Measurement of the Sixth Overtone Band of NO Using Cavity-Enhanced Frequency Modulation Spectroscopy

David OsbornJoakim Bood and Andrew McIlroy

Combustion Research FacilitySandia National Laboratories

Livermore, CA 94551USA

Absorption methods provide quantitative measurements

• Measures absolute number densities

• Unaffected by quenching or predissociation

Enhance absorption signal

Reduce background noiseto the quantum limit

S

NmaximizedGoal:

Sensitivity enhancement by an optical cavity

cavLFWHMFSR

F2

The effective absorption length is increased by a factor 2F/the minimum detectable absorption is reduced by 2F/

Lcav = Total cavity losses

• High reflectivity mirrors (R>99.99%)• High cavity finesse (F>10000)• Large number of passes (>10000)• Long effective pathlengths (>1 km)

FSR

FWHM

Optical cavity Cavity modes

Cavity Ringdown Technique

Trigger

R R

L

0

1)(

11

c

• Cavity decay time is a function of the intracavity absorption

• Time-domain measurement avoids laser technical noise very high sensitivity

Large time delay betweenMeasurements of () and 0

Shot-noise limit: the highest possible sensitivity in direct absorption

L = absorption cell pathlength (cm)= absorption coefficient (cm-1)e = electron charge (C)B = detection bandwidth (Hz) = detector responsivity (A/W)P = photon power at detector (W)

P

eBL

2

min

P

e

LBL

LD

21)( min

min (cm-1 Hz-1/2)

• Shot noise = Noise due to quantum fluctuations of the photons arriving at the detector

• The experiment will have the least noise when this is the dominant noise source

• If Beer’s Law is written as:

•Minimum detectable absorption:

LeII 0

Basic Principle and Setup for FM spectroscopy

EOMLaser

Phase shifter

RF Source RF amp.

Low-passfilter

c c

c-m

c+m

Mixer

Signal

Simultaneous comparison betweenon-resonant and off-resonant cases

c-m

c c+m

Sample PD

• High modulation frequency (to decrease N)

• High finesse (to increase S)

Cavity Enhancement + Frequency Modulation

High modulation frequency

No signal!

Low modulation frequency

Technical noise(1/f-noise)

Apparent dilemma:The modulation frequency is limited by the cavity linewidth

No modulation

FM to AMconversion

Empty cavity (FSR = )Transmission is pure FMZero backgroundFrequency noise immune

One mode shifted by moleculesFM unbalanced and signal appears(due to differential change of modes)

FSR

Moleculardispersion

Laser EOM ()

Detection at =FSR

The NICE-OHMS* solutionNoise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy

*J. Ye, L. S. Ma, and J. L. Hall, Opt. Lett. 21, 1000 (1996).

AOM2

PID2

/4

PID3AO

M3

EOM3

EOM2

EOM1

AOM1

541 MHzLock-in

PID1

NICE-OHMSsignal

Laser

4 MHz

Ref

ere

nce

etal

on

High-finesse cavity

/2

OI

PBS/2

PBS1

PBS2 BS

BSPD1

PD2 BS

ML

BD1

BD2

WavemeterBS

PID 4

Electrical

Optical

Experimental Design

Linewidth 1 x 10-7 cm-1

Empty Cavity Parameters

Free spectral range (FSR)

541 MHz

Finesse (F) 20000

Cavity FWHM 27 kHz

Cavity transmission 15%

Effective path length 3.5 km

Balanced detection reduces residual amplitude modulation

Residual amplitude modulation (RAM) background signal

Balanceddetector I1-I2

Bea

m 2

2

Requirements for efficient suppression of RAM:1. Equal amplitudes at the two detectors (balanced)2. The two signals must be in phase

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6 Balanced detection

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6 Single detector

Scans over an NO line

ND filter disc

BS

Beam 1

1

Accidental etalon

Imperfections in EOMAccidental etalons

Calibration measurement in C2H2 Transition: P(11) in the (11 + 33) band

T = 296 KP = 77 mTorr

p = 1.80(7)10-5 cm‑1torr‑1

at 296 K

= 210-11 cm-1Hz-1/2Dmin

= 610-13 cm-1Hz-1/2Dshot

12646.96 12647.00 12647.04 12647.08

-3

0

3

Re

s. (

V)

Wavenumber (cm-1)

-6

-3

0

3

6

Inte

nsi

ty (

V)

12646.96 12647.00 12647.04 12647.08

-1.9

-1.8

-1.7

-1.6

-1.5

-1.4 Exp Fit

Inte

nsi

ty (

V)

Cavity-enhanceddirect absorption

NICE-OHMS

12440 12460 12480 12500 12520 12540 12560

-0.4

-0.2

0.0

0.2

0.4Experiment

FM

Inte

nsi

ty (

arb

. un

it)

Wavenumber (cm-1)

P-branch R-branch

12440 12460 12480 12500 12520 12540 12560

-0.4

-0.2

0.0

0.2

0.4Calculation

FM

Inte

nsi

ty (

arb

. un

it)

Spectrum of NO(7), 21/2 - 21/2 sub-band

Line positions predicted from C. Amiot andJ. Verges, J. Mol. Spec.81, 424 (1980)

12562.92 12562.96 12563.00 12563.04 12563.08

-0.2

0.0

0.2

Re

s. (

V)

Inte

nsi

ty (

V)

Wavenumber (cm-1)

-0.4

0.0

0.4 NICE-OHMS

Exp Fit

Inte

nsi

ty (

V)

-2.1

-2.0

-1.9

-1.8Direct absorption

NO(7 0) spectrum R(7.5) in the 21/2 - 21/2 sub-band

T = 296 KP = 67 Torr

Cavity-enhanceddirect absorption

NICE-OHMS

dTNL

S )(ln1

Line Intensity

S = 1.24(37) x 10-27 cm/molecule = 5.2 x 10-26 cm2/molecule

Sensitivity ~ 1 ppt(parts per ten)

Fitting Intensity Data

Line intensity(cm/molecule)

)()(2

3

8

4

1)(

2

70

22"

3

0

mFmm

m

Q

e

hcmS

rot

kT

E

Honl-Londonfactor

Constants

Boltzmann fraction

vibrational transitiondipole moment

F(m) = 1 + am + bm2

Herman-Wallis factor

|70| = 3.09(47) x 10-6 Debyea = -0.0085(26)b = 0.00125(45)

~

Extracting the Dipole Moment Function

(r - re) = M0 + M1(r-re) + M2(r-re)2 + M3(r-re)3 +…

2'"

2

",' )()()( drrrrrrrR eveevvv

DW = M. Drabbels and A. M. Wodtke, J. Chem. Phys. 106, 3024 (1997).LBP = S. R. Langhoff, C. W. Bauschlicher, and H. Partridge, Chem. Phys. Lett. 223, 416 (1994).

= 0

= 7

= 21

Extracting the Dipole Moment Function

(r - re) = M0 + M1(r-re) + M2(r-re)2 + M3(r-re)3 +… 2'"

2

",' )()()( drrrrrrrR eveevvv

Transient NICE-OHMS

Probelaser

EOM Demod.

Pumplaser

NICE-OHMS is a cw technique

• Pump laser dissociates precursor• Probe laser tuned to an absorption line of the species to study

Monitor complete time evolution of reactant or product

The First Transient NICE-OHMS Signal

-0.04 -0.02 0.00 0.02 0.04 0.06 0.08

-0.02

0.00

0.02

0.04

0.06

0.08

[NH

2] (a

rb.

unit)

Bandwidth = 10 kHz

NICE-OHMS signal

[NH

2] (a

rb.

unit)

Time (sec)

-0.04 -0.02 0.00 0.02 0.04 0.06 0.08

1.9

2.0

2.1

2.2

2.3Direct absorption signal

Monitored NH2 transition

X2B1(170 000) 3313 3422

540.12681~ cm-1

PNH3=10 mTorr, T = 296 K(Single-shot signal)

Cavity-enhanced direct absorption

NICE-OHMS signal

NH2 signals at higher pressures

-0.04 -0.02 0.00 0.02 0.04 0.06 0.08NH

2 si

gnal

(ar

b. u

nit)

Time (sec)

NICE-OHMS signal

-0.04 -0.02 0.00 0.02 0.04 0.06 0.08NH

2 si

gnal

(ar

b. u

nit)

Direct absorption signalMonitored NH2 transition

X2B1(170 000) 3313 3422

540.12681~ cm-1

PNH3=150 mTorr, T = 296 K

Summary

• NICE-OHMS combines FM spectroscopy and cavity enhancement into a single spectrometer that provides ultrahigh sensitivity (210-11 cm-1Hz-1/2 demonstrated).•Balanced detection techniques reduce residual amplitude modulation.

• We have made the first measurements of the (7 0) vibrational band of NO, and determined a new electric dipole moment function.

• A transient species (NH2) has been measured for the first time using NICE-OHMS. These initial results indicate the potential for NICE-OHMS asa powerful chemical diagnostic technique.

Laser Phasemodulator

Demod.

NICE-OHMS

Outline

Motivation Basic concept of cavity-enhanced frequency modulation

spectroscopy (NICE-OHMS) Experimental arrangement Measurement of the 6th overtone of NO Recent results and future projects

- Transient measurement of NH2

- Flame diagnostics Summary

Motivations for ultrasensitive spectroscopy

To probe weak transitions: Molecular spectroscopy – Overtone/combination bands can be explored

– Fundamental tests: forbidden transitions, perturbations, etc.

To measure low concentrations– Trace gas monitoring

– Medical diagnostics

– Chemical kinetics

– Flame chemistry

Example: A• + RH AH + R (1) Rate = k1[A•][RH] 1st order in [A•]

A• + A• products (2) Rate = k2[A•]2 2nd order in [A•]

k1

k2

Acknowledgments

Dr. Joakim Bood and Dr. Andrew McIlroy

Mr. Paul Fugazzi

Mr. Gary Wilke

Dr. Ray Bambha

Dr. Jun Ye (JILA, University of Colorado, and NIST, Boulder)

Dr. Richard Fox (NIST, Boulder)

Financial Support: Division of Chemical Sciences, Geosciences, and Biosciences,

the Office of Basic Energy Sciences of the United States Department of Energy.

• Cavity ringdown provides quantitative measurements

• Problems- Large time delay between on and

off-resonant measurements

- Need large dynamic range to monitor intensity decay

- Low frequency (1/f)

noise dominates

Cavity Ringdown in Flames

Thoman Jr, J.W., McIlroy A., J. Phys. Chem. A 104, 4953 (2000)

130

140

150

160

170

180

23,440 23,450 23,460

To

tal C

av

ity

Lo

ss

(p

pm

/pas

s)

Wavenumber (cm-1)

12CH (1,1) R(8) 12CH (0,0) R(8)12CH (2,2) R(12)

13CH (0,0) R(8)13CH (1,1) R(8)

Backgroundnoise

•Differential measurement•Monitor E field, not intensity•Detect at high frequency

First test of Transient NICE-OHMS: Detection of NH2

Phot

on F

lux

1400013000120001100010000

Wavenumber (cm-1

)

current mirrors

NH2 A X

Flash photolysis using Excimer laser (193 nm):

NH3 H + NH2

Time-resolved FTIR spectrum

Challenges of Transient NICE-OHMS

All cavity fringes will be shifted due to change in refractive index upon photodissociation.

– Instantaneous (sub-ps) number density increase as NH3 H + NH2

– Slower temperature rise as excess energy equilibrates with bath gas. Temperature rise (at constant pressure) lowers density

FSR

P = -800 HzT = 13 kHz

0.5 mTorr NH3, 1mJ/cm2 @ 193 nm1.6 x 1011 molecules/cm3 NH2

before photolysis after photolysis

Experimental DesignC

W 5

32 N

d:Y

AG

CW Ti:Sapphire

PD1

Intensityservo

AOM driver

BS

/4

Opt. Isol.

Etalon (FSR = 300 MHz)

Freq.servo

Function gen. 2, = 4 MHz

PD4

PD2

PD3

To AOMDriver (VCO)

To laser’sRef. cavity

Phase trimmer

3 PZTs

LP

Phase trim.FunctionGenerator 1 = 540 MHz

NICE-OHMS signal

BP

EOM 1

AOMEOM 3

Z EOM 2

AOM

ToEOM3

Lock-in

error

servocontroller

Dither

Function gen.

/2

Laser-to-cavity lock

FSR tracking

Laser-to-cavity locking is a big issue– Heat from the flame will introduce thermal motion to the flame chamber

– The cavity mirror mounts have to be decoupled from the motion

of the flame chamber external stabilization of the mirror mounts is needed

– Laser lock must withstand refractive index changes

– Mirrors of lower reflectivity and cavity locked to laser (instead of vice versa)

• Cavity twice as long as the previously used cavity– Sidebands have to be lined up with cavity modes located

two modes apart from the central mode

Challenges of NICE-OHMSfor flame diagnostics

Low-pressure flame experiment

Invar rod

Bellow

Mirror mount

Mirror mount

FM Spectroscopy ApplicationsFM Spectroscopy demonstrated for detection of stable molecules

Bjorklund, G. C., Opt. Lett. 5, 15 (1980)

Gas-phase kinetics using time-resolved FM spectroscopyNorth et al., Int. J. Chem. Kinet. 29, 127 (1997)

Quantitative concentration measurements in shock tube kinetic experimentsVotsmeier et al., Int. J. Chem. Kinet. 31, 323 (1999)Votsmeier et al., Int. J. Chem. Kinet. 31, 445 (1999)Friedrichs et al., J. Chem. Phys. 4, 5778 (2002)

Deppe et al., Ber. Bunsenges. Phys. Chem. 102, 1474 (1998)Friedrichs and Wagner, Z. Phys. Chem. 214, 1723 (2000)

Chemical kinetics using flash photolysis/CW FM spectroscopyPilgrim et al., J. Phys. Chem. A 101, 1873 (1997)

Flame measurements using Intra-Cavity Laser Absorption SpectroscopyCheskis, S., J. Chem. Phys. 102, 1851 (1995)

Vibrational transition dipole moment for various overtones

0 1 2 3 4 5 6 7 81E-6

1E-5

1E-4

1E-3

0.01

0.1V

ibr.

tra

nsiti

on d

ipol

e m

omen

t (D

ebye

)

v'

Coudert et al. (1995) Mandin et al. (1997) Snels et al. (1999) Lee and Ogilivie (1988) Our measurement

12830

10

v

v

2550

30

v

v

670

50

v

v

Basic Principle and Setup for FM spectroscopy

EOMLaser

Phase shifter

RF Source RF amp.

Low-passfilter

PD

c c

c-m

c+m

Sample

Mixer

c-m

c c+m

Signal

Simultaneous comparison betweenon-resonant and off-resonant cases