Ortho-para iodine separation revisited

Chemical Physics 132 ( 1989) 209-2 17 North-Holland. Amsterdam

ORTHO-PARA IODINE SEPARATION REVISITED J.L. BOOTH, F.W. DALBY, S. PARMAR

’ and J. VANDERLINDE

’

Physics Department, University ofBritish Columbia, Vancouver, British Columbia, Canada V6T 2A6 Received

23 November

1987; in final form 26 August

1988

An attempt was made to alter the ratio of ortho-iodine to para-iodine based on the techniques and results reported by Balykin et al. No enrichment of para-iodine was produced by pumping test cells containing either pure iodine vapour or a mixture of iodine and 2-hexene with an argon ion laser beam. However, differential collisional quenching of the fluorescence from different excited state vibrational levels was seen which mimicked the effect which we attempted to observe.

1. General remarks

iodine as a diatomic molecule would surely fail, and consequently an alternatively means of effecting an ortho-para separation in I2 must be used. Just such an alternative is suggested by the methods of laser isotope separation and two different schemes have been reported. The first makes use of the well-known [ 8,9] predissociation of iodine molecules in the excited B 3110+Ustate. Selective laser excitation of ortho-iodine molecules to a predissociating B311,,+” level preferentially destroys these molecules. Neglecting all other effects, iodine atoms would recombine to produce ortho- and para-iodine in the ratio of their statistical weights, 7 : 5. The continuous destruction of the ortho molecules with only 7/ 12 of the atoms recombining to the same species would yield a net increase in the para-ortho ratio. An enrichment was reported by Bazhutin et al. [ lo] who interpreted a decrease of fluorescence yield in response to argon ion laser pumping of I2 as such a ratio shift. A more careful investigation of the solution to the rate equations and constants they provide, however, leads one to conclude that no appreciable shift in the ortho to para ratio would occur. Instead, a large free atom concentration would be established accounting for the loss of fluorescence yield. In a later paper [ 111 some of the same authors, now monitoring both the fluorescence yield from para-I2 lines pumped by the 50 17 8, argon ion laser line and from the ortho-I2 pumped by the much stronger 5 145 8, argon ion laser line, concluded that iodine was being driven into the walls of their glass vessel. The fluores-

It has long been known [ 1,2] that homonuclear diatomic molecules exist in one of two almost non-interconverting nuclear spin states labelled ortho and para (ortho being the one with greater statistical weight). An enrichment of one or the other of these states has been reported only for hydrogen and iodine. A simple method for the preparation of para-hydrogen, developed by Bonhoeffer and Harteck [ 3-6 1, consists of cooling hydrogen in the presence of activated charcoal. This catalyst allows the molecule to relax to its lowest energy state, a para-hydrogen state. On reheating, the molecules are found to occupy only para states. The ortho to para relaxation in the gas contained in a glass vessel occurs on the time scale of months primarily due to interactions with paramagnetic impurities. Similarly, ortho-para relaxation in D2 requires many months [ 7 1. For other homonuclear molecules the temperature required to produce a reasonable change in the ortho to para ratio is related to the rotational constant of the molecule. For iodine, then, one expects to observe a significant effect only at a temperature that is roughly a factor of 2000 lower than for hydrogen, i.e. at about 5 mK. At this temperature the description of ’ Permanent

address: D.G.A. Inc., 4526 Telephone Road, Suite 205, Ventura, CA 93003, USA. 2 Permanent address: Physics Department, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3.

0301-0104/89/$03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )

B.V.

210

J.L. Booth et al. /Ortho-para iodine separation

cence of both species decreased, but as the ortho-I1 quantum yield decreased more the authors claimed some enrichment (up to a factor of 2) of para-iodine over ortho-iodine. A second method of producing an ortho-para ratio shift was reported by Badger and Urmston [ 12 ] and more recently by Letokhov et al. [ 11,13,14]. It consists of selectively existing ortho-I, in the presence of 2-hexene which acts as a scavenger for the excited state iodine molecules. With the aim of investigating ortho-para relaxation in iodine we attempted to reproduce the earlier results via both the above techniques.

ARGON ION LASER

J / BEAM /-/ SPLITTER\/ PM.2

DYE LASER

2. Experimental The experiments described below are based upon the earlier reported successes and are divided into two sections: ( 1) pure iodine and (2) mixtures of iodine and scavengers. 2.1. Iodine alone The T-shaped Pyrex cell, of length 15 cm, inner diameter 0.6 cm, and volume 6 cm3, was baked for several days and evacuated to a residual pressure of 6 x IO-’ Torr. Iodine (BDH assured), further purilied via trap to trap distillation, was used to fill the test cell with dry Iz vapour. The vapour pressure in the test cell was fixed by immersing a container with the purified crystals, attached to the vacuum line, into a cold bath and allowing about ten minutes for iodine vapour to diffuse to the test cell. The cell was then sealed and removed from the vacuum system. A Coherent CR- 15SG argon ion laser tuned to 5 145 A was employed to selectively excite the B 3II,+,+X ‘Cc (43’-0”) P( 13) and R( 15) ortho transitions of I1 (and, as will be discussed later, also non-selectively excited iodine molecules into the ‘II,” and B. state continua). At the same time, the fluorescence induced by light from a Coherent CR699-21 scanning ring dye laser was used to monitor the ortho- and para-iodine populations in a different section of the cell. The experimental arrangement is shown in fig. 1. About 800 mW of the argon ion laser 5 145 A radiation was used to excite the iodine in the test cell while the remainder was used to pump the

L-J

Fig. 1. A T-shaped cell, containing either pure iodine or a mixture of iodine and 2-hexene, was simultaneously irradiated with the argon ion laser beam and a scanning ring dye laser beam. The induced fluorescence in the cell was observed with EMI photomultipliers and recorded on a strip chart recorder.

dye laser. A 30 mW dye laser beam (power density z 1 W/cm2) was continually scanned over a 30 GHz region near 17408 cm-‘, allowing us to monitor the 15’-O”R(30), 15’-0” P(25), 18’-1” P(84),andthe 19’- 1 )I P ( 12 1) transitions during the argon ion laser excitation of the test cell. The u’ = 15 vibrational level is only slightly predissociated at low J (~6%) and the R( 30) : P( 25) fluorescence ratio provides a reliable measure of the para/ortho ratio of 12. The argon ion laser induced fluorescence was monitored by an EM1 9558B photomultiplier while the dye laser induced fluorescence was observed with an EM1 9558QB photomultiplier each ofwhich was preceded by a Coming CS2-62 red pass filter to eliminate stray laser light. Prior to irradiating the test cell with the argon ion laser, the dye laser fluorescence yield was measured at 15 min intervals over a 1 h period and it was observed that there was no significant decrease in the fluorescence in the absence of 5 145 A radiation. The cell was pumped by the argon ion laser beam for a 140 min period during which time the dye laser induced fluorescence of the ortho- and para-iodine disappeared at equal rates. The cell was then left in

J.L. Booth et al. /Ortho-para

iodine separation

the dark for 12 h and, again, the dye laser induced fluorescence was observed. The signal levels were identical to those previously seen at the time pumping with the argon ion laser was stopped. Heating the cell with a hot air gun, on the other hand, resulted in complete recovery of the fluorescence suggesting that the iodine was loosely bound to the Pyrex walls of the test cell. The results obtained from a cell containing 2.5 f 0.3 mTorr of iodine vapour are shown in figs. 2 and 3. Following Bazhutin et al. [ 111 we define an enrichment factor, E, as E= para& (2) : ortho-I, (t) para-1, ( 0 ) : ortho& ( 0 )

(1)

and obtain E= 1.00 rt 0.01 (i.e. no enrichment was

TIME

(MINI

Fig. 3. The fluorescence induced in a cell containing 2.520.3 mTorr of iodine vapour simultaneously irradiated by an 800 mW argon ion laser beam and a continually scanning dye laser over a 1 cm-’ region containing both ortho- and para-iodine transitions allowed us to monitor any changes in the relative amounts of ortho- and para-iodine present in the cell during the experiment. The ratios, (m) 15,-O” P(25) : 15,-O” R(30), (+) 15,-O” P(25) : 18,-l” P(84),and (0) 15,-O” P(25) : 19,-l” P(121), normalized by their initial values, indicate that no enhancement of the para-iodine was realized. The variation in the third ratio is large owing to the weakness of the 19’- 1VP ( 12 1) transition.

o?.

seen) after two hours of pumping in contradiction with their value of 2 after irradiating the test cell for roughly 120 min. 2.2. Iodine + scavenger

100

50

TIME

(MIN)

Fig. 2. The normalized fluorescence of a cell containing 2.5 f 0.3 mTorr of pure iodine vapour induced by an 800 mW argon ion laser beam ( + ) decayed rapidly over a 140 min irradiation period. The insets show the spectra induced by scanning the dye laseroverthe (a) 15’-0” R(30), (b) 15,-O” P(25), (c) 18’-1” P(84), and (d) 19’-1” P(121), transitions. Notice how the ratio of peak height at time t to its initial peak height decreases equally for each line indicating that no ortho-para separation has occurred.

Badger and Urmston [ 121 claimed that 2-hexene reacts rapidly with excited state iodine but only very slowly with ground state iodine. This provides a means of selectively removing either ortho- or paraiodine from the system. Hence, provided that no radical chains form during the reaction one might well expect to obtain an ortho-para separation. 2-hexene obtained from the Aldrich Chemical Company (99%+ ) and further purified by vacuum

J.L. Booth et al. /Ortho-para

212

distillation was used in the preparation of our test cells. Guided by the results of Letokhov et al. tests were made with cells containing 1.7 k 0.2 Torr of 2hexene and 30 + 3 mTorr of iodine. The dark reaction was monitored as previously described and was found to be negligible. The experiment was carried out as already described and the results are shown in fig. 4. As can be seen, the total fluorescence intensity of each peak decreased, but no enrichment was observed. We have also done some experiments using other scavengers (namely acetylene, nitric oxide, nitrosyl chloride, and ethyl iodide) and found no enrichment.

iodine separation

3. Conclusions 3.1. Pure iodine We have observed no enrichment of para-iodine. This is in obvious contradiction with the conclusions of Letokhov et al. [ 11,13,14], who, under similar conditions, reported an enrichment factor of almost 2. This difference clearly needs to be addressed. The first question that arises is the validity of the label ortho or para for a given state. It is well known [ 8,9 1, that, in the absence of hyperfine effects, the exclusion principle leads to the prediction that, for a homonuclear, diatomic molecule, one will observe non-interconverting odd and even nuclear spin states, so-called ortho and para states. When hyperfine coupling is taken into account, the symmetry which allows one to separate the states into ortho and para species no longer exists, and one obtains an admixture of the two. For molecular iodine, the hypertine effects are small and may be treated as a perturbation. Hence, the new eigenfunction for the Hamiltonian may be written as IY“ortho”)=jI//ortho)+Ce,I~,para).

(2)

For iodine, it has been shown [ 8,9,15 ] that the coupling is between ortho and para levels of electronic states of opposite u and g character and is given by E, =

(uparal&Igortho) AE

(31

where the matrix element is about l/30 cm -I, and AE is the energy difference between the two coupled states. For the ground electronic state, X ‘C: , the closest state to which it may couple lies about 1O4cm- ’ above it and, consequently, 1X ‘Z; “ortho”) ’

I 0

k

I

IO

I

I 20

I

I 30 TIME

I

I 40

1

I

50

I

1-1 I 60

70

(MIN)

Fig. 4. The decay of the induced fluorescence of iodine in a cell containing 30 & 3 mTorr of iodine vapour and 1.7?0.2 Torr of 2-hexene is shown above. As before, the rate of decay of the argon ion induced signal, ( q ), is identical to that of the 15’-0” P(25) line, ( + ), and the 15’-0” R( 30) line, (+), indicating no orthopara separation. The plot shows the ratio fluorescence of each peak at time t to its initial height.

= 1X ‘Cz ortho)

+3X10-6]y’upara).

(41

Near the B 31”10+U electronic state dissociation limit, V’= 87, many electronic states lie close together and significant u-g ortho-para mixing occurs and has been observed by Pique et al. [ 15-l 7 1. In our work, however, we were concerned with low-lying states, u’ < 43 which are about 1O3 cm- ’ below the closest admixing levels. This yields a coupling strength of E 3 X 1Ow5 which demonstrates that, for our experi-

J. L. Booth et al. / Ortho-para iodine separation

ments, the ortho and para character of a given state is well preserved. To interconvert between the two species, one may consider three mechanisms: ( 1) a radiative decay, (2) collisions between molecules and paramagnetic partners, and, (3) atomic exchange upon collision between iodine molecules and iodine atoms. The first was considered by Raich and Good [ 18 ] for molecular hydrogen in which the radiative decay is able to occur because of the hyperfine mixing of ortho character in one electronic state with para character from another state. They found that for hydrogen, in the electronic ground state, the lifetime for an ortho to para conversion (J= 1 to J= 0) is of the order of 5 X 1O’* year. Their ideas are readily adapted to iodine, allowing one to estimate the lifetime for such a spontaneous emission to be about 5 x 1O9year. Thus, the probability of a transition from an ortho to a para state via dipole radiation in the X ‘Cl electronic state is completely negligible. The second method of relaxation, namely paramagnetic collisional scrambling, is much more important. Such collisional relaxation of para-hydrogen by nitric oxide and oxygen was studied by Gieb and Harteck [ 19 ] and Farkas [ 201 with oxygen being the slightly more efficient converter. It was found that in the presence of atmosphere pressure of oxygen, parahydrogen has a half-life of about 100 s. This represents a conversion rate on the order of one in every 10’ ’ collisions. For hydrogen, the relaxation mechanism is necessarily magnetic and Wigner [ 2 1 ] introduced a theoretical basis for such a relaxation which was later refined by Kalker and Teller [ 221. Simply put, a paramagnetic species colliding with a molecule introduces an inhomogeneous magnetic field to the system. This field gradient can induce a change of nuclear spin orientation in one of the nuclei while leaving the other undisturbed. This may result in a transition between an ortho and a para state (or vice versa). To obtain a rough estimate for the ortho-para relaxation of iodine relative to that of hydrogen one first assumes that the two partners do not stick together. The duration of a collision is estimated to be the time required for the paramagnetic partner, travelling at thermal velocity, ZJto traverse the geometric length of the molecule. Next, the collisional distance is taken to be the radius of the molecule. Placing the

213

appropriate values for these parameters for iodine and hydrogen into the Wigner equations [ 211 one obtains the ratio of collisional ortho-para conversion for iodine compared to hydrogen as Tz 100. For iodine, however, one must also consider electric quadrupole relaxation processes which should further increase the collisional transition rate. Thus one expects that roughly one in every lo9 collisions between an iodine molecule and a paramagnetic partner should result in an ortho-para conversion. Finally, ortho-para hydrogen conversion by atomic hydrogen due to direct exchange reactions, H+d-H2-+pH2+H,

(5)

was also studied by Gieb and Harteck [ 191 and Farkas [ 201. Farkas found this reaction rate to be of the order of lo5 g/mole s, several orders of magnitude greater than for the other paramagnetic species (i.e. a conversion every 3 X 1O4 collisions as compared to the previous value of once every 10” collisions). Such an exchange process will depend upon the relative sizes of the two partners and their velocities. For such a reaction, one expects the cross section for iodine to be at least four times as large as for hydrogen from physical size considerations, and the time for a collision for iodine to be roughly ten times longer. This translates into an exchange rate for iodine which is at least forty times that of hydrogen, implying that in the presence of 3 mTorr of atomic iodine one expects that the half-life of ortho- or paraiodine will be of the order of 3 s. Thus, to achieve maximal ortho-para conversion one would like to maintain an extremely low concentration of iodine atoms. In order to explain the apparent disagreement between our results and those of Letokhov et al. one must examine the differences between the two sets of experiments. Letokhov et al. based their findings on the observed difference in fluorescence yield induced by an intense argon ion laser line at 5 145 8, which was used to selectively destroy ortho-iodine, and from a weaker argon ion line at 50 17 A, which primarily excited para-iodine. In their paper of 1976 [ 111, they reported that the fluorescence from both these argon ion laser lines decayed but at unequal rates (in previous papers they stated that the 5017 8, quantum yield did not vary over the course of the experiment ). This claim is quite significant, especially in

214

J.L. Booth et al. /Ortho-para

iodine separation

view of some of our earlier observations. When we first began this work, only moderate attention was paid to degassing our I2 cells. As a result, it was seen that over time, in the absence of pumping, the relative quantum yield of fluorescence from different vibrational bands varied, as illustrated in fig. 5. This variation disappeared when more scrupulous care was taken in ensuring that the cells were clean and thoroughly degassed before filling them with iodine vapour. It is postulated that impurities degassed from the walls of the cell were responsible for this differential quenching of vibrational states. Consider the time dependence of the population of an excited state,

where n* is the number density iodine molecules, n is the number state molecules, B,,,, is the laser the radiative decay rate, r, is the and rc is the collisional decay pressed as

dn* =B,,rI(n-n*)-~Rn*-I’pn*-~cn* dt

S=

(6)

(7)

where [P] and [I21 are the concentrations of degassed impurities and iodine molecules, respectively. Taking dn*/dt =O and knowing that the signal observed, S, is proportional to the radiative decay rate times the excited state population, one obtains

B,,, IrR n

B,,.z+r,+r, +r,

.

(8)

Initially, when little outgassing has occurred, re may be neglected provided the concentration of iodine molecules is sufficiently low and the ratio of the signals of a line with no predissociation to one with r,=r, is

b

S~Ml S,,=,

s,,=,, &,=,

174&n-l

= 1;

a 1+a

(9)

where a = T,JB,,,. I. When the collisional decay rate is no longer negligible, i.e. significant outgassing has occurred so that k, [P] is now appreciable,

b

,74&m-’ p* t =0 min

i

ro=k,[P]+k,lI,]

of the excited state density of the lower excitation rate, rR is predissociation rate, rate. r, may be ex-

%

t= 48 hrs

Fig. 5. In cells containing low pressure of iodine vapour which were not carefully evacuated and degassed the relative fluorescence yields from different vibrational transitions appeared to vary. A typical result is shown above for a cell prepared by evacuation down to a residual pressure of roughly 1.6~ 1O-’ Torr and then filled with 3.3 & 0.3 mTorr of iodine vapour. Notice that although the relative peak heights of lines from different vibrational transitions seemed to vary over time (e.g. peak b versus peak c), peaks belonging to the same vibrational levels did not (e.g. peaks a and b). This variation is attributed to differential quenching of the vibrational bands by material outgassed by the cell.

=1+

a 1+a+h’

(10)

where b=TJB,,J. Thus, by comparing formulas (9) and (10) one predicts an apparent shift in the two ratios. In particular if one signal originates from a para state with high predissociation while the other is due to an ortho level with less predissociation one may misinterpret the above effect as a pseudo-enhancement of the para population. For iodine, it is well known that the predissociation of the B 3110+Ustate is highly dependent upon the vibrational and rotational quantum numbers of the level considered [ 9 1. For a state with high predissociation the effects of collisional quenching are minimal, while states with low predissociation are affected to a much greater extent. The hypothesis of a pseudo-enhancement of para-iodine owing to differential quenching of vibrational states agrees with our observations, as indicated in fig. 5. The highly pre-

J. L. Booth et al. / Ortho-para iodine separation

dissociated transitions, lines c and d, appeared to grow in size relative to the less predissociated transitions, a and b. Note also that the relative sizes of lines a and b, corresponding to transitions from the same vibrational bands (BtX (15’-0”) R(30) and P(25) respectively) did not change. Hence if one uses the fluorescence from one rovibrational transition to monitor the ortho-iodine population and the fluorescence from another, very different, transition to detect the para-iodine, then it is possible to mistake the apparent shift discussed above for an actual orthopara separation. This effect, which, to the best of our knowledge, has not previously been observed, is of some interest independent of the problem considered here. Finally, one sees that if the molecular iodine concentration is large enough to make the second term in rc ( = k2 [I21 ) dominant, then no differential quenching of the iodine fluorescence will be observed. Balykin et al. [ 111 reported that they evacuated their cells down to about low4 Torr before filling them with iodine. This was precisely the initial background pressure range in which we observed the largest differential quenching for cells containing a few mT of iodine vapour. Furthermore, Balykin et al. [ 111 reported that the enhancement ratio as determined by relative fluorescence yields decreased as the initial concentration of iodine vapour was increased, also in agreement with our collisional quenching hypothesis. Subsequent investigations led us to another important observation: when a cell containing 3 mTorr of iodine vapour was irradiated with the 5 145 A laser light the fluorescence decayed in much the way that Balykin et al. reported [ Ill. However, when an identical cell was irradiated with 5750 8, dye laser light which excited a more highly predissociating state (BeX (19’-l”)P(121)),thefluorescencedecayed much more slowly over a one hour period ( x 5% versus z 55%). The argon ion laser, delivering x 800 mW of power to the iodine cell, was estimated to be running on roughly 400 longitudinal modes over its bandwidth ( x 0.2 cm- ’ ) and may, therefore, be considered a continuous source. The dye laser, on the other hand, producing x 150 mW and having a bandwidth of 7 x 10m5 cm-‘, operated on a single mode. One would expect the two laser beams to have had similar effects. In order to explain this seeming

21.5

contradiction one notes that although the B-X (43 ‘‘0”) P(13) andR(15) lineslieabout lOOOcm-‘below the B-state dissociation limit, there is absorption to the B-state continuum as well to the lIIlu continuum in this region [23,24]. From Tellinghuisen’s values of the apparent extinction coefficient, E”, at 5 145 8, one may estimate that the continuum absorption was about 1 mW (an absorption of roughly 0.124%). We were able to estimate the absorption coefficient of the B-X (43’-0”) P( 13) and R(15) transitions for our experiments to be about 0.245% or 2 mW absorbed out of the argon ion laser beam. However, we also expect that at such a high argon ion laser power the bound to bound transitions should be highly saturated and, therefore, our estimate of the B-X absorption is too large. In fact, we directly measured the absorption of argon ion laser radiation by iodine vapour in a cell containing about 280 mTorr of iodine and by extrapolation found that a total of only about 1 mW of power was absorbed instead of the theoretically predicted 3 mW which assumed no saturation. This means that the non-selective processes, which are not subject to saturation, should overpower the selective ones when the argon laser intensity is high. Furthermore, even at low power the selective bound to bound absorptions are only about 43Ohpredissociated so that the ratio of selective to non-selective excitation is still only roughly 1 : 1. Thus one would not expect to observe much enhancement of the para to ortho ratio in molecular iodine by pumping the iodine vapour with an ion argon laser tuned to 5 145 A. The kinetics for our system cannot be determined from our measurements as we monitored only the molecular iodine and not the atomic iodine population. We tried to fit the argon ion laser induced fluorescence as a function of time to several analytical expressions which included a sum of two exponentials, an exponential plus a term in 1/t. The best result was obtained when fitting to an expression of the form l/3

I,(t)=&+

( > -!!L 1+/3t

’

(11)

where the various coefficients, no, n2, (Y,p, are simply parameters used to fit the data. As such, they are not sufficient to allow much insight into the kinetics of the reactions. Consequently, we are unable to say how

216

J.L. Booth et al. /Ortho-para iodine separation

the scrambling of ortho and para molecules place, either through exchange reactions, I+o-I*-+I~

took

)

13-‘p-I* +1 ) or via 1+0-I*-+p-1,+1.

(13)

3.2. Iodine and 2-hexene Based upon the above the considerations it is obvious that one should try to minimize the production of iodine atoms or free radicals when attempting to separate ortho- and para-iodine. If one could find a scavenger that reacts only with the excited state molecules by directly removing them from the system, a 100% separation could be effected, provided the resulting product was inert. The findings reported by Letokhov et al. [ 11,13,14] using 2-hexene as a scavenger were promising. In ref. [ 111 they reported that in mixtures of iodine vapour and 2.2 Torr of 2-hexene pumped by 2.2 W of 5145 8, laser light the enrichment factor started at 2 when the iodine vapour pressure was 300 mTorr and increased to a value close to 4 when the iodine vapour pressure was reduced to 40 mTorr. Our findings failed to support these claims. In a mixture of 1.Ok 0.2 Torr of 2-hexene and 30 If: 3 mTorr of iodine vapour we observed no dark reaction between the two gases and only a slow reaction when the cell was irradiated by the dye tuned to either the (14’-0”) P(25) (~4% predissociated) or the (19’-1”) P(121) (~55% predissociated) transitions. However, when the cell was excited by an 800 mW argon ion laser beam at 5 145 8, a vigorous reaction took place over a 10 to 15 min time scale. The argon ion laser beam excitation results in the formation of both ground and excited state iodine atoms along with some excited state molecules. The dye laser tuned to the ( 19’- 1” ) P ( 12 1) transition creates ground state atoms in addition to excited state molecules and tuned to the ( 14’-0” ) P ( 25 ) excitation primarily produces excited iodine molecules. If the reaction involved 2-hexene and excited state iodine molecules with no radical chain formation one would expect that the dye laser beam would have had an effect similar to that of the argon

ion laser. Further, if the 2-hexene reacted with ground state iodine atoms which are produced by predissociation induced by the dye laser beam absorption then the ( 19’ - 1” ) P ( 12 1) irradiation should have had a much larger effect than the ( 14’-0” ) P (25) irradiation, but this was not observed. We conclude, then, that the reaction mechanism involved excited state iodine atoms which were produced only by the argon ion laser excitation and entailed a radical chain as outlined by Balykin et al. [ 111. 1;+1+1*, 1*+x+x1

(14a) )

(14b)

x1+1-+x12,

(14c)

xI+I~+xI~+I)

(14d)

1+1*x*.

(14e)

Clearly the kinetics of low-pressure iodine in the presence of a Pyrex surface is not well understood leaving the door open for further research. We are confident, however, that neither of the techniques described in this paper is capable of separating orthoand para-iodine. Finally, one technique which has not been attempted but which is suggested by the work of Piquet et al. [ 15-17 ] is the following: Suppose one were able to excite iodine molecules from the ground state in which ortho and para character is well defined to a state which has large u-g ortho-para mixing but which does not lead to any direct dissociation or predissociation. Then by continual, selective excitation of an ortho-iodine X ‘Cc state one could eventually completely deplete the ortho population. As the ortho molecule is excited, it acquires some character and this “para” excited state could decay, probably in two steps, back to a ground state para level. The rate of such a separation would be limited by the amount of ortho-para mixing in the excited state molecule and by the relative rates of spontaneous emission of this level back down to the ground state by the “ortho” part of the wavefunction as compared to the decay of the “para” part to an intermediate level. Acknowledgement We would like to thank Dr. N. Basco of the UBC Chemistry Department for the loan of some equip-

J.L. Booih et al. / Ortho-para iodine separation

ment used in this work. We would also like to thank C. Ahlborn and K. Mah for their helpful discussions and assistance and E. Williams for the glass blowing required throughout this research. Finally, we thank A. Chanda, R. Abraham, and U. Narger for reading this manuscript and their subsequent valuable comments. This work was supported by the Natural Science and Engineering Research Council of Canada.

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