High Precision, Sensitive, Near-IR Spectroscopy in a Fast Ion Beam

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 Experimental laboratory spectra needed to aid in theoretical, observational work. C3H2 C3H C3H3+ e e H2 C3H+ C+ C2H2 C2H e C2H4 C2H3+ e C2H5+ e C+ CH4 CH3+ e CH3OCH3 CH5+ C2H5CN CH3OH, e CH3CN, e CH3OH H2O, e H2 CH3CN HCN, e CH3+ CO, e NH3, e CH2CO CH3NH2 H2 e N, e CH2+ CH HCN H2O H2 H3O+ e CH+ OH H2O+ H2 C OH+ HCO+ H2 H3+ O CO H2 H2+

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

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

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

7. First Generation Ion Beam Instrument Direct Laser Absorption Spectroscopy in Fast Ion Beams –DLASFIB. Pioneered by Saykally group in late 1980s– early 1990s.1,2,3 Studied HF+, HN2+, HCO+, H3O+, NH4+ in the mid-infrared, with no supersonic expansion. Lacked sensitivity to see larger or more complex ions, especially at high temperature. Coe, J. V. et al. J. Chem. Phys.1989, 90, 3893–3902. 1Coe et al., J. Chem. Phys.1989, 90, 3893–3902. 2Owrutsky et al., J. Phys. Chem.1989, 93, 5960–5963. 3Keim et al., J. Chem. Phys.1990, 93, 3111–3119.

8. Ion Beam Setup

9. Ion Beam Setup

10. Ion Beam Setup Cold Cathode Ion Source Note: No rotational cooling Source Chamber Beam Deflector7 Mass Spec Detector Time-of-Flight Region Cavity Mirror Detector 4Kreckel, H. et al. Rev. Sci. Instrum.2010, 81, 063304.

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

12. Heterodyne Detection Absorption Dispersion - +

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

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

15. Doppler Splitting Amount of shifts depend on the mass of the ion Ion Beam nred nblue n0 Relative Frequency

16. First Spectroscopic Target • Obtain rovibronic spectral transitions of A 2Pu – X 2Sg+ 1–0 Meinel band of N2+ • Near-infrared transitions probed with commercial tunable titanium–sapphire laser (700–980 nm) • N2+ formed in cold cathode ion source; no rotational cooling

17. Experimental N2+ Signal Absorption Lock-In Amplifier Output Fractional Absorption (× 10−7) Dispersion Lock-In Amplifier Output No absorption observed! A 2Pu – X 2Sg+, qQ22(14.5) line Frequency (cm−1) NICE-OHMS absorption signal strongly affected by saturation; saturation of the ions decrease the absorption to below the noise

18. Spectral Signals A 2Pu – X 2Sg+, qQ22(14.5) line, red- and blue-shifted • 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

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 2Pu – X 2Sg+, qQ22(14.5) line, red- and blue-shifted Average the line centers Average the line centers Only ~8 MHz from linecenter obtained in N2+ positive column work.6 Confident in improvements in the mid-IR. 6Siller, B. M. et al. Opt. Express2011, Accepted.

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 N2+ – 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 HN2+ in • the ion beam Increasing N2 Supersonic Expansion Discharge Source7 • Enable rotational cooling H3+ HN2+ Time-of-flight mass spectra of hydrogenic ion beam with increasing amounts of N2 • 750 K with cold cathode; • <100 K with supersonic source? • Vibrational spectroscopy of rotationally cooled molecular ions (CH5+, C3H3+, etc.) 7Crabtree, K. N. et al. Rev. Sci. Instrum.2010, 81, 086103. Supersonic expansion discharge source

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