Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR

James N. Hodges, Kyle N. Crabtree, & Benjamin J. McCall WI06 – June 20, 2012 University of Illinois at Urbana-Champaign

Outline Motivation Spectroscopic Techniques for Ions: N2

+

Mid-IR Instrument H3

+ Spectroscopy Conclusions

AstrochemistryIons reactive

intermediates in ISM

~20 ions have been observed

Many carbo-cations have transitions in mid-IR

Lab spectra help observations

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

O

H2

H2

HCO+

CO

HCNCH3NH2

CH3CN

C2H5CN

N, e

NH3, e

HCN, eCH3

CN, e

eCO, e

H2O, e

CH3OH, e

CHCH2CO

CH3OH

CH3OCH3

CH3+

C2H5+e

C2H4

e

C3H2

eC3H

eC2H

B.J. McCall. Ph.D. Thesis, U. Chicago, 2001.

Indirect THz Spectroscopy

Combination differences extract energy spacings for rotational levels.

Useful for ions with transitions in the THz region - Herschel, SOFIA

Fundamental ScienceFluxional species

Spectrum remains unassigned

WI07 up next!

White et al. Science, 284, 135 (1999).

CH5+

MotivationGeneral, Sensitive, High Precision, Mid-IR Spectrometer for Molecular Ions

General – Multiple Ions of InterestSensitive – Weak Transitions & Trace DetectionHigh Precision – Reduced Uncertainty in Combination

Differences

Velocity Modulation SpectroscopyCations go to cathode

Plasma Discharge Cell

+HV -HV

S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted

Plasma Discharge Cell

+HV -HV

Plasma Discharge Cell

+HV -HV

Laser

Detector

S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted

Plasma Discharge Cell

-HV +HV

Plasma Discharge Cell

Laser

DetectorPlasma Discharge Cell

Laser

Detector

S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted AC Driven – Absorption Profile ModulatedVelocity Modulation Provides Ion-Neutral Discrimination

Plasma Discharge CellPlasma Discharge Cell

Laser

DetectorPlasma Discharge Cell

Laser

Detector

S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).

Velocity Modulation of N2+

Heterodyne Spectroscopy

Creates fm-triplet with spacing typically in the rfMixers demodulate rf signal Sensitive to relative sizes/phases of sidebandsAbsorption/Dispersion - 90o Phase Separation“Zero background”Operation at rf frequencies reduces 1/f noise

LaserDetector

EOM

Signal

Velocity Modulation of N2+

Velocity Modulation & Heterodyne at 1 GHz

Cavity EnhancementLaser

CavityDetector

Enhances Pathlength

Increases Intracavity Power

Allows saturation of rovibrational transitions – sub-Doppler features

Requires active locking to maintain resonance – PDH locking

Velocity Modulation in a CavityVelocity Modulation Provides Ion-Neutral Discrimination

Velocity Modulation

Ion Signal Encoded at 2x the Plasma Frequency

Velocity Modulation Provides Ion-Neutral Discrimination

Cavity Enhanced Velocity Modulation Spectroscopy of N2+

PZT Detector

Detector

EOMLaser Lock-In Amplifier2f

B. M. Siller et al., Opt. Lett., 35, 1266-1268. (2010)

NICE-OHVMS

Large Signal Small Noise

Cavity Enhancement

Heterodyne Spectroscopy

NICE-OHVMS

N oiseI mmuneC avityE nhanced-O pticalH eterodyneV elocityM odulationS pectroscopy

Velocity Modulation

Sensitivity to Ions

B. M. Siller et al., Opt. Exp., 19, 24822-24827. (2011)

NICE-OHVMSHeterodyne sidebands at the cavity FSR allows the combination of heterodyne spectroscopy with a cavity. Cavity Modes

Laser

NICE-OHVMS

Lock-In Amplifier

AbsorptionSignal

PlasmaFrequency

Detector

Lock-In Amplifier

DispersionSignal

90° PhaseShift

1 × Cavity FSR

Laser EOM

2f

Comparison of Techniques on N2+

NICE-OHVMS

Mid-IR Instrument

Optical Parametric Oscillator (OPO)

High optical powerSaturation of rovibrational transitions

Spans 3.2 – 3.9 μm range

OPO Light Generation

Yb Doped Fiber Laser OPOEOM Am

p

1064 nm

OPO Light Generation

Signal 1.5-1.6 m

Pump1064 nm

Idler 3.2-3.9 m

Periodically Poled Li:NbO3

Ion Production/Velocity Modulation

~AC HV40 kHz

L-N2 InL-N2 Out Gas In

Liquid Nitrogen Cooled Positive Column Discharge Cell- ”Black Widow”

Ion Production

Mid-IR Instrument

OPO

YDFL

EOMLock-In

Amplifier

AbsorptionSignal

Lock-In Amplifier

DispersionSignal

Wave-meter

40 kHzPlasma

Frequency

80 MHz1 × Cavity FSR

90o Phase Shift

IPS

2f

ni = np - ns

H3+ Spectra

Sensitivity = 2 x 10-9 cm-1 Hz-1/2

Shot Noise Limit = 8 x 10-11 cm-1 Hz-1/2

Sig

nal

H3+ Spectra

S/N ~ 500Precision of Line Center ~ 300 kHz

Sig

nal

Summary & Conclusions

• Constructed a general high precision mid-IR spectrometer• Demonstrated the first NICE-OHVMS spectra of H3

+

• 1.5 orders of magnitude from the shot noise limit

Acknowledgements

McCall Group with Special thanks to:Brian Siller &Joseph Kelly

NSF GRF# DGE 11-44245 FLLWSpringborn Endowment