AD-AO79 613 AEROSPACE CORP EL SEGUNDO CA AEROPHYSICS LAB F/g 7/402 (1 DELTA)-? ATOM KINETIC STIJDIES.(U)NOV 79 Rt F HEIDNER. C E GARDNER F04701-79-C-0060

UNCLASSIFI ED TR-008015606)-2 SD-TR-79-6 NL

". fllfl/ll/lflEE2hIIEIIIhhEE

m o2h hh80h

0 (A)-I Atom Kinetic Studies

R. F. HEIDNER and C. E. GARDNER

El Segundo, Calif. 90245

DDC8 November 1979

,~JANIgOInterim Report LL]

APPROVED FOR PUBLIC RELEASE;DISTRIBUTION UNLIMITED

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AIR FORCE WEAPONS LABORATORYKirtland Air Force Base, N. Mex. 87117

SPACE DIVISIONAIR FORCE SYSTEMS COMMAND

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This interim report was submitted by The Aerospace Corporation,

El Segundo, CA 90245, under Contract No. F04701-79-C-0080 with the

Space Division, Deputy for Technology, P.O. Box 92960, Worldway Postal

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and stimulation of ideas.

mes C. Garya, Lt, USAF .eph . Ca, Lt 4 ol, USAF,--Poject Officfr 6hief, Advanced Technology Division

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Electronically Excited Oxygen I AElectronic Energy Transfer1Flow-Tube Kinetic Studies Ip-A -?~Gas Mixing 'rCW Iodine Laser ( 4 AW o s

20. ,ABSTRACT (Continue an reverse side If necessary end identify by black number)

A discharge flow !.ystem has been used to refine kinetic rate measurementsfor the O2(1LA) + 17' energy-pooling process and for the iodine- catalyzed re-moval of O2(1A). Particular attentio{A/ras givenithe effects of I2-carrier gas(Ar) on the emission intensity of O2 (itA) and Oa2it). A heated 12 saturator anda saturator bypass technique were developed to minimize such e-urc-s. Thedata reported here indicate that processes proportional tck,(I2)A) are im-portant at [Oa(1A)I and [IYI densities, which are appropriate lra cw O2 (1 tA)-Iatom transfer laser.,

FOM 7 UNCLASSIFIED

CONTENTS

I. INTRODUCTION 7............................... 7

II. EXPERIMENTAL .............................. 11

III. RESULTS .................................... 17

A. Influence of Injected Ar on 0 (1A, 1Z) IntensityM easurem ents ................................. 17

IB. Energy Pooling of I with O2( A) ................ 20

IC. Iodine Catalyzed Removal of O ( A) .............. 26

IV. DISCUSSION .................................. 35

V CONCLUSIONS ................................ 39

APPENDIXES

A. Chemical Kinetics of the 2( 1A)-I Atom System ........ 41

B. Rate Coefficients for the Kinetic Model ofAppendix A ............................... 43

Acces5of r

'... ..

-1-

FIGURES

1. Electronic Energy Levels in O -12 System ................. 8

2. 12 Dissociation Cycle in 0 2(A)-I Atom Transfer System . . . 9

3. Schematic of O-I Diagnostic Methods ...... ............... IZ

4. Vapor-Pressure Curve for 1. ...................... 14

5. Variation of 0(A, 2 ) Emission with Ar Injection .......... 186. Effect of Ar on 16340 Versus [O2( A)]2 ................ 19

7. Determination of k 2 without Saturator Bypass Technique. . . . 23

8. Determination of k 2 by Use of Saturator Bypass Technique . . 25

9. Time-Dependent O 2 (1A) Dimol and 02( L) Emission in thePresence of 1 ................................. Z8

10. Time-Dependent First-Order Decay of O(I A)(X = 1. 27 4m) Versus 12 .......................... 29

Ii. Determination of kef f with First-Order Decay ofO2( A) Assumed ........... .............................. 31

12. Time-Dependent Second-Order Decay of 0 (1A)(X = 1. 27 Lm) Versus 12............' .......................... 32

13. Determination of k + k' with Second-Order DecayKinetics Assumed @or Z( .A) ...................... ... 33

-3-

i TABLES

I . Spectroscopy of the 02-1 Atom Transfer System .. .. .. ...... 13

I. Flow System Reactant Contact Time. .. .. ... .......... 16

4 I.Comparison of Kinetic Results .. .. .. ...... ......... 34

5

I. INTRODUCTION

Workers at AFWL recently demonstrated an efficient, chemically

pumped, cw electronic transition laser that is based on the o?('A)-I atom

transfer system.

k 1 3 2

o0(A) + .1( / o3Z) +I P.)

The intent in this technical report is to supplement and, where necessary, to

revise the conclusions reached in earlier studies on this transfer system. 2

The nomenclature and rate-coefficient numbering scheme introduced in the

pioneering studies by Derwent and Thrush 3 - 7 are used in this report. For

reference, the relevant spectroscopic energy levels and the logic flow chart

of the OZ(IA)-I atom transfer system are reproduced from Reference 2 and

appear as Figs. I and 2.

1 W. E. McDermott, N. R. Pchelkin, D. J. Benard, and R. R. Bousek,"An Electronic Transition Chemical La.er," App1. Phys. Lett. 32,469 (1978).

2 R. F. Heidner III, J. G. Coffer, and C. E. Gardner, O2(IA) - I AtomEnergy-Transfer Studies: CW Inversion on 1. 315 pim I-Atom Transition,TR-0078(36i0)-i, The Aerospace Corporation, El Segundo, Calif.(15 Dec. 1977).

3 R. G. Derwent, D. R. Kearns, and B. A. Thrush, "The Excitation ofIodine by Singlet Molecular Oxygen," Chem. Phys. Lett. 6, 115 (1970).

4R. G. Derwent and B. A. Thrush, "Measurements on Oz(IAg) and O2(IE+)in Discharge Flow Systems," Trans. Faraday Soc. 67, 2036 (1971).

5 R. G. Derwent and B. A. Thrush, "The Radiative Lifetime of theMetastable Iodine Atom I(5 2PI/ 2 )," Chem. Phys. Lett. 9, 591 (1971).

R. G. Derwent and B. A. Thrush, "Excitation of Iodine by SingletMolecular Oxygen," J. Chem. Faraday Soc. II 68, 720 (1972).

7 R. G. Derwent and B. A. Thrush, "Excitation of Iodine by SingletMolecular Oxygen," Disc. Faraday Soc. 53, 162 (1972).

-7-

H 20

C? 15 -0211+)

3-7E 5239 cm-1

02(161

11803 cm1

5

7603 cm-1 7882 CM1

0 02 1--2

Fig. 1. Electronic Energy Levels in 0 1~ System

-8-

17

02 ('A) 02 ('A) ---(1' A)41!1 + 0 31

-9-j

022

/I +IAL02111

f t

02 ig. 02.1 12 Disocat+i Cycle in* 2,Atom Tranfer System :

In this study, refined experimental methods were used, and conclusions

quite different from those of Derwent and Thrush were reached. These new

observations are of critical importance in the continued development of the

cw I*' laser that is based on energy transfer from O 2 (IA).

Io

-.10-

ti II. EXPERIMENTAL

The experimental apparatus is depicted in Fig. 3. 02(1A) is produced

by a microwave discharge in pure 02, and the 0 atoms that are simultaneously

produced recombine on a heated HgO surface. The absolute concentration of0 2 (1 A) was measured by means of an isother-1 al calorimeter method that is

described in detail in Reference 2. Spectro: opic methods were calibrated

against the probe measurements. The relevant spectroscopic features of the

0z-1 atom system are summarized in Table I.

The 12 carrier gas was Ar. It flowed at atmospheric pressure

through an 12 saturator enclosed in an oven. A three-station digital thermo-

couple was used to monitor the temperature of the saturator. The Ar + 12

was metered by means of a stainless steel needle valve held at a temperature

several degrees warmer than the saturator. Partial pressures of I in Ar

were calculated from the vapor pressure data of Gillespie and Fraser (Fig. 4).

Mole fractions of 12 in Ar of (5 - 10) X 10-3 were obtained at saturator tem-

peratures of 60 to 70°C.

The quality of the kinetic data was improved markedly when a "saturator-

bypass" arrangement was used to inject the Ar + 12 into the 0 2 flow.

This arrangement permitted a constant flow of Ar to be continuously injected

into the 02 stream and a measured fraction to be saturated with I Thus,

the partial pressure of Ar in the flow remained constant while the concentra-

tion of injected 12 was varied.

Two methods of obtaining decay plots of 0 2 ( A), 02(Ir), and I' were

used. The Ar + 12 could be shunted to one of three fixed injectors (A, B, or

C), with the optical detectors used to monitor a fixed position on the flow

tube. Alternatively, a single injector was used, and the detectors were

8L. J. Gillespie and L. H. D. Fraser, "The Normal Vapor Pressure of

Crystalline Iodine," J. Am. Chem. Soc. 58, 2260 (1936).

A r

D A CHART SCP

I I ~THERMOMETERTRNIT

LOCK-IN PHOTONCOUNTER

SATURA ORp

r-- Ge GaAsI j PMT

0. I25 m 0.25 mI IIR VISIBLI

Fig. 3. Schematic of 0 -I Diansi ehd

2Nogwsi Mthd

S. Sfn Ln Sn -. n Smcm Nl N 70'

41 >4 O

0 S0

v ou -u 0 "S ' I

I-, .o IS' '0X

41 "d:I n 0

w 0 u

+1 N D 0o s

M "D

41 ~ ~ ~ ~ b u ' 41 I, 1"41~ ~~~ ISSN 4o 10'

41t > a S.41 41 4

o~~ +1 "g N 04-0~

Q '0 -o x 1<

0 Z.

o o~

0413 r

'a+ Uo U .0 414 z'O Z '.

4-'0 40 4 * .5 -. v -

:* Ua U PQ oro to. Z X*

be +1 bo r' c Ca a~ 41G 1

aS ~ ~ m 411 ' 5

0 41 ?: -

$4~M 'a14 Id.-4'Ix a'

0c .04 ' 0. u to0

IS' Co1 ' U -0 'aaUO~ ~~ ~ -0+ N.u

0 4

10 .10.0

1.0

0.12.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

T_1x 10 3K_1

Fig. 4. Vapor-Pressure Curve for 1 2 (Ref. 8)

- 14-

translated along the flow tube (Positions 1, Z, 3, and 4) on a movable platform

mounted on a ball bushing rail system (Thompson Co.). The relationship of

the injector-detector positions to the contact time of the reactants is given in

Table II, e. g., B-2 indicates that Injector B was used with the detector at

Position 2.

The flow tube was made of square quartz (2.0 X 2.0 cm i.d.) and was

internally coated with Halocarbon wax. Deposits of iodine oxides could be

observed occasionally; however, mild external heating from a heat gun

restored the original properties of the Halocarbon surface.

-15-

Table H. Flow System Reactant Contact Timea

Contact Time, msecDetecto rPositionb Injector Position

A B C

1 52 90 128

z 73 iii 149

3 94 132 170

4 115 153 191

aTable entries must be divided by the volume

flow rate V(l/sec) to obtain the contact time inmilliseconds. Typically, V = 1.0 1/sec.

bSee Fig. 3.

-16-

III. RESULTS

A. INFLUENCE OF INJECTED Ar ON O2( A, im)

INTENSITY MEASUREMENTS

In earlier experiments carried out in this Laboratory, 2 He was used as

a carrier gas for IZ instead of Ar. However, severe gas dynamic effects are

possible when He is injected into 02P because the viscosities are so different.

It was hypothesized that the use of an Ar carrier would greatly alleviatc this

problem with only a small change in the partial pressure of Oz -

This hypothesis was explicitly tested by injecting Ar counter to the O?flow through L-shaped injectors (Fig. 3). Clyne evaluated the mixing times

characteristic of similar arrangements. From his studies, a mixing time of

5 rnsec was predicted. The partial pressures of 02 and Ar were calculated

on the basis of complete mixing with the conventional formula

1 PTOT (Z)

where the rh. are molar flow rates.1

The decay times of 2(1A) were essentially unchanged by the injection

of Ar into the flow of active O. The intensity of both the "dimoll emissionand the Oz(IT) emission, both proportional to [O2(iA)]Z, decreased markedly

with Ar addition; however, the ratio of these two intensities remained roughly

constant (Fig. 5). Theoretically, a plot of 16340 versus [O 2 (1 A)]? calculated

from Eq. (2) should be a straight line; but, as demonstrated in Fig. 6, there is

9 M. A. A. Clyne and H. W. Cruse, ''Rates of Elementary Reactions Involvingthe BrO(X 2 Tr) and IO(X 2 7T) Radicals," Trans, Faraday Soc. 66, 2214 (1970).

-17-

4

6f340

3

'7619

CO

'6340

00.5 1.0

P r Torr

Fig. 5.Variation of 0,('A, ~ Emission withAr Injection

-18-

0.9-

_0.8-

CnC

B0.6 -3 A

1.0 0.95 0.90 0.85 0.80

2 2 calculated

Fig. 6. ]Effect of Ar on 1 6340 Versus[02(az

-19-

a substantial deviation from linearity. Three possible explanations for this

behavior are:

1. The O 2 and Ar are not completely mixed.

2. The increase in system pressure with Ar addition has aneffect on the 0 2 (IA) production mechanism.

3. The addition of Ar has an effect on the dimol emissionmechanism.

The first explanation is supported by the fact that the dimol emission in-

creases as a constant mass flow of Ar is injected farther upstream (A - B - C).

The measured intensity deviation from the gas dynamic predictions (25% low

for hAr /rho? = 0.4) is rather small. The second explanation given may be

valid also since the system pressure increases by approximately 25% for

rhAr/rho 2 = 0.4. Evidence for the third possibility has been cited recently

at AFWL;* however, as demonstrated in Fig. 5. this effect is minor in the pre-

sent system unless compensating errors exist for 02( 1E) and dimol emission.

The correct evaluation of the kinetic effect of I? (and therefore I atoms)

on electronically excited 02 requires that the amount of Ar carrier be limited

to approximately 10 to 20% of the 02 mass flow. This constraint mandates

the saturation of the buffer gas stream with IZ at elevated temperatures. The

"1saturator bypass" technique is equally important since it permits the mixing

of Ar with O? to be identical as the partial pressure of 12 in the Ar is increased.

Tiie total pressure is, thus, held constant, and, therefore, the discharge produc-

tion of Oz(IA) should also be constant.

IB. ENERGY POOLING OF I: WITH 02(.-

It is a generally accepted fact that O2(1I) is enhanced by the addition of

I2 to a stream of electronically excited 02. The mechanism is as follows:

D. Benard, private communication, 1978.

-20-

k 3?

0O 2 ( A ) + I --- + 0 213 : (3) -

k5

O2(IA) I* - 2; 4

Unfortunately, earlier work in this Laboratory and also by Derwent and

Thrush 3 - 7 failed to give the general analytic expression for the function

0 2 (ilZ) = f(1 2 ), which correctly describes the time-dependent removal of 0 2 (tA)

by I atoms. The simplest analytic approximation was derived in an earlier

study, Eq. (B-13) of Reference 2, where 1Z10 represents the steady-state

-1= 2 (5)

10 k 9 (k 5 + k 6 ) [Oz(3M)]

concentration, which results from O(I A) energy pooling with no I., i. e,

k

O( 1A) + 0 2 (1 A) 0 02(1 Z) + Oz(3Z) (6)

Two processes reduce the [0 2 (1 A)] between the Ar + I? injector and the1 0fluorescence region, thus reducing O2( Z) 0 . They are:

1. The previously discussed gas dynamic perturbations causedby the Ar carrier gas introduced with the I Z .

2. The I-atom catalyzed removal of 0 2 (IA), which is discussedin the Section III. C.

Thus, a variation of Eq. (B-i ) of Reference 2 must be applied in order to

properly evaluate k2 . Either of the following two expressions will provide

proper analytical forms for determining k 2 :

-12 -

~Z 1640k k,\ 6340 k (k5 + k6) [i(3E

( .kk [i] (8)TO '1.- 27 / k 9 :k(k 5 + k 6 ) [02( 31)]

Two experimental approaches were applied to measuring k 2 . In the first

approach, considerable data reduction and system modeling are required.

It is presented in order to demonstrate the errors introduced by gas dynamic

problems.

From Eq. (B-6) of Reference 2, the expression can be written

1 0 / \i/Z2(-64."27 = exp(kff[I])t (9)

0(tN) 16340) 1 1.27

where keff is a measured rate coefficient equal to [kIk6 /(k 5 + k 6 )] + k7

(Section UI.C), and t is the time between 12 injection and 02('Z) monitoring.

The 16340 cannot be monitored directly since 12 (B -. X) emission is too great

to permit the higher values of 12 addition, which are of interest. The It. 27

can be monitored directly, however. Figure 7 is a plot of the three stages of

data reduction: (1) the raw (I7619/I0619 - i) versus [I]/[02(3Z)] data,

[Eq. (5)] (Curve A); (2) the data corrected for the gas dynamic reduction of

the [0 2 ('A)]2 by use of the results presented in Fig. 6 (Curve B); and (3)

doubly corrected data that incorporate the gas dynamic correction and the

correction derived from Eqs. (8) and (9) (Curve C). This last plot is roughl-

linear and yields a value k2 = 2. 5 x 1010 cm 3 /mol-sec.

2 Z

k .....-..-

PI (Tord x 103

0.5 1.0 1.5 2.0 2.5

5/C

krducio of . -... , -- -

3-Z3

0.25 0.50 0.75 1.0

[111[3213}), X 103

Fig. 7. Determination of k2 without Saturator Bypass Technique.

A, raw data; B, data corrected for gas dynamic effects;

C, dat.a corrected for gas dynamic effects and for linear

k ef f reduction of 0 2(I

The second experimental method is much more reliable. As described

in the experimental section, the "saturator-bypass" method of controlling theAr concentration was used. A constant-mass flow rate of Ar was injected

into the active 02, and a varying fraction of this flow was passed through the

I? saturator. Thus, the total pressure in the system remained constant

(to approximately 0. 3%), and gas dynamic corrections to partial pressures

were eliminated. The fundamental emission signal of 0 2 (1 A) at . 27 ILm is

not overlapped by any other spectral feature except for I" at 1.315 pLm. This

latter problem was eliminated by the use of very narrow slits on the near

infrared monochromator. Thus, experimental values of I0 . 27/I1.27 could

be provided as input to Eq. (8). At PTOT = 3.67 Torr, k2 = 5.5 X 1010

cm 3 /mol-sec was determined, and at PTOT = 1. 69 Torr the result was

k = 4. 6 X 1010 cm 3 /mol-sec (Fig. 8). Therefore, the best value of the

I -O2(iA) energy pooling rate is

k2 = (5.0 ± 1.0) x 1010 cm 3 /mol-sec (10)

where the error reflects the uncertainty in the amount of 12 added, but not

the uncertainty in the quantity ki/k 9 (k 5 + k6 ) required to solve Eq. (8). The

result is approximately three times faster than that reported by Derwent

and Thrush, 7 i.e., k 2 = (1.6 ± 0. 2) x 1010 cm 3 /mol-sec. The quantity

k i /(k 5 + k 6 ) was shown to be cloqmely approximated by the equilibrium con-

stant for Process (i),2,7 i.e., KEQ= k,/k 5 L k 1 /(k 5 + k 6 ). The 0 2 (IA)

ene rgy-pooling rate

k 9

02(1A) + o (1A) -. o2(1M) + oz(32:) (6)

4has been reliably measured only once, and any error in that quantity is

reflected in the reported value of k 2 .

-24-

PlITorr) x 103

0.3 0.6 0.9 1.2 1.5 1.8

8-

6-

N4

1 2 3 4 5 6

11/10 2( 3E) I x 104

Fig. 8. Determination of k 2 by Use of Saturator BypassTechnique. A, raw data; B, raw data correctedby measured (I0 7/I.27), Eq. (8).

-25-

The absolute value of k? can be viewed in two perspectives from a laser

systems point of view. 0 2 (iZ7) is critical to the operation of the cw 07(1&)-1

atom transfer laser since it dissociates 12 with high efficiency. The larger

value of k 2 reported here aids this initiation process and removes I? as a

potential deactivator of I . Eventually, k2 could serve as a pressure-scaling

limitation on the transfer laser since this energy-pooling process is second

order in 02(iA). Note that the "Oz(l Z) catastrophe" observed in the previous

study2 is largely eliminated for discharged 02 by means of the heated satu-

rator and the "saturator-bypass" method. The 'T, catastrophe has been

observed in real time in related experiments;'.- however, for discharged

oxygen, it must have an onset at added 12 pressures greater than 2 mTorr.

C. IODINE-CATALYZED REMOVAL OF Oz(IA)

Derwent and Thrush 7 reported that the measured removal rate of O 2 ('A)

in the presence of I was first order in [O(IA)] and first order in [I]. The

analytic expression for this kinetic behavior was presented as Eqs. (B-5) and

(B-6) in Reference 2.

-dO2 ( 1 A) [ kik6 ](B-6,

dt k + k6 7 ][][02(A)] Ref. 2)

That analysis results in a first-order decay of Oz( iA) given by

L0z( A) J f

*2

R. F. Heidner III, unpublished results.

-26-

The improved experimental procedures described previously were employed

to collect data on the decay of 1. 27- tm and 6340-A emission in the presenceof added I. These data indicate that second-order decay processes in

0 2 (1 A) are not negligible at Oz(iA) and I-atom densities appropriate for atransfer laser device.

The most interesting second-order loss process for 02( A) is the

energy-pooling reaction with I.

k 2

0 o2( A)+ o 02( Z)+ 1 (4)

The collisions that do not result in 02(1Z) must be considered also, e.g.,

!k/

OZ( A) + 1 0 2( Z) + 1 (12)

The decay of 0 2 (IA) monitored by the dimol emission at 6340 A is shown in

Fig. 9. Curvature in the plots is evident although contrary to the prediction

of Eq. (ii). A second discrepancy with theory is that these plots do not

extrapolate well to the intensity at t = 0. Unfortunately, dimol emission is

a poor diagnostic at high 12 addition because of interfering I 2 (B - X) emis-

sion. Thus, the experiments were continued with 0 2 (1 A) fundamental

emission at 1.27 pLm. In changing of diagnostics, considerable sensitivity

to second-order effects is lost because the dimol emission is proportional

to [0 2 (IA)] 2 , and because the dirnol detector (RCA GaAs phototube) is very

stable compared to the intrinsic Ge detector used for . 27-ptn radiation.

Nevertheless, Fig. 10 is a plot of 11. 27 versus time as a function of added

12, with it assumed that first-order kinetics apply. Curvature is not evident

within experimental error; however, the intercepts are not constant, and,therefore, the linearity may be an artifact of the small dynamic range of

-27-

10-9~ =20

9- 2 -

8-~%

x 44*A B b'Lq C

20

__ AOD7619

Aen6340

0 25 50 75 100 125 150 175t, msec

Fig. 9. Time-Dependent 02 A) Dimol and 02()Emission in Presence of 12. 1 + V -72.6 x5 jo-li mol/cm 3 , 02(3L) = 1. 6 x 10mol/cm 3 ; 0(A1 1. 5 x 1to 8 mol/cm3 .

28-

40

Cd,

1098

7

60 2 0 4 0 6 0 8 0 100 120 140t, msec

Fig.10.Time-Dependent Firs t-Order Decay of 02('A)Fi. (X = 1. 27 m) Versus 12, 1 + I* (mol/cm 3 ):

A, 0; B, 4.05 X 10- 1; C, 1. 06 x 10- 10; D,1. 54 x 10- W; EF, 1. 09 x 10-10; F, 2.*10 X 1-0G, 3. 39 x 10- 10;P I-T 4. 38 x f0_1

-29-

0 2 (IA) concentrations measured. If the data are plotted according to Eq. 11),

an empirical first-order removal rate of keff = 1.5 X 1010 cm3/mol-sec is

derived from Fig. it. Derwent and Thrush 7 reported keff = (8.0 ± 1.0)x 1010cm 3 /mol- sec.

When the same data are plotted on the basis of second-order kinetics

(Fig. 1U), the result is extremely interesting. The decay of 0 2 (1 A) is

written as follows:

0z( A) [02(I A)A[?

d dt [02(tA)][I*I = 2(k2 + k')KEQ 3 (13)dt 2 2 EQ [0 ( 3 )

[- [O2 (A)] 0 = 2(k2 + kz)KEQ [I] ] (14)

The intercept variation in Fig. 12 may be within experimental error,

and the linearity is no worse than that in Fig. 10, in which first-order

kinetics is assumed. With the measured value of [0 2 (IA)]5 1 inserted into

Eq. (14), the quantity 2(k + k') :- 1. 3 X 10t cm3/mol-sec is determined(Fig. 13). Given the assumptions involved, this value is rather close to the

value of k? = (5.0 ± 1.0) X 1010 cm 3 /mol-sec, which was determined by acompletely different method. Data over a much larger dynamic range would

be required in order to accurately deconvolute the first- and second-order

components of the decay of 0 A). The results of these studies are

compared with earlier results in Table III.

-30-

5

5 3 0

0 k ef 1.52 x1010CM /mol-sec

2 2

15 mlarr

00 1 2 3 4

1*)+ (111 x 10 10 Mal/cm 3

Fig. If. Determination of keffwith First-OrderDecay of 020A) Assumed

14

10 280 mlorr12

_E

C 1

C14

0= 20466-0 102tme

2-2

IA

141

-12

-I 10

10 0

C 2 8

4- 0

[I]I402 13) x g

Fig. 13. Determination of k2 + k'z with Second-OrderDecay Kinetics Assumred for 0 2('IA)

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-40

0 .0

- 0 -4

13

t4-

- - 00

0* 0

0~~~ '1.00 -

--41

* . 4-w,+ 4)

0 •0 k

a 0

. 0

SIllI

k 4

.44 .0oM

-34-

Lo

IV. DISC USSION

The efficient operation of the cw 02 (iA)-I atom transfer laser requires

a kinetic environment consisting of high concentrations of OZ(IA) and I".

Since I/I must be greater than 0. 5 in order to exhibit laser action, kinetic

studies must distinguish the effects of ground-state and excited-state I atoms.

I to I ratios higher than those used by Derwent and Thrush7 were used in

these studies, which may explain the differences in Table III. Measurements

I 'of the energy-pooling rate k2 between 02( A) and I tend to be low unless

the accelerated decay of 0 2 ( A) by iodine is explicitly accounted for by Eq. (8).

Thus, the present value of (5.0 ± 1. 0) x 1010 cm 3 /mol-sec is strongly

favored over earlier measurements. 2,' 7 Even this value could be systemati-

cally in error if k9 were incorrectly determined. The large value for

k? is supported by the rapid dissociation of injected 12 by 0 2 (1 2-)

Oz( Z ) + 12k 5(3 Z) + 21 (15)

The iodine-catalyzed removal of 0 2 (iA) is a much more complicated

question. The results of the present study support a substantial contribution

to the decay of 0 2 (1 A), which is proportional to [0 2 (IA)]. The following is

a numerical example of the difference between first- and second-order

kinetics. The experimental conditions are typical of the recent laser i experi-

ments; in addition, the I*: and O2(IA) densities are comparable to the present

studie s.

[02( 1 A]o = 1. 5 X 10-8 mol/cm 3 (280 mTorr)

[I] + [i'] = 3.0 x 10-jo mol/cm 3 (6 mTorr)

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[O2(i A)]

[O2(3z)] 0.4

The first-order half-life of O(iA) can be expressed as

= (In Z)(kefI]) " I (16)

= 62 msec (Derwent and Thrush)

= 340 msec (this work)

The second-order half-life of 0 2 (IA) can be expressed as

~1/2 =jZk1 ')I'1- (17)

= Z8. 3 msec (this work)

The predicted second-order half-life for 0(IA) would appear to be the cor-

rect order of magnitude to describe the laser operation. Even the large keff

for first-order decay given by Derwent and Thrush 7 results in a relatively

slow decay. Three scaling cases must be considered:

i. The [0.(IA)]/[0 2 (3Z)] 0 could be increased, which wouldimprove the mass efficiency of the laser but would alsoincrease the decay rate by increasing [ii ] prior to theoptical cavity. Fast, efficient mixing would be imperative.

2. The [O2 (lA)]/[O2 (3 Z)]0 ratio could remain constant, but thetotal O pressure could be increased. The decay rate wouldremain constant if the [I] + [I " ] remained constant.

3. The [I] + [1*1 could be increased to enhance the gain andimprove the energy extraction. This technique could accel-erate the OZ(/A) loss rate to unacceptable levels.

-36-

Gain length scaling may be required rather than [I*] scaling in order to

increase overall system gain. Scaling from [O?(,A)] /[O2)] = 0.4 to

O2(iA)]/[O ) = increases the decay rate of O2(iA) less than a factor

of 2 and should be pursued if technically feasible. Increasing the pressure

of the 0 2 (IA) is very desirable unless the Derwent and Thrush4 value for

k 9 is much lower than the actual value. Experiments on the decay rate of

SO(1 A in pure oxygen system strongly support the Derwent and Thrush

value.

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1

V. CONCLUSIONS

Modification of the Derwent and Thrush3 - 7 kinetic rate package is

necessary because the Oz(A)-I* energy-pooling rate is substantially faster

than these authors have proposed. Furthermore, collisions between O(I A)

and I*", resulting in kinetics that are second order in 0z(IA), may dominate

the removal of OZ(1A) at densities appropriate to the transfer laser. This

area of the kinetics, which includes temperature-dependent measurements,

is of critical importance in accurate laser modeling.

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APPENDIX A

CHEMICAL KINETICS OF THE O2 (1 A)-I ATOM SYSTEMa

I k 1 31+ 2 ( A) -4 1' + 2 . (A-i)

kz

i + Oz( 1A) - I + 2(1 Z ) (A-2)

1 2 31 + Oz( A) - I + 2(3z) (A-Z')

k3

I* a I + h,) (A-3)

0 2 ) O(3Z) + h\) (A-4)

Z37 k 5 OIA

1+ 0 (Z) --- + 2 A) (A-5)

:" + O 2(3Z) --- + O(3Z) (A-6)

i k73 (A- 7)1 +0 2 A) -0 + 0 2( A)

kI: + M . I + M (A-8)

0( A) + 0( 1 A) 2 O*('2-) + 00Th (A-9)

0 2 2;) + wall 0 O( A) + wall (A-10)

a Where possible, the above reactions are numbered as they appear

in Reference 7.

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*kI + wall Ip + wall (A-i11)

.1 k1 12 302 A) + wall 0bO(2 + wall (A -12)

k

14 14

02 ( Z) + o2m 0, 2( A) + 2 (A- 4)

12Z)+ 0 ( Z) + 2 (A- 15)

I17 ) + 0 A) 770+ +z ? + ) (A-17)

k18or I + wall b* 1/21 2 + wall (A -18)

-42-

APPENDIX B

RATE COEFFICIENTS FOR THE KINETIC MODEL OF APPENDIX A

Process k Reference

k ~(4.6 ± 1. 5) X t1 cm 3/mol- sec

10 3k 2 (t. 6 ± 0. 2) X 10 cm /mol- sec 7

(5 t 1) x 10 tocm 3/mol-sec This Work

k + '6 0t cm 3final-sec This Work

k37.8 sec- 2

k40. 077 sec 13 32

k5(1. 6 ± 0. 5) X 101 cm /mol- sec 2

k 6 3.4 x 10 tocm 3/mol- sec 2

k 7 8 x 10 t cm 3/mol-sec 2

k7 3

k91.2 x 10 cm 3/mol-sec 2

k 1 0 0 Yoi x to- 2

kl YH 1 1 2

k 13~

14 34k 151x 10 14cm 3/mol-sec 2

k 1 6 4 x 10 13 cm 3 /mol-sec 2

k 8y 1 8 (Pyrex)- 1; Y18 (Halocarbon)ft 10- 2

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