic Target Obtain rovibronic spectral transitions of Meinel band of N 2 + Near-infrared transitions probed with commercial tunable titanium–sapphire laser (700–980 nm) N 2 + formed in cold cathode ion source; no rotational cooling

13. Experimental N 2 + Signal Frequency (cm −1 ) Fractional Absorption (× 10 −7 ) No absorption observed! Absorption Dispersion Absorption signal strongly attenuated by saturation. 3 Not observable! Saturation parameters: 30,000 carrier, 6300 sidebands. Dispersion signal attenuated by a factor of 2 due to saturation. 3 Ma et al. J. Opt. Soc. Am. B 2008, 25, 1144–1155.

14. Spectral Signals Obtain line centers, linewidths, and amplitudes from fits FWHM ≈ 120 MHz (at 4 kV) From Mills et al. J. Chem. Phys. 2011, 135, 224201.

15. TOF MS Mass spectrum of nitrogenic ion beam. Energy spread in inset corresponds to an expected linewidth of 120 MHz. From Mills et al. J. Chem. Phys. 2011, 135, 224201.

16. Spectral Signals Obtain line centers, linewidths, and amplitudes from fits FWHM ≈ 120 MHz (at 4 kV) Noise equivalent absorption ~ 2 × 10 −11 cm −1 Hz −1/2 (50× lower than last ion beam instrument) 1 Within ~1.5 times the shot noise limit! From Mills et al. J. Chem. Phys. 2011, 135, 224201. 1 Coe et al. J. Chem. Phys. 1989, 90, 3893.

17. Ultra-High Resolution Spectroscopy Rough calibration with Bristol wavelength meter (~70 MHz precision) Precisely calibrate with MenloSystems optical frequency comb (<1 MHz accuracy)

18. Frequency Comb Calibrated Spectra Only ~8 MHz from line center obtained in N 2 + positive column work. 4 Confident in improvements in the mid-IR. 4 Siller, B. M. et al. Opt. Express 2011, 19, 24822. Average the line centers

19. Summary and Conclusions Ion Beam Spectroscopy – effective in studying molecular ions. High sensitivity spectroscopy used to study ion beam – high S/N, Doppler splitting. Spectroscopy on rovibronic transitions of N 2 + – first direct spectroscopy of electronic transition in fast ion beam. Accurate frequency calibration with optical frequency comb.

20. Present and Future Work Ro-vibrational spectroscopy in the mid- IR Integration of supersonic cooling Stay tuned to MG05 for more information!

21. Acknowledgments McCall Research Group Machine Shop Electronics Shop Jim Coe Rich Saykally Sources of Funding –Air Force –NASA –Dreyfus –Packard –NSF – Sloan –Research Corp. – Springborn Endowment