The rate of homogeneous recombination of fluorine atoms

CHEMICAL PHYSICS LE-ITERS

VOlUme 25, number Z

THE RATE OF HOMOGENEOUS

RECOMBINATION

P.S. GANGULI Deparrmenr

of Chemistry,

Emory

15 March 1974

OF FLUORINE

ATOMS

and M. KAUFMAN University.

Received 21 December

A rkmta.

Georgia 30322, USA

1973

The recombination rate of fluorine atoms ha‘s been determined in a Teflon-coated flow reactor at 295-K and pressures of lo-81 torr with Ar as the carrier gas. Concentrations of atomic fluorine are measured by titrating with Cl2 and using the intensity of chlorine atom recombination emission as an end oint indicator. The data is fit with a thirdP order homogeneous recombination rate constant of 2.9 X 1013 cm6/mole sec. Possible systematic errors in the titration lead us to claim only a factor of two accuracy for the result. The measured rate constant is ca. a factor of 10 less than values calculated by two different theories and ca. a factor of 100 less kn the accepted rate constants for homogeneous recombination of most other atoms under similar conditions.

combine,

There have been no measurements reported of the rate of recombination of fluorine atoms despite the practical importance of this information for the design of HF lasers and its theoretical interest as a prototype for recombination uncomplicated by the influence of excited molecular states. Such experiments have been retarded by the fact that, due to the low optical absorption coefficient of F3 and the extreme corrosiveness of F, there existed no simple and convenient method of measuring the concentration of ei*&er atomic or molecular fluorine. Recently a chemiluminescent titration of atomic fluorine with Cl, has been suggested by Schatz and Kaufman [ 1] and employed by Nordine and Rosne&. This titration is based on the observation that the reaction F + Cl, + ClF f- Cl (M = -3 * 2 kcal/mole [3] ) is very-rapid [4] , but the reaction Cl + F, + ClF + F (A.fX= -23 + 2 kpl/mgle [3] ) is extremely sIow*z_ Thus when CI, is added to a flowing gas containing atomic fluorine, the fluorine atoms are rapidly converted into chlorine atoms, with no interference from F2_ Light is emitted as chlorine atoms re$ Nordie and Rosner have carefully investigated the titration at rapid flow rates and low pressures. The authors gxatefully. acknowledge a preliminary communication of some of their rek&s. %%Emplcjying molecular beam ankysis [S] we have deter&id that the r&e constant for this reaction must be less than 1 X-10' cm3/moleseeat 295°K.

-

the

and the intensity

concentration

of this light is a measure of of atomic chlorine that is produced

by the titration.

’

In our experiments atomic fluorine is generated by a microwave discharge (50 W of 2450 MHZ power in an Evenson-type cavity [6] 7) in dilute F,--Ar mixtures flowing through an alumina tube_ The-discharged gases then enter a 19-mm diameter Teflon-coated quartz .tube in which they flow through two light traps and then pass a moveable Teflon inlet where Cl, is added. (Rapid mixing is promoted by the Use of a multiperforated spiral of Teflon capillary tubing on the end of the inlet.) The emitted light is viewed through a Wratten 16A filter by a 93 1 A photomultiplier tube (giving an effective bandpass of 5 160-6200 A). At low pressure the inlet -,. is located 3 cm upstream from the phototube..However, at higher pressure 8 cm is needed to ensure good mixing. Recombination measurements are made by employj& two photomultiplier-filter cpmbinations separated by a distance of either .14.2 cm or 44.0 cm. Fig. 1. illustrates some typical titration curves:At low pressure (1 .l torr - fig: I a) Fe em,issioti intensity increase? roughly as the square of the added Cl, (atid . .thus .a$ the square of the Cl atoms produced [7] ):a&d then abruptly levels off at the endpoint *_;c\t hi& p& :_. _: 1 Commerckly z&ilabk from the Opthos Inskumeni Cd.‘- ._. .:. * Footnqte see n&i page. :-.--___. _... .. ;. .. .L

Volume

CHEMICAL PHYSICS LETTERS

25. number 2

-d[FJ/df=2k[F]‘[hl]

0

2

4 6 0 Cl, Flow (Std

2.0 cm3/

4.0 set)

60

80

Fig. I. Cl2 titration of atomic fluorine. I = intensity in arbitrary units. Arrows mark the Cl2 flow chosen as the endpoint. (3) total pressure I. 1 torr, (b) tot31 pressure 51 torr. sure (59 torr 1 fig. I b) the emission approaches its maximum value more slowly and the endpoint is more difficult to locate. The difference between the two curves is most likely due to atomic chlorine recombination between the Cl2 inlet and the phototube. At high pressure both the recombination rate and the time for recombination (due to the larger inlet-phototube distance) are larger. At a pressure of 5 torr a calibration of emission intensity versus [Cl] was employed to verify that [Cl] existing at the endpoint could be correctly obtained from the Cl, flow needed to reach the endpoint (to within 5 15%). The calibration was prepared by monitoring the emission from discharged Cl,-Ar mixtures whose atomic chlorine concentration was determined by NOCl titration [7]. In addition, at 1.1 torr atomic fluorine concentrations measured by the chemi-

luminescent titration with Cl2 were found to agree to within 15% with those obtained by an H, titration which employed molecular beam analysis as an endpoint indicator [S] . Table I lists the results of 20 measurements of the recombination rate taken at 295°K and pressures in the range 10-81 torr. The rate data is fit to the form l

At still larger flows of Cta the emission intensity decreases. undoubtedly due to factors such as vibrational relaxation and quenching of excited Cl2 and changes in flow patterns.

222

+k,v[F].

in which k is the homogeneous recombination rate constant and k, is the wall loss (assumed iirst order) rate constant. The first I3 measurements were made using a Teflon-coated tube in which wall loss was negligible, and from each of these runs a value of k could be directly calculated. Upon recoating the tube, we were unable to completely passivate the walls for loss of fluorine atoms. The last seven measurements were thus taken with a single wall coating and each run was solved painvise with the other six to give values of k and k_,. The average of these six values is reported in table 1. The mean value of the homogeneous rate constant is 2.9 X lOi cm6/mo1e2 set, with a standard deviation of the mean of 0.2 X 1013 cm6/mole2 sec. Some systematic variation in the calculated values of k is noted, with measurements at low pressure giving high rate constants in those experiments in which walI losses are not negligible. These differences could result from a larger portion (up to 3%) of the third bodies being F and Fz in these experiments. Both F and F2 are probably more efficient chaperons than is Ar. However, since these are also the experiments in which k is determined with the least accuracy, we choose to ignore the variation at this time. Systematic errors may also result from

the Cl, chemiluminescent titration not having been validated over the pressure range of the recombination measurements, and thus we arbitrarily set an error limit on our result of a factor of two. It is interesting that the measured value of k is ca. a factor of 100 lower than the accepted rate constants for recombination of Cl, Br, I, 0, and H in Ar, and a factor of 40 lower than that for N in Ar [9]. Theoretical approaches predict homogeneous recombination rate constants for fluorine atoms in Ar which are ca. a factor of 10 higher than our determination. For example, Lloyd [IO]has employed the Benson and Fueno theory [I 1] with four different potential energy funciLions to calculate k values for F in Ar at 300°K that span the range (1.8-6.0) X 1014 cm6/mole* sec. In addition, Keck’s modified phase-space theory predicts a homogeneous rate constant

of 2.4 X lOI4 cm6/mole2

secO_

o We would like to thank J-C. Keck and V.H. Shui for calcul&ng k for us according to the latest modification of their theory. See ref. ! 12 ] _ The potential parameters einployed ~etho_~giveninref.[9]..

Volume 25. number 2

15 March I974

CHEMLCAL PHYSICS LETTERS Table 1 a)

RatedataforFi-F+Ar-F2+Ar Run

Total pressure (torr)

IArl

J

53

282

1.10

0.94

0.47

0.107

2.55

2

61

323

1.55

I.18

0.42

0.118

2.61

[Fli

Time @CC)

IF2li

IFI f

kw

k =) i 013 cm6

(10”rnole/cm3)

Csec-’ 1

mole’ set

3

61

324

1.34

1.07

I.01

0.116

2.45

4

61

324

I.46

1.15

0.39

0.116

2.44

5

62

329

2.07

1.46

3.34

0.117

2.60

64

340

I.37

1.07

0.33

0.116

2.59

7

67

357

1.64

1.28

0.65

0.112

2.16

8

80

425

2.04

1.47

2.0

0.118

1.91

9

81

431

I.98

1.38

1.7

0.119

2.94

10

10

50

0.66

0.64

1.62

0.128

3.29

11

13.9

62

1.76

1.56

b)

0.241

2.37

12

14.1

62

1.59

1.38

b)

0.244

3.07

13

14.4

73 74

b) 1.82

2.50

14

1.79 0.295

0.284

14

2.20 0.721

0.337

4.16

15

23

I22

0.689

0.274

I.83

0.326

3.43

2.5

16

15

80

0.674

0.276

2.47

0.334

4.35

2.4

6

.

2.3

17

19

101

0.716

0.282

2.72

0.326

4.7%

2.4

I8

31

165

0.764

0.302

2.47

0.310

2.86

2.5

19

41

220

0.742

0.284

1.99

0.310

2.70

2.5

20

50

267

1.073

0.35 1

3.40

0.326

2.63

2.5

?? Phototube seuaration: runs l-13.14.2 cm: runs 14-20.44.0 b, Measurements of totai F2 flow w&e not made in these r&s. c) k,,= 2.9X 1013 cm6/mole2 sec.

cm.

We have also made some preliminary measurements of k withN2 asa third body and find it to be ca. a factor of 10 more effective chaperon than Ar. Since in C1, Br, and I recombination, N, is only 25% more effective than Ar [ 131, it is likely that the low value of k .for fluorine atoms in Ar is due to an unusually weak F,--Ar &&action. The low value of the F2 -t-Cl rate constant, which makes the chemiluminescent titration possible, strongly suggests that the primarily repulsive interaction between F, and Ar must hold for F2 and CI as well. The authors gratefully acknowl@ge support of this work’by the Office of Naval Research under coti-, .., t&t NOO14-73-C-0219. ‘. ;

References [I] 121

(31 [4] IS] [6]

[7 f -

[i]

G. Schatz and hf. Kaufman, J. Phys. Cbem. 76 (1972) 3586. P.C. Nordine and D.E. Rosner, Department of Engineering and Applied Science, Yale University, New H&en. Connecticut.06520, USA, private communication. Nat. Stand. Ref. Data Ser., Nar. Bur. Std., No. 31 (1970). J. Warnatz, H.Gg. Wagner and C. Zetzsch, Ber. Bunsenges.. Physik. Chem., 75 (1971) 119. .M. Kaufman and C-E. Kolb, ~em.‘lnstr. 3 (1971) 175. F.C. Fehsenfeld, KM. Evenson and H-P. Broida,.Rev_ Sci. Instr. 36 (1965) 294; N.A.A. CXyne.and D.H. Stednian, Trans. Fara&y Sot. ‘. :. 64 (1968) 1816. C.E. Kolb and M. IG~&~I, J,:%ys. C&em.‘76 (197ij’947;,-

.

_’

. _. .. .’

..,.,

.’

:. : -._j -223

I

Votume 25, nwiber

2

‘CHEMICAL

J-W’. BozzeUi snd M. Kaufman, J_ Pbys. Cbem. 77 (1973) 1748. [9] V.H. Shui, J.P. Appleton and J.C. Keck, Thirteenth Symposium on Combustion, The Combustion institute 2 I (197 11, and references therein. IO] AX. Lloyd, Intern. J. Chem. Kinetics 3 (1971) 39. 111 S.\V_ Benson and T. Fueno, J. Chem. Phys. 36 (1962) 1597.

PHYSICS

LETI-ERS

. ..< I5 March 1974 .T:$ r; .‘I* i.2

?:$ [I21

V-H. Shui. J.P. Appleton and J-C- Keck, J. Cbem. Phys. :+:

53 (1970) 2547; V.H. Shui, J. Chem. Phys. 57 (1972) i704; J.C. Keck, Advan. At. Mot. Phys. 8 (1972) 39.

[ 131 R.P. Widman and 1325.

$2 .,.li .__:

:, .,,

B.A. DeGmff, J. Phys. Cbem- 77 (1973f

,;

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