1. High Precision, Sensitive, Near-IR Spectroscopy in a Fast Ion Beam Michael Porambo, Holger Kreckel, Andrew Mills, Manori Perera, Brian Siller, Benjamin J. McCall MWAM 2011 University of Illinois at Urbana-Champaign 22 October 2011

2. Outline Introduction Description of Instrument Results Summary Future Work

3. Molecular Ions in Astrochemistry 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 Experimental laboratory spectra needed to aid in theoretical, observational work.

4. Ion Production Techniques Oka, Saykally, McCall Maier, Nesbitt   Ion-neutral discrimination Low rotational temperature Narrow linewidth Compatible with cavity-enhanced spectroscopy Positive Column Supersonic Expansion Hollow Cathode Hirota, Amano  High ion column density  We want…

5. Sensitive, Cooled, Resolved Ion BEam Spectrometer – SCRIBES

6. Ion Production Techniques Oka, Saykally, McCall Maier, Nesbitt   Ion-neutral discrimination Low rotational temperature Narrow linewidth Compatible with cavity-enhanced spectroscopy Mass spectrometry of laser-probed ions Spectral identification of ion mass Velocity Modulation Supersonic Expansion SCRIBES  Hollow Cathode Hirota, Amano  High ion column density  McCall So we want…

7. First Generation Ion Beam Instrument Coe, J. V. et al. J. Chem. Phys. 1989, 90, 3893–3902. Direct Laser Absorption Spectroscopy in Fast Ion Beams –DLASFIB. Pioneered by Saykally group in late 1980s– early 1990s. 1,2,3 Studied HF +, HN 2 +, HCO +, H 3 O +, NH 4 + in the mid-infrared, with no supersonic expansion. Lacked sensitivity to see larger or more complex ions, especially at high temperature. 1 Coe et al., J. Chem. Phys. 1989, 90, 3893–3902. 2 Owrutsky et al., J. Phys. Chem. 1989, 93, 5960–5963. 3 Keim et al., J. Chem. Phys. 1990, 93, 3111–3119.

8. Ion Beam Setup

10. Source Chamber Cold Cathode Ion Source Note: No rotational cooling Beam Deflector 7 4 Kreckel, H. et al. Rev. Sci. Instrum. 2010, 81, 063304. Time-of-Flight Region Mass Spec Detector Cavity Mirror Detector

11. Spectroscopy: Heterodyne Detection Single Carrier Frequency Carrier + Other Frequencies Electro-optic modulator EOM Analyte 113 MHz Relative Frequency (MHz)

12. Heterodyne Detection Absorption Dispersion + -

13. Cavity Enhancement Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) 5,6 Analyte Optical cavity increases pathlength by factor of ~100 EOM 5 Ye, Ph.D. Dissertation, University of Colorado Department of Physics, 1997. 6 Foltynowicz et al., Appl. Phys. B 2008, 92, 313–326. Relative Frequency (MHz) Cavity Modes Laser Frequencies

14. Spectroscopy Layout PZT EOM ~113 MHz Lock-In Amplifier Vel. Mod. ~ 4 kHz Dispersion Absorption Detector Dispersion Absorption

15. Ion Beam Doppler Splitting 0 red blue Amount of shifts depend on the mass of the ion Relative Frequency

16. First Spectroscopic Target Obtain rovibronic spectral transitions of A 2  u – X 2  g + 1–0 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

17. Experimental N 2 + Signal A 2  u – X 2  g +, q Q 22 (14.5) line Frequency (cm −1 ) Fractional Absorption (× 10 −7 ) No absorption observed! Absorption Lock-In Amplifier Output Dispersion Lock-In Amplifier Output NICE-OHMS absorption signal strongly affected by saturation; saturation of the ions decrease the absorption to below the noise

18. Spectral Signals FWHM ≈ 120 MHz (at 4 kV) Noise equivalent absorption ~ 4 × 10 −11 cm −1 Hz −1/2 (50× lower than last ion beam instrument) Within ~1.5 times the shot noise limit A 2  u – X 2  g +, q Q 22 (14.5) line, red- and blue-shifted

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

20. Frequency Comb Calibrated Spectra A 2  u – X 2  g +, q Q 22 (14.5) line, red- and blue-shifted Only ~8 MHz from linecenter obtained in N 2 + positive column work. 6 Confident in improvements in the mid-IR. 6 Siller, B. M. et al. Opt. Express 2011, Accepted. Average the line centers

21. Summary and Conclusions Fast ion beam spectroscopy can be very effective for general molecular ion spectroscopy. Integrated NICE-OHMS and velocity modulation spectroscopy for performing sensitive measurements of ion beam. Operational spectroscopy on rovibronic transitions of the Meinel band of N 2 + – first direct spectroscopy of electronic transition in fast ion beam. Performed precisely calibrated measurements with optical frequency comb to get line centers to an accuracy of ~8 MHz.

22. Present and Future Work Vibrational spectroscopy in the mid-IR Finished construction of mid-IR DFG laser at 3.0 µm. Produced HN 2 + in the ion beam Supersonic Expansion Discharge Source 7 Enable rotational cooling H3+H3+ HN 2 + Increasing N 2 Time-of-flight mass spectra of hydrogenic ion beam with increasing amounts of N 2 750 K with cold cathode; <100 K with supersonic source? Vibrational spectroscopy of rotationally cooled molecular ions (CH 5 +, C 3 H 3 +, etc.) Supersonic expansion discharge source 7 Crabtree, K. N. et al. Rev. Sci. Instrum. 2010, 81, 086103.

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

24. Absorption and Dispersion Absorption and dispersion related by the Kramers- Kronig relations. Example for Gaussian absorption profile: