248-nm laser photolysis of chbr3/o-atom mixtures: … a: approved for public release, distribution...
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Ghanshyam L. VaghjianiAir Force Research Laboratory
AFRL/PRSP10 E Saturn Blvd
Edwards AFB, CA 93524, USATel: 661 275 5657Fax: 661 275 6245
Email: [email protected]
248-nm Laser Photolysis of CHBr3/O-atom Mixtures:Kinetic Evidence for UV CO(A)-Chemiluminescence
in the Reaction of Methylidyne Radicals WithAtomic Oxygen
Distribution A: Approved for public release, distribution is unlimited
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1. REPORT DATE APR 2005 2. REPORT TYPE
3. DATES COVERED -
4. TITLE AND SUBTITLE 248-nm Laser Photolysis of CHBr3/O-atom Mistures: Kinetic Evidencefor UV CO(A)-Chemiluminescence in the Reaction of MethylidyneRadicals with Atomic Oxygen
5a. CONTRACT NUMBER
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) Ghanshyam Vaghjiani
5d. PROJECT NUMBER 2303
5e. TASK NUMBER 0423
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Air Force Research Laboratory (AFMC),AFRL/PRSP,10 E. SaturnBlvd.,Edwards AFB,CA,93524-7680
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12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
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15. SUBJECT TERMS
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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18
Distribution A: Approved for public release, distribution is unlimited
• Strong Emissions From:• O(1D), O(1S)• NH(A)• OH(A)• CO(a)
• Cause of Chemiluminescence:• Plume-Atmospheric Interactions
• Source Chemistry:• Precursors? & Formation?• O-atom Reactions• Other Reactions
• Space Experiment
AFRL’s Motivation
Observation Platforms Thrusters
Space Shuttle Space Shuttle Mir Space Station Progress-M MSX Soyuz-TM
• Spacecraft Atmospheric Interactions• Chemiluminescent Combustion Processes
Distribution A: Approved for public release, distribution is unlimited
UV/Vis Plumes
Spectral DataRadiance Data
⇔ Plume Data ⇔
Laboratory Studies
⇔⇔
Chemiluminescent Processes
Identify Spacecraft Atmospheric Interactions
⇔
Modeling Studies
⇔
Distribution A: Approved for public release, distribution is unlimited
Proposed CO(a) Source Chemistry
• Unreacted Fuel → → Precursor(s)
• Precursor(s) + O → Products
→ CO(a(v’)) + R
→ CH2 + OR’
→ CH + OR’’
k = 1.3 x 10-10
• CH2 + O → Products → CO(a(v’≤8), a’(v’≤5), d(v’≤1)) + H2
k = 9.5 x 10-11
• CH + O → Products → CO(a(v’≤8), a’(v’≤5), d(v’=0)) + H
CO + H2 (Main)2H + CO (~ 20%)H + HCO (HCO*)CH + OH (~ 6%)
{{CO + H (Main)
HCO (HCO*)HCO+ + e- (~ 0.03%)C + OH (? +ve Ea)
Distribution A: Approved for public release, distribution is unlimited
Apparatus
He/CHBr3
He/O2(O-atoms)
248-nm Laser
Calorimeter
Pumpuv/vis-Spectrometer
vuv-Spectrometer
Distribution A: Approved for public release, distribution is unlimited
Very Weak CO(A) Signal in O2 only Strong OH(A) Signal in O2 only
k = (2.3-5.9) x 10 -11
• CH + O2 → Products → CO(a(v’≤4), a’(v’=0)) + OH
Comparison of CO & OH-Chemiluminescence
CO + OH (~ 20%)CO2 + H (~ 30%)HCO + O (~ 20%)H + CO + O (~ 30%){
0
100
200
300
400
500
-400
-300
-200
-100
0
100
130 140 150 160 170 180 190
CO
-ch
emilu
min
esce
nce
sig
nal
in O
/O2 (
cou
nts
)
CO
-ch
emilu
min
esce
nce
sig
nal
in O
2 (c
ou
nts
)wavelength (nm)
0
100
200
300
400
500
0
100
200
300
400
500
260 270 280 290 300 310 320
OH
-ch
emilu
min
esce
nce
sig
nal
in O
/O2 (
cou
nts
)
OH
-ch
emilu
min
esce
nce
sig
nal
in O
2 (c
ou
nts
)
wavelength (nm)
Weakened OH(A) Signal in O/O2Strong CO(A) Signal in O/O2
CO + OH(A) (~ 0.48%)
Distribution A: Approved for public release, distribution is unlimited
Bromoform Photolysis
CH(4Σ-) + Br2 Br + HBr + C CH(2Π) + Br2 (Br + Br)
CH(4Σ-) + Br hν CH(2Π) + Br hν CBr + HBr (H + Br) ↑hν (~25%) (~75%) hν(∆H=- 24.8) CHBr + Br2 ← hν + CHBr3 → Br + CHBr2 (∆H=- 49.6) hν ↓hν ↓(?) hν hν HBr + C CBr + H CBr2 + HBr CBr2 + H hν
Br + CHBr → CH(A) + Br hν hνλ CBr + H C + HBr
hν CH + Br
431nm CH
10
100
1000
190 200 210 220 230 240 250 260
λ (nm)
σ (
10-2
0 cm
2 mol
ec-1
)
Distribution A: Approved for public release, distribution is unlimited
• Chemiluminescence Intensity Varied as (Laser Fluence) 2
∆Ho298K(kcal mol-1)
C(3P) + O(3P) → CO(A1Π) (-71.8)
CHBr + O(3P) → HBr(X1Σ+) + CO(A1Π) (+1.3)
CH + O(3P) → H(2S) + CO(A1Π) (+9.2)
CBr + O(3P) → Br(2P3/2) + CO(A1Π) (+3.8)
CBr2 + O(3P) → Br2(1Σ+g) + CO(A1Π) (+29.1)
• Diatomics or Triatomics Need to be Internally Excited
CO(A) Source Reactions
Distribution A: Approved for public release, distribution is unlimited
1
10
100
1000
104
105
0 500 1000 1500 2000
165.
7-n
m C
O c
hem
ilum
ines
cen
ce (c
ou
nts
)
time (µs)
Excess CH4 Present
CHBr3 Photolysis, Excess O-atoms, 2 Torr He
Time Resolved CO(A)-Chemiluminescence
q Bimolecular Reaction RateCoefficients of Added SubstrateWhen CH4 Present
↓
kO2 = (2.2 ± 0.3) x 10-11
kN2O < 7 x 10-14
kNO = (3.4 ± 0.5) x 10-11
kH2 < 2 x 10-13
kCH4 < 6 x 10-14
↓
q (C + O) Not the Source
Distribution A: Approved for public release, distribution is unlimited
CHBr3 Versus CBr4 Photolysis
0
50
100
150
200
250
140 160 180 200 220 240 260
wavelength (nm)
CO
-ch
emilu
min
esce
nce
(co
un
ts)
CHBr3 Photolysis
CBr4 Photolysis
Excess O-atoms, 2 Torr He
q Stronger VUV Signal inCHBr3 Photolysis
↓
(CH# (or CHBr#) + O) Important
q Signal in CBr4 PhotolysisVaries as (Fluence)2
↓
(CBr2# + O) not Important,
Since Br2* Signal Varies as(Fluence)1
Distribution A: Approved for public release, distribution is unlimited
CBr4 Photolysis
0
50
100
150
200
250
300
350
200 220 240 260 280 300
chem
ilum
ines
cen
ce (c
ou
nts
)
wavelength (nm)
Laser Off
Br2* ∝ (Fluence)1
Excess O-atoms, 2 Torr He
CO* ∝ (Fluence)2
q CBr2 + O → CO + Br2*
q CBr2 Formed inAbsence of Photolysis
q CBr2 Formed inPhotolysis
q CBr + O → CO* + Br
↓
CBr2 + O → CO* + Br2not Important
Distribution A: Approved for public release, distribution is unlimited
CHBr3 Versus CBr4 Photolysis
1
10
100
1000
104
105
0 200 400 600 800 1000
165.
7-n
m c
hem
ilum
ines
cen
ce (c
ou
nts
)
time (µsec)
CBr4 Photolysis
CHBr3 Photolysis
Excess O-atoms, Excess CH4, 2 Torr He
q CHBr3kO2
= (2.2 ± 0.3) x 10 -11
q CBr4kO2
= (2.4 ± 0.4) x 10-12
↓
(CBr# + O) Source is not asImportant as (CH# + O) in CHBr3Photolysis
q CHBr# has Very ShortLifetime (~ 5 µs) andk(CHBr + O2) < 2 x 10-14
↓
(CHBr# + O) Source notImportant in CHBr3 Photolysis
Distribution A: Approved for public release, distribution is unlimited
CH(a4Σ-) + O Reaction Rate Coefficient
0
5000
10000
15000
20000
25000
30000
0 0.5 1 1.5
(k' -
ko2
[O2] le
ft)
(s-1
)
[O] (1 x 1014
molec cm-3)
Excess CH4, 2 Torr He
q k(CH(a) + O) = (1.35 ± 0.47) x 10-10
Previously:q k(CH(X) + O) = (9.5 ± 1.4) x 10-11
Distribution A: Approved for public release, distribution is unlimited
282.2-nm Signal
1
10
100
1000
104
105
0 200 400 600 800 1000
282.
2-n
m c
hem
ilum
ines
cen
ce (
cou
nts
)
time (µs)
CHBr3 Photolysis, 2 Torr He
q Absence of O-atoms
X-trace: (O2, 8.8 x 1014)∆-trace: (O2) + (CH4, 5.0 x 1015)
↓CH(X2Π) + O2 → CO + OH(A)
CH(a4Σ−) + O2 → CO + OH(A)
q 5.0 x 1013 of O-atoms
•-trace: (O2, 8.8 x 1014)? -trace: (O2) + (CH4, 5.0 x 1015)
↓CBr2 + O → CO + Br2(D)
(CBr2 + CH4) Slow Reaction
Distribution A: Approved for public release, distribution is unlimited
Br2*-Chemiluminescence
260 270 280 290 300
Br 2
-che
milu
min
esce
nce
afte
r la
ser
ph
oto
lysi
s (c
ou
nts
)
Br2 -ch
emilu
min
escence b
efore laser
ph
oto
lysis (cou
nts)
wavelength (nm)
0.3 0.2 0.1 0
0
1
2
3
4
5
Laser off
Laser on
CHBr3, Excess CH
4 & O-atoms, 2 Torr He
q Laser off
CHBr3 + O ? CBr3 + OHCBr3 + O • CBr2 + BrO
↓CBr2 + O • Br2
* + CO
q Laser on
Br2* ∝ (Fluence)1
↓CHBr3 + hν → CHBr2* + BrCHBr3 + hν → CBr2 + HBrCHBr2* + hν → CBr2 + HCHBr2* + O → Br2
* + HCOCHBr2 + O → CBr2 + OHCHBr2* + O → CBr2 + OH(A)CHBr* + O → CBr + OH(A)
⁄
⁄
?
⁄
⁄
Distribution A: Approved for public release, distribution is unlimited
Time Resolved Br2*-Signal
0.1
1
10
100
1000
104
0 200 400 600 800 1000
289.
9-n
m c
hem
ilum
ines
cen
ce s
ign
al (
cou
nts
)
time (µs)
-2 0 2 ln[laser fluence]
ln
[in
t. in
ten
sity
]-7
-4
-1
CHBr3 Photolysis, Excess CH
4, Excess O-atoms, 2 Torr He
q Fast Br2* Rise
q Also:kO2
< 9 x 10-14
kCH4 < 7 x 10-14
kO = (5.4 ± 1.0) x 10 -11
↓CHBr3 + hν → CBr2 + HBr
Less ImportantCBr3 + hν → CBr2 + Br
q Since:CBr4 + hν → CBr3* + Br
↓ φ? CBr2 + Br
CBr4 + hν → CBr2 + Br2 { φ?
{
Distribution A: Approved for public release, distribution is unlimited
CO* Production Mechanism
50
100
150
200
250
ener
gy
(kca
l m
ol-1
)
reaction path
CO(X,a,a',d,A) + H
(X,0)
(a,5)
(a,8)
(a,11)
(a',0)
(a',5)
(a',9)
(a,0)
(d,0)
(d,4) (A,0)(A,1)(A,2)
CH(X,a) + O {HCO}* HCO(X,A,B,C) +HCO(X) OH(X) + C
(X,0)
(X,0)(X,1)(X,2)
(X,3)
Min
Max
(a,0)(a,1)(a,2)
(X,0,0,0)
(B,0,0,0)
(C,0,0,0)
(X,0,0,0)
(A,0,0,0)
reactantsintermediate(s)
products
~
~
~~
~ ~ ~ ~CH# + O → {HCO}* → CO* + H
MCO* → CO(X,a,a’,d,A)
Distribution A: Approved for public release, distribution is unlimited
Conclusions
• 248-nm Photolysis of CHBr3/O-atom MixturesStrong Emissions From:
• CO(A), CO(a)• OH(A) when O2 Present• NH(A) when NO Present• Br2(D)
Kinetic & Laser Fluence Trend Analyses of the Chemiluminescence:• CH + O• CH + O2 (NO)• CBr2 + O
• 248-nm Photolysis of CBr4/O-atom Mixtures• CBr + O• CBr2 + O
• Thermospheric O-atoms + Plume Fragments (CH) → UV Emissions