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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
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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
~~
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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
~~
~ ~
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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
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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
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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)~
~
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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
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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
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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
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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
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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
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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
~ ~
~
~ ~
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Comparison between CD3O2 and CH3O2
See Patrick Dupré’s talk (RF10)
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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
~ ~
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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
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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)
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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
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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
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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
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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
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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
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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
~~
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IR Range Coverage from Ti:Sa
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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
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High-Resolution Ti:Sa Laser System
quadrupole amplification (in a seededcavity)
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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
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Characteristics of CRDS setup
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A 2A' - X 2A" Transition of CH3O2 and CD3O2
~ ~
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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
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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)
~ ~
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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
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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
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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
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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