Photoionization of a 50 cSt viscosity polydimethylsiloxane silicone oil

Ra&at Phys Chem Vol 23, No 1-2, pp 117 119, 1984 printed in Great Bntam

0146-5724/84 $ 3 0 0 + 00 Pergamon Press Ltd

PHOTOIONIZATION OF A 50 cSt VISCOSITY POLYDIMETHYLSILOXANE SILICONE OIL J. CASANOVAS,R. GROB, J. P. GUELFUCCIand R. LAOU Slo Ho] Centre de Physique Atomlque, Laboratolre Assoc16 au CNRS N ° 277, Unlvermt6Paul Sabatler, 118 route de Narbonne, 31062 Toulouse Cedex, France (Received 3 February 1983, accepted 15 March 1983)

Abstract--We have studied the influenceof incident photon energy (6 ~
for the studied silicone oll and for three values of the apphed electric field. As in similar studies<4-u) the extrapolation of these experimental curves down to the lowest photon wavelength giving rise to a detectable electrical signal (-~ 10t4A) gives the photoionization energy threshold value I~'qp of the studied liqmd 7

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The average number of polymeric units n is 30 The fluid was used either as recewed (except for degassing and drying) or after purification by passage through freshly actxvated silica gel. During the experiments the samples were maintained under ultra-pure Nv All the measurements were carried out at room temperature (23 + I°C). 2. RESULTS Figure 1 shows the variations in normahzed photocurrent against photon wavelength or photon energy

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FIG 1 Photocurrent dependence on photon wavelength or photon energy for the Rhrne-Poulenc 604 V 50 mhconeoll t = 2 3 _ 1 ° C , Curve 1 E = 3 2 2 k V c m ', Curve 2 E = 8 . 9 2 k V c m ~,Curve 3 E = 1 4 6 2 k V c m -~ 117

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Figures 2 and 3, give the p h o t o c u r r e n t as a function of the applied electric field for different p h o t o n wavelengths F r o m these figures a n d from similar ones o b t a i n e d for oil samples o f different purities, the following points can be noted" (1) The shape o f the I i f ( 2 ) or I i f ( E ) curves does not depend on the sample purity (2) The/leqp value deduced from the I = l (2) curves does not depend on strength o f the a p p h e d electric field, at least between 0 a n d 15 kV cm ~ (see inset in Fig 1) We o b t a i n e d IL~,q p = 72 9 + 0 05 eV (3) C o n t r a r y to what we observed in h y d r o c a r b o n p h o t o l o n l z a t l o n ~ ~7t a n d to o b s e r v a t i o n s m a d e in p h o t o t o n l z a t i o n studies of some T M P D - s o l v e n t mixtures °2~ (at least for p h o t o n wavelengths lower t h a n 230 nm), the 1 - / ( 2 ) curves c o r r e s p o n d i n g to the &fferent electric field values we studied are not

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FI(, 2 Photocurrent dependence on applied electric field strength for the Rh6n~Poulenc 604V 50 silicone oil t = 2 3 _ + 1 C, Curve 1 ) . = 1 2 1 6nm, Curve 2 2 =1283nm, Curve 3 ) . = 1 3 6 2 n m , Curve 4 , i = 1 4 0 6 n m , Curve 5 2 = 1 4 3 . 7 n m

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proportional, except between 158 and 170 nm (see Fig 1) The lower the p h o t o n wavelength, the greater this effect becomes (4) The I = / ( E ) curves go g h r o u g h the origin This b e h a v l o u r is identical with that of the same type of curves o b t a i n e d in hquld alkane p h o t o i o n l z a t l o n ' ~ or in electron p h o t o m j e c t l o n studies in llqmd hydroc a r b o n s ~ ,4, DISCUSSION In Fig 4 a possible kinetic scheme for slhcone oil molecules excited above the p h o t o l o n l z a t l o n thre.shold is given M is a silicone oil molecule, M * and M * * are an excited and a superexclted state of this molecule qS~ is the probability for M * * to give the geminate ion pair ( M ' , e ) P(E) IS the field dependent charge escape prob,lblhty This scheme certainly depicts quite well the oil p h o t o l o n l z a t l o n mechanism and the electric field effect on the geminate ion pairs but, it does not take into account the particular conditions of charge creatmn or charge collection inherent to the type of experiments described m this paper Indeed, owing to the very short ranges of the low energy p h o t o n s used in these experiments, the ion pairs are created very near the radiation entrance window This implies that the ions are not only subjected to geminate r e c o m b i n a t i o n (bulk process) and to applied electric field but also to Image potentials (wall process) The shape of the I = I(E) curves and particularly the fact that these curves go t h r o u g h the origin seems to indicate the image potentials play a p r o m i n e n t part in the r e c o m b i n a t i o n of the charges ~ t 4 ~ The measured p h o t o l o n l z a t l o n current, for a given wavelength, therefore essentially represents the product qS~P'(E), P'(E) being the field d e p e n d e n t escape p r o b a b i h t y of a charge towards its image charge If we assume that the applied electric field has no effect on q~ , the observed differences in the shape of the I = 1(2) curves according to the electric field value can only be explauled by a P'I E) variation with / As P'(E) depends on the separation distance between an ion and its image charge, z e in fact on the p e n e t r a t i o n d e p t h o f the p h o t o n s in the oil and, as It is p r o b a b l e that the higher the p h o t o n energy the shorter its course in the hquld, it is not then sur-

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FIG 3 Photocurrent dependence on apphed electric field strength for the Rh6ne-Poulenc 604V50 slhcone off t = 23 + I~'C, Curve 6 2 = 148.8 nm, Curve 7 ).=151nm, Curve 8 ). = 1 5 4 1 n m , Curve 9 2 = 1 5 4 8 n m , Curve 10 2 = 1 5 7 9 n m

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FI(, 4 Kinetic ~cheme for the excited slhcone oll molecules above the pholoumJT,itlon threshold

Photoiomzatlon of a 50 cSt viscosity polydlmethylslloxane- silicone oll prising that for a given value of E the escape probablhty P ' ( E ) varies with 2 and that, P ' ( E ) decreases with 2 The electric field effect observed in silicone oil photoIonlzatlon does not exist m the case of hydrocarbons, although similar charge creation and charge collection c o n d m o n s exist in these compounds Indeed, in hydrocarbons, ~5'71 the I = f ( 2 ) curves corresponding to distinct values of the applied electric field (0 < E ~< 15 k V c m 1) are proportional, even for the lower values of the field. This indicates that ~b± is not affected by the electric field and that, in this case, P'(E) is independent of 2. This difference m the behavlour of a slhcone oil and a hydrocarbon is probably due to the photon energy range covered in each c o m p o u n d above the photolonlzatxon energy threshold value - 8 5-10.2 eV for a hydrocarbon, 7 3-10 2 eV for the silicone oil. On the other hand, in the photolonizatlon experiments performed on TMPD-solvent mixtures °2) the I = f ( 2 ) curves corresponding to distinct values of the electric field are only proportional for 2 lower than 230 nm (1.e hv > 5.4 eV) The electric-field effect observed for higher values of 2 was correlated with different initial thermahzation distances r, of the geminate ion pairs ( M +, e ), according to the value of 2 (r decreasing as 2 approaches the photoionization threshold wavlength) and consequently to a variation of P ( E ) with ,~ Taking into account the fundamentally different charge creation conditions in these experiments and in ours, the fact that in Bullot's experiments t~2) the electric-field effect increases as 2 approaches the photolonlzatlon threshold value of T M P D , while in our expenments the electric-field effect does not exist (within experimental errors) for the values of 2 close

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to the photolonlzatlon threshold but becomes important as 2 decreases confirms the preponderance, in our conditions, of wall processes over bulk processes We believe that, if our assumptions are true, the observed electric-field effect would become unimportant (or even disappear) for higher electric field strengths Studies along these lines are now in progress in our laboratory.

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