Cavity EnhancedVelocity Modulation Spectroscopy

Brian Siller, Michael Porambo & Benjamin McCallChemistry Department

University of Illinois at Urbana-Champaign

Applications◦ Astrochemistry◦ Fundamental physics

Goals◦ Completely general (direct absorption)◦ High resolution

Ion Spectroscopy

Molecular ions are important to interstellar chemistry

Ions important as reaction intermediates

>150 Molecules observed in ISM

Only ~20 are ions Need laboratory data to

provide astronomers with spectral targets

Ions & Astrochemistry

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

Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR

Support for Herschel, SOFIA, and ALMA THz observatories

Indirect Terahertz Spectroscopy

60-670 µm 0.3-1600 µm 3-400 µm

0

80

60

40

20

0

2.01.51.00.50.0-0.5-1.0

80

60

40

20

0

2.01.51.00.50.0-0.5-1.0

3320

3300

3280

3260

3240

2.01.51.00.50.0-0.5-1.0

3320

3300

3280

3260

3240

2.01.51.00.50.0-0.5-1.0

12

3

4

5

6

01

2

3

4J’

cm-1

cm-1

J”

IR TransitionsEven Combination differencesOdd Combination Differences1-0 Rotational TransitionReconstructed Rotational Transitions

Indirect Terahertz Spectroscopy

CH5+ is a prototypical carbocation

◦ SN1 reaction intermediates◦ Highly fluctional structure◦ Spectrum completely unassigned

Fundamental Physics

E.T. White, J. Tang, and T. Oka, “CH5+: The Infrared Spectrum Observed”, Science, 284, 135-137 (1999).

Animation from Joel Bowman, Emory University

Positive Column◦ High ion density◦ Simple setup

Direct Absorption Techniques

Ion Beam◦ Rigorous ion-neutral

discrimination◦ Mass-dependent Doppler

shift

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode

Velocity Modulation Spectroscopy

Plasma Discharge Cell

+1kV -1kV

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Velocity Modulation Spectroscopy

Plasma Discharge Cell

+1kV -1kV

Laser

Detector

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Velocity Modulation Spectroscopy

Plasma Discharge Cell

-1kV +1kV

Laser

Detector

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Drive with AC voltage◦ Ion Doppler profile alternates red/blue shift◦ Laser at fixed wavelength◦ Demodulate detector signal at modulation frequency

Velocity Modulation Spectroscopy

Plasma Discharge Cell Detector

Laser

Velocity Modulation Spectroscopy

0 1

Want strongest absorption possible Signal enhanced by modified White cell

◦ Laser passes through cell unidirectionally◦ Can get up to ~8 passes through cell

Velocity Modulation Spectroscopy

Plasma Discharge Cell

Laser

Detector

Also want lowest noise possible, so combine with heterodyne spectroscopy

Single-pass direct absorption

Single-pass Heterodyne @ 1GHz

Velocity Modulation of N2+

0

1

2

Doppler-broadened lines◦ Blended lines◦ Limited determination of line centers

Sensitivity◦ Limited path length through plasma

Velocity Modulation Limitations

Improve by combining with cavity enhanced absorption spectroscopy

CavityTransmission

Error Signal

Pound-Drever-Hall Locking

Ti:Sapph Laser

EOMPZT

Lock Box

30MHz

Detector

Detector

AOM

PolarizingBeamsplitter

QuarterWave Plate

0.1-60kHz <100Hz

CEVMS Setup

Lock-In Amplifier

Transformer

Cavity Mirror Mounts

Audio Amplifier

Laser

40 kHz

CEVMS Setup

Doppler profile shifts back and forth Red-shift with respect to one direction of the

laser corresponds to blue shift with respect to the other direction

Net absorption is the sum of the absorption in each direction

Extracting N2+ Absorption Signal

Abso

rptio

n St

reng

th (A

rb. U

nits

)

Relative Frequency (GHz)

Demodulate detected signal at twice the modulation frequency (2f)

Can observe and distinguish ions and neutrals◦ Ions are velocity modulated◦ Excited neutrals are concentration modulated◦ Ground state neutrals are not modulated at all

Ions and excited neutrals are observed to be ~75° out of phase with one another

Extracting N2+ Absorption Signal

Typical Scan of Nitrogen Plasma Cavity Finesse 150 30mW laser power

N2+ Meinel Band

N2* first positive band

Second time a Lamb dip of a molecular ion has been observed (first was DBr+ in laser magnetic resonance technique)1

Used 2 lock-in amplifiers for N2

+/N2*

1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010)

Line centers determined to within 1 MHz with optical frequency comb

Sensitivity limited by plasma noise

Precision & Accuracy 0

1

2

A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)

Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy

NICE-OHMS

Cavity Modes

Laser Spectrum

J. Ye, L. S. Ma, and J. L. Hall, JOSA B, 15, 6-15 (1998)

Experimental Setup

Ti:Sapph Laser

EOMPZT

Lock Box

30MHz

Detector

Detector

AOM

PolarizingBeamsplitter

QuarterWave Plate

Experimental Setup

Ti:Sapph Laser EOM

PZT Detector

AbsorptionSignal

Lock-In Amplifier

40 kHzPlasma

Frequency

Experimental SetupTi:Sapph Laser EOM

PZT DetectorEOM

113 MHzCavity FSR

DispersionSignal

Lock-In Amplifier

90°PhaseShift

X Y X Y

Results

AbsorptionDispersion Lock-In XLock-In Y

113 MHz Sidebands

1 Cavity FSR

Lock-In XLock-In Y

No center Lamb dip in absorption

AbsorptionDispersion

Spectra calibrated with optical frequency comb

Frequency precision to <1 MHz!

Sub-Doppler fit based on pseudo-Voigt absorption and dispersion profiles

(6 absorption, 7 dispersion)

Line center from fit: 326,187,572.2 ± 0.1 MHz

After accounting for systematic problems, line center measured to within uncertainty of ~300 kHz!

AbsorptionDispersion

113MHz

Ultra-High Resolution Spectroscopy

Technique Comparison

VMS OHVMS

CEVMS NICE-OHVMSNICE-OHVMS

Better sensitivity than traditional VMS◦ Increased path length through plasma◦ Decreased noise from heterodyne modulation

Retained ion-neutral discrimination Sub-Doppler resolution

◦ Better precision & absolute accuracy with comb◦ Resolve blended lines

Can use same optical setup for ion beam spectroscopy

NICE-OHVMS Summary

Ion BeamInstrument

AbsorptionSignal

Lock-In Amplifier

40 kHzPlasma

Frequency

Experimental Setup

Ti:Sapph Laser EOM

PZT DetectorEOM

DispersionSignal

Lock-In Amplifier

X Y X Y

drift tube (overlap) variable apertureselectrostatic deflector 1

steerers

Einzel lens 1

Einzel lens 2

electrostatic deflector 2

TOF beam modulation electrodes

wire beam profile monitors

retractableFaraday cup

electronmultiplierTOF detector

ion source

Brewsterwindow

Brewster windowFaradaycup

S _ R I Be S

Ion sourceIon opticsCurrent measurementsCo-linearity with laserMass spectrometerLaser couplingVelocity modulation ±5V ~ ±100MHz

Laser Ion Beam Spectrometer

Ground 4kV 2kV

Ion Beam Results Ion density ~5×106 cm-3

Cavity finesse ~450 Lock-in τ=10s

4kV float voltage ±5V modulation ~120MHz

linewidth

2

21'McqV

Ion mass

Float voltage

Positive Column◦ High ion density◦ Simpler setup◦ Direct measurement of

transition rest frequency

Unique Advantages Ion Beam

◦ Rigorous ion-neutral discrimination◦ Simultaneous mass spectroscopy◦ Mass identification of each spectral

line◦ No Doppler-broadened component

of lineshape

Positive Column◦ Mid-IR OPO system

~1W mid-IR idler power Pump and signal lasers referenced to optical frequency

comb◦ Liquid-N2 cooled discharge cell

Ion Beam◦ Mid-IR DFG laser

Ti:Sapph referenced to comb Nd:YAG locked to I2 hyperfine transition

◦ Supersonic expansion discharge source

Current Work

McCall Group◦ Ben McCall◦ Michael Porambo

Funding

Acknowledgements