High resolution cavity ringdown spectroscopy of jet-cooled deuterated methyl peroxy (CD3O2) in the near IR

Shenghai Wu, Patrick Rupper, Patrick Dupré and Terry A. Miller

Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, Columbus, OH 43210

Alkyl peroxy radicals play a key role as intermediates in oxidation of hydrocarbons (atmospheric as well as combustion chemistry)

Methyl peroxy is simplest alkyl peroxy radical → starting point for spectroscopic characterization

Ambient cell cavity ring-down spectroscopy (CRDS) Several peroxy radicals have been studied in our lab → near IR A – X

electronic transition is sensitive, species-specific diagnostic Rotational structure is only partially resolved (congestion due to

different rotational lines and different conformers)

High resolution, rotationally resolved IR CRDS of methyl peroxy under jet-cooled conditions would be of great value

Jet-cooled Peroxy Radicals (CD3O2) Motivations

~~

Strong, but non-selective electronic transition in the UV (B 2A’’ – X 2A’’)

Weak, very selective transition in the near IR (A 2A’ – X 2A’’)

Room temperature CRDS (pulsed1 and cw2) spectrum at moderate resolution (photolysis of acetone) → overlap of several rotational transitions (congestion), no spin-rotational structure resolvable

Negative-Ion PES3 (instrument resolution 40 cm-1)

Recently ionization detection techniques: TOFMS with moderate resolution laser (supersonic jet expansion4 and effusive beam5) → parent cation only stable for CH3O2

Methyl Peroxy: Spectroscopic Background

1 Miller group and Y. P. Lee group (JCP 112 (2000) 10695, JCP (2007)) 2 D. B. Atkinson, J. L. Spillman, JPCA 106 (2002) 8891

3 G. B. Ellison, M. Okumura and coworkers, JACS 123 (2001) 9585 4 Bernstein group (H. B. Fu, Y. J. Hu, and E. R. Bernstein, JCP 125 (2006) 014310) 5 G. Meloni, P. Zou, S. J. Klippenstein, M. Ahmed, S. R. Leone, C. A. Taatjes, and

D. L. Osborn, J. Am. Chem. Soc. 128 (2006) 13559

~~

~ ~

Ti:Sa ringcw laser

Ti:Sa Amplifier

(2 crystals)

Nd:YAG pulse laser

Raman Cell

PD

InGaAsDetector

Ring-down cavity with slit-jet(absorption length ℓ = 5 cm)

L = 67 cm

Vacuum Pump

1 m single pass, 13 atm H2730 - 930 nm, ~ 1 MHz

50 - 100 mJ ~ 8 - 30 MHz (FT limited)

ℓ

Nd:YAGcw laser

1st Stokes, ~ 1.3 m (NIR), ~ 2 mJ

SRS ~ 220 MHz (limited by power and pressure broadening in H2) ΔDoppler (slit jet) ~ 250 MHz

R ~ 99.995 – 99.999% @ 1.3 m

SRS (stimulated Raman scattering)

20 Hz, ns, 350 mJ

slit-jet: longer absorption path-length less divergence of molecular density in the optical cavity

S. Wu, P. Dupré and T. A. Miller, Phys. Chem. Chem. Phys. 8 (2006) 1682

P. Dupré and T. A. Miller, Rev. Sci. Instrum. 78 (2007) 033102

Experimental Setup

IR Beam

9 mm

-HV

• radical densities of 1012 - 1013 molecules/cm3 (10 mm downstream, probed)• rotational temperature of 15 - 30 K• plasma voltage ~ 500 V, I 1 A (~ 400 mA typical), 100 µs length• dc and/or rf discharge, discharge localized between electrode plates, increased signal compared to longitudinal geometry

Previous similar slit-jet designs:D.J. Nesbitt group, Chem. Phys. Lett. 258 (1996) 207R.J. Saykally group, Rev. Sci. Instrum. 67 (1996) 410

Pulsed Supersonic Slit-jet and Discharge Expansion

5 cm

5 mm

10 mm

Electrode Electrode

carrier gas (300 – 700 Torr Ne) + precursor CD3I (1%) and O2 (10%)

Viton Poppet

0 leq αmin Δ(αl)√Δt(ringdown

time)(absorption equivalent

length)

(experimental sensitivity (standard deviation in

one pass absorbance))

(noise equivalent absorption)

300 µs

6 km 0.02 ppm 4.5 ppb Hz-1/2

Main reactions in our discharge environment

CD3I → CD3 + I (discharge)CD3 + O2 + [M] → CD3O2 + [M]

CD3 + CD3 → C2D6

O2 → 2 O (discharge)CD3 + O → D2CO + DCD3 + CD3O2 → 2 CD3O2 CD3O2 → products

Hamiltonian

2 2,

,

2,1

( )2

Rot

a b c

SR

a b c

H H

AN BN CN

H

N S S N

• Unpaired electron → coupling of electron spin with molecular rotation• Hund’s case b (absence of external magnetic fields, ignoring hyperfine interaction)• Structure due to internal rotation (tunneling) not resolvable for CD3O2

→ consider only overall rotation (asymmetric rigid rotor) and spin-rotation

N = rotational angular momentum S = electron spin angular momentumA,B,C = rotational constants in the principal axes of inertia (a,b,c) = reduced spin-rotation tensorial components (Cs symmetry → 4 independent components)~

~

Spectra of CD3O2 (RT and Jet-cooled)

room temperature

jet-cooled

• Cs symmetry → pure c-type transition moment

• close to a prolate symmetric top (ΔKaΔJKa”)

• [CD3O2] = 3.7(3.1) x 1012 cm-3 → interplay between RT (cross-section) and jet-cooled investigation

Jet-cooled CRDS Spectrum of CD3O2

• 10 % O2 and ~ 1% CD3I in Ne• dc discharge: 350 mA• stepsize: 50 MHz, • RD time average: 4

A 2A’ ← X 2A”, vibrationless band 000

~ ~

rQ0 pQ1

pQ2

pQ3

rQ1

rQ2

• spread out over ~ 30 cm-1

• > 1000 lines, 350 of which due to single transition• small background (0.2 ppm) → due to precursor molecule

Jet-cooled CRDS Spectrum of CD3O2

A 2A’ ← X 2A” • simulation1 using 15 fitted parameters • T = 15.5 K• Voigt profile (Lorentzian 330 MHz, Gaussian 390 MHz)• vast majority could be simulated (rP1 enhanced at 7373.2 cm-1)• spin-rotation doublet well resolved • opposite extension of J progression (B,C rotational constants)

1SpecView simulation package, V.L.Stakhursky, T.A.Miller, 56th Symposium, 2001

pQ1 rQ0 J”= 0.5 5.5 10.5

J”=1.5 5.5 10.5

10.5 5.5

1.55.5

0.5

10.5

~ ~

J”=N”+1/2

J”=N”-1/2

Jet-cooled CRDS Spectrum of CD3O2

A 2A’ ← X 2A”, pP1 band

J’’=1.5J’’=1.5

~ ~

two other branches in this region (not labelled)

pQ2 around 7370 cm-1

rP0 around 7371.5 cm-1

J”=N”-1/2J”=N”+1/2

Assignment of the CRDS Spectrum

Experimental spectra1% CD3I + 10% O2 in Ne diluent

300 mA discharge current

0

2

4

6

8

7360 7365 7370 7375 7380 7385

wave numbers / cm-1

ppm

– K = 1– K = -1|K”| = 0

|K”| = 1

|K”| = 2

|K”| = 3

|K”| = 4

Experiment

Simulations at T = 15 K

Q

QP

PR

R

Spectroscopic Constants of CD3O2

cm-1 X A

A 1.296 1.182

B 0.332 0.340

C 0.290 0.288

~ ~

1 M. B. Pushkarsky, S. J. Zalyubovsky, T. A. Miller, JCP 112 (2000) 10695

7373.2739(15)T00

-0.0029(15)-0.0003(15)εcc

1.1781(10)1.2932(10)A

0.32707(11)0.32079(11)B

0.28387(11)0.28546(11)C

0.0695(15)-0.0718(15)εaa

0.0107(14)-0.0091(14)εbb

0.0218(22)0.0138(22)|εba+εab|/2

AX

CD3O2cm-1

~ ~

~

~

~

~~

• 350 lines have been used in the fit (with N up to 10 and K up to 4)

• Standard deviation of the fit is 0.0018 cm-1

• T00 = 7373.2739(15) cm-1 is consistent with the value determined from room temperature1 spectra, i.e., T00 = 7372.6(8) cm-1

• Rotational constants in remarkable good agreement with those derived from room temperature spectra and also from ab initio (< 6 %)

→ benchmark calculations

• |ε”aa| |ε’aa| only change in sign, εbb, εcc are small, in agreement with c-type transitions

• ε << A,B,C → spectrum dominated by rotational structure

• 350 lines have been used in the fit (with N up to 10 and K up to 4)

• Standard deviation of the fit is 0.0018 cm-1

• T00 = 7373.2739(15) cm-1 is consistent with the value determined from room temperature1 spectra, i.e., T00 = 7372.6(8) cm-1

• Rotational constants in remarkable good agreement with those derived from room temperature spectra and also from ab initio (< 6 %)

→ benchmark calculations

• |ε”aa| |ε’aa| only change in sign, εbb, εcc are small, in agreement with c-type transitions

• ε << A,B,C → spectrum dominated by rotational structure

~ ~

~

~ ~

Comparison between CD3O2 and CH3O2

See Patrick Dupré’s talk (RF10)

Conclusions

Studied A – X vibrationless transition (weak) of deuterated methyl peroxy radical (CD3O2) via pulsed CRDS in the near IR (1.36 µm)

Observed the spectrum under jet-cooled conditions (Trot ~ 15 K) by combining a narrow-bandwidth laser source (~ 220 MHz) with a supersonic slit-jet expansion and electric discharge (dc or rf)

Rotational and spin-rotational structure have been resolved in the spectra and corresponding spectroscopic constants (15 for ground and excited state) were determined

~ ~

Acknowledgments

Prof. Terry A. Miller Shenghai Wu, Patrick Dupré Gabriel Just and Prof. Anne B. McCoy Miller group members Machine shop: Jerry Hoff, Larry Antal, Joshua Shannon

NSF for funding

Peroxy Transition

a//

a/

y

x

- 2

- 4

- 6

- 8

- 10

eVO2CH3O2CH3

O2 : a1g - X3g

A2A/~ X2A//~

CH3O2 :

RO2 - R perturbation

HOMO (non bonding on O atoms, in plane)

SOMO (antibonding (*), out of plane)

Cavity Ringdown Absorption Spectroscopy

R

L

A = Nσl

A = L/cτabsorber - L/cτ0

Time

Inte

nsit

y

0

absorber

τabsorber

lNσ+= cL )/(

R1-( )0τ cL )/(

R1 -=

CRDS: - absorption technique with good sensitivity- immune to the noise caused by source fluctuations- absolute determination of the absorption cross-section

l

Direct Absorption Measurement

t (s)

Tra

nsm

issi

on s

igna

l

0 100 200 300

0.0

0.5

1.0

0t

e

t

e

0min

0

1

( ) ( )c

0

( )( )

L Ll

c c

0 ln (1 )

L L

c R c R

0 = ring-down time for empty cavity() = ring-down time in presence of absorbing medium = absorption, min = minimum detectable absorptionL = length of cavity, l = medium absorption length R = mirror reflectivity, c = speed of light

Non-exponential Decay

laser > = Doppler

The beginning of the decay reflects the medium absorption The end of the decay reflects the empty cavity absorption Ringdown time depends on angular frequencies of the incoming EM field

The non-linear response of the absorption medium The absorption is saturated at the very beginning of the decay The later part of the decay is approximated by the linear absorption

The chemical or physical dynamics faster than Multi-exponential decay To analyze the decay as a function of time

Number Density

/ 4 ln 2peakI

obsI I

AbsN

min

min

/ 4 ln 2

/obs

I

NN

S N

Observed number density:

Minimum detectable number density:

AbsI observed rotational (or vibrational) integrated absorption per pass, i.e. the integrated area of the spectral line (or spectral band) integrated absorption cross-section, ( Aref() d / Aref(0)) * peak

min/ ( ) /( )peakS N

example CH3

I = 2.1 x 10-19 cm/moleculel = 5 cmAbsI = 13 x 10-6 cm-1 → Nobs = 1.2 x 1013 cm-3

Rotation-vibration Hamiltonian

2

2 2 2

,

( ) ( )

1( )

2a b c

R SR

H F p N V

AN BN CN N S S N

H H

HR: rotational Hamiltonian (asymmetric top, rigid rotor)

HRT: spin-rotation Hamiltonian

z, a

p = internal rotation angular momentum

= internal rotation angle

F = internal rotational constant

A,B,C = rotational constants

N = rotational angular momentum

= axis of internal motion

Structure due to internal rotation not resolvable for CD3O2 → consider only overall rotation (asymmetric rigid rotor) and spin-rotation

Nuclear spin statistics: 11:16 for A and E levels

CH3OO - X-state

(degrees)

-150 -100 -50 0 50 100 150

V( )

(cm-1 )

0

100

200

300

400

500

600CH3OO - A-state

(degrees)

-150 -100 -50 0 50 100 150

V( )

(cm-1)

0

200

400

600

800

1000

1200

CH3O2: Internal Rotation

5E

4E

4A1

3A2

0E, 0A1

1E, 1A2

• barrier to internal rotation around the C-O bond is substantially higher in the A than the X state (~1100 cm-1 vs. ~300 cm-1)• small methyl torsional frequencies in the ground state (132 cm-1 for CH3O2 and 107 cm-1 for CD3O2): location of vibrational levels lead to typical sequence and also some atypical transitions

~~

IR Range Coverage from Ti:Sa

Ti:Sa ringcw laser

Ti:Sa Amplifier

(2 crystals)

Nd:YAG pulse laser

Raman Cell (H2)

PD

InGaAs orInSbDetector

Ring-down cavity with slit-jet(absorption length ℓ = 5 cm)

L = 67 cm

Vacuum Pump

1 m single pass730 - 930 nm, ~ 1 MHz

50 - 110 mJ ~ 10 - 30 MHz

ℓ

Nd:YAGcw laser

1st Stokes, ~ 1.3 m (NIR), ~ 2 mJ2nd Stokes, ~ 3 m (MIR), ~ 200 JSRS: ~ 180 MHz @ NIR ~ 220 MHz @ MIR (limited by power and pressure broadening in H2)

R ~ 99.995% @ 1.3 m ~ 99.96% @ 3 m

SRS (stimulated Raman scattering)

ns, 350 mJ

slit-jet: longer absorption path-length less divergence of molecular density in the optical cavity

Doppler (slit jet) ~ 100 MHz @ MIR ~ 200 MHz @ NIRD. Anderson, S. Davis, T. Zwier andD. Nesbitt, Chem. Phys. Lett. 258, 207 (1996)

S. Wu, P. Dupré and T. A. Miller, Phys. Chem. Chem. Phys. 8 (2006) 1682

P. Dupré and T. A. Miller, Rev. Sci. Instrum. 78 (2007) 033102

Experimental Setup

High-Resolution Ti:Sa Laser System

quadrupole amplification (in a seededcavity)

Longitudinal Discharge

- HV

-HVDesign adapted from:D.J. Nesbitt group Chem. Phys. Lett. 258 (1996) 207R.J. Saykally groupRev. Sci. Instrum. 67 (1996) 410

Discharge Expansions

Transversal Discharge

Characteristics of CRDS setup

A 2A' - X 2A" Transition of CH3O2 and CD3O2

~ ~

Jet-cooled CRDS Spectrum of CD3O2

A 2A’ ← X 2A”, rR0 band~ ~

3.5

J”=2.5

other branch in this region (not labelled)→ rR1

J”=N”-1/2

J”=N”+1/2

Methyl peroxy - CH3O2

Wave number / cm-17000 7500 8000 8500 9000

Lo

ss p

er

pa

ss / p

pm

0

200

400

600

800

C.-Y. Chung et al., JCP, accepted, 2007

COO bend

801

O-O stretch

Nv’’v’

701

origin

000

1222

1211

methyl torsion

A 2A' - X 2A" Transition of CH3O2 Under Ambient Conditions (RT)

~ ~

Rotational Contour of the Origin Band of CH3O2 (RT)

7375

- overlap of several rotational transitions

- no spin-rotational structure resolvable

- high-resolution spectra of methyl peroxy under jet-cooled conditions would be of great value

Rotational Temperature

• we can influence the rotational temperature in the jet by precursor seeding concentration• we can vary Trot by a factor of two (15 K to 28 K)• “over-seeding” increases density number, which is correlated to a background level increase

Trot ~ 28 K

Trot ~ 15 K

Rotational Temperature

• 350 lines have been used in the fit (with N up to 10 and K up to 4)

• Standard deviation of the fit is 0.0018 cm-1

• T0 = 7373.2739(15) cm-1 is consistent with the value determined from room temperature1 spectra, i.e., T0 = 7372.6(8) cm-1

• Rotational constants in remarkable good agreement with those derived from room temperature spectra

• Linewidth (Voigt profile) 330 MHz Lorentzian → finite lifetime ~ 1.5 ns of electronic transition 390 MHz Gaussian → Doppler (~ 300 MHz) plus source (~ 250 MHz) linewidth

• 350 lines have been used in the fit (with N up to 10 and K up to 4)

• Standard deviation of the fit is 0.0018 cm-1

• T0 = 7373.2739(15) cm-1 is consistent with the value determined from room temperature1 spectra, i.e., T0 = 7372.6(8) cm-1

• Rotational constants in remarkable good agreement with those derived from room temperature spectra

• Linewidth (Voigt profile) 330 MHz Lorentzian → finite lifetime ~ 1.5 ns of electronic transition 390 MHz Gaussian → Doppler (~ 300 MHz) plus source (~ 250 MHz) linewidth

Kinetics

- detection capability of produced molecules in the MIR (CH3, C2H6, H2CO) and NIR (CH3O2) - using literature absorption cross-sections and experimental absorbances → molecular densities- using literature reaction rate constants for kinetics studies

Reaction time / µs

Den

sity

num

ber

/ 10

14 c

m-3

[CD3O2] = 3.7(3.1) x 1012 cm-3 (probe region)

CH3CH3

C2H6C2H6

H2CO

CH3O2

no oxygen presentwith oxygen present