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Gabriel M. P. Just, Patrick Rupper, Dmitry G. Melnik and Terry A. Miller
EXPERIMENTAL PROGRESS FOR HIGH RESOLUTION CAVITY
RINGDOWN SPECTROSCOPY OF JET-COOLED REACTIVE INTERMEDIATES
Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry)
Peroxy Radicals: Motivations
Atmospheric and Combustion interest
The low temperature combustion of hydrocarbons is a critical process in the overall degradation of our atmosphere quality leading to the formation of the peroxy radicals which, by reacting with the NO radical upset the NO NO2 balance and leads to the formation of O3 in the troposhere.
The formation of peroxy radicals is believed to be partially responsible for the negative temperature coefficient (NTC) behavior of hydrocarbon combustion observed from 550-700 K.
Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry)
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)
Peroxy Radicals: Motivations
Alkyl peroxy radicals play a key role as intermediates in the oxidation of hydrocarbons (atmospheric as well as combustion chemistry)
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
Cavity Ringdown Spectroscopy
R
L
A = σ Nl
A = L/cτabsorber - L/cτ0
Time
Intensity
0tabsorber
Sensitivity of Technique:If R = 99.999% and L = 135 cm
then τ0 = 550 µs
Leff = 165.0 km ~ 100 Miles ~ Columbus – Cleveland
l = 5 cmleff = 6.1 km
τabs
σ Nl+= cL )/(
R1-( )
τ0
cL )/(R1 -
=
l
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
Spectra improvement
It is known that the methyl peroxy radical (CH3O2) has a tunneling splitting which is due to the methyl torsion1. This tunneling splitting was estimated to be about 2-3 GHz for CH3O2 and about 200 MHz for CD3O2
1G.M.P.Just, A.B.McCoy, and T.A.Miller JCP 127, 044310 (2007)
cm-1
7000 7200 7400 7600 7800 8000
ab
sorp
tion
/ p
pm
0
100
200
300
400
500
600
CRDS Spectroscopy of CD3O2 at RT
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
600
801121
1
801122
280
1
000
cm-1
7365 7370 7375 7380
pp
m p
er
pa
ss
-2
0
2
4
6
8
SRS
DFM
CD3O2 using DFM
cm-1
7373.4 7373.6 7373.8 7374.0 7374.2
pp
m p
er
pa
ss
-2
0
2
4
6
8
SRS
DFM
More characterization of the laser source For characterization purposes and more
importantly spectroscopic purposes, we decided to change frequency range in order to go to the MIR using DFM by using not a BBO crystal but a LiNbO3 crystal and the fundamental of a Nd:YAG laser
cm-1
3000 3005 3010 3015 3020
ppm
per
pas
s
200
400
600
800
1000
1200
1400
1600
MIR Linewidth CH3I Absorption
cm-1
3006.8 3007.0 3007.2 3007.4 3007.6 3007.8
ppm
per
pas
s
200
400
600
800
1000
1200
1400
cm-1
7287.0 7287.2 7287.4 7287.6 7287.8 7288.0 7288.2
ppm
per
pas
s
0
50
100
150
200
250
300
350
62 MHz
71 MHz 67 MHz
65 MHz
82 MHz
83 MHz
75 MHz
147 MHz
142 MHz
212 111
221 110
Estimating the source linewidth
2
2
2,2
22,
2
42.2
)(2
SourceNIRDoppler
MIR
SourceNIRDopplerNIR
NIR MIR
ΔνDoppler 128 MHz 53 MHz
ΔνSource 69 MHz 49 MHz
Conclusion and Future Work
We can obtain an experimantal linewidth of about 145 MHz in the NIR and of about 70 MHz in the MIR (nearly Doppler limited).
The improvement in linewidth (from 250 MHz for SRS to 145 MHz width DFM in the NIR) allowed us to resolve the tunneling splitting in CD3O2 which wasn’t the case using SRS.
From these investigation, we can estimate that our source linewidth is about 69 MHz in the NIR and 49 MHz in the MIR
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 $$$
