Cavity-Enhanced Velocity Modulation Spectroscopy

Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCallUniversity 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

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

Heterodyne (FM) Spectroscopy

Laser

RF

FM Laser

+-

Most environmental noise scales like 1/f Velocity modulation is ~40kHz

◦ Much better than direct DC detection◦ Still room for improvement

Frequency Modulation (FM) ◦ Modulate laser frequency at RF (≳100MHz)◦ Demodulate detector signal

RF Carrier

Audio

FM Signal

Single-pass direct absorption

Single-pass Heterodyne @ 1GHz

Velocity Modulation of N2+

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

EOMPZT

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

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).

N2+

◦ Velocity directly dependent on voltage◦ No significant phase shift with respect to voltage

N2*

◦ 78° phase shift with respect to N2+ signal

◦ Peak N2* density occurs when rate of formation

equals rate of destruction

Phase Analysis

V (kV)

t (μs)

Peak N2*

Density

N2+

◦ Velocity directly dependent on voltage◦ No significant phase shift with respect to voltage

N2*

◦ 78° phase shift with respect to N2+ signal

◦ Peak N2* density occurs when rate of formation

equals rate of destruction◦ Analogous to Earth’s heating/cooling cycle with the

sun Sun is brightest at noon (peak voltage and N2

+ velocity) Hottest time of day is 5pm (peak N2

* density) 5 hour time delay in 24 hour day = 75° phase shift

Phase Analysis

Line centers determined to within 1 MHz with optical frequency comb

Precision & Accuracy

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

Support for Herschel & Sofia THz observatories

Indirect Terahertz Spectroscopy

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

0

1

2

3

4J’

cm-1

cm-1

J”

IR Transitions

Even Combination differences

Odd Combination Differences

1-0 Rotational Transition

Reconstructed Rotational Transitions

Indirect Terahertz Spectroscopy

Sensitivity Limited by Plasma Noise

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

NICE-OHMS

• 3rd derivative Doppler lineshape• Lamb dips from each laser frequency

Direct Absorption Heterodyne

SinglePass

CavityEnhanced

Velocity Modulation Techniques

McCall Group Funding

◦ Air Force◦ NASA◦ Dreyfus◦ Packard◦ NSF◦ Sloan

Acknowledgements