Volume 79,number Z. CHEMICAL PHYSICS LETTERS I5 AprlI 1981

A CO~~pA~SO~ OF THE HF LASER-EDUCED MULT~HOTON DE~OMPOS~rON

OF METHANOL AND 2,2,2-TR~FLUOROE~ANOL*

Darlene ANDERSON *, Robert D. McALPKNE, DX EVANS and HM. ADAMS Physrcol Chernntry Branch. Atonuc Energy of Canada Lrmted Research Company. Chalk Rarer Nuclear Laboratorres, Chalk Rwer. Ontarro, C&a& KOJ IJO

Recetved 12 December 1980

HF laser-mduccd multrphoton decompos!tlon (MPD) of 2,2,2-trlfluoroethaol, for the fiuence and pressure ranges 4-107 J/cm2 and 0 l-4 0 kPa respecrively. was studred and compared wth results for methanol As for multIphoton absorption (at low pressures), XIPD 1s more easily achieved for TFE than for methanol at comparable pressure and flJence

I introduction

Recently the multlphoton abscrptrcn (MPA) of HF laser photons by molecules contammg a common in- frared chromophore, the hydroxyl group, was studed [l] as a function of laser ra&ant energy fluence (E;) and reactant pressure (P). The ObJectlve of tlus study was to ldentrfy parameters which controlled the effi- crency of MPA and thereby to determme condlttons for wfuch efficient multrphcton decompcsltion (MPD) rtught be anticipated. The OH-contammg molecules were found to &wde mto two groups accordmg to theu size. For the large molecules, ethanol and 2,2,2-t+ fluorcethanol (TFE), a(E,P) (the absorptmn cross sec- tlon), and E((n), P) (the fluence requxred to give a par- trcular average excrtatlon, (n) for pressure P) obeyed the fcllowmg emprrrcal forms.

rr(E,P) = iY.!i+P , (1)

E(h),P) = ~‘(‘+b)((n)hciit/)l/~l+~), (2)

where LZ, b and K are empirical constants, and EiL is the wavenumber of the laser lme. Howevei, a(E,P) and E(bz >,I’) were more complex for the small molecules,

l Issued as AECL Number 7192 * National Summer Student Program 1980. Present address

Department of Chemistry, Uruverstty of Brrtrsh Columbia, Vancouver, British Columbia, Canada

CH,OH and CD,OH, and were not expressible III a sunple empirical form. The major difference between the large and small molecules was observed at low pres- sures (near “collwonless” condlttons). Under these con-

dhons, MPA for the small molecules was found to be considerably more difficult (required larger values of E for the me values of 0~) and P) than for the large molecules. As pressure WIS mcreased (self-colhslons were allowed to occur durmg the laser pulse) the dls- tmction between the large and small OH-conta~g molecules decreased.

The MPA results for the OH-containmg molecules lead to the expectation that “colhsionless” MPD would by very much more &fficult to a&eve for a small

molecule such as methanol than for a large molecule such as TFE. In order to test thus hypcthesa, we have undertaken a comparison study of the HF laser-induced MPD of methanol and TFE.

Due to practical hmitatmns, we are not able to study reactant pressures sufficiently low to a&eve “coihsion- less” conditions. However by obse&g the pressure dependence of product yrelds, it is stti possible both to tier the relative “colhsionless” MPD reactivitles and

also to study the effects of co&Ions on tis relative reactivity. These collisional effects can provide infcrma- tion of fund~en~ interest and are important for such key potent& applicatrons of MPD processes as laser isotope separation.

0 009-26 14/S 1 /OOOO-0000/S 02 50 0 North-HoUand Publishing Company 337

Volume 79. number 2 CHEMICAL PHYSICS LETTERS 15 Aprtl 1981

2 Experimental

A Lumomcs model 2 12A laser, previously described [2,3], was used for these studlzs For TFE, the HF P1(7) hne (UL = 3644.16 cm-l) [4] and for CH30H the HF P1(6) hne (VL = 3693 50 cm-l) [4] were used for all lrradlatlons reported here

Analysts of the non-condensable products (H, CO and CH,) by mass spectrometry was previously Is-

cussed [2,3] For several of the experunents, the reac- tant concentration was also measured (with an accuracy of about 28%) followmg laser Irradiation, usmg a

Per!un-Elmer model 2 1 infrared spectrophotometer tuned to the reactant OH-stretch mode. From measure- ments of both reactant depletion and product forma- tlon, the stoichometnc ratios (the ratio of the product molecules formed to reactant molecules decomposed) were obtamed.

Table 1 The effect of ~,7,2-trlfluoroethanol 1n1u.11 pressure on \aIues 01 I'll, and YCo [Y, = (number of product molecules/pulse)/(in~rlal

reactant prcssurc)] Expcrlments were pertbrmed uslny the HF PI (7) lmc focused by y 20 cm focal length anh-reflectance coated

Cc lens Tcmpcrature \xas 22 5 1°C _________~. _ _.~_ ._ ----

Pressure tt1

__~__ ___~ - --- -.---- ____

0 10

0 10

0 10

0 20

0 30

0 30

O-IO

071

1 00

1 7-o

1 20

1 23

1 33

I 33

1 33

1 35

1 59

1 59

1 59

1 78

1 78

1 78

1 79

2 00

2 00

2 67

267

267

3.33

4 00

3000

3000

3000

1500

2000

2000

1500

1000

700

1000

1000

1000

500

500

500

500

1000

1000

1000

300

500

500

500

300

300

100

200

200

200

200

1 o-l8 No

_ --_.

16’4 YH2 IO-l4 Y(-0 (molecules/ (molecules/

pulse kPa) pulse hPa) __ ____

0 879 0 12 12

0901 0 12 15

0 879 0 10 10

1 76 0 18 0 85

2 64 0 43 18

2 70 O-II 16

361 0 50 19

640 1 1 25

8 79 0 98 7-S

108 0 97 29

10 6 0 89 27

11 1 16 44

117 10 28

11 7 0 94 30

120 11 34

119 14 29

14 0 12 28

14 3 10 28

14 0 0 59 7-8

16 1 16 38

16 1 21 48

1s 7 18 47

15 8 10 36

176 io 4.1

17 6 12 41

24 1 0 54 25

24 1 16 4.9

23 5 14 38

29 3 0 82 31

35 2 11 33

338

Volume 79, number 2 CHEMICAL PHYSICS LETTERS 15 Apnl 1981

3. Results and discussion

3 1. Pressure dependence and MPD reachbrz kitletics

The pressure dependence of Yt+=, Yco and Yctq4 [Y, = (number of product molecules, x, Per pulse)/(mi- tral reactant pressure)] for the HF laser-mduced MPD of methanol was described m ref. [2]. Extrapo~ting the dependence of Y,(P) to zero reactant pressure gives

YH~(O)G~ X 10 =, YCo(0)d I X IO’” and YmJO) < 1 X 1011 For 0 17 <I’< 10 k.Pa, YJP) 1s hear in P, and above 10 kPa, begins to level off [2].

For TFE, measurements of YH~ and YCO, for the pressure range 0.10 <P 4 4.00 kPa are gven m table 1, and are plotted m fig 1. The scatter m these results IS larger than was observed for methanoi, and the func-

tlonal form of Yx versus P (approbated by the sold bnes in fig. I) 1s less clear. However, by eutrapolatlon, Y&(o) < 2 X fO13 and Yco(O) = (1 .O _t 0.2) X 10L4 for ?‘FE. Consequently, the “colhsionless” MPD yield of CO is a factor of 100 larger far TFE than 1s the maxunum “colhnonless” MPD yield for any non-con- densable product from methanol.

The general rate law for MPD IS pven by

KINETIC COLLlS10&S 1294K3 PER LASER PULSE

2.2 2 - TRIFLUORETHANOL PRESSURE (kPol

Fzg 1 The pressure dependence of YH~ (top graph) and Yco (bottom graph) for hfPD of 2,2,2-~iffuoroe~~o~ mduced by the HF P1 (7) hne focused by a 20 cm focal length fens

-dN,/d~z = QV” + (terms dependmg on product

concentrations), (3)

where N IS the number of reactant molecules, ttt IS the number of laser pulses, tz IS the reactton urder and k,, IS the tzth order rate constant.

The first term of (3) arnes from reactant MPD, whrle the second term nnses from such posslbrtties as product-product or product-reactant reachons, quenchmg due to product and further MPD of products. If the second term can be neglected (which IS probably vahd, at least, for small reactant depletion), (3) can be mtegrated to give

tt=l, WI

Y?Z 0 ( ,z _ r&pt, tt F 1, WI

where No and N,R are respectively the number of re- actant molecules in the cell mltlally and after m iaser pulses.

For TFE (mltlal pressure 1 00 kpa) measured values of N,,! and the number of product molecules obtamed after nz laser pulses are plotted m fig. 2. Attempted fits

FIN 2 The number of 2,2,2-trlfluoroethanol (m&ally at a pres- sure of 1.00 kFb) and product molecules present m the reactmn 0el.l (of volume 12 9 +- 0 4 cm3) as a funcUon of the number of laser pulses [HF laser Pt (7) ltnel .2,2,2-trhoroethanol IS shown as 0, II, as A and CO as 1. The dotted bne, m each use, IS a fit of the data to a fist-order form [eq (4a)l with kl = 7 X lCF5 put& and the sohd hne m each case, ES a fit to a second-order form with &z = 2 5 x 1513 moiecufe pul~?

339

Volume 79. number 2 CHEXlIC4L PHYSICS LETTERS 15 April 1981

of TFE and product concentration versus t?r for first- and second-order forms of (4) are &so shown m fig 2. ?Uthough both forms appear to fit the low-converston results (nr < 5000 pulses. N,,,/N, > 0 7), neither form appears to adequately descrtbe the hrgh-conversron resulrs (W > 5000 pulses, iV,~~/N~ < 0 7) The mabrhty to dntmptsh between first and second order demon- strates tnterpretrve problems that can arose when e-x- perunents, run at a stngle mrtrai reactant pressure (or over too restncted a pressure range), are used to deter- mme MPD krnettc orders. The mabrhty of enher fiisr- or second-order krnetrcs to adequately descrrbe the entne range of nz values may be due to the neglected terms for eq (3) becoming rmportant for htgh conver- s10ns.

dent of pressure for methanol (for the pressure range 0 169-14.95 kPa,theseare63 +4%Hz,24 k3%C.Q and I2 +-3%CH4, we assume that the storchrometrrc rattos are also pressure mdependent. For TFE at mitral pressures of 1 00 and 1.33 kPa, we obtain storchlometnc

ratros of 0 10 -i- 0 02 for I-12 and 021 + 0.03 for CO. Smce the non-condensable product proportions seem to change for P < 0 03 kPa (as can be verified from table l), it seems hkely that at least one of the storchto- metrrc rattos, for TFE. changes wtth pressures.

From the nle~surements plotted III frg 2, stotchlo- metric ratios were calculated For methanol, at an lm- tin1 pressure of IO 67 ~PJ these are 0 35 + 0 72 for Hz,0 14+008forCOandO06rO04forCH~ Smce the non-condensable product proportrons xe mdepen-

In ref [3-j, product yrelds for MPD of methanol for varrous rmtral reactant pressures were reported. Usmg those data, and the determtned stoichrometnc ratros, values of N,?, were obtained From N,r , No and nr (grven m table 21, values of k, and X-z were cakulated for each uutrai methanol pressure, and are plotted on a log-log scale tn fig 3 The term X-, IS nearly mdepen- dent of pressure, while kl = P, mdlcatmg that k3 is a true rate constant, and the HF laser-mduced MPb of methanol IS second order m reactant pressure_ We at-

Table 2 C~lucs ofk, andk; [eqs (a)] calculated for the EIF Itier-Induced MPD of methanol from data reported UI ref. [7-l C\perlnlents

mdlcattd b) (+) \xcrr’ pcriormcd at n rempemtutc of 16 5*C All others \\ere performed at 22 + 1°C _- ---__--~ --~____ ____ __-_____

Intt~sl methanol pressure

(IJW ~--__- ~-___--_

0 169

0473

0 674

0 943

1 347

1 350

2 694

4 045

5 388

6 736

8 082

8 760

9 430

10 107

20 773@-)

10 776

10 783

13 47(e)

i4.95(+)

I?1 1O-‘8 IS0 lO-‘8 h’nl IO5 K, fpulse-’ )

ioz4 li’:! (molecule pulse-’ )

20000 0 816 0 788 0 175 291

10350 2 05 1 95 0 4x3 2 67

5000 3 26 3 17 0 560 I 74

3300 4 55 4 40 1 02 2 27

x00 651 6 21 1 89 2 97

2500 6 52 6 21 1 9.5 3 06

2600 I30 117 4 0.5 3 29

500 19 5 18 8 3 18 3 82

500 26 0 24 9 8 65 3 40

500 32 5 30 8 10 7 3 40

500 39 0 37 0 10 5 2 77

500 42 3 39 9 II 7 2 84

500 45 5 42 4. 14 1 3 21

500 48 8 45 7 13 1 2 78

400 52 0 490 f4 9 2 94

500 52 0 49 1 115 2 27

500 52 1 48 5 14 3 3 11

400 65 1 61.0 16.3 2 58

400 72.2 680 15 0 2 14

340

Volume 79, number 2 CHEMICAL PHYSKS LETTERS 15 Aprti 1981

I Illllll I I Iillll I Illlltfz

d’ IO ( I illfill I lttlfill

10 t ICI ID

FIN 3 Values ofk, nndkz [eqs (4)] for HF laser [PI(~) he] mduced MPD of methanol at VICTIOUS mltwl reactant pressures k, = P whtie X-, IS independent of P demonstratmg that this MPD IS best described as second order for the pressure range studted

tnbute the second-order behavtor to coUtstons piaymg

a major role m the HF laser-mduced MPD of methanol Prevtous studres carried out over a more restricted pres- sure range (0.07-I .73 kPa) concluded that the CO2 laser-mduced MPD of methanol was first order IS] . However, for thrs small pressure range, It nnght be dif- ficult to drstingu~sh first- and second-order kmetics.

For TFE, the storchrometric ratios probably change for P < 0 3 kPa and the results of Yx versus Pare more scattered than was the case for methanol. Consequently, applymg a procedure sirntlar to that outhned above for methanol, does not provrde such clear results. However, forTFEO3GPG4kPawefindkt aFandkZaP-d, where c and d are each between MI.5 and 1. ‘l&s result

hkely indxates that the HF laser-induced MPD of TFE

over this pressure range is best described as between frost and second order. Such kinetics would result, for example, rf there were a transttion, over the pressure range, from prnnanly fist-order (“colhsionless”) to second-order (colhsionally assisted) kmetlcs,

The HF laser beam was focused with a 114 cm focal length CaF, lens to produce a focal cylinder of area

2.6 X lOa cm2 and length 3.4 cm. Into thrs focal cyhnder wds placed a 0.9 cm long cell (volume 15.6 -+ 1 .O cm3). The fluence (which was varied by the use of filters) was constant for the n-radiated reactant mole- c&es wtthm the focal volume. Values off, the fraction of the trradiated molecules decomposed per pulse, for TFE at pressures of 1 .OO and 1.33 kpa, and for me&&a- no1 at a pressure of 10.67 kPa are grven 111 table 3 and are plotted m fig. 4. For TFE, the 133 kPa results (at least for E > 60 J/cmz) do appear to gwe slightly smaller f than the 1 00 kPa results for the same E. However the fluence uncertatnttes (due to pulse-to-pulse vana- trons tn the laser radtant energy) are too large to con- clude confidently that these drfferences (whrch from fig. 1 would not be expected to be large) are real. We have thus chosen to consrder all of the TFE results

0 1Ll 20 30 JO 50 i?a 7a

I 1 I I I I I 1 I 1

IIll I I I I I I I I I I I 0 50 IO0 150

FLUENCE (1 ( cm’)

Ftg. 4. The fraction, f, of uradlated molecules decomposed/ pulse as a function of fluence for 2,Z,Z-~~uo~oe~anol at a pressure of 1 00 kF% fo), 1 33 kPa (A) and for methanol at a pressure of 10.67 kPa (X) The sobd ime IS a fit of the TPE results to the formf= e -O/E The average number of photons absorbed per molecule, 00. for 2,2,2+nfluoroethanol (from ref. [I I) is shown for the reader’s converuence.

341

Volume 79, number 2 CHEMICAL PHYSICS LE-lTERS 15 April 1981

Table ? The et fecr of radtant cnerp) !luence on the HT I~ser mduced MPD of 2,2,2-trtfluoroethanol and methanol

___---- _-__-~

Fluence (J/cm’)

Ml IO-” YH2 lo-tZ YCO 10-t’ YCH, f (molecules/ (molecules/ (molecules/ pulse kPn) pulse bPa) pulse kP3)

~- -

I 00 bPa of 7,,2.7-tntluoroethnnol, HF PI (7) fme

95 _’ 17 4000 25 46

88+ 15 3000 31 66

81 i 17 4000 20 38

59% IO 6000 11 21

58% II 6000 8 3 16

26 I 4 25000 049 0 92

21. + 5 10000 0 32 0 81

1-i ? 3 20030 0 15 0 21

043

057

0 35

0 19

0 is

85x 16’

64x lO-3

23x 10-3

I 33 hPa ot 2.2 ?-trxfhmroethanol, HF Pt (7) lme

107 r 19 2000 26 37 Cl 1 040

86r 15 2000 16 25 G53 0 26

77t 30 2000 18 30 G3.5 0 29

642 11 3000 8 1 15 c54 0 14

28 z 4 10000 I4 15 < 0 22 19x Id’

16 f 3 31500 0 13 0 16 =z 0 022 !9X lo-3

10 67 lPa of methanol, HF PI (6) hnc

-16 i 8 5000 5 0 061 60 1 7 x 10-Z

39= 7 5000 0 36 - 051 3 2 x Lo-3 -__- ---_ -___ -----._

together. The results fit weli the equation

f = ,-E,IE 1 G?

which Fuss [t;] used to fit vah~es off versus E for SF6 under coklonless conditions. The sohd lme U-I fig. 4 ts a plot of eq_ (5) for E0 = 90 J/cmf! As E mcreases,

log f approaches zero (f approaches 1) m a manner slm&r to that reported for other systems [6-8]

Values of 02) versus E, obtamed from ref. [l] , are shown m fig. 4 for the two imtlal pressures of TFE studled. We are just able to detect decomposlhon for a threshold fluence ---lS J/cm?- - Thus corresponds to (n) = 5-10. Between 250 and 500 photons are absorbed for each molecule decomposed. For f = l/e, E. 52: 90 J/cm” and 110-140 photons are absorbed for each molecule decomposed. Smce probably 8-10 HF laser photons collectively have sufficient energy to decom- pose a TFE molecule, less than 7% of the photon en- ergy is used m chermcal conversion. The rem-g en-

ergy must be colhaonally degraded durmg and following the laser pulse.

342

The two measured values off (table 3, fig 4) for MPD of 10 67 Mza of CH,OH are almost a factor of IO Iess than those of TFE for the same E. However, for a focused laser beam, YX mcreased linearly with pressure for methanol 121. Correspondingly, one would expect f to mcrease urlth pressure, and consequently values off for CH,OH are probably =0x)1 those for TFE when fluence and reactant pressures are comparable. Attempts to measure f for methanol pressures much below 10 67 k.Fa were unsuccessful due to the small product yields obtained for fured fluence irradlatlons usmg cylindrical geometry.

4. Conclusions

The various studies reported III this paper support the view that collisionless HF laser-induced MPD is more easily achieved (occurs at lower fluences) in TFE than m methanol. The extrapolated zero-pressure yield for CO from TFE 1s more than loo-fold that of any of the

Volume 79, number 2 CHERilCAL PHYSICS LEI-l?ERS 15 4pril 1981

non-condensable products from methanol. Even for very low pressures, the MPD process in methanol ap- pears to be mainly coUrsrona!ly assisted. In additton to colhsionless MPD, colbsronaly assrsted processes are occurnng in TFE. However, the levellmg off of the yreld versus P occurs sooner m TFE than m methanol, indrcatmg that the pressure range over which they are effective IS not as large as m methanol. Possibly col- hsronal quenchmg processes are more efficient for the large molecule, TFE, than for the small molecule, methanol.

Values off, corrected to the same pressure and fluence, also mdrcate that TFE 1s at least IOO-fold more easily decomposed than 1s methanol.

References

[ 11 R D hIcAlpme. D K Evans and F K hlcClusby. J Chem Phys 73 (1980) 1153

121 R D McAipme D.K EIIans and F K hlcClusky,Chem Phys 39 (1979) 263

[3] D K Evans, R D hlcAlpme nnd F.K hfcClusky, Chem Phys 32 (1978) 81

[4] T F. Deutsch, Appl Phys Letters 10 (1967) 234 [5] S E. Blalkowsbl and W A Gudlory, J Chem Phls 67

(1977) 2061 [61 W Fuss.Chem Phys 36 (1979) 135 [7] K hl Lenry, J L Lyman, L B Asprey and S hl Freund,

J Chem Phys 68 (1978) 1671 (81 D hl CO\, R B. Hall J A Horslry, G Xl Kramer, P

Rabmowtz and A Kaldor, Saence 20.5 (1979) 390

343