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Mid-IR Direct Absorption/Dispersion Spectroscopy of a Fast Ion Beam

Mid-IR Direct Absorption/Dispersion Spectroscopy of a Fast Ion Beam. Brian Siller Michael Porambo Benjamin McCall. Near-IR Work. N 2 + is a good test molecule Not much else in the near-IR. Mid-IR Motivation. Astrochemistry >160 Molecules observed in ISM Only ~20 are ions

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Mid-IR Direct Absorption/Dispersion Spectroscopy of a Fast Ion Beam

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  1. Mid-IR Direct Absorption/Dispersion Spectroscopy of a Fast Ion Beam Brian Siller Michael Porambo Benjamin McCall

  2. Near-IR Work • N2+ is a good test molecule • Not much else in the near-IR

  3. Mid-IR Motivation • Astrochemistry • >160 Molecules observed in ISM • Only ~20 are ions • Fundamental Physics • CH5+ • Carbocation reactive intermediates CH5+ From White et al. Science, 1999, 284, 135–137. C6H6 C6H7+ e H2 C6H5+ C2H2 C4H3+ H C4H2+ C3H2 C3H C 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 H2O, e CH3OH H2 HCN, e CH3CN CH3+ CO, e NH3, e CH2CO CH3NH2 e H2 N, e CH2+ CH HCN H2O H2 H3O+ e CH+ OH H2O+ H2 C OH+ HCO+ H2 H3+ O CO H2 H2+

  4. ExperimentalSetup fast Locking Electronics slow Ion Beam Instrument Quarter Wave Plate AOM 20MHz Polarizing Beamsplitter EOM2 EOM1 PZT Ti:Sapph Laser Lock-In Amplifier Lock-In Amplifier 113MHz 40 kHz Drift Tube Modulation Cavity Modes Laser Spectrum Absorption Signal Dispersion Signal

  5. Ion Beam Instrument AOM PPLN EOM2 EOM1 Ti:Sapph Laser Cavity Modes Nd:YAG Laser Laser Spectrum RAM: Residual Amplitude Modulation

  6. NICE-OHMS Implementations • Near-IR (0.7-1.7µm) • Nd:YAG1 • Ti:Sapph2 • ECDL3 • EDFL4 • Mid-IR (8.5µm) • QCL5 • Our DFG system represents the first* broadly tunable mid-IR NICE-OHMS spectrometer *see talks WI06 and WI07 on our OPO-based system for the other “first” of this kind 1J. Ye, et al., JOSAB 15, p.6 (1998) 2L. Ma, et al., JOSAB 16, p.2255 (1999) 3C. Ishibashi, et al., J. Mol. Spec. 200, p.147 (2000) 4F. Schmidt, et al., JOSAB 24, p.1392 (2007) 5M. Taubman, et al., Spectrochim. Acta A 60, 3457 (2004)

  7. Spectroscopy Characterization • The first broadly tunable mid-IR NICE-OHMS system (2.8-4.8 µm) • Doppler broadened methane 30mW coupled into cavity Finesse ~ 300 Saturation parameter ~ 14 Absorption Dispersion

  8. Sub-Doppler Spectroscopy • wm-NICE-OHMS of methane • ~3 MHz dither, 1f detection Dispersion Absorption

  9. Sensitivity Analysis • fm-NICE-OHMS: 7×10-9 cm-1 • wm-NICE-OHMS: 8×10-10cm-1

  10. Frequency Calibration • Calibrate Ti:Sapph with frequency comb • Lock Nd:YAG to I2 hyperfine transition

  11. Ion Beam Integration N2+ Δt ~ 4 ns ΔE ~ 4 V Δν ~ 70 MHz Cathode in Front N+ Anode in Front +7 kV +4 kV 0 kV +1 kV +4 kV 0 kV H2+ H3+ NO Collisions! H2+ + H2 → H3+ + H H+ H2+ H+ H3+

  12. Future Work • Obtain and optimize H3+ absorption signal • Supersonic expansion discharge source • 750 K rotational temperature with cold cathode • <100 K with supersonic source • Vibrational spectroscopy of rotationally cooled molecular ions (CH5+, C3H3+, etc.) Crabtree, K. N. et al. Rev. Sci. Instrum.2010, 81, 086103.

  13. Acknowledgments McCall Research Group Jessica Pearson Joe Puhr Craig Riccardo Funding NSF NASA Air Force Packard Dreyfus Sloan

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