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 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.

4. 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

5. 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

6. NICE-OHMS Implementations
Near-IR ( µm)
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 ( µm)
Doppler broadened methane
Absorption
Dispersion
30mW coupled
into cavity
Finesse ~ 300
Saturation
parameter ~ 14

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: 8×10-10 cm-1

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

11. 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

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,

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