248-nm laser photolysis of chbr3/o-atom mixtures: … a: approved for public release, distribution...

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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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

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6. AUTHOR(S) Ghanshyam Vaghjiani

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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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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18


• 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


UV/Vis Plumes

Spectral DataRadiance Data

⇔ Plume Data ⇔

Laboratory Studies

⇔⇔

Chemiluminescent Processes

Identify Spacecraft Atmospheric Interactions

⇔

Modeling Studies

⇔


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)


Apparatus

He/CHBr3

He/O2(O-atoms)

248-nm Laser

Calorimeter

Pumpuv/vis-Spectrometer

vuv-Spectrometer


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%)


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

)


• 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


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


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


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


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


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


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


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)

⁄

⁄

?

⁄

⁄


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?

{


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)


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

248-nm laser photolysis of chbr3/o-atom mixtures: … a: approved for public release, distribution...

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