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C0MIIUSICATIOS.S TO THE h l T o R
are nnt, accurate enough to assess the importance nf a charge deiisity dependent term and that evideiice should be sought nu ail experimental basis such as that being attempted by Roltnn. We did not meau to imply otherwise in our previous publication and welcome this oppnrtuiiit,yof clarifying our position. Coss
CHEYII-.\L
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J. E. Bmon R. I!. GILqON
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CHARLOTPESVILLE, \‘lnGlNl.\ RECEIVED
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Holographic Interferometry in Electrochemical Studies
Sir: 1iiterferomet.ry has heeu employed for a cnusiderable time as a meaus for studyiug electnichemical reactious.1~2 .\Inre recently, schlicrcri nptical techniques and conventinnal iutrrfernmetry have hecu applied to iiivestigatiniis o f single-inn diffusinu cnustaiits, t.hermal cniiductivity (if traiispareiit liquids, cniicentratiou gr:rdient,s in the electrolyte at working electrodes, elcctrnorgaiiic reactions, and related areas by O’Rrien aiid However, despite the recoguized value in electrcchemical investigatinns, classical interferometry tuid schlieren nualytical techuiques are seldom used because nf t h r inhereiit difficulties sssocinted with cell size, cniifigumtiniis, nptical iiihnmtigent.ities, etc. This paper iutrnduces the :rpplicxtinn of holngraphic interferometry t.n electrochemical studies aud examiues some of the advaiitxges nfferrd by holographic tech-
uiques. The theory of the hnlogr:iphic process has beeu extensively discussed iii the literature6 atid will tint, be detailed herein. However, it is siguificiiiit t o uote that a hologram is able to record a cnmplex npticnl wave so that when the holngram is later rcilliimiuwtcd, t.here is created another optical wave identic:il with the wigiiial in both amplitude arid phase. It is this prnperty which makes it possible to use a hologram t o geiierate either t.he test beam or comparison beam or both in :tu iiiterfernmeter. Holographic interferometry may be coiisidered as a form of cnmmnu path interfernmrtry except. that, the test and compnrisoti beams arescpamted iii time. Two techiiiques-diiubleexposiire interferometry aiid “stored beam” iiiterfcri)mctry--h(ith nf which have beeu described by Hefliiigrr, Wuerker, :tiid Jhoks,’.8 are nf iiiterest iii their :ipplic:itioii to elrctrnchemical prnccsses. Dnublcexposure holographic interfernmetry iiivolves the sequeiitial recordiiig of two holograms on the same photographic plate. Wheu the plate is drveloped, the two holngraphic imagrs arc recniistructcd : i d viewed simultaneously. Since the hdngr:im precisely recreates the opt.icel waves which existed :it the times nf the two expnsures, the appearance of iiiterferruce friuges is n measure nf the differences in the two recorded waves. If the photngraphic plate mid the elemcuts of the holocamera remain absolutely statinnary duriiig and bet.ween the t,wo exposures, an infinite friiige pntteru is produced. However, if t h r p h t e or OIIC of the optical elements is mnved so as tu ch:iiige the tiugle betweeu the beams, finite friuges result. 1’hot.ographs taken nf double-exposed, iiifiiiite friiige holngr:ipliic interferograms nf the :ui~~dic dissnlutiini of irnu are shown in Figure 1. The 03-iii. diameter iniu (1:errovac E) rod, pressed-fitted iuto :i I-iu. wd. Teflnii tube, is mnuiited iii air H-ccll coiitiiiiiiiig 1 P sulfuric acid. The iroii-electnde c~imp:irtnieiit is cyliadricsl with the elcctriide nxis iinrmnl to the cell cnmpartmeut axis. The ciinirrii iuid Iiiser tixis lire parallel to the cyliiider :ixis.
n i
(1) A. G . Swnnrcev. Z. P h w i k . Chrrn.. A168, 45 (1834). (2) K. Ihl :xnd It. Muller. Z. B l r k ~ m e h ~ m59, . . 671 (1955). (3) It. N. O’Rrien nnd C. Itoanfield. J . Phya. (%cm.. 6 7 , CA4 (1903). (4) IC. N . O’Rrien. J . Elmtmd~c.m.Snr.. I l l , l:300 (IRCA): Rrr. sei. insir.. 35, BUR (i9rA): J . mrimmrm. 113, RXH (1900). (5) It. N. O’Brien. C. A. Ito.enfielcl. K . Kinoxhiln. \V. 1’. \-iiksrnyshy”. : m I J. Lejn, Can. J . Chcm.. 43, 3504 (1905). ( 0 ) E. N. Leith iwd J. Vrmtnieks, J . Opt. SM. Am.. 52, 1125 (1902): 53. 1377 (1963): 54. 1295 (IOCAI. ( i )L. 0. Heflinger. It. F. \Vuerker. and It. E. Brooks. And. Phus.. 37, 642 (1908).
sor..
.I.
:,.
Figure 1. 1’I~ot~~gr:q~lw laken of duuhle-exposed, infinite fringe holographic interfemymmr of iron disvht,ion in 1 F sulfuric acid: n, 1 min at. 200 ma; b, 1 min at 20 ma. Electrode: 0.5-in. dinmeter,
Ferrovac E iron.
(8) 11.
E. Brooks, L. 0.Heflinger.
LEtlerd. 7 , 248 (1905).
:mil
It. F. \Vuerker, Appl. Phua.
COJIJIUXICATIOSS TO THE EDITOR
An auxiliary platinntn electrode is in a separate compartment which is out of view iii the figures. The interferogram i n 1:igure l a represent s the dissolution of irnn after 1 mill at. 200 nxi and 1:igure lb, after 1 min at 20 ma. The hemispheres on the electrndc surface are hydrngen bubbles resulting from hydrngen evolution at open-circuit condit.ions. The true t.hree-dimensional property characteristic of holngrams is lost. in the photograph. However, by photographing t.he hologram from different angles and planes of focus or by visually observing the hologram, the three-dimensional nature can be realized. In the case of double exposure interferometry, the holographic record represents nnly one instant of time, and it is not possible t o view changes a s they occur in the subject. without making new holograms. However, if only the comparison beam is holographically recorded and t.he reconstructed comparison beam is compared with the actual subject and test beam, then the real time behavinr of t.he subject can be seen interfernmct.ricnl1.y. Again, it is possible to select either infinite or fi1iit.e fringe patterns as described above. “Stored beam” hnlographic interfernmetry offers not. only real time infnrmation, but, provides a means for acquiring t.ime-lapse or moviiig picture records of the course of rcaetion. A series of phntographs shnwing a time-lapse sequencc made 1vit.h a “stored heam” holographic int.erferometc?*is shown in Figure 2. A finite fringe pattern
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is prnduced by slightly tilting the photographic plate which contains :irccnrd nf the cornparison beam. The electrochemical cell consisted of 0.5-in. thick copper eleetrndes separated by a I-cm gap which contained a 0.1 F cupric sulfate solution. Over-all cell dimensions are 2 X 2 X 0.3 in. with an electrode area of G em2. The fringe pattern represents the density or concentration prnfile of copper sulfate in thesolution. During the passage of current, the cathode region becomes depleted in copper ions, whereas the anodic region becomes enriched with copper ions. From the functional relationship between concentration and index of refraction,3one fringe shift corresponds to a change in concentration of 0.144 g/l.or 0.0009 F cnpper sulfate. Figures 2a through 2d show concentration change with time in a cathode-over-anode configuration. After 20 min of electriilysis, the directinn nf current was reversed and Figures 2e to 2h show the resulting changes in the concentration profiles with time. The concentration profile is almost restored to its initial condition at the end of reversed current period of about 20 min. The fringe patterns can be used not nnly to calculate diffusion coefficients nf reaction specics,d-5but also for detecting and analyzing the effects of active sites and environmental parameters tin electrochemical processes. The holographic hterfernmeters used in the cxperiments described in this paper were designed and constructed at TRW Systems Group.’~* Although rigidity of the optical system is required in holographic interV d u m 71, Xtrmber 9 August 1967
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ferometry, 110 elaborate, vibration-damping optical mounts were necessary in the experimental setup. A conventional, lathe-type optical bench was used on a heavy laboratory bench. There are several advantages which holographic interferometry has to offer over classical methods of interferometry. I n the first place, alignment and preparatory procedures are far less critical than for many conventional forms of interferometry. The common path nature of the holographic interferometer provides significant flexibility. It permits the use of imperfect optical elements. Irregularly shaped electrochemical cells of ordinary laboratory glass may be used in the scene without in any way degrading the fringes produced. Furthermore, it is possible to place a ground glass diffuser behind the scene so that the light is caused to traverse the scene in many directions, thereby permitting three-dimensional information to be recorded. During reconstruction, the image can be examined from diff ererit directions and the interference pattern corresponding to each of these directions analyzed. Thus, it is possible to investigate such matters as the localization of regions of electrochemical activity, the nature of flows which do not have rotational symmetry, and the performance of asymmetric electrodes. Holographic interferometry enables one to observe changes which occur in a subject as a function of time, thus producing a differential interferogram. Because those aspects of the subject which do not change do not affect the interference pattern, one has a technique for measuring subtle changes in very complex subjects incapable of being explored with any of the classical methods of interferometry.
COAIRIG~ICATIONSTO THE
EDITOR
ing the variation of trans-2-butene yield from the sensitization of cis-2-butene (or cis-%butene from trans2-butene) in the absence and in the presence of varying amounts of other olefinic quencher molecules. The result of our experiment indicates that triplet benzene is an electrophilic donor and furthermore it provides new data of current interest in benzene photochemistry. The gaseous samples were handled in a vacuum line free of mercury and grease. A Hanovia mercury resonance lamp provided 2,537-A radiation, and a Corning 68-7-54 filter was used to filter out radiations with wavelengths shorter than -2400 A. The photolysis was carried out in a cylindrical quartz cell with two flat end windows. The pressure of benzene was 2.50 mm and that of cis-2-butene was 1.00 mm, while the quencher molecules were in varying amounts for the sets of runs presented here. The conversion of cis-2butene was typically less than 5%, and the product analysis was carried out with a gas chromatograph equipped with a thermistor detector. The separation of 2-butene isomers was complete with either a GO-ft or 22-ft, 0.25-in. o.d., dimethylsulfolane column at room temperature. The notation used by Cundall, et ~ l . , ~inb kinetic expressions of sensitization and isomerization will be adopted here for convenience. Only two competing reactions of interest are shown below
TI
+ C.B. -% G + &To T~ + Q G + Q'
(1)
(11)
where TI is the triplet benzene, C.B. is cis-2-butene, cis-To the cis-butene triplet intermediate, G the ground state benzene, Q the olefinic quencher, Q' the quencher PHYSIC 4~ RESEARCH CESTER C. K N O X TRW SYSTEMS GROVP R. R. S A Y ~ N O triplet intermediate, and k3c and k,, the bimolecular TRW Isc. E. T. SEO rate constants for reactions I and 11,respectively. The REDONDO BEICH,CALIFORNII90278 H. P. SILVERMAN yield of fmns-2-butene is used as a measure of the &-To RECEIVED 11 i~ 15, 1967 yield or the extent of reaction 1, since cis-To mill either give cis-Zbutene back or eventually isomerize to trans2-butene. Two more reactions involving the triplet benzene should be considered in the kinetic treatment, Benzene Photosensitized cis-trans Isomerization
of 2-Butenes: Competitive Quenching Study
Siy: The excited singlet benzene (lBqu)produced in the gas phase is known to undergo fluorescence and intersystem crossover to a triplet ~ t a t e , l -and ~ a triplet energy transfer process is responsible for the cis-trans isomerization of 2-butenes by benzene photosensitizaWe have been studying the tion in the gas competitive efficiencies of various olefinic molecules for quenching triplet benzene in the gas phase by measurThe Journal o j Physical Chemistry
(1) H . Ishikawa and (1962).
W. -1.N o j e s , J r , J . Chem Phys , 37, 583
(2) G. B. Kistiakowsky and C. S Pnrmenter, ibzd, 42, 2942 (1965). (3) W.A. Noyes, Jr., IY.A . 31ulnc, and D A Harter, ibid , 44, 2100 (1966). (4) (a) R. B. Cundall and T. F. Palmer, Trans. Faraday SOC, 5 6 , 1211 (1960), (b) R B Cundall, F. J. Fletcher, and D G. l l i l n e , J . Chem. Phys., 39, 3536 (1963); Trans. Faraday S O C ,60, 1146 (1964); (r) R. B Cundall and A. S Davies, i h z d , 62, 1151 (1966). (5) (a) S. Sato, K . Kikuchi, and 31 Tanak:i, J . C'hem. Phys , 39, 239 (1963); (b) A1. Tanaka, T. Terumi, and S Snto. R d l . Chem. SOC. Japan, 38, 1645 (1965). (6) P. Sigal, J . Chem. Phys , 42, 1953 (1965), 46, 1043 (1967) (7)
W A. Koyes, J r , and D A . Harter, % b i d ,46,
674 (1967)
