Nanosecond pulse radiolysis effect studied spectrographically

Volume 6, number 2

NANOSECOND

CHEMICAL PRYSICS LETTERS

PULSE

RADIOLYSIS

EFFECT

I.5 Juiy 1970

STUDIED

SPECTROGRAPHrCALLY D. C. WALKER and S. C. WALLACE

Received 18 Nay I970

Absorption spectra ham been obtained for the benzene excimer and br solvated electrons in wa%r, methanol and glycerol by a techniqueutilising a 3 nanosecond&wenkov light flash as the i&err& source for spectrographic detection during pulse radiolysis.

In this letter we outirie and demonstrate the potentiality of a novef technique which may be regarded as yielding “self-portraiW - spectra arising from self-absorption of the system’s own cerenkov light during nanosecond pulse radiolysis. In genera1 spectrographic methods have been superceded by speciro~hotometric techniques for studying the absorption spectra of very short-lived chemical species because suitably short spectroflash light sources were not cavailablc. In order to overcome this problem of time resolution we have made the flash of Corenkov radiation act as light source ior absorption spectrographic analysis. In this way the spectra of species present only during a 3 nanosecond electron pulse are recorded. (In their elegant stroboscopic spectrophotometric technique Hunt and co-workers 0 ] employ the Cerenkov light flash from one pulse as light source for the next electron pulse.) The model 130/2667 Febetron accelerator, which produces a 3 X IO-9 see pulse of 0.5 MeV electrons at about 50 A mm-2 mean current density, is ideally suited foe this purpose, The electron energy exceeds the Cerenkov threshold by only about 0.25 Me’7 and consequently the ‘Cexenkov light (about 5 photons of visible light per electron) is produced predominantly at the front edge of the cell and thus almost all of it has to penetrate regions of high concentration of chemical species (typically = 20 000 per electron), partic-

~ularly during the latter half of the pulse. Further-

more the mean electron-pulse doserate is so large (1015 rad see-l) that the high concentration of absorbing species yield large optical densities and enough light is created that only one pulse is needed to darken,the photographic film in a . medium resoIution spectrograph. Typically a

mean concentration of lO+M is created half way through the guise for a species produced with G w i. Since the effeCt&e Gp@icaL Light path is RS1 mm, optical densities 3 OS are obtained for species having decadic molar extinction coefficients (E) in excess of fOOOM-l cm’l. The method can probably be used to study strongly absorbing species with mean lifetimes down to 1O‘1o sec. However, it will be complicated by any system showing significant radiation-induced fluorescence or phosphorescence. The experimental arrangement is depicted in fig. 1 (a). A cylindrical stainless GeeI ceU contains a thin (I/1000 inch) stainless steel foil electron window at one end and a quartz optical window sealed at the other. About 40 mm in dia.meter and 5 mm deep the celI incorporates entrance and exit ports for easy replenishment of deaerated solutions from a ghr.ss He-pressurised flow system. Light emerging from the quartz window can be restricted by an iris and is transmitted by a lens system onto the slit of a grating spectrograph for visible and IR studiesl or to a quartz prism spectrograph for W spectrai analysis. Colour glass filters can be inserted for rough spectral seIection aztd to avoid unwanted diffraction orders, Fig. l(b) indicates qualitativeiy tie intensity of Cerenkov light produced (curve A) and the distribntion of chemical species produced (curve.B) ~3 a function of depth fn the ceil. The &renkov light is emitted in a broad cone predomin~t~y in the forward direction [2] am%has a spectral distribution given by a smooth intensity variation from the far W to the far IR given by f- I/k2 [3]. The time dependence of the processes is indicated fr?.fig. l(c). Curve C is the electron pulse

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CHEMICAL

PHYSICS LETT&S

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July 1970

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Fig. 1. (a) Irradfaiion cell. @) IntensiQl-depth relationships for Eerenkov light produced (curve A) and dose deposited (curve B) for OS MeV electrons in-water. (Qualitative sketch only with arbitrary intensity scales.) (c) Intensity-time relationship expected for Cerenkov light (curve C). build-up of absorption chemical species (curve D) arid transmitted light (curve E). (Again arbitraqr intensity scales are used.) current profile’ and, because &etenkov radiation is eniitted very quickly (< lo-l2 set), C also represeuts the light intensity proZile. Curve D shows the build-up of chemical species during the pulse for species having b’mean lifetime substantially Ionger‘than the pulse duration. During the early part of the pulse most of the light will be trans_ m#ed.whereas it will. be mainly absorbed during the latter part when a strongly absorbing species is produced. Thus curve E may depict the timeprofile of transmitted. light. The integrated inter&@ under curve E will be proportional to total light recorded by Ihe .spedtrcgraph when-absorbing.species are pregent, wherkas the area under C is proportional to toti. light recorded for puke. Cerenkov emission In our experiments an’absoi@ion spectrxim correkponds ,to the difference between these two. ’For the. hydra&d ekctron the pure Cerenkov .&mission sp@rum was dbtained by irradiati .. .-a 6pd H2SO4 sblutzor, (half-life of .sq fi: 6 X 1032 : -. ~~‘iata Provided by th8 t&&factur8s of tha.Fehetron, -Field Einisaion_Corporatio& McMinnvilIe, Oregon.

set, e aq being the only species produced vAich absorbs visible light), whereas Eerenkov attenuated by eaq was obtained in pure deaerated water. These two spectra are shown in fig. 2 as curves C yd E respectively. Their difference, shotin as F, represents the absorption spectrum of e& and agrees ne71 with published data [4]. (These spectra have been obtained from the mea-

sured density of the film and converted to relatnumbers of photons at the various wavelengths by use of gharacteristic curves obtained on the ive

for each 10 nm wavelength interval.) Fig. 3 shows the absorption spectra of radiation-produced species obtained by this technique in pure liquid glycerol and methanol. These specfiIm

-tra are seen to correspond.ve+y

an electron scivenger. Howeyer any uncertainti in the Ce+nkov bawotind spectrum can be vey rified &her by fitting‘the curve $0 the.Reoretical spectra variation.(Z= I/AZ [qj. or’? dir&t

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closely t@the

ab;kor@tion spectra of solvated electrons obtained by mitirosecond pulse -radi,o!ysis tec_hniques in t.h&e nedia [5], The pGe &erenkov spectra were qbtained in .&ese two sjrstems ,bySadding 2M CHC13,

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Volume

S, number 2

CHEMICAL

15 July 197Q

PHYSICS LETTERS

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Fig. 2. Absorption

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obtained as the diEt?rence between pure 6ererikov light transmitted by

5M H2SO4 solution (C) and partially absorbed Cerenkov emission

in pure water (?3). The dashed line indicates the known absorption spectrum of e& obtained by micro-second p&e radioLysis techniques E4j.

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Fig. 3. Ai&rption spectra if glycerol (@j and methanol Ir) the background ~ereakcv emission wss obtained in 2a;ICHC13 sok+ons of these iiipuds. I(?pticaldensity is log @erenkov)-log’(transmitied Ii&t). Arrows indicate wave-length of absorption maxima obtained in ref. [5j &XTthese- Xiqufds. ,.I 113

,-- -Ligu-m* e, n+bir 2 .

CH&ICA~ PHYSICS LETTERS

.15 JuIy 1970

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WAVELENGYH fnm! . I&. 4, Absorptionspectrumobtainedin pure &id b&zehey Eerenkov background was deduced from the emission from cyo:ohexane &king appropriate small corrections for refractive index and density. _The dashed line indicates the spectrum observed by Cooper and Thomas and attributed to the benzene excimer. coniparison,

correcting

for a refractive

index

and density change, with pure cyclohexane which appears to produce no absorbing species during radiolysis. Further evidence that &is technique performs as supposed is afforded b;: the absorption spectpm shown in fig. 4 oHa.ined from irradiated pure liquid benzene. 3 is very similar to that reported by Cooper and Thomas [63 a’nd attributed to the lElu ,- jl3& transition of the benzene ‘kxcimer. (Half-life 18 nsec.) It is apparent from fig. l(c) that the time profile of C and E should be ~~stin~ishab~e experimentally. Replacing the spectrograph by a monochro.mate* - photomultip3er - oscilloscope combitiation we hope to obta%lsufficient time resolutiqn-to actually observe the shortening of the transmitted light puke as postulated. We are also attempting to calc+~? the ‘leffective optical path length!‘.of this system as indicated in fig. l(b) from dose-depth and Cerenkov distribution da’&, .

1X4.:,_-._’

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This should then permit evaluations of GX E data to be made. Financial support of the Xational Research Council of Canada is mcst gratefully acknciwledged. REFERENCES [I] M. J, Bronskili and%W.Hunt, J. Phys. Chom. 72 (1968) 3762; M. J. Bronsktll, R. K. Wolff and J. W. HW3t.J. whys. Chum. 73 (1969) 1175. [Z] E. &5.Shaede and D.C. Walker, Intern. J,Rad. Phys.‘ Chcm. 1 (1369) 307. 131L M, Fra& and L Tamm, DoM. Acad. Nauk SSm 14 f1937) 109.

[4] J. P.Keene, Radiation Ros. 22 (19.64)i. . [S] F2$ai and M. C. Sauer. J, Chem.Phys. 44 (19fi61

. M. C. Sauer, S. Arai and L.M. Dorfman,J. Chem. Phys. 42 (1965)708. fti]R. Cooper’andJ. K Thomas, J. Chsm.Phys. 46 (1908) 5097:

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