Kinetics and detection of F(2P) atoms in a discharge flow system

Chemrwl Physm 79 (19X3) 35 1-3hO North-Holland Puhhshrng Comp.my

KINETICS

AND

351

DETECTION

OF F(‘P)

ATOMS

1N A DISCHARGE

FLOW

SYSTEhI

A nr\* techmque for the derectwn of F(’ P) .nom\ m J dlschdr_gs flop s>\tem I\ dcscnhsd The bsr rexnon F - Brl - BrI‘- Br ha\ heen used m the presence of excecs Br, to rapIdI> comert the F ~torn, to BrF Th‘hrBrF 1, then dete~kd h\ lawr-mduccd fluorescence rxclted on the 5-O or 7-O handheads of the B'lI(O-)-X1\‘- trdn\luon u\mg d pukd tundhle due Ibrr L\lng this sensmke spectroscopic marher techmqur concentrdtlonk of F( ‘P) 2 3 x 10’ cm- ’ can he .ictrLted Thlr method ha. ken used to measure the rates of nxcuon of F(‘P) .mxn< x\:th CH, .md CHF, under pseudo-im\r order condmom \xtrh [Reagem] % [F] The folloumg rate constdnts were ohtamed .n 29s K nnd 1 Torr prs
1. introduction The measurement of absolute rate data for reactrons mvolvmg ground state F( ‘P) atoms has been hmdered by the difficulty of detectmg F( ‘P) atoms directly with sufficient sensitivity to allow the use of the low concentrations necessary for accurate rate measurements of rapid reactions under ideal pseudo-ftrst order condrtions [l] The techniques of resonance absorption and fluorescence. although both very sensitive for many atomic specres. are made experimentally difficult in the case of F( 'P, ) detection by the need to work in the far vacuum UV at 95 nm. below the cut-off vvavelength of LiF amdow material at 105 nm [2]. Both F-atom resonance fluorescence [3] and absorptton [4] have. however, been used to measure absolute rate constants, collimated hole structures replacing the windows The relatively low transmissron and small acceptance angle of the collimated hole structures limrted the sensttivity (IF]>, 1 X IO” cme3) Direct detection of F atoms by mass sprctrometry has been used [S] but. due to the low potential (15.8 eV) for the process F2 + e-+ F‘+ + Deceased 0301-0104/83/0000-0000/$03

00 Q North-Holland

F-T c- compared to the lomsatlon potential of F (17.4 eV). rcqutrrs a source of F atoms free of Fz (e-g_ by the reaction N A NF2 - N2 + 2F. [5_6]) Thts problem may be avorded by using F atoms m excess and monitoring the other reagent [7X]. [F] bemg determmed b> using a titration reaction Several such redctrons hale been suggested and of these the reaction F + Cl, -+ ClF - Cl (k = 1.6 x lo-“’ cm-’ molecule-’ s-t [X7]) has been partrcularly successful_ smce the second+ reaction Fz T Cl + ClF+ F is ~10~1. haling k < 5 x 10-l“ cm’ molscule’ s- ’ [9.3]. enablm, 0 a dtschnrge In F, to be used as a source of F atoms. Chlorine dtom recombinatton emissron [lo] or mass spectrometric detectron of Cl, or ClF may then be used to determme absolute [F] Care must be taken. ho\\-

ever. that the substantial

O-atom

impurity

from an

F,/He discharge in a sthca tube does not affect the medsurements EPR has been used but suffers from poor senstttvtty ([F]a 2.5 X IO” cm-” [ll]) and spatlal dlscrmtmdtton \vhrch may pm\ ent d pseudo-frrst order study and necessrtate modellmg of the redctron [12.13] Several photolysis techniques have used HF (v’ ) 0) product chemtlummescence to follow the reaction. the F atoms bemg produced either bv

L

h

t 1900 V) The computer was used to accumulate br~ween 1000 and 4000 laser shots. thus averagmg

out short term fluctuation, m the laser power and producmg d decay trace which was stored on floppy discs Pretrlggermg of the transient recorder enabled accurate determmatlon and subtrxtron of the c\\ bxlqyound by cl BASIC routme. ~hlch then mtegratcd the decays to produce a rqgwl mtrnslty I The Idser-dependent bachcaused by incomplete dlscrnninatlon ground. agJmst scattered Lser light. was small and could he determined b> measurmg I with no F cltoms

pre\ent 3. RCWI~I\

The v,xlatlon of the fluorescence signal I WJS m\etsugated .IS J function of [F],,. [Br,] and the reaction rime. r. bet\\een addition of the Br, and ohsrrv.mon of BrF fluorescence at the laser axis A ldsrr WXJ~IOI~ spectrum of BrF (B-‘ll(O’)x’ ‘1: T) H.IS recorded m the region 530-485 nm. aho\\mg d srrles of red-degraded bands The laser wets then tuned to either the 5-O or 7-O bandhead and the undispersed fluorescence Intensity. 1. WIS momtored

Fig Z (a) Vanauon of fluorescence mtenslty I (arbltary umls) Hlth IF], under condltlons of excess Br2. [Brz] = 6x lOI cmm3 usmg excIta1Ion on the 5-O bandhead of BrF (B-X) and boxcar detection (b) Smular plot of I versus [Br-] tahen under condruons of excess F atoms [F] = 1 x III’3 cme3. usmg excltanon on the 7-O bandhead and the lrarwent recorder/signal averager dccrcuon system.

The variation

of the fluorescence

signal I with

both [F] (Br2 bemg m excess) and [Br,] (F atoms bemg m excess)

could be used to demonstrate

dependence

of I on [F] and [BrJ

the

Fig 2a shows a plot of I agamst [F],, taken usmg the boxcar detector (30 ps gate. 10 ps delay) with excltatlon of BrF at 504 nm on the 5-O bandhead The detection sensitivity is 5 X 10” cm-’ and the plots of I versus [F] were linear over the enure range of [F] used ([F] < 2 x IO" cm-‘) This rather high detectlon limit was unposed by the background sqgnal due to Br, fluorescence arising from any weab super-radiance at X > 511 nm, and by the limited counting time (time constant) of the boxcar. Fig 2b shows d plot of I versus [Br,]. with F in excess. taken using excltatlon on the 7-O bandhead and the transient recorder to dccumulate decays. Again the dependence is hnear, ~lthin the scatter of the data caused by varIatlon m the laser power (typically -C + 10%). The detection hmlt for [F]. usmg this detection method and [Br,] X- [F]. was typically 3 x 109 crnm3 (signal: background = I). This sensltivlty was obtained by accumulatmg up to 4000 shots and integrating the resulting decay traces between 2 5 and IS ps after the laser pulse, so using only that part of the decay curve with the highest signal to noise ratio The background slgnal was the same. whether determined by removmg the Brz or F,, or by stoppmg the discharge. indicating that it was caused by residual scattered light from the laser and not by an impurity. Under the conditions of excess Brz used to detect F atoms m the kmetlc studies, the linearity of the plots of I versus [F] could be shown to be independent of the (fixed) [Br,] and of both laser power and reaction time. At the laser powers used the fluorescence signal I was roughly proportlonal to the laser power. showing that the transition was not appreciably saturated. The fluorescence decays from BrF (B). excited on u’ = 7, showed non-exponentlal behaviour, an initial rapld decay being followed by a more slowly decaymg section. The ti’ = 7 level is predlssociated at J 2 29 [21] hence rapld rotational redistribution mto these levels caused the Initial fast decay, slower vlbratlonal relaxation into the stable o’ levels causing the lifetime to increase at longer times 1221 Since linear

cxamrnrd by addmg Br, at different points along the tube No vari,ttton of I could be detected mdrcLltmg that \\a11 loss of F atoms was negligible (x-,, -C 8 SC’)_

0

05

10

15

20

25

[BrF] is small. all quenching and energy transfer processes are dominated by He bath gas (= 1 Torr) and the excess Br, ([Br?]< 1 x IO” cm-“). hence the form of the decay curve IS independent of [F] or [BrF] This 1s necessary in order for I to be lmear with [F] and can be further demonstrated by changing the Integration hmits used m the BASIC routme. no change m the hneartty bemg found. The use of long reaction times (20 ms) and lo\\ concentrattons of Br, ([BrJ = [F]) enabled the absolute F atom concentration to be determined hy titration wtth Br, Fig 3 shows the vartatton of LIF signal I with [Br,]. The mittal sectton of the curve rises lmearly w&h [BrJ. F atoms being m excess. unttl. when [F] = [Br,]. the curve tads off to tts hmttmg value due to complete removal of F atoms and further increase in [Br,] has no effect on the fluorescence signal I. Extrapolation of the initial linear section of the curve to the lmuting value of I enabled absolute [F] to be determined_ Since the flow rate of F2 was hnown. this allowed the dtssociatton efficiency of F2 in the mtcro\vavs dtscharge to be calculated as (SO + 15%). For the dilute F2/He mixtures used (< 0 02%) this \\as Independent of the ratio F,/He and this figure has been used throughout to calculate approGmate values of [F], from the flow rates of Fz and He. only relative [F] being measured during the kinettc runs. The rate of wall loss of F atoms could be

The ktnettc measurements ~verre tahen under pseudo-first order condrtrons wrtb [RH] 3 [F],,_ Reagent RH v\as added at a serteh of mlet Jets and .t ftled flow of Br, (t>picall> [Br,]= 4 X lo’-’ cnie7) was added mrmedr~tel~ before the fluorescence chamber. The [Br?] dnd the point of addttron \vere kept constant during the determmatton of each mdtvrdual fu-St-order rate. Values of the fluorescence intensity I,. for different F i RH reaction times I. were ratroed to that of the shortert reaction tmlr used (I,,). thts being arbrtrarrl? defined as time zero. Thts procedure beeps [RH] constant. preventrng qucnchmg b> RH affecting I_ The value of I(, \\.Is chsched dt the begmning. middle and end of each separate first order determinationenablmg an! variation in Lrser po\\er to be detected Provided that thts \\ds -C 205. values of I,, v.cre interpolated beween the measurements. hence correcting for an> slog s>stematic draft in laser po\\cr. This procedure is superior to using a separate po\\er detector to mcdsure the laser po\\sr. and normahsmg to this. smce it automaticalh Jccounts for any sdturdtton of the transitton An? abrupt changes m laser

po\\er could be detected as dtscontinunres m the frrst order decay plots of In (I,,/I) agamst time .md were dtscarded from the analysis. Typical pseudo-frrst order decay plots. for the rwctton F + CH,. are shown m ftg 4 These were shown to be Independent of the trme between .tddrtion of Br, and observatton of BrF. demonstr.umg the absence of the process Br + F2 - Br + BrF. The plots were analysed usmg a weighted least-mean squares program and a small correction (usually < 5%) was applied for axtal dtffusion The rate constants quoted are the mean values and the error _given IS lo. The results are SubJect to an

[CM,[/lO” full’) 3 54 -I-II 448 4 15 3 hS 371 403 406 3 57 3 62 142 468 -I 75 4 65 349 3 53 7 66 2 30 706 1 SS 17s -I 25 146 1 69 1 90 2 16 2 43 26s 2 93 3 36 3 63 3 96 444

addtttonal uncertamty of 10 to 15% to allow for the systematrc errors present m a flow tube elperrment such as diffusion. variatton in flow proftle and flow rate calibrattons 3-3

Renctror~ F-I- CH<:

The reaction of F atoms with CHF, was studied using the boxcar detector and excttatton on the BrF 5-O bandhead at 504 nm. Ftg 5 shows a summary plot of ftrst-order decay rate of F( ‘P) against [CHF>]. [F], was typrcally 9 X IO” cm-’ and [CHF3] was varted between 1.4 and 4.8 x 10IJ

[F[,/lO”

h’

(cm-“)

(5-

48 59 63 56 53 50 50 67 59 67 4s 50 50 53 3s 37 ‘8 24 22 19 50 43 13 14 17 19 21 23 25 29 32 34 38

7 7 7 7 7 7 s 6 6 5 9 9 10 9 9 10 9 IO 9 10 10 IO 11 1’ 11 11 12 12 12 12 11 12 11

ab Mean ~aluc of A, = (1 52&O 1O)X1O-‘3

cm3

molecule-’

’ 1

53 7 63 S 630 59 5 56 2 53 8 59 5 61 3 506 549 5s 2 67.9 740

6S 1 50 7 504 45 2 37 6 35 7 31 3 75 1 63 0 210 30 3 30 6 33 9 3s 3 39 9 43.7 486 55.7 60 7 690 s-’

k,/lO-” (cm’ n~nlecuIc- ’ s- ’ ) I 52

145 141 1.43 1 45 1 45 I 48 151 142 1 52 1 32 1 45 1 56 1 46 I 45 143 1 70 1 63 1 72 I 66 1 57 1.48 144 1.80 1 61 1 57 1 58 1 49 149 1.45 1 53 1.53 1 55

, -_ lij_(_____

summztsed in tabk 1. The mean value of k: &it 298 K WAS (1 5 -+ 0 1) x IO-” cm’ mole~ulc-’ s-‘-

-0.

.

l

.

.-

.

.

2j,/ 0

For the study of the raprd rexnon of F .uoms ~tth meth.me. ewtt‘ttlon on the 7-O bandhead of

-

, 10

FlS 5 Summa5

,

IC;F,l I ld3,cm3 LO

30

20

plot of L’ ( = -cl In[F]/dr)

13rF md signal ~\engmg. using the mmslen~ corder. \\a\ used to proxrde sensttne detrctron

50

~g~m\t [CHF,]

cm-‘. the upper hnut bemg Imposed by the throughput of the flowmeter. Moderate flow speeds (9 ms-‘) ensured adequate F atom decay down the tube and the fnst-order plots obtamed were linear over more than two hfettmes. The data are

I-dble 2 Summaq

of measurements

on I., (F&CH,)

ar 29s K A)

[CH,]/IO” -3 (cm )

IFl,/10”

3 74 37s

36 19

10 20

209 171 I 72 2 13 z 25 23s 2 6s 2 76 3 21 3 95 5 31 5 33 4S1 6 14 5 77 5 39 473 4 28 3 43 5 62 3 13 2 32

1.1 I I 09 09 14 i4 14 19 19 22 47 35 35 5’ 43 36 29 2.9 75 35 12 16

19 16 20 23 17 1s 10 15 17 IS II 15 14 12 13 15 16 15 1-l 15 t-t 14

a’ Man

WLl/IFl,

(cm-‘)

~.Aw of AZ = (6 6*04)x10-”

cm’ molewls-’

a-l_

rc-

of the ION concrntr‘~ttons of F atoms used. TypicA conditions \vere [FJ,, = 1 X IO” cm- ’ and [CH,] betl\een 1.7 and 6.2 x 10” CIII-‘ xxrth a Irne.lr flo\v Leloclty of 25 rns-‘. The rdtto [CH.]/[F],, wets ahx~ags greater th,m 10 and no correctton 1~~s necessxy for consumptton of CH,. Good pseudo-first-order plots were obtamsd o~t’r up to three hfetrmes. Fig 6 shoxvs thr xxrdtton of X with [CH,] and the data .u-e summartsed m t_thie 2. The mean value of k, at 29s K ~a\ (6.6 f 0 4) X lo-”

cm’ molecule-’

s-‘_

. . /

ICHJ 0 0

1

l-16 6 Summq

2

3

I

r.

plot of I,’ ( = -d InfF]/dr)

.

10”cni3 5

6

;~gnrnst [CH,]

4. Discussion Table 3 summdr’ses the previous absolute determmat’ons of X gyh and k_:yh and includes several ‘nd’rect and relat’ve determ’nations For the react’on F + CHF, good agreement is found w’th the v,‘lue of Clyne et al. [7] (k, = 1.9 X IO-” cm’ molecule- ’ s- ’ ). obtained usmg mass spectrometric samphng from a drscharge flow system under cond’tions of excess F atoms Agreement IS also close w’th the other direct measurement due to Goldberg and Schne’der [ 131 who used EPR detec-

Table 3 Rare deiermmatlon RGKt10n

F+CHF,

F+CH,

tion. with modell’ng of the cav’ty response and flow profile ‘n a d’scharge flow system. to determine X-, = 1.5 X IO-” cm’ molecule-’ s-‘. Reasonable agreement 1s found with the relative measurements of both Root and co-workers [23]. X, = 2 0 X lo-” cm’ molecule-’ s- ’ from “F nuclear reco’l stud’es. and of Setser and co-workers [24]. k, = 2 1 X IO- I7 cm’ molecule-’ s- ‘. from relat’ve HF product chem’lum’nescence distr’bution measurements in a flowing afterglow. In the latter case the agreement may be fortuitous since. for CHF3. the authors observed an anomalous variat’on of HF (L)’ > 0) em’ssion with CHFJ concentratton and suggested that the data might not reflect the reactant removal rate. However. for relat’ve rate determinations not employing the HF product chemiluminescence technique. the good agreement found between the three direct determinations of X-, suggests that this reaction may be reliably used to provide a more conven’ent standard than the F + CH, redct’on for calibration of the slower reactions of F atoms. The mean of the three d’rect determ’nation is Xiyh = (1.6 + 0.4) X IO-” cm3 molecule-’ s- ‘. The agreement between the d’rect determmations of kz9”. for the reaction F + CHJ. is also good. Wagner and co-workers [25] obtamed k, = 8.0 X lo-” cm’ molecule-’ s- ’ us’ng mass spectrometry m a d’scharge flow system w’th [CH,] >

for A 2 and X q at 298 K hlechod mass spectrometry IF] z= [CHF,] EPR F + NO chemllum’nescence HF product chem’lum’nescence ‘sF nuclear recoil this uork mass spectrome’ry [CH,] B [F) mass spectrometry [F] 30 [CH,] resonance absorption SF, muluphoton dlssoc’auon/ IR chem’iummescence F+ NO chemlluminescence laser pulse delay laser pulse delay

‘“F nuclearreco’l

‘h’s v.ork

h ZYh(cm3 molrculeI 9x IO--”

15x 1o-‘7 32x10-I3 2 1x10-‘x 20x 1o-‘q

15x 1o-‘3 8 ox lo-”

’ s- ’ )

RIZf 171 [I31 1271 P41

123) [Zj]

60x10-” 75x10-”

171 [41

5 7x lo-” S-10 x10-” 7.1 x IO--” 43x lo-” 6 1X10-” 6.6X IO--”

WI 1271 1291 WI ~31

[F]. F a1oms betng detected dtrectly III the absence of F2- Clyne et al. 171 obtamed k, = 6.0 x lo-” cm’ molecule-’ s- ’ using the method described above and Clyne and Nip [4] determmed X-, = 7.5 x lo-” cm’ molecule’ s- ’ usmg resonance absorption in a flow system Recently Fasano and Nogar [26] have used the multiphoton dissoctatton of SF,-product chemtlummescence technique_ to measure k, = 5.7 X IO-” cm’ molecule-’ s-’ In all cases agreement wth the present determmatlon of k, = (6 6 * 0.4) X lo-” cm’ molecule-’ s-I 1s reasonable and the mean of these fne direct determmatlons IS k”” 2 = (6.7 f 1.3) x 10” cm’ molecule- ’ s- ‘. The agreemrnt with the lels direct techniques [27-291 is rather more varied. x is also shown in table 3. The detection hmit of [BrF] >, 3 X 10’ cm-’ is set by scattered hght from the laser pulse and careful redesign of the fluorescence cell and collection optics could further reduce thts hmit. as would greater laser power, the transltlon not at present bemg saturated Thts may be compared with the detectton hmlt for OH by LIF of [OH] > 10’ cm-’ [30]. The OH radical IS a very much stronger absorber than BrF. havmg a hfetime 7” of 0.7 ps as compared with 60 ps for BrF. In addltlon the fluorescence from OH hes at shorter wavelengths than that of BrF. m a region where the photomultipher detectlon efflclency is greater. both effects contributing to the lower detection limit of OH. The good sensitivity shown by pulsed LIF detectlon of a comparattvely weah absorber such its BrF demonstrates the advantage of LIF as a detection system for kmetlc studies of many free radicals. Also the htgh detectton sensttivity for F( ‘P) makes this technique Ideally suited to the measurement of the more rapid F atom reactlons and of reacttons such as F + CHzCl, where secondary reactive products dre formed. Further xxorh on such reactlons will be reported later. Achnoaledgement We acknowledge gratefully support of this work by the Science Research Council and the US Air Force Offlce of Scientific Research (Grant AFOSR-78-3507). AH wishes to thank ICI (Mond

Divtsion) and the SERC for d CASE Alx.xd and Drs D. Husain and A.J. 4IcRobert for helpful adwce and critlclsm of thlh manuscript_

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