J . Phys. Chem. 1993, 97, 10352-10357
10352
Effect of Electron Quenchers on the Thermalization Range Parameter and Ionization Quantum Yield of Photoelectrons Produced by VUV Irradiation of 2,Z-Dimethylbutane and 2,2,4-Trimethylpentane. Comparison with yray Irradiation J. P. Guelfucci,' J. Casanovas, M. L. Huertas, and J. Salon Luboratoire des dtcharges dans les gaz, URA du CNRS no. 277, Centre de Physique Atomique, Universitt Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France Received: March 9, 1993; In Final Form: June 29, 1993'
The ionization quantum yield and thermalization range parameter of photoelectrons were extracted by a threeparameter adjustment from experimental curves of the photocurrent as a function of an applied electric field. Results were obtained at VUV photon energies of 0.5-2 eV above the threshold energy in 2,2-dimethyl butane and at 10.03 eV in 2,2,4-trimethylpentane. These values were compared to those obtained when electron quenchers were added to the solvent. The same technique was carried out when excess electrons were produced by C060y-ray irradiation. With VUV irradiation, the drop in photocurrent arises mainly from the drop in the ionization quantum yield, whereas with y-rays it is rather the thermalization distance that is affected. These measurements support the preferential interaction of the quenchers with the excited states produced by VUV irradiation and also support scavenging a t the epithermal stage of the geminate pair resulting from y-ray irradiation.
I. Introduction In the liquid phase, electrons created by photoionization of a dissolved molecule (i.e., TMPD) or of the liquid itself are actually ejected from the molecule as soon as the energy of the incident photon is greater than the ionization threshold of the molecule. This last empiricalexpression illustrates the difficulty of defining an actual ionization potential, as pointed out by Funabashi:' inadequacy for unique physical signification of the ionization potential in a condensed system, since photoelectrons remain in the Coulombic field of their parent ion. In a classical description of the photoionization process, it is generally accepted2 that photoionization is initiated by vertical absorption of the incident photon. At this stage, there is no difference between the gas phase and the liquid phase. However, due mainly to an adiabatic polarization of the surrounding molecules,3 the ionization threshold is shifted from the gas phase to the liquid phase according to the well-known formula
+ + vo
I, = zg P+
I , and I, are the ionization energies in the gas phase and in the liquid phase, PC is the polarization energy by the positive ion, and VOis the electron conduction level. The experimental law ZN
= k(hv - hy,)"
giving the normalized photocurrent IN against the excess energy above the ionization threshold, IC = hv,, was determined for pure alkanes,4 tetramethylsilane? and solvated molecules.6 The fact that this power law was observed whether the molecule was initially in its ground state or in an excited state7is for some authors7 an indication of direct, band-to-band, transitions. For others? without totally excluding the possible presence of excited states before actual ejection of the electron, this law is only an aspect of the Onsager theory concerning recombination of the geminate pair composed of the electron and its parent positive ion. However, there are some good indirect indications that excited states occur in the photoionizationprocess of pure alkanes9 and solvated molecules.IO The existenceof these states-described as spatially extended"-is inferred from the simultaneous *Abstract published in Advance ACS Abstracts, September 1, 1993.
0022-3654193f 2097-IO352$04.OO/O
z
s
4
' 5
6
7
'8
L
L' 8.4 8.6 8.8 9 9.2 9.4 9.8 9.8 10 Figure 1. First derivative of the photocurrent spectrum of cyclohexane17 irradiated by VUV photons. Tentative Rydbcrg assignment along a d series (quantum defect 6 = 0.18), a p series (6 = 0.46), or an s series (6 = 0.82).
lowering of the photocurrent and the fluorescence yield when electron quenchers are introduced into solution as well as from the Stern-Volmer behavior of the reverse of the photocurrent on changing the concentrationof the quenchers.12 Thisinterpretation has been q~esti0ned.l~ Until recently, the only direct indication of the presence of excited statessituated above the ionization threshold in the liquid state-was the photoconduction spectra of solvated mole c u l e ~ , which ~ ~ . ~present ~ shoulders at energies close to those of the UV absorption peaks in the gas phase. Considering the energies at which they are situated, these shoulders could even arise from Rydberg series, although it seems that no Rydberg structure could be hoped for in the liquid phase considering the effect of pressure in the gas phase.16 More recently, our team has reported17 the VUV photoconduction spectra of pure cyclohexane (Figure 1) and established (Table I) the values of characteristicenergiesclose to those of the Rydberg seriesobserved in the gas phase.'* Currently, we are working to confirm these results. The work presented here is a continuation of previous studies concerning first the effect of electron quenchers on the photo@ 1993 American Chemical Society
Effect of Electron Quenchers Produced by VUV Irradiation
TABLE I: Tentative Rydberg Assignment of Absorption W V Spectrum of Cyclohexane in Gas Phase and Photocurrent Spectrum of Cyclohexane in Liquid Phase along s Series (Quantum Defects 6 = 0.18) 4d gas
liquid
8.95 8.95
(4d)" 9.09 9.09
5d 9.30 9.31
(Sd), 9.43 9.44
6d 9.48 9.46
(6d)" 9.62 9.66
The Journal of Physical Chemistry, Vol. 97,No. 40, 1993 10353
2. Analysis Technique. In previous papers,17J9we reported the possibility of adjusting the experimental curves of In vs E by the expression22
7d 9.59 9.52
conduction ~ u r r e n t ~ and . ~ . 'second ~ the possibility of estimating the photoionization quantum yield C$i and the thermalizationrange parameter B3 of the photoelectron^^^ from the curve of IN, the normalized photocurrent, vs E, the applied electric field. It is implicitly accepted, if using parameters C$i and B3, that the process of ionization is composed of a series of distinct steps: excitation, photoelectron release, thermalization, and finally recombination with the parent ion or, alternatively, collectionby the applied electric field. If this classical pattern is maintained in the interaction with electron quenchers, the interaction will have different effects according to the stage at which it occurs. The emitted photoelectrons in the presence of quenchers could be thermalized at shorter distances from their parent ions than in the absence of quenchers either by actual capture or by loss of energy due to interactions with intramolecular transitions of guest molecules. In this case, a drop of B3 will be measured in the presence of quencher whereas C$i should be unchanged. On the other hand, if the main action of these quenchers is a deexcitation of excited states, as suggested by some experiments,20 C$i will be mainly affected. First, we present the analysis of the curves of IN vs E for 2,2dimethylbutane (D22B) over an energy range covering 0.5-2 eV above the ionization threshold and for 2,2,4-trimethylpentane (224TMP) at 10.03 eV. The results are compared with those obtained when the electron scavengers ( C C 4 and C7F14 at 0.1 mol/L) are added to the solvent. The second part of the paper examines whether the scavenging has a different effect on parameters B3 and Ge (equivalentto C$i) when irradiation iscarried out with COm y-rays instead of VUV photons. The aim of the experiment is to check that the mechanism of interaction of the scavengers with the geminate pair really is different.
11. Thermalization Range Parameter and W V Photoionization Quantum Yield in the Presence and Absence of Electron Scavengers 1. Experimental Section. The experimental setup and techniques used are described elsewhere!-1' The setup was enlarged by incorporation of a sampling system for the photocurrent and for the signal proportional to the photon flux. Although actually plotting the photocurrent spectrum by means of the setup developed is not strictly necessary to obtain the curves of IN vs E, its use allows spectra to be plotted at several fields (from 2 to 30 kVcm-1) and thus the values at the required energies to be obtained. In the same aim of giving access to a sufficiently large range of photon energies, we used a resonance lamp filled with H2/Ar (100 Pa) excited by a Raytron microwave generator. It supplies a relatively regular and stable spectrum at wavelengths above 136 nm. However, the flux produced at higher energies falls off sharply and also faded during an experimental run, especially around the line at 121.6 nm. At these highest energies, experimental limitations are obvious. The three-parameter adjustment method used for the curves of Z = f(E) is very sensitive to the slightest deformation of the curve, and any error in the estimation of the applied electric field affects the reliability of the parameters determined. Thus, we evaluated the interelectrode distance by the method described by Casanovas21based on extrapolation to zero field, or zero current, of the curve of the ionization current of hexane by W o photons versus the applied field. Moreover, the currents are low enough (10-lO A with a stability of the order of 10-14 A) for any deformation of the field by space charge to be excluded.
where k is a constant depending on the incident photon flux, 4i is the quantum yield for the production of a thermalized electron (here called photoionizationquantum yield), and PI(Bl,E) is the one-dimensionalOnsager probability of escaping recombination at the front electrode. This probability depends on Dl(x,BI), the spatial distribution, at thermalization, of charge distances x from the front electrode. It is assumed to be the same for both positive ions and ionizing events. Therefore, the attenuation function of the photon flow is
Furthermore,P3(B3,E),the three-dimensional Onsager probability of escaping geminate recombination, depends on D3(r,B3), the spatial distribution function of the thermalization distances r of the thermalized electron around its parent ion. B3 is the thermalization range parameter (often called the thermalization distance even though for each electronthe thermalization distance is, in fact, r). For the presentation of the results, we chose to use the function
3 exp( PE3(n = 2): D3(r,B3) = 7
i)
2B3 The utilization of this modified exponential functionlg gives quantum yield values that are not only realistic but close to those obtained through other techniq~es.~~9~4 Moreover, the simulations carried out with Monte Carlo method~25$~~ show that the function we chose is one which best accounts for the real distribution of thermalized photoelectrons when the slowing down process only involves a small number of interactions with a high momentum exchange. 3. Variation of the Parameters vs Excess Energy. Although thereis a certain dispersion in the results (Figure 2), the following remarks can be made concerning D22B. a. Attenuation Parameter BI. The attenuation parameter, B1, of the photon flow can be compared to values extrapolated from gas data16,27if one considers that the attenuation remains exponential and takes into account the increase of absorbing centers with no other condensation effect.19 In this approximation, B1 is estimated by using
where 6 = molar extinction coefficient in gas phase, M = molar mass of the compound, and d = density of the liquid. Thevalues of B1 deduced from the molar extinction coefficient are between 23.5 nm (at 10.65 eV) and 29.3 nm (at 9.57 eV). The B1 values measured (Figure 2c) range between 23 and 28 nm. Although our values present high dispersion with respect to the montonous curve deduced from the molar extinction coefficients, it can be considered that, on average,the three-parameter adjustment of the IN vs E curvesgives results which are not simply qualitative. b. Thermalization Range Parameter Bj. The thermalization range parameter B3 only increases slightly with the excess energy. This supports the idea that most of the thermalization path occurs at subvibrational energies,2* even if the role of intramolecular interactions at higher energies must be considered.8 The only values for thermalization distances in D22B available in the literature29 were obtained from the UV ionization current of TMPD dissolved in D22B using a two-parameter adjustment. In order to compare the values obtained in ref 29 with our values,
Guelfucci et al.
10354 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 11
0.5-
o
E
I
o
&io
0.4-
E
o
.I-
e
TI aJ *Ih
0.3-
7
5
0.2' 0
c, E I a 0
0
0.10
0l 6.6
e 9
9.2
9.4
9.6
v
10.2 10.4
10
9.8
(w/cm)
Figure 3. Effect of electron quenchers (0.1 mol L-I) on photocurrentvs applied electric field: 0,pure D-2-2-B; +, D-2-2-B/C7F14; 0 , D-2-2B/CCL. h
E
E
v
p3
m
L aJ c,
PI L
I
n aJ CI)
E
4
0
az
6.6
9
9.2
9.6
9.4
9.6
10.2 10.4
10
h
E
Y
4
m aJ
u
E
I c,
m
.I-
TI E 0
.r
w
I L c,
aJ
E
5 ] ,
aJ
n
0
8.8
,
9
,
,
9.2
,
,
,
9.4
,
9.6
,
,
,
9.8
,
10
,
. 10.2
,
1
eV
10.4
Figure 2. Parameters deduced from the three-parameter adjustment of photocurrent (vs applied electric field) in D-2-2-B irradiated by VUV photons at different energies. The adjustment was carried out assuming a PE3(n=2) distribution function of the thermalized electrons: (a) photoionization quantum yield &, (b) thermalization range parameter E3 (nm), (c) attenuation parameter of photon flux E1 (nm). 0, experimental points; A, El extrapolated from gas data.
TABLE Ik Thermalization Range Parameter (A) of Photoelectrons Produced in D22B through UV Photoionization of TMPJP or VUV Photoionization of Pure Solvent. excess D22B (A) D22B/TMPD (A) energy (eV) E G E G 1.28 99 165 66 120 1.43 1.58
104
174
16
129
96 133 IO 125 a Assuming Gaussian (G) or exponential (E) distributions. our curveswere reajusted with the same Gaussian and exponential distribution functions used in ref 29. Table I1 shows that the results are close for a given excess energy above the ionization threshold.
The decrease in B3 observed (Figure 2b) above 10 eV could be attributed, as suggested by Lee and Lipsky,30 to the fact that the electronemitted carries away part of the excess energy,leaving the positive ion in an excited state. The experimental limits and the accuracy of our three-parameter adjustment method do not allow us to state firmly that the slight decrease actually is due to this process. c. The quantum yield 4i is a rapidly increasing function of the excess eenergy. However, the method is not accurate enough to resolve the excited states detected by measurement of photocurrent. In summary, it is the quantum yield which is the essential factor in the variation of photocurrent with energy. It is also the determinant factor in the change of photocurrent (at comparable excess energies) whichoccurson goingfromoneliquidt0an0ther.l~ 4. Action of Electron Quenchers. a. 2,2-Dimethylbutane. As mentioned above, we plotted the photoconduction spectra at several fields and then drew the values required from the spectra. The addition of electron quenchers introduced a further source of inaccuracy since, first, the current dropped (Figure 3) and, second, the dark current drifted during experimentation. The drift was considered to be linear. The problem was overcome in a second series of experimentswhere the dielectric field was varied while keeping the energy fixed. No notable difference was observed between the two series of measurments, so the results were pooled without distinction (Figure 4a and 4b). At the same energy, the variation of BI resulting from the addition of quenchers is less than 30%. Among the values of E3 and $i, we selected those resulting from measurements in which the variation of B1 is less than 20% (Table 111). It appears that the action of the scavengers essentially produces a lowering of the ionization quantum yield whereas the values of B3 fluctuate without it being possible to determine whether they actually do fall. b. 2,2,4- Trimethylpentane. The same measurements were made using a lamp filled with krypton at 10.03 eV (Table IV). Here, the high stable lamp flux gives greater experimental accuracy. The value of BI for pure solvent is 260 A instead of 264 A extrapolated from gas data. It remains constant within 12% when CC14 is added. +i falls by almost half while B3 is slightly decreased (16%), which could indicate to a lesser extent scavenging of the electrons at the epithermal stage. For the same solvent, Lee and Lipsky31 have estimated thermalization distances of photoelectrons ejected from guest molecules (TMPD). In absence of quenchers, their measurements are in agreement with ours and with those of Choi et al.32 At 1.13 eV above the ionization threshold, the reported vlaues of B3 were 30,432 and 21.5 A.31 At 1.73 eV above the threshold, the reported values of B3 are 36 ,432 and 44 A (this work). However, the effect of quenchers on B3 was radically different: estimated
Effect of Electron Quenchers Produced by VUV Irradiation 0.M
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10355 TABLE IW Effect of CQ (0.1 M) on @I, l?, (A), and J& (A) Resulting from W V Irradiation of 224TMP at 10.03 eV Assumine a P E & F ~ ~Function
.
224TMP .r
0
U c
1I
W
.r
h
J + E m a
0.20
0 10
t
i
+ *
+
0 0 . 0 0 ; 8.8
TMP + CC4 (0.1 M)
E
+
, , , , ,
,
9
9.2
3
O
9.6
9.4
,
, , ,
,
9.8
10
10 2
, ,
I
+
90
, 10.1
+
80
60
-
50
-
70
m m L c W ,
E
40
-
30
-
Ir)
W 0)
E
0
0
I
0
O +
0
0
0
+
0
0
0
+ 0
c1
0. m
m ai
I
+
0
10
2o
o
I !
8.8
,
, 9
,
,
,
9.2
,
,
9A
,
,
9.6
,
,
9.8
,
, 10
,
, 10.2
IO.&
(b)
Figure 4. Effect of electron quenchers (0.1 mol L-l) on parameters $i (a) and B3 (b) at different energies: 0,pure D-2-2-B; 0,D-2-2-B/CC4.
+, D-2-2B/C7F14;
TABLE IIk Values of 41,B1 (A), and 4& (A) Resulting from WV Irradiation of Pure D22B and D22B/Quencher Assuming a PE&=2) Function. hv (ev) 9.15 9.50 9.54 9.69 mean
D22B + C7F14 BI (A) ai B3(A) 39.9 342 0.045 39.4 391 0.163 68.0 321 0.074 73.0 349 0.082 55.0 360 0.091 D22B
L..
9.15 9.22 9.29 9.50 9.58 9.65 9.80 9.88 mean
337 422 462 425 381 316 369 375 386
D22B Bi (A) ai 379 0.085 379 0.230 353 0.220 313 0.230 356 0.193
+ CClr
0.032 0.023 0.045 0.120 0.180 0.125 0.110 0.132 0.096
Ba(A) 45 48 48 55
49
D22B 52.8 75.0 47.6 41.0 30.5 62.7 67.0 67.5 55.5
379 368 368 379 313 260 299 299 333
0.085 0.130 0.130 0.230 0.230 0.330 0.560 0.560 0.280
ai
B3(A) B1 (A)
-1
*
ai B3(A) B1 (A)
45 44 44 48 55 55
47 47 48
In these experiments, the variation of BI from pure liquid to liquid/ quencher is less than 20%.
by adjustment of the curves of the photocurrent versus electric field, addition of perfluoro-n-heptane (0.1 M) caused a decrease of B3 from 27 to 18.5 A, whereas (this parameter falls by almost half in our works, using CCl, as quencher) was not affected. c. Discussion and Conclusion. There is a high risk when using adjustment with several parameters of obtaining a shift from one parameter value to another. Even in the relatively favorable case (UV irradiation of TMPD) where the adjustment only uses the
0.97 82 260 0.60 65 224
0.5 44 260 0.28 37 229
0.39 135 265 0.21 116 227
probability of escape from the parent ion P3(B3,E), the limits of the method are clearly illustrated in the following example. When the function PE3(n=2) is calculated at B3 = 40 A, the probability of escape in 224TMP is 0.0939 at E = 0 kV cm-1 and 0.275 at E = 40 kV cm-l. Thus, IPJ(~O)I/P~(O)I = 2.93 at B3 = 40 A. This value becomes 3.25 at B3 = 30 A. That is to say, it varies by 10% whereas B3 varies by 25%. At a higher field, the variation of the function P3(B3,E)vs the field is still less sensitive to change in B3. Consequently, any experimentalerror, especially a systematic deformation of the curve Z =f(E) resulting from an inaccurate estimation of the applied field due to an incorrect measurement of the interelectrodedistance, can lead to erroneous values of B3. The introduction of an additional parameter B1 via function Pl(BI,E) therefore might seem hazardous and the conclusions drawn from it even more so. Yet, the following observation can be made: (1) The method gives a good separation of the values of B1 and B3I9since, for a given curve, we obtain the same value of B1 using different distribution functions for the calculation of P3(B3,E). This probably results from the fact that the functions PI(B1,E) and P3(B3,E) have quite different shapes, especially at low field. (2) Apart from the set of values (BI, B3, and 4i) which gave the best adjustment, other quite different values also give an adjustment that is almost as g0od.lg For example, although the best adjustment (typified by a standard deviation of u = 0.85%) is obtained with the set of values BI = 251 A, B3 = 46 A, and 4i = 0.47 for 224TMP, adjustment is still acceptable ( u = 2%) with the values B1 = 338 A, B3 = 38 A, and 4i = 0.58. Nevertheless, in a space with coordinates BI,B3, and 4,the cloud of points allowing an adjustment that is better than a given value of u is very dense in the vicinity of the optimal trio. A statistical test should enable this observation to be rationalized. As we did not have such a test at our disposal, we adopted the following selection criteria: (i) The standard deviation u was lower than 1.5% for the pure solvents (for most energies it was lower than 1% and often 0.5%). (ii) For measurements using electron quenchers, the adjustment was not usually as good ( u from 1% to 5%). We therefore only chose values of B3 and 4i giving a variation of B1 of less than 30% (Figure 4) or 20% (Table 111) when going from the pure solvent to a mixture of solvent and quencher. (3) Furthermore, as mentioned above, the fact that B1 determined in this way is close to that obtained from the molar extinction coefficients in the gas phase is a good indication of the validity of the adjustment. This is why, although the level of experimental accuracy should require a more quantitative estimation, the present studies seemed to support the hypothesis of an interaction between the quenchers and the excited states of the alkane molecules. This conclusion, as we have already mentioned, tends to oppose that of Lee and Lipsky3I concerning TMPD in solution in 224TMP.
III. Effect of Electron Scavengers on High-Ewrgy Photon Ionization Parameters 1. Presentation. It is known that the efficiency of scavengers depends on how the photoelectrons are produced. As illustrated in Figure 5, the curve of the reciprocal of the photocurrent against
' Guelfucci et al.
10356 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
:::1.2
-
1.1 1
-
0.9
-
0.8 0.7
0.6 0.5
-
-
0.4
-
0,3
1
0.2
(W/cm)
0.1
0
Figure 5. Effect of the concentrationof an electronquencher (hereC ~ F I ~ ) on the reciprocal photocurrent in the case of VUV irradiation (curve A) and on the reciprocal of the free ion yield (Gfi), in the case of y-ray irradiation (curve B).
scavenger concentration is linear whereas that of the reciprocal of the yield Gs, the free ion yield of Coda y-rays, is parabolic9 (Gfi is the number of pairs of ions collected for a given field for 100 eV of energy deposited into the medium). Without going back over the whole discussion, we interpreted this difference as indicating an interaction between the quenchers with an excited state in the first case and with the electron, in its thermalization process, in the~econdcase.~ In thesame way that three-parameter adjustment of the curves of IN against E allows discrimination between parameters B3and I&, two-parameter adjustment (since ionization occurs in volume) of the curves of free ion yield, Gn, against field, E, allows discrimination of B3 and Gtotsince
, 0
10
This function is considered to be suitable when the thermalization path includes a large number of high momentum exchange
40
D-2-2-B, at different concentrations: 0 , pure D-2-2-B; +, D-2-2-B/ C7Fl4 (0.014 mol L-I); 0, D-2-2-B/C7Fl4 (0.052 mol L-l); A, D-2-2B/C7F14 (0.133 mol L-I).
TABLE V Effect of C7F14 and CCb on , C and & (A) when D22B Is Irradiated by y-rays from Co@ D22B PE2 PE3(n=l) G 4.96 4.10 2.72 pure A :) 80 68.7 159.6 C7F14 (0.014 M) C7Fl4 (0.052 M)
A :) GtGt
B3(A) C7F14 (0.133 M) C c 4 (0.0057 M)
Gm B3(A) Got
Bj(A) Gtot
B3(A)
of ion pairs at thermalization for 100 eV of energy deposited into the medium. Once more, the variation of the parameters Gtot and B3 must be able to take account of the way the scavengers interact with the geminate pairs. 2. Experimental. The curves from which we determined parameters G,, and 83 were established a few years ago. Reference should be made to previous publication^^^^^^ for the description of the setup and of the experimental protocol. We shall simply mention here that we used an activity source that was sufficiently weak to limit electron-positive ion recombination other than geminate (if this is possible since whatever the number of initial ionization events, not geminated recombination occurs in spurs produced by secondary electrons resulting of a same primary ionization). The induced currents were lower than 3 X 10-l0 A (at 30 kV cm-I), and the residual currents of the pure solvents are about 5 X 10-13 A with a stability of l&14 A. In the presence of scavengers, the dark current rose considerably. We therefore had to limit the applied field to 35 kV cm-1. To make the comparison more relevant, we also analyzed this range of fields for pure solvents even though measurments were made up to 200 kV cm-I. Gn was determined from the current collected using hexane as second standard (the values of Gno obtained by the various authors are in very good agreement for this liquid) and making the corrections required for the different numbers of C and H atoms. 3. Choice of Distribution Function. The curves of Gn against E obtained withvarious scavenger concentrations (Figure 6 ) were analyzed using the following distribution function:
30
Figure 6. Evolution of the free ion yield Gfi in D-2-2-B irradiated by y-rays of Co60 vs the applied field E resulting from additionof C7Fl4 into
CCI4 (0.0105 M)
Gtotis the free ion yield before recombination, Le., the number
20
CC4 (0.0408 M)
Got
B3(A) C c 4 (0.153 M)
Gtot
B3(A)
6.97 57 6.40 54.4 6.35 49.5 7.5 61.7 6.34 64.3 6.68 56.4 6.27 49.1
5.72 49.2 4.97 48.3 5.02 43.4 5.81 53.4 5.26 55.1 5.42 48.9 5.02 42.9
3.22 124.3 2.88 120 2.70 111.6 3.44 131.9 3.14 135.2 3.08 122.8 3.67 111
interactions.8 However, the use of this function often gives total free ion yields (Gtot) which are too low compared with those deduced from the yield of photolysis prod~cts.~3
(
n! PE,(n = 1): D3(B3,r)= L)n+'L#'e4r/B3) B3 When n = 1, the use of this function also assumes a large number of collisions and an exponential attenuation of the electron flux with a single efficient cross section.34
PE,(n=2): as above with n = 2 This is a function that is often used when ionization of the liquid itself or of solvated molecules is produced by VUV photons (see section 11) or UV photons.31
This modified exponential is purely empirical, but, for some liquids, it leads to more satisfactory total free ion yields.33 4. Results and Discussion. The results concerning D22B and 224TMP are presented in Tables VI and V. With the exception of one value (CCL4 a t 0.0057 M in D22B), 8 3 decreases steadily as the scavenger concentration is increased, whereas Gw does not vary to any large extent. Note, however that the value of Gbt obtained with scavengers is greater than that obtained without. This anomaly probably arises from the fact that the two series of measurements were carried out separately, a few years ago, and that the calculation of GtM(or of Gfi) from the current detected is strongly dependent on the experimental conditions. To remove any uncertainty, we performed the measurements on D22B and the mixture D22B/CC& (0.2 mol/L) once more, paying particular
Effect of Electron Quenchers Produced by VUV Irradiation
TABLE VI: Effect of C7F14 on C, and & (A) When 224TMP Is Irradiated by y-rays from Co60 224TMP pure
Got
E3 (A)
C7F14 (0.0065M)
C7Fl4 (0.0125M)
C ~ F(0.052 I ~ M)
Gtd
B3 (A) Gm
&(A) Gbt
(A) E3 (A) B3
CCl4 (0.11 M)
Got
PEz
PE3(n=l)
5.45 65.5 7.65 49.5 7.28 49 6.78 46.6 6.92 43.1
5.03 52.5 6.13 43.1 5.83 42.9 5.43 40.6 5.46 37.8
G 2.77 136 3.35 110 3.22 108.9 2.92 104.5 2.82 99
TABLE W: Effect of CCq (0.2 M) on C, and & (A) when D22B Is Irradiated by y-Rays from Cow
D22B D22B+ Cc4
5.70 5.53
84.8 63.6
4.62 4.48
72.9 54.6
2.65 2.24
169 136
attention to keeping the experimental conditions the same. The results (TableVII) show a clear decreaseof B3 wheras Gt,remains constant. Considering these results, it could be proposed that the quenchers act on the electron in the process of thermalization and not on the superexcitedstates, which have too short a lifetime to interact with the additives. If this interaction only involved electrons with an energy of a few tens of electronvolts, lower than the intramolecular transitions, no difference should be seen in the determination of the 8 3 parameters with the different types of irradiation. Yet, in spite of the large fluctuations which limit the correct evaluation of this parameter in the presence of quencher, it seems that the decrease is not as great with VUV irradiation: more or less zero for D22B and 15% for 224TMP instead of 25-30% for comparable quencher concentrationswhen the irradiation is of the high-energy type. If this reduced effect of the scavengers on B3 under VUV irradiation were confirmed, it would be reasonable to propose that even though the subvibrational stage remains determinant in as far as the thermalization distance and its modifications are concerned all the external part of the slowing down process concerning the interactions of electrons of higher energy with the molecules of the medium (momentum exchange and interactions with intramolecular transitions) must have some influenceon the thermalization parameters as suggested by the difference in the shape of the distribution function according to the type of irradiation. Apart from any experimental uncertainty,it remains that, under VUV irradiation, & drops dramatically in the presence of quenchers whereas under T r a y irradiation Gtotshows no notable change.
IV. Conclusion The object of this study was to obtain a better definition of the conditions in which an electron is transferred from a molecule into the continuum of a nonpolar liquid medium, in particular if it involves a mechanism of ionization that is direct or that occurs via excited states at energies greater than the ionizationthreshold.
The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10357 Apart from themost “direct”test, Le., plotting photoconduction spectra, it is possible, using a three-parameter adjustment technique (with VUV ionization) or a two-parameter technique (irradiation by high-energy photons) on the curves of photocurrent againstapplied electric field, to discriminatebetween the variations of the different parameters involved in the classical description of the photoionization process. The drop in photocurrent resulting from the introduction of scavengers into the solution seems to have two different origins. In the case of VUV irradiation it is mainly due to the drop in the ionization quantum yield, whereas with irradiation by y-photons from Co6Oit is rather the thermalization distance that is affected. These measurements therefore support the preferential interaction of the scavengerswith the possible excited states produced by VUV irradiation, whereas they probably act at the epithermal stage with electrons produced by y-ray irradiation. The comparison of these various studies should, however, be continued to enable a firm conclusion to be reached.
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.-_.
AX?
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