Chemical Physics 79 (1983) 277-288 North-Holland Puhiishing Company

277

DEUTERIUM SEPARATION AND INFRARED PHOTOCHEMISTRY IN CF,HCI

J. MOSER. P. MORAND. R. DUPERREX * and H. VAN DEN BERGH

Insrirur de Chimie Phpiqrrc. EC& Po!r:cchrriqur F~d~ruk Luu.wnne Sa-irxrluod

Received 28 March 1983

Deutrrium separation has been demons1ratcd by studying the infixed mulripholon dissouiarion of CF2DCl Itnd CF,HCI/CFzDCI mixtures. 11 is shown how the parametrrs which control the infrllrcd pholochcmisrc- of CF,DCI such ;1s rhs

temperature and the pressure. as well as the wavelengrh. the energy fiuencc and rhe intensity of rhs laser pulse. c;in bs optimised for deuterium separation. Infrared analysis of the disappearing CF,DCl 2nd CF?HCI peciks indicates isotopic selectivities in excess of 104. Dissociation yields of up IO 5%. are observed at flusnccs around 6 J cm-’ with 30 ns pulses at pressures in the range 0.3 to 150 Torr. A new route is presented for redeuterAon of the drutcrium-drplswd CF,HCl IO natural abundance by way of high-temperature exchange in the _eas phase.

1. Introduction

During the past few decades a multitude of new methods have been devised for the separation of isotopes [l]. If we consider the more efficient of these techniques we find large energy consump- tions per separated atom/molecule. Often the en- ergy required to separate isotopes can be as much as 10’ times the minimum free energy required for this process due to the entropy of mixing. Thus there remains much room for further improve- ment_

The advent of the laser as a source of well-col- limated high-intensity radiation of narrow spectral bandwidth has stimulated the discovery of many new schemes of laser isotope separation (LIS) [2-S]. One particular method of LIS is infrared multiphoton dissociation (IRMPD) [6.7]. Many isotopes have been separated on a laboratory scale using IRMPD [S]. and its possibilities for separa- tion on a larger scale have been demonstrated for 13C. 32S and 33S [9,10].

The work we describe here is concerned with deuterium separation from hydrogen. Large quan-

* Prcsenr address: IBM Research Laboratories. San Jose. CA.

USA.

0301-0104/83/0000-0000/$03.00 @ North-Holland

tities of druterium are needed as a moderator in

hca\y-water nuclear reactors. Furthermore the

separation of deuterium presents a particularly

challenging problem for two reasons: First. for use in a heavy-water reactor \ve must enrich from natural abundance D in H of about 1 in 7000 to at least 99-752 pure D,O. This implies an enrichment of = 10”. Second_ fairly efficient chemical meth- ods for deuterium separation have been known for some time [I I]. Thus any new method such as IRMPD must be able to compete favourably with these established methods which have already been optimissd over many years for the larse-scale pro- duction of D,O.

Several groups are trying to find a workable photochemical separation for deutcrium by IRMPD [12-201. and it appears that rhc most

favourable case studied so far is the IRMPD of

CF,H. Even so it is not clear if in this molecule the isotopic selectivity is quite high enough for it to be able to compete for instance with the much used Girdler-Sulfide process [ll]. One might remark that one of the disadvantages of the latter is the presence of large quantities ot highly toxic H,S gas at the plant site.

We wish to report on the IRMPD of the CF,HCl for deutrrium separation. Encouraging results are

The Ino folhnving nicthnds have hcen eni- pL,c‘d. FirsI. perflu~my3rop~ he oxide is lie;lIed IL) t SO-700°C in Ihc presence of 3 large esce.is I = IUO- ItlW times) of DC1 in ;i plass bulb aI :I I.1121 prc33ure of less I!ian I aIni. After I5 Ii the hor twlb is c~r~llcd ;ud rhe s_as hrougtlt into COIIIXI

xx1111 ;I >t)luIiL)n <li XaOH in mxwr to get rid of the IX‘1 aat rttc Cr_l..COF formed in rhr deccxnpmi- Iilw af IflC tlcxafluoropr~~p~le~~e oxide_ The i'f DCI is k:n dried hy passing it rhrough ac1ivaIcd silica gel. frerzr-pumped and checked for IsoIopic purily by infrared (IR) absorption spsc- I r<bcnpg. When srarting with DC1 (995 pure).

made from ihe reaction between D,O and brnzoyt chloride heated under ~~;ICUUIII for = 1 h xvith a hoI air gun. \ve obtain CF,DCt \vith better than 98% D. The gas-phase reactions are

.O ISO-Z~HPC

CT- CF-CF, ---t :CF2 + CF,-COF. it1 15h

.A secwld method for preparing CF2DCl is Ihc rc:IcIicm hcrmxxn CF. HCI and again a large cscc~~ c>l’ DCt in :I quartz huth at X!5-500°C. Hcrc \vt‘ liavc Ilic rcac~inn secl~~ence

51

c-t-. HCI - :CF. t HCt. (-1) _125_‘,&,‘(~ -

I1 :C‘F Z - (aces> DCI) s CF2DCl. (5)

kvticrc the reaction time 31 stcvated Icnipcr:iIurr’ \v;is hst\vccn 2 min and = I Ii depending on Ihc Icmperaturc. Again the fornxt~ion of C,F, by re;Lc- rim (3) is nearly carnplstely avoided hy keeping the decomposition r;iIe of CF,HCI low and lx

using a large excess of DCI (ar leas 100 x ). The CF?DCl is distilled under V;ICUUI~ and freeze- pumped. The isotopic purity of the retaction prod-

uc1 is found IO be better Ihan 955;;.

X1os.I irradiations are carried out in Pyres cells \viIh inIernat diameter 20 mm or 40 mm and an optical path lsngrh of 10 cm. fitted \virh NaCI xvindcnvs glued \viIh Araldits. These windows are either at 90” to the incoming I:Iser beam or at Bwvster‘s angle. The latter hold significantly higher radiation flwnces (intensities). These rex- tion cells are equipped with ;1 T&ton stopcock. The high-Irmperarurs measurements are curried out in a brass cell with internal dixmrtrr 30 num and 9 cm optical path length which is also equipped Lvith NaCI end windows at 90” IO the laser beam. Heating is done electrically and the temprrrrture is measured IO f 4°C.

2RO J. Moser er al. / Dewcrium separation and infrared phorochrmisr~ in CF2 HCI

7 ? Pressures A._.

Pressures are measured absolutely with a capac- itive manometer and concentrations are checked with a Perkin Elmer model 521 IR absorption spectrometer.

_‘. 4. Isotopic selectivity

This quantity is defined here as

isotopic selectivity =/(CF,DCl)/f(CF2HCl), (6)

where f(M) is the fraction of the isotope hl de- composed per laser pulse in the irradiated volume. j(M) is determined from the relation

[Ml,= [Mlotl -f)“. (7)

where the subscripts 0 and II indicate respectively the initial concentration of M and the concentra-

tion after II laser pulses. We have defined the isotopic selectivity in this way as it is independent of the number of laser pulses. Previously the selec- tivity has been defined by

p = D/H(final)

D/H(initial) ’ (8)

which does depend on the number of pulses [14].

The Beer-Lambert law has been verified for all applied conditions.

2.5. The temporal shape of the laser pulse

The time dependence of the laser pulse intensity is measured with a photon drag detector and dis- played on a 100 MHz recording oscilloscope. The laser pulse length can be adjusted with a “passive” plasma switch which consists of a 35 cm long 5 cm internal diameter Pyrex tube with two NaCl end windows positioned between two 20 cm focal length lenses. The laser beam is focused into the center of this tube and rendered parallel again after leaving the tube. Varying the pressure of the better than 99.99% pure He in the tube changes the pulse length of the transmitted pulse. Some typical pulses obtained in this way are shown in fig. 1. The pulse-to-pulse repeatability of the plasma switch is quite good, and is shown in figs.

Fig. 2. Fluence profiles measured horizontally (s) and verti- cally (_v) across the center of a bum pattern with 7.5 J cm-’ on the 10R28 line. The three different bum patterns kdicated are obtained at different sensitivities. showing how arbirrary a bum pattern can be. These measurements were made a: 130 cm from a 2 m focal length KC1 lens. by moving a 1.1 mm diameter pinhole across the beam in front of a thermopile.

lb-ld. By varying the He pressure between 150

and 400 Torr, the pulse length (fwhm) can be varied between about 30 and 180 ns without tail- ing (see figs. le-lh).

2.6. Spatial energy jhence distribution

Ideally certain IRMPD experiments should be carried out with a homogeneous spatial energy fluence distribution in the laser beam (and a square-wave temporal distribution). Here energy fluence is defined as F = J’Idt where I is the light intensity per unit area. The use of such tailored

Fig. 3. Digital histogram of the fluence distribution in the laser beam shown in fig. 2.

laser beams would facilitate the sometimes in-

volved deconvolution of IRMPD results [27.28]. which would lead to better reproducibility of cx-

perimental results between laboratories. Corrrc-

tions of the spatial energy fluence distribution are

not carried out here. as our beam has a fairly

homogeneous distribution, and corrections are ex-

pected to be relatively small. Typical fluence pro-

files as measured across the beam horizontally and

vertically are shown in fig. 2. These data are obtained by moving a 1.1 mm diameter pinhole

across the beam in front of a thermopile to mea- sure the “local” energy fluence. Fig. 3 shows a

two-dimensional histogram of the beam. The aver-

age fluence over the whole area is taken to be 100 in relative units. The total area is limited arbi-

trarily by the sensitivity of the detection system.

3. Results and discussion

Let us now consider some of the main require-

ments for an effective isotope separation process

using IRMPD and the relation between these

requirements and the basic photophysics of

CF,HCl/CF,DCI mixtures. Of course we must keep in mind that several of these points may be interrelated_

(1) Isoiopic selecriviry: The IRMPD should be

as selective as possible. If one wants to attain the

enrichment from natural abundance to better than

99.7% D in one step, a selectivity in the order of

lo‘-’ is required. The possibility of finding such a high isotopic selectivity in IRMPD depends in the

first instance on the spectral properties of the

isotopomers. Hence we show the IR spectra of

CF2HCl and CF,DCl between 700 and 1500 cm-’

in fig. 4. The region of interest here is that attaina- ble by the CO? laser which we use. which is

roughly situated between 900 and 1100 cm-‘. The

peak in the CF,DCI spectrum around 1015 cm-’

appears to be ideal as little or no absorption is

evident in CF:HCl near this wavelength. To a

large extent it 1s this difference in IR absorption near the selected wavelength of the laser that is

used to optimise the isotopic selectivity. We can

thus choose the experimental conditions so that

the CF,DCl will dissociate efficiently (i.e. it is just

a c&se B or reaction threshold bo:rlsneck molecule

[29.30]). whereas CF,HCl will dissociate quite inef-

ficiently (i.e. it is typical of a case C or early

bottleneck molecule [29.30]).

From an economic point of \-iew selcctivities in ~SCCSS of only lo-’ arc required. as enrichment

beyond 1% D abundance can be done well \vith

classical methods. High photon selectivity (dis-

cussed below) and high reaction yields are more

important economic criteria.

(2) Cheup phoro~~s: WC use a CO, laser which is an efficient. ~vcll-developed snd reliable photon

source between 9 and 11 pm [_;I].

(3) Phoron efJcienq-: This is quite a complicated

point with many iruenvoven facets. First. we try to

look for a process in Lvhich we act on that minor

isotope (CF?DCl in this case). Second. the IRMPD of this isotope must be efficient at the applied

conditions as mentioned above (case B). Simplified

this implies several conditions_ The density of states

J

Fig. 5. (a) Absorption of 2 Torr of CF2DCI a~ room temprra- ture wim optical path length 10 cm. (b) Absorption of 760 Torr CPzHCl a1 room lcmperaturr ( -) clnd 200°C (- - -_) also with I = IO cm.

of the molecule to be excited should not be too small, and there must be a reasonable oscillator strength available near the wavelength of excita- tion. The extent to which these conditions must be fulfilled depends on the laser beam itself [29]. As WC shall see below, reaction yields of 5% have been obtained at the rather modest fluence of 6 J cm-‘.

A third factor important to photon efficiency is that the absorption of photons by the major com- ponent of the mixture (CF,HCI) must be abso- lutely minimal, so that no photons are wasted in excitation of this isotope. This is a particularly stringent condition as the CF2HCl is present in a

n

R l

l 1 I I I I I

988 984 980 976

Wavenumber /cd

Fig. 6. IRMPD “spectrum” of CFaDCI between 976 and 988 cm-’ at 298 K (0) and 473 K (I). taken with laser pulses of 30 ns fwhm and 6 J cm-‘.

7000 fold excess. Fig. 5 shows the IR absorption spectra of 2 Torr of CF,DCI compared with that of 760 Torr of CF2HCI at 25 and 200°C. As indicated. in the region near 985 cm-’ which shows minimal absorption of the major isotope, one must hope for a maximum in the IRMPD yield for the minor isotopr CF;DCl. Hence we have measured the latter, and it IS shown in fig. 6 as a function of the wavelength at 298 and 473 K for a fixed energy fluence of 6 J cm-‘. In these experiments a pulse length of = 30 ns fwhm was used at all wavelengths. As we hoped. the maxi- mum in the IRMPD yield of CF,DCl at both temperatures are red-shifted with respect to the low-intensity infrared absorption spectrum just so much that they are close to 985 cm-‘. where

CF,HCl absorption is minimal. Preliminary measurements of the low-fluence

photon selectivity (3 J cm-‘) gave values of ( )I) o/(t~) r, between 30 and 500 depending on the total gas pressure. and we are currently investigat- ing this quantity in more detail. (II) is the average number of photons absorbed per isotopomer. Effi- cient use of photons requires (II) D/(tz)k, to be = 5000 or larger. Although it may not necessarily be the case here, the value of (u)D/(tl) ,, can often be improved by using multiple-wavelength excitation.

At this point it may be worthwhiie to look at the temperature dependence of the IRMPD of CF,HCI and CF,DCI. The temperature depen-

loo

1140 1120 1100 lO%J 1060

v/cm-’

Fig. 7. Low-intensity transmission spectra of CF,HCl at (a) 298 K and (b) 523 K. The spectral resolution is 0.5 cm-‘, the pressure 1.3 Torr. IRMPD “spectra” of 0.3 Torr CFaHCl at (c) 298 K and (d) 523 K. The laser pulse was 300 ns long (70 ns fwhm) and has a fluence of 4.5 J cm-‘. The fraction dissoci- ated per laser pulse in the irradiated volume is indicated in relalive units.

dence of the IRMPD process has been discussed previously in the literature in SOIW detail [32.33]. The measured temperature effects arc sl~ow~l for CF,DCI in fig. 6. and for CF,HCl in fig. 7. One important reason for such tenlperature drpcn- dences in IRMPD is the change in the distribution

of rovibrational states. At different tenlpcraturcs different sets of rovibrational states will be pumped. resulting in a changed IRMPD effi- ciency. III fig. 7 we see a slight red-shift of the low-intensity absorption sprctrum upon ternpcra- ture increase. A larger red-shift is found between the low-intensity absorption spectra and the IRMPD spectra. If we speculate that the shoulder and peak observed in the two IR spectra near 1107 c111- ’ are reflected in the shoulder near 1079 CIII- ’ and the peak near 1087 cm- in the 1RMPD spectra (at respectively 523 and 29s K). this red- shift is = 20 cm-’ at 298 K and 28 cm-’ at 523 K. If this speculation is correct the IRMPD sprc- trum of CFzHCl is not much wider than the

A

_- . .

. I.

. I .=

2.0 I I I

0 02 0.4 Q6

PCF&O”

Fig. 8. Plots of the fraction of CF?DCl dissociated per laser pulse (/) against the CF,DCI pressure are used IO establishe the essentially collision-free region of IRMPD for (A) a long

laser pulse of = 2 ps duration. and (B) a shorter laser pulst of

= 60 ns duration.

corresponding nornxd IR ahsorption spectrum of this compound. This observation may be con- trasted with observations 011 SF, where a IWJC~ Wider IRMPD spectrum ~1s ohservcd as ccun-

pared to the corresponding low intensity IR absorption spectrum [34]. One csplanation for this difference hetwwn CF,HCl on one hand and SF, on the other is that the latter is probably a case B tnolecule at the applied conditions. \vhereas the former is predominantly a case C ~nolrculc. As case B is characterised bv a coupliug which is greater than the mean distance between states ( V B S ) [29]. and the frequency range (.I) whew the interaction between irradiation field and mole- cule is significantly large. OIW might cspect such an interaction over a wide frequrncy range in the case of SF,. In case C 1n01ecules (1’ -X 6) the frequency range where the interaction is signifi- cant is smaller. and hence much less spectral broadening is expected in going from the ioxv-in- tensity absorption spectrum to the IRhlPD sprc- trum. In this context we might rspsct that at even higher intensities in the case of CF, HCl or CF, DC1 the IRMPD spectrum might he broadened as case

Fig. 9. The intensity dependence of the IRXIPD yield as a function of the laser energy fluencc in ths decomposition of CF2 DC1 under essentially collision-free conditions. The reac- tion yields obtained with a short laser pulse of 60 ns dumtion (0). i.e. the high intensity yields. are _esnerally larger than the yields obtzained with a Ionyx laser pulse of = 300 ns duration (W of lower svcmge intensity. A rough correction for the flucncs distribution of the beam show that the corrected curves merge at the higher flucnccs rather than cross. and that the high-intensity yield is slways bigger rhan the yield ZII lower

intensity. Given the scatter in the experimental data an exact correction for the flusncc distribution is not possible 1271.

B is approached. In general such a spectral widen- ing will be possible for molecules where the

absorption spectrum changes strongly with varying

internal energy.

It should be mentioned that these IRMPD

spectra, as well as some other IRMPD data shown below. are established under what we call essen-

tially collision-free conditions. What we mean by

this is best shown by looking at fig. 8 where the

essentially collision-free regions are established for a “long” laser pulse (> 95% of the energy dis-

sipated in = 2 ps, see fig. la) and a “short” laser

pulse (> 95% dissipated in = 60 ns. see figs. 1 b.

lc. Id and 1 h). The regions at pressures below

respectively 150 mTorr for the long pulse and 1

Torr for the short pulse where the IRMPD yield is

essentially independent of the pressure are consid-

ered as essentially collision free. Although colli-

sions probably do occur during the IRMPD pro-

cess in these low-pressure limiting regions. they do

not influence the total yield in an appreciable way.

In using shorter laser pulses to photodissociate

CF,DCl and hence higher laser intensities at the same energy fluence, the reaction yield of the case

C molecules increases as we approach case B. This

brensiry dependence of the IRMPD yield shown in

fig. 9, for the collision-free case, increases at lower

fluences as may be expected. At higher fluences and intensities, as case B is approached. it disap-

pears_ Similar intensity dependences have been

observed and discussed before [35-381. LRF plots,

which are often used to represent such data as

shown in fig. 9 [39-411, are not used here because

at these rather low yields it is doubtful that steady

state is attained in the IRMPD. If steady state is

not attained. although a LRF plot may still give an approximately straight-line relation, it is not sensi-

ble. The main conclusion to be drawn from the

intensity dependence shown in fig. 9 is that for

most of the applied fluences, and in particular for

the lower fluences, an enhancement in the IRMPD

yield may be expected if we would use even shorter

laser pulses. This will be necessary if we wish to do

experiments at pressures near 1 atm as will be

discussed below.

A fourth factor concerning photon efficiency is

that the reaction products themselves should pre-

ferably not absorb IR laser photons. This should

not only be so for the stable products which have

been thermaiised. but also for reaction products produced in excited states which might absorb at

quite different wavelengths. It should also be men-

tioned here that of course the reaction products

should be stable so that back reaction cannot occur. If not. the reaction intermediates must be scavenged chemically or removed from the reac-

tion mixture by other means.

Photon efficiency also implies using as few as

possible photons to dissociate one molecule. There

are two aspects which must be considered here.

First. the endothermicity of the dissociation pro-

cess must be selected to be as small as possible.

The case of CFzHCl which dissociates mainly to

:CF, and HCl at the applied conditions. has a

reaction threshold of only = 55 kcal mole-’ [42]

which corresponds to = 20 photons per molecule.

Second, as few photons as possible should be

absorbed above the reaction theshold by the disso-

ciating molecules. The number of photons ab-

sorbed above the reaction threshold depends es-

sentially on the competition between optical pumping and unimolecular reaction rate constants

[43]. For a given molecule, and hence a given set of

unimoIecuIar rate constants, which depend on in- ternal energy and angular momentum quantum number [44], the faster the optical pumping the higher the level of internal energy from which the

molecules will dissociate. As fairly fast optical

pumping, i.e. high laser intensity, is required for

effective IRMPD at atmospheric pressure, only molecules which have appreciable unimolecular

decomposition rate constants near, the reaction

threshold will decompose before being excited

highly above the reaction threshold. This is the so-called small-molecule criterion. Smaller mole-

cules tend to have a relatively small density of

states, p(E, J) for a given amount of internal

energy E. The density of states is related in a

particularly simple way to the unimolecular de- composition rate constant just at the reaction threshold where [33]

kc&,, J) = (d/h)p(E,, J). (9

d is the reaction path degeneracy which is equal to

1 here. In small molecules with few internal de-

grees of freedom the unimolecular rate constants

!? b

fl

90- C

I I I I I , 3000 2000 1000

Wavenumber / cni’

Fig. 10. (a) Transmission spectrum of 0.3 Torr CF,DCI with 10.5 Torr of CF2HCI. The bands characteristic of fust either one of the isotopes are indicted. (b) The same bands expanded

5 X _ (c) The same mixture after irradiation with 500 laser

pulses of 70 ns fwhm and 6.3 J cm-’ at the IORX line. The

CF, DC1 has disappeared essentially completely whereas the

CF, HCI remains essentially constant.

will furthermore increase rapidly with increasing internal energy above the reaction threshold. Thus even molecules undergoing very fast optical pump- ing will start to undergo unimolecular reaction just above the reaction threshold.

Another criterion for the efficient use of pho- tons. as has been mentioned above, is fast optical pumping. If isotope separation is to be carried out at relatively high pressures near 1 atm. collisional effects may start to interfere with the IRMPD process unless the duration of the optical pulse is quite short. Although a small number of collisions may be beneficial to the IRMPD yield for case C molecules [43,45], a large number of collisions will always be detrimental to the reaction yield. Fur- thermore collisions can influence the isotopic selectivity of IRMPD in two ways. First. selectiv- ity can be diminished by collisional scrambling. Second. selectivity can be enhanced at higher pressures by stronger quenching of the slower

I 1 ,

50 150

P(CF2HCIl/Torr

Fig. I I. The fraction of CF,DCI dissociated per Iaser pulse in

the irradiated volume as a function of the CF,HCI pressure

added under different conditions. The pulse Iength is 30 ns fwhm. the fluence is 6 J cm-‘. and the CF,DCI pressure is 0.4

Tow. 0 Xkasursmenrs ar rhc IORX line and 19s K. C

measurements at the 10R34 line and 173 K. ti measurements at the IORZS line and 473 K.

pumped isotope [46]. for instance by an added inert gas. That collisional deactivation is an im- portant reason for going to shorter laser pulses can be seen easily from an attempt \ve made for the isotopically selective IRMPD of natural abun- dance CF,DCl in CF2HCl at a total pressure of 20 Torr. This experiment was done with 2 ps laser pulses of = 6 J cm-’ at the lOR2S line. Selective IRMPD was accomplished. but the fraction disso- ciated of the deuterated substance was less than lo-’ per laser pulse. Fig. lci shows an experiment with a shorter laser pulse (70 ns fwhm. 300 ns total length [33.43]) also at the lOR2S line. The fluencs is 6.3 J cm-’ and the temperature 29s K. The mixture with 0.3 Torr CF,DCl and 10.5 Torr CF2HCl is irradiated with %O laser pulses. The CF2DCl disappears essentially completely whereas the CF,HCl does not change at all within the uncertainty of the esperiment. Results obtained using an even shorter pulse of = 30 ns fwhm. also at a fluence of 6 J cm-‘. are shown in fig. 11. Fractional yields between 0.01 and 0.05 are now obsetxed for pressures between 20 and 150 Torr. again showing the expected enhancement in yield as decreasing the laser pulse length at constant fluence. Fig. 11 also illustrates some of the interre- lated aspects of IRMPD namely the temperature. the wavelength and the pressure. Excitation of 0.4 Torr of CF2DCl at the IORZS line and room temperature shows an increase of the reaction

yield when we add up to 40 Torr of CF,HCI. This effect is characteristic of predominance of case C molecules. Upon adding more than 40 Torr CF, HCI, collisional deactivation starts to compete strongly with optical pumping. and the reaction yield decreases linearly on a semi-logarithmic plot as has been observed previously [28.43]. At the same wavelength, but at higher temperature (473 K). the reaction yield decreases over the whole range of applied pressures. This is typical of pre- dominantly case B molecules. The more efficient optical pumping at 463 K which is associated with case B behavior is probably caused to some extent by the change in the rovibrational population of the molecules prior to excitation. At the higher temperature the collision frequency is also some- what enhanced, but this probably results in only a minor effect. Changing the wavelength at 473 K to the R34 line near 984.43 cm-’ results in slightly higher dissociation yields for CF2HCl partial pres- sures below = 40 Torr. This is also reflected in the IRMPD spectrum shown in fig. 6. For optical pumping on the R34 line however the slope of the plot of log/versus the CF,HCl pressure is larger, i.e. the optical pumping competes less well with vibrational deactivation at this wavelength [28.43,45]. A possible explanation for both phe- nomena, i.e. higher reaction yields at low pressures and slower optical pumping. when comparing the experiments at the R34 line with those at the R28 line. is that at the R34 line a larger fraction of the CF, DC1 molecules interact with the laser beam, and that these molecules are pumped rather slowly especially near the reaction threshold. The fraction of the molecules interacting with radiation at the R28 line is smaller resulting in smaller low-pres- sure yields. but the molecules which interact with the radiation do so more effectively in particular near the reaction threshold where competition with collisional dactivation is most effective [43]. The measurements of fig. 8 indicate that if laser pulses of 3 ns (fwhm) were to be used in the IRMPD of CF,DCl near 985 cm-‘, one can expect essentially no decrease in the IRMPD yield due to collisional deactivation.

Data such as shown in fig. 11 can be used to obtain a rough estimate of the cross section for photon absorption at the wavelength of interest

near the reaction threshold [43] using the equation

o,,l/kv = PcZ,,{P,,,). (10)

where (I RTB = u$-,-~ - u&-~ is the difference be- tween the coefficients for optical absorption and stimulated emission at the reaction threshold bot- tleneck (RTB). I is the laser intensity per unit area. /3, is a collision efficiency. Z,, is the Lennard-Jones collision frequency. and P,,? is the quenching half pressure. For the experiments at 473 K with P,,2

= 60 Torr. & = 1. I is = 200 MW cm-‘, hv = 1.95 x IO-” J/photon. and Z,, = 3.05 X lOI cm” mol-’ s-I_ we obtain uRTB = 6 x IO-‘” cm’. With

&X3 = %B/[l - (f,,/&,* ,>I (11)

and p(,n = 19.J)/p( 111 = 18, J ) = 1.44. where )II is the number of IR photons absorbed per molecule. one obtains for the effective absorption coefficient at the applied conditions a& = 2 x 10mi4 cm’_ This is somewhat larger than the low-intensity low-resolution absorption coefficient of CF,DCl at the same wavelength. which is not unreasonable given the crudeness of the approximation inherent in eq. (10).

For all the data shown in fig. 11 the isotopic selectivity was measured to be in excess of 10’. No isotopic selectivity measurements have been made at significantly higher fluences where much higher reaction yields can be expected, as we then would have to work with focused laser radiation under much less well-defined conditions.

Such high selectivities are measured in the fol- lowing way: f(CF?DCl) is established after a few tens of laser pulses, when enough of this iso- topomer remains for an accurate measurement. f(CF?HCl) is measured after many more laser pulses, sometimes as many as 104, to establish an upper limit for this quantity. In most of these measurements where no visible decrease in the CF,HCl concentration is observed, only an upper limit for J(CF2HCl) can be established from the number of laser pulses and the known accuracy of the measurement. To some extent this way of measuring selectivity distorts the picture in which we claim to measure f for the two isotopomers in a mixture, because after a relatively small number of laser pulses hardly any CF2DC1 is left, and the

probability of energy scrambling or thermal con- tributions is reduced. Still, it appears that during the first hundreds of pulses. when plenty of ex- cited CF,DCl is present and hence collisional en- ergy scrambling between isotopomers can easily take place at the pulse durations and some of the pressures considered. this scrambling does not contribute significantly to the IRMPD of CF,HCl. The results furthermore imply that any thermal contribution to the reaction is completely negligi- ble. at all conditions, and even at high pressures. (4) Concerning rhe working molecule: The working molecule should be cheap. CFzHCl is produced in quantities exceeding 30000 tons per year for the manufacturing of teflon. Hence it is not expensive and easy to come by. It is a non-toxic, stable and volatile gas at room temperature. Its boiling point at -40.78”C makes it relatively easy to separate from HCl which boils at - 114.24”C after redeu- teration to natural abundance. (5) The CF,HCl which is depleted photochemi- tally in its deuterium content must be redeurerared

to natural abundance. The most obvious way for such an exchange is by contact with water which is an essentially inexhaustable source of deuterium at natural abundance. Unfortunately the usual method employed for such an exchange with many freons 121-261 with NaOH and DMSO gave faster hydrolysis than exchange. Many other methods in both acidic and basic media using different cata- lysts were also not successful 126,471. A possible procedure for redeuteration of CF>HCI to natural abundance is by high-temperature exchange in the gas phase. The depleted CF2HCI is brought into contact with a large excess of HCl at natural abundance at temperatures in excess of 42S°C. This causes the following reaction sequence

CF,HCL(depleted) M 4’5-gooc :CFz + HCI. (12)

:CFz + HCI large excess and at natural abundance

z CF?HCl(nna), (13)

where nna stands for nearly natural abundance_ The side reaction (3) which is the combination of two difluorocarbene radicals to give tetrafluoro- ethylene is avoided by keeping the number of

decomposition cycles for the CF,HCl molecules low. by having a large excess of HCl present. and by keeping the steady-state concentration of :CF, low_ The latter is probably accomplished by work- ing in the right range of temperatures. The as- sumption here is that if the decomposition rate of CF,HCl increases faster with increasing tempera- ture than the :CF, insertion rate in HCl. and the :CF2 combination rate constant being only weakly dependent on the temperature [33]. the steady-state concentration of difluorocarbene radicals will in- crease with increasing temperature. The redeutera- tion was tested using CF,HCl with a 15 fold excess of DCl. At 450°C the reaction was completed in = 1 h. and at 550°C within 5 min. In a final step the HCl which is now slighrly depleted in dsu- terium must be redeuteratsd in exchange with water. This proved to be an efficient process even if CaCI, or H,SO, lvere added to the water to decrease the solubiliry of the HCI. The method proposed here for redeuteration to natural abun- dance is probably quite energy consuming as large quantities of gas must be heated to high tempera- tures. Some of this energy can be recuperated by heat exchange_ For a D,O production in the order of a few tons per year a photochemical unit could be coupied directly to a freon 22 plant. and redeu- teration would not be necessary.

All the IRMPD experiments reported above were carried out with radiation at a single laser line. It is likely that improvements in selectivity and in reaction yieId can be attained in many cases when the irradiation is carried out at the same energy fluenccs and intensities. but at two (or more) different wavelengths 1491.

4. Conclusion

Photochemical deuterium separation has been investigated in mixtures of CF?HCl and CF,DCl with powerful COz laser pulses. IR analysis of the disappearance of these two isotopomers indicates selectivities in excess of 10’ for reaction yields up to 5%. This high selectivity in the reaction yield is even attained in experiments with a 400 fold cx- cess of CF,HCl over CF2DCl at a total pressure of 150 Tori with a pulse length of 30 ns. The opti-

288 J. Moser er al. / Deureriunr separarion and infrared photochemisr~ in CFzHCI

malisation of the photochemical isotope separation is discussed in terms of the basic IR photochem- istry of the system. A new method for redeutera- tion of the photochemically depleted CF,HCI to natural abundance, by way of high-temperature gaseous exchange with HCI, is proposed. The selectivity in absorption was found to be smaller than expected.

Acknowledgement

The authors are grateful to Martin Quack for several helpful discussions, and to the Swiss Fonds National for funding. We would also like to thank E. Grunwald for furnishing some CF,DCl for spectroscopic purposes. and DC. England of Du Pont de Nemours for a sample of perfluoropro- pyleneoxide.

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