Mid-IR Direct Absorption/Dispersion Spectroscopy of a Fast Ion BeamBrian SillerMichael PoramboBenjamin McCall

Near-IR Work• N2

+ is a good test molecule• Not much else in the near-IR

• Astrochemistry– >160 Molecules observed in ISM– Only ~20 are ions

• Fundamental Physics– CH5

+

– Carbocation reactive intermediates

Mid-IR Motivation

H2+

H3+

CH+

CH2+

CH3+

CH5+

CH4

C2H3+

C2H2

C3H+

C3H3+

C4H2+

C4H3+

C6H5+

C6H7+ C6H6

H2

H2

H2

H2

H2

C

e

C+

e

C+

C

H

C2H2

H2

e

OH+H2O+

H3O+H2O

OHe

OH2

H2

HCO+

CO

HCNCH3NH2

CH3CN

C2H5CN

N, eNH3, e

HCN, eCH 3CN, e

eCO, e

H2O, e

CH3OH, e

CHCH2CO

CH3OH

CH3OCH3

CH3+

C2H5+e

C2H4

eC3H2

eC3H

eC2H

CH5+

From White et al. Science, 1999, 284, 135–137.

Ti:Sapph Laser

PolarizingBeamsplitter

QuarterWave Plate

AOM

Locking Electronics

Ion BeamInstrument

PZT

20MHz

EOM1 EOM2

113MHz

AbsorptionSignal

Lock-In Amplifier

40 kHzDrift Tube

Modulation

DispersionSignal

Lock-In Amplifier

CavityModes

LaserSpectrum

ExperimentalSetup

fastslow

CavityModes

LaserSpectrum

Ion BeamInstrument

Ti:Sapph Laser

Nd:YAG Laser

EOM1 EOM2

PPLN

AOM

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

Absorption

Dispersion

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

H2+

H3+H+ H2

+

H3+

H+

Δt ~ 4 nsΔE ~ 4 VΔν ~ 70 MHz

+7 kV +4 kV 0 kV

Cathode in Front

+1 kV +4 kV 0 kV

Anode in Front

N+

N2+

NOCollisions!H2

+ + H2 → H3+ + H

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.

AcknowledgmentsMcCall Research Group

Jessica PearsonJoe PuhrCraig Riccardo

FundingNSF NASAAir Force PackardDreyfus Sloan