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

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

Mid-IR Motivation Astrochemistry Fundamental Physics >160 Molecules observed in ISM Only ~20 are ions Fundamental Physics CH5+ Carbocation reactive intermediates H2+ H3+ CH+ CH2+ CH3+ CH5+ CH4 C2H3+ C2H2 C3H+ C3H3+ C4H2+ C4H3+ C6H5+ C6H7+ C6H6 H2 C e C+ H OH+ H2O+ H3O+ H2O OH O HCO+ CO HCN CH3NH2 CH3CN C2H5CN N, e NH3, e HCN, e CH3CN, e CO, e H2O, e CH3OH, e CH CH2CO CH3OH CH3OCH3 C2H5+ C2H4 C3H2 C3H C2H CH5+ From White et al. Science, 1999, 284, 135–137.

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

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

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)

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

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

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

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

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

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.

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