1. ANALYSIS OF THE CAVITY RINGDOWN SPECTRA OF THE SMALLEST JET-COOLED ALKYL PEROXY RADICALS
Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller

2. Peroxy Radicals: Motivations
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

3. Ring-down cavity with slit-jet
Experimental Setup
20 Hz, ns, 150 mJ
Nd:YAG pulse laser mJ
Dn ~ MHz
(FT limited)
20 Hz, ns, 350 mJ
Nd:YAG pulse laser
Ti:Sa
Amplifier
(2 crystals)
SRS (stimulated Raman scattering)
nm,  ~ 1 MHz
1 m single pass, 13 atm H2
Nd:YAG
cw laser
Ti:Sa ring
cw laser
Raman Cell
BBO
P. Dupré and T. A. Miller,
Rev. Sci. Instrum. 78 (2007)
BBO, ~ 1.3 mm (NIR), ~ mJ
DnBBO < 100 MHz
(specification of the laser)
Ring-down cavity with slit-jet
(absorption length ℓ = 5 cm)
L = 135 cm
1st Stokes, ~ 1.3 mm (NIR), ~ 2 mJ
DnSRS ~ 200 MHz
(limited by power and pressure
broadening in H2) ℓ PD
InGaAs
Detector
R ~ – 1.3 mm
Vacuum Pump
S. Wu, P. Dupré and T. A. Miller,
Phys. Chem. Chem. Phys. 8 (2006) 1682
slit-jet: longer absorption path-length
less divergence of molecular density in the optical cavity

4. Pulsed Supersonic Slit-jet and Discharge Expansion
carrier gas (300 – 700 Torr Ne)
+ precursor RI (1%) and O2 (10%)
Viton Poppet
9 mm
5 mm
Electrode
Electrode
-HV
10 mm
5 cm
IR Beam
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)
radical densities of molecules/cm3 (10 mm downstream, probed)
rotational temperature of 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

5. CH3O2 (Methyl Peroxy Radical)
In the NIR

6. CH3O2 weak, σ ~ 10-20 cm2/molecule A state - bound selective ~ ~ b ~ ~
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).

7. CRDS Spectroscopy of CD3O2 at RT
100
200
300
400
600
Experimental Data
SX = 1.1, SA = 1.0
Predicted tunneling (A/E) splittings (strong dependence upon the mass) for the vibrationless band
for CH3O2: 2 – 3 GHz
for CD3O2: 100 – 200 MHz
1211
1222
801
Exp
1233
Sim
7000
7200
7400
7600
7800
8000
wave numbers / cm-1
G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, (2007)
C.-Y.Chung, C.-W.Cheng, Y.-P.Lee, H.-S.Liao, E.N.Sharp, P.Rupper, and T.A.Miller, JCP 127, (2007)

8. Jet-cooled CRDS Spectrum of CD3O2
A 2A’ ← X 2A”, vibrationless band 000 ~ r0 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” p1 Q
10 % O2 and ~ 1% CD3I in Ne
dc discharge: 350 mA
stepsize: 50 MHz
RD time average: 4
S. Wu, P. Dupre, P. Rupper and T. A. Miller, J. Chem. Phys., (2007)

9. Jet-cooled CRDS Spectrum of CD3O2 - Q branch -
A 2A’ ← X 2A” ~
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 p1 r0 Q Q
J”=N”-1/2
J”= 0.5
5.5
10.5
10.5
5.5
0.5
J”=1.5
5.5
10.5
10.5
5.5
1.5
J”=N”+1/2
+1.5 ppm
1SpecView simulation package,
V.L.Stakhursky, T.A.Miller, 56th MSS Symposium, 2001

10. Jet-cooled CRDS Spectrum of CD3O2 - P branch -
J”=N”+1/2
J”=N”-1/2
J’’=1.5
J’’=1.5
two other branches in
this region (not labelled)
p2Q around 7370 cm-1
r0P around cm-1
+1.5 ppm
A 2A’ ← X 2A”, p1P band ~

11. Tunneling splitting
H=Hrot+HSR+HTR+T00

12. Jet-cooled CRDS Spectrum of CH3O2

13. Jet-cooled CRDS Spectrum of CH3O2 - Q branch -
More complicated
Spectrum due to the
fact that the tunneling
effect is of the same
order of magnitude that
the spin-rotation

14. C2H5O2 (Ethyl Peroxy Radical)

15. CRDS Spectrum of C2H5O2 at RT
P.Rupper, E.N.Sharp, G.Tarczay, and T.A.Miller, JPCA 111, 832 (2007) Cs C1
T conformer
G conformer
absorption / ppm
ΔEX(T-G) = 81 cm-1 NT/NG=e-6 at 20K
wave numbers / cm-1

16. Jet-cooled CRDS Spectrum of C2H5O2 - G Conformer -
~ 90 K
~20 K

17. Jet-cooled CRDS Spectrum of C2H5O2 - T Conformer -
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
~ 90 K
~ 20 K
3 point smoothing was applied

18. C2H5O2 T Conformer pQ rQ Experiment Simulation with Trot = 90 K K” = 01
J=N -1/2
J=N +1/2 rQ
Experiment
Simulation with
Trot = 90 K
~ ~ X A
1.0988(5)
1.0666(4) B
0.1473(2)
0.1477(2) C
0.1365(2)
0.1364(2)
- 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

19. C3H7O2 (Propyl Peroxy Radical)

20. Propyl Peroxy Assignments at RT
T1G2
(90 cm-1)
1-propyl peroxy
T1T2
(104 cm-1)
* CH3O2
G1G2
(0 cm-1) *
G1T2
(27 cm-1)
G1’G2
(147 cm-1)
2-propyl peroxy
T (149 cm-1)
G (0 cm-1)
→ generality to observe peroxy radicals of this size
jet-cooled with high resolution using our CRDS setup
Tarczay et al., Chem. Phys. Lett. 406, 81 (2005).

21. C6H5O2 (Phenyl Peroxy Radical)

23. CH3O2 (Methyl Peroxy Radical)
In the MIR

24. 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, (2007)

26. ν9 CH3 as
ν1 CH3 ss

27. 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

28. Aknowledgment Dr Miller The Miller group: NSF $$$
Dr Patrick Rupper (Switzerland)
Dr Erin Sharp (JILA)
Ming-Wei Chen
Dr Dmitry Melnik
Dr Philip Thomas
Dr Linsen Pei
Rabi ChhantyalPun
Dr Shenghai Wu (U. of Minnesota)
NSF $$$