Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCallChemistry Department, University of Illinois at Urbana-Champaign

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

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

Ion Spectroscopy Techniques

Ion-neutral discrimination

Low rotational temperature

Narrow linewidth

Compatible with cavity-enhanced spectroscopy

VelocityModulation

Supersonic

Expansion

Hollow Cathode

High ion column density

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

Optical cavity acts as a multipass cell◦ Number of passes =◦ For finesse of 300, get ~200 passes

Must actively lock laser wavelength/cavity length to be in resonance with one another

DC signal on detector is extremely noisy◦ Velocity modulation with lock-in amplifier

minimizes effect of noise on signal detection

Cavity Enhanced Absorption Spectroscopy (CEAS)

Laser

CavityDetector

CavityTransmission

Error Signal

Pound-Drever-Hall Locking

Ti:Sapph Laser

EOM

PZT

Lock Box

30MHz

Detector

Detector

AOM

PolarizingBeamsplitter

QuarterWave Plate

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

rpti

on S

trength

(A

rb.

Unit

s)

Relative Frequency (GHz)

V (kV)

t (μs)

Extracting N2+ Absorption Signal

Abso

rpti

on

Relative Frequency

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 & Accuracy0

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

Large Signal Small Noise

Cavity Enhancement

Heterodyne Spectroscopy

NICE-OHMS

Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy

NICE-OHMS

Cavity Modes

Laser Spectrum

Experimental Setup

Ti:Sapph Laser

EOM

PZT

Lock Box

30MHz

Detector

Detector

AOM

PolarizingBeamsplitter

QuarterWave Plate

Experimental Setup

Ti:Sapph Laser

EOM

PZT Detector

Lock-In Amplifier

Signal

40 kHzPlasma

Frequency

Experimental SetupTi:Sapph Laser

EOM

PZT Detector

EOM

N × Cavity FSR(113 MHz) N oise

I mmuneC avityE nhanced-O pticalH eterodyneV elocityM odulationS pectroscopy

NICE-OHVMS

• Sidebands spaced at 9 cavity FSRs (1 GHz)• 3rd derivative-like Doppler lineshape• Lamb dips from each laser frequency and

combination of laser frequencies

0

1

2

3

See talk MI10 for more thorough analysis

NICE-OHVMS

• Retain ion-neutral discrimination

N2* N2

+

Velocity Modulation Techniques

Increased path length through plasma Better sensitivity due to heterodyne

modulation Retained ion-neutral discrimination Sub-Doppler resolution due to optical

saturation◦ 50 MHz Lamb dip widths◦ Resolve blended lines◦ Better precision & absolute accuracy with comb

NICE-OHVMS Summary

McCall Group◦ Ben McCall◦ Andrew Mills◦ Michael Porambo

Funding

Acknowledgements