Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller

ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET-

COOLED ALKYL PEROXY RADICALS

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

Methyl peroxy is smallest 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

electronic transition is sensitive, species-specific diagnostic

Rotational structure is only partially resolved (congestion due to overlap of different rotational lines and different conformers)

High resolution, rotationally resolved IR CRDS of alkyl peroxy radicals under jet-cooled conditions would be of great value provide molecular parameters to characterize radicals and benchmark

quantum chemistry calculations identify directly spectra of different isomers and conformers

Peroxy Radicals: Motivations

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 = 135 cm

Vacuum Pump

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

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

ℓ

Nd:YAGcw laser

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

DnSRS ~ 200 MHz (limited by power and pressure broadening in H2)

R ~ 99.995 – 99.999% @ 1.3 mm

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 SetupNd:YAG pulse laser

20 Hz, ns, 150 mJ

BBO

BBO, ~ 1.3 mm (NIR), ~ 2 - 3 mJ

DnBBO < 100 MHz (specification of the laser)

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), 220 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, 207 (1996)R.J. Saykally group, Rev. Sci. Instrum. 67, 410 (1996)

Pulsed Supersonic Slit-jet and Discharge Expansion

5 cm

5 mm

10 mm

Electrode Electrode

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

Viton Poppet

CH3O2 (Methyl Peroxy Radical)

In the NIR

CH3O2

a) Jafri et al., J. Am. Chem. Soc. 112, 2586 (1990).b) O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997).

a

b

~

~

~

weak, σ ~ 10-20 cm2/moleculeA state - boundselective

~

cm-1

7000 7200 7400 7600 7800 8000

ab

sorp

tion

/ p

pm

0

100

200

300

400

500

600

Exp

Sim

CRDS Spectroscopy of CD3O2 at RTPredicted tunneling (A/E) splittings (strong dependence upon the mass) for the vibrationless band for CH3O2: 2 – 3 GHz for CD3O2: 100 – 200 MHz

G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, 044310 (2007)C.-Y.Chung, C.-W.Cheng, Y.-P.Lee, H.-S.Liao, E.N.Sharp, P.Rupper, and T.A.Miller, JCP 127, 044311 (2007)

1222

1233

1211

7000 7200 7400 7600 7800 8000

wave numbers / cm-1

0

100

200

300

400

600Experimental Data

SX = 1.1, SA = 1.0

801121

1

801122

280

1

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

~ ~

r0 Q p1 Q

• Cs symmetry → pure c-type transition moment• close to a prolate symmetric top (ΔK ΔJ)• spread out over ~ 30 cm-1

• > 1000 lines, 350 of which due to single transition

K”

S. Wu, P. Dupre, P. Rupper and T. A. Miller, J. Chem. Phys., 127 224305 (2007)

Jet-cooled CRDS Spectrum of CD3O2

- Q branch -• simulation1 using 15 fitted parameters (350 lines have been used in the fit, N up to 10, K up to 4) • H=Hrot+HSR+T00

• T = 15.5 K•linewidth (Voigt profile) 300 MHz Lorentzian → finite lifetime ~ 1.5 ns of electronic transition 300 MHz Gaussian → Doppler plus source linewidth

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

Q Q 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

A 2A’ ← X 2A”~ ~

J”=N”+1/2

J”=N”-1/2p1 r0

+1.5 ppm

Tunneling splitting

cm-1

7370.9 7371.0 7371.1 7371.2 7371.3

pp

m p

er

pass

1

2

3

4

5

A LevelsE Levels

SRS+2 ppm

DFM

• H=Hrot+HSR+HTR+T00

Jet-cooled CRDS Spectrum of CH3O2

Wave numbers / cm-1

7370 7380 7390

ab

sorb

an

ce / p

ass

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Jet-cooled CRDS Spectrum of CH3O2

- Q branch -

Wave numbers / cm-1

7382 7383 7383 7384 7384

abso

rban

ce /

pass

More complicated Spectrum due to the fact that the tunneling effect is of the same order of magnitude that the spin-rotation

C2H5O2 (Ethyl Peroxy Radical)

P.Rupper, E.N.Sharp, G.Tarczay, and T.A.Miller, JPCA 111, 832 (2007)

Cs

C1

T conformer G conformer

ΔEX(T-G) = 81 cm-1 NT/NG=e-6 at 20K

CRDS Spectrum of C2H5O2 at RT

wave numbers / cm-1

ab

sorp

tion

/ p

pm

cm-1

7580 7585 7590 7595 7600

ab

sorp

tion

/pa

ss

0

1

2

3

4

5

Jet-cooled CRDS Spectrum of C2H5O2

- G Conformer -

~ 90 K

~20 K

• able to vary/control the rotational temperature in the jet• non-thermalized conformer population in the jet (T conformer < 0.2 % at 20 K) → Trot Tpopulation

→ conformer not at eq and not at the statistical limit. → conformers are not relaxed Tconf ~ 78 K~ 20 K

~ 90 K

Jet-cooled CRDS Spectrum of C2H5O2

- T Conformer -

3 point smoothing was applied

C2H5O2 T Conformer

Experiment

Simulation withTrot = 90 K

K” = 015

5

rQpQ

- asymmetric rotor with spin-rotation interaction- similarity of rotational constants in ground and excited states- upper and lower spin rotation components (J = N ± 1/2) group together for several N

X A

A 1.0988(5) 1.0666(4)

B 0.1473(2) 0.1477(2)

C 0.1365(2) 0.1364(2)

~ ~

J=N -1/2 J=N +1/2

C3H7O2 (Propyl Peroxy Radical)

Propyl Peroxy Assignments at RT

cm-1

7300 7400 7500 7600 77000

10

20

30

40

ppm

cm-1

7300 7400 7500 7600 7700

ppm

-20

0

20

40

60

80

100

120

140

160

T1T2, T1G2G1G2

G1T2, G1G2'1-propyl peroxy

2-propyl peroxy

G

T

T1G2

(90 cm-1)1-propyl peroxy

2-propyl peroxy

T1T2

(104 cm-1)

G1T2

(27 cm-1)

G1G2

(0 cm-1)

G1’G2

(147 cm-1)

G (0 cm-1) T (149 cm-1)

Tarczay et al., Chem. Phys. Lett. 406, 81 (2005).

*

* CH3O2

→ generality to observe peroxy radicals of this size jet-cooled with high resolution using our CRDS setup

C6H5O2 (Phenyl Peroxy Radical)

Wave numbers / cm-1

7488 7490 7492 7494 7496 7498 7500 7502

abso

rptio

n / p

pm

0

10

20

30

40

Simulation at 20K

Iodobenzene as precursor

CH3O2 (Methyl Peroxy Radical)

In the MIR

Going to the MIR

At the end of 2007 Y.P. Lee group published the observation at room temperature of the fundamental C-H stretch of the methyl peroxy radical using a step FTIR1

We are looking now to obtain the first jet-cooled High Res. Spectrum in the MIR of CH3O2 .

1 Huang et al. JCP 127, 234318 (2007)

cm-1

3000 3001 3002 3003 3004 3005

pp

m p

er

pass

250

300

350

400

450

500

O2 e- On

O2 e- Off

StickPlot vs Simulation

cm-1

2990 3000 3010 3020 3030

ν9 CH3 asν1 CH3 ss

Conclusion and Future Work

We have successfully observed and start to analyze C(H/D)3O2 C2(H/D)5O2 C3H7O2 and C6H5O2.

We have obtain preliminary spectra in the MIR (79 lines) that might belong to CH3O2 and need to be pursued as well as the vibrational transition of other radicals

Aknowledgment Dr Miller The Miller group:

Dr Patrick Rupper (Switzerland) Dr Erin Sharp (JILA) Ming-Wei Chen Dr Dmitry Melnik Dr Philip Thomas Dr Linsen Pei Rabi ChhantyalPun Dr Shenghai Wu (U. of Minnesota)

NSF $$$