[731090]

(731091)

[731106]

(740029]

[740092]

{740153}

[740159]

(741012)

[741030]

[741035]

ion) yield in liquid methanol, INT. J. RADIAT.
PHYS. CHEM. 5: 521-3 (1973).

Brown, B.J.; Barker, N.T.; Sangster, D.F., Electron
solvation processes in » alcohol-hydrocarbon
mixtures, AUST. J. CHEM. 26(10): 2089-99 (1973).

Shaede, E.A.; Dorfman, L.M., Flynn, G.J.; Walker,
D.C., Spectrum, kinetics, and radiation chemical
yield of solvated electrons in hexamethylphosphoric
triamide, CAN. J. CHEM. 51(23): 3905-13 (1973).

Marevtsev, V.S.;  Vannikov, A.V., Optical
absorption spectra of the solvated electron in
irradiated moncethanolamine, HIGH ENERGY
CHEM. 7(2): 104-8 (1973), Transl. from Khim.
Vys. Energ. 7(2): 119-24 (1973),

Miyajima, M.; Takahashi, T.; Konno, S.; Hamada,
T.; Kubota, S.; Shibamura, H.; Doke, T., Average
encrgy expended per ion pair in liquid argon,
PHYS. REV. PT. A %3): 1438-43 (1974).

Kroh, J.; Piekarska, J., Non—homogeneous kinetics
of ionic reactions in liquid n—hexane and 2,2,4-
uimethylpentane, BULL. ACAD. POL. SCI. SER.
SCL. CHIM. 22(2): 167-72 (1974).

Takahashi, T.; Konno, S.; Doke, T.; The average
energies, W, required to form an’ ion pair in
liquefied rare gaecs, J. PHYS. C: SOLID STATE
PHYS. 7(1): 230-40 (1974).

Belloni, J.; Cordier, P.; Delaire, J., Transient
spectrum from nanosecond pulse- radiolysis of
liquid ammonia, CHEM. PHYS. LETT. 27(2): 241-
4.1974).

Robinson, M.G.; Freeman, G.R., Electron mobilities
and ranges. in liquid C,—C, hydrocarbons and in
xenon: Effects of temperature and field strength,
CAN. J. CHEM. 523): 440-6 (1974).

Shaede, E.A.; Kurihara, H.; Dorfman, L.M.
Observation of initial ion-pairs in pulse~irradiated
weakly polar liquids, INT. J. RADIAT. PHYS.
CHEM. 6: 47-54 (1974).

Brandon, J.R.; Firestone, R.F., Solvated electrons
in alcohol-alkane and alcohol—amine solutions, J.
PHYS. CHEM. 78(8): 792-6 (1974).

16

[741062]

(741084)

[741094]

(741100)

[741110]

{748021}

[749139]

(751047)

751054]

[758020]

Perkey, L.M.; Hentz, R.R., Pulse
radiolysis of liquids at high pressures. V.
Absorption spectrum and yield of the solvated
electron in liquid ammonia at 23°C and pressures
up to 6.7 kbar, J. CHEM. PHYS. 60(11): 4383-9
(1974).

Brown, B.J.; Barker, N.T.; Sangster, D.F., Pulse~
radiolysis of binary quid alcohol systems
containing benzene, AUST. J. CHEM. 27(5): 1129—
32 (1974).

Seddon, W.A.; Fletcher, J.W.; Sopchyshyn, F.C.,
Jeveak, J., Pulse radiolysis of liquid methylamine,
ethylamine, and ammonia: the yield of solvated
electrons, CAN. J. CHEM. 52: 3269-73 (1974).

Kira, A.; Thomas, J.K., Formation of ions and
excited states in the pulse radiolysis of
benzonitrile, J. PHYS. CHEM. 78(21): 2094-6
(1974).

Shinsaka, K.; Freeman, C.R., Epithermal electron
ranges and thermal electron mobilities in liquid
aromatic hydrocarbons, CAN. J. CHEM. 52(20):
3495-506 (1974).

Mozumder, A., Effect of an external electric field on
the yield of free ions. II. The initial distribution of
ion pairs in liquid hydrocarbons, J. CHEM. PHYS.
60(11): 4305-10 (1974).

Ertl, G.; Kueppers, J., Low energy electrons and
surface chemistry, Verlag Chemie G.m.b.H.,
Weinheim, 1974, Chapter 1.

Dodelet, J.—P.; Freeman, G.R., Electron mobilities
and ranges in liquid ethers: jion-like and
conduction band mobilities, CAN. J. CHEM. 53(9):
1263-74 (1975).

Dixon, R.S., Lopata, V.J., Solvated electron yields
in pulse-irradiated liquid 1~propanol, J. CHEM.
PHYS. 62(11): 4573-4 (1975).

Dodelet, J. ‘reeman, G.R., Electric field effect
on free ion yields in x-irradiated liquids: a
multipair spur model, INT. J. RADIATION PHYS.
CHEM. 7(2/3): 183-92 (1975).



oot A TM
= NEOPENTAT oie
oe TETRAMETHYL BUTANE | 4
Le} * 2,24/4- TETRAMETHYLPENTANE 4 [
* 288 TETRAMETHYLOCTANE O10 2 PROPADIENE 4
Lal 2 2,2,3,3- TETRAMETHYLPENTANE. 4 L 3 Ei uM
“es THEME, Tt
ooer + =i, n 4
. © OCTADIENE-1,7 °
hab a 4 G b 4
i e
’ . 0.06 4
Lop , . 4 L rete 4
Sri . 0.04 oe o
Osh ay . . 4 L 2 “e 4
° v o ope- | #8 ° 4
0.6f ao . on 4 bo 4
a Fe
v o 200 300 400
oat 2° . 4 TEMPERATURE, °K
°° Ficure 5. G7, for some dienes, from [731054].
0.2b eo 4
FA
300 400 480
TEMPERATURE, °K
Ficure 3. Gj; for some saturated hydrocarbons, from [720269].
at tot
0.66}- | 0.46 + ACETYLENE |
@ PROPYNE
F 4 + © BUTYNE-2 4
0.58} * TRANS-BUTENE-2 4 03 = ° Ht - 4
© CIS-BUTENE -2 a * HEXYNE-S *
L = ISOBUTENE 4 L 4
2 2-METHYLBUTENE-2 °
0.50, + 2.3-DIMETHYL BUTENE-2 4 0.30) 4
v HEXEN o .
| 5 SRANS-HEXENE-2 4 oi OF 4
4 TRANS-HEXENE-3 a
0.42/- v CIS-HEXENE ~3 4 022+ a 4
¢ CYCLOHEXENE :
Gr L 4 L . 4
e o 6 4
0.34 . eo O14 . :
L oq L og e 4
0.26 s ° Fa i °
L ° vow L . 4
a ‘ po Ti po
0.18b ° , 3 4 200 300 400
° . ° . TEMPERATURE.°K
L ob yg 4
io | Ficure 6. Gj, for some alkynes, from [731054].
°
, 3
oe 2 4
eroe
L n n 4 4 4 — 7]
200 300 400

TEMPERATURE, °K

Ficure 4. Gj, for some singly—unsaturated hydrocarbons, from
{741012}.

10


Yields of Free Ions Formed in Liquids by Radiation.*

A. O. Allen

Department of Chemistry
Brookhaven National Laboratory
Upton, N.Y. 11973

Free ions are those produced by ionizing radiation which escape initial recombination.
Yields of free-ion pairs are tabulated for liquid alkanes, alkenes, alkynes, dienes and
aromatic hydrocarbons, and other non-polar and polar compounds, including alcohols,
ethers, esters, halides, amines, nitriles, etc. Total ion yields for a few liquids are compared
with gas phase ion yiclds. Theoretical treatinents of these data are discussed.

Key words: Electrons; free ions; ion yields; liquid; non-polar systems; polar systems; radia-
tion chemistry; radiation physics.

1. Definitions

When X-rays, gamma rays or fast charged particles traverse a gas, positive and negative ions are
formed in equal numbers, and (in the absence’ of an applied electric field) become uniformly distributed
through the volume by diffusion, and eventually disappear by pairwise general recombination. If a D.C.
electric field is applied, the ions drift to the electrodes and give up their charge to the external circuit.
At ordinary radiation intensities, a relatively modest field strength suffices to collect all the ions. The
number of ions of either sign produced by unit energy input is called the ion yield and is a characteristic
property of each gaseous substance.

In a liquid under radiation, the charge collected by an applied field is usually only a small fraction
of that produced by.the same radiation absorbed in the same amount of the same material in gaseous
form. The cause of this difference was correctly described nearly seventy years ago by G. Jaffé
[080002]'. In a gas, ionization knocks out an electron with sufficient momentum to escape from the
electric field of the positive ion it leaves behind; but in a liquid, collisions with surrounding molecules
rapidly reduce its initial outward momentum so that it has only a small escape probability. Usually the
electron rapidly returns to its parent ion and recombines with it. This process is called initial
recombination or geminate recombination. The number of electrons that escape initial recombination,
per unit energy input by the radiation, is called the free-ion yield. Since an applied electric field tends to
pull the electron out of the field of the positive ion, the free—ion yield increases with the applied field. The
field strength that a liquid can stand is limited by dielectric breakdown; in most liquids, the free—ion
yield at the maximum usable field is still only a small fraction of the ion yield found in the gaseous form
of the same substance. Only in monatomic liquids (liquefied rare gases) has complete collection of the
ions been reported.

The escaped electron can (1) remain as a quasi—free electron in a delocalized state in a conduction
band; (2) aggregate solvent molecules about itself to form a solvated electron; (3) react with a solvent
molecule, or an impurity or solute molecule to form a new negatively charged molecule, or negative ion.

* This is a data review prepared for, and in cooperation with, the Radiation Chemistry Data Center of the Radiation Laboratory, University of Notre Dame, Notre Dame,
Indiana 46556, The Laboratory is operated under contract with the Energy Research and Development Administration. The work ofthe Center is supported in part by the National
Bureau of Standards, Office of Standard Reference Data

"Numbers in brackets indicate the literature references at the end of this paper.


[680042]

[680047]

[680061]

[680227]

[680376]

{680610}

[680821]

[690179]

[690185}

[690240]

[690348]

(6903871

{690429}

(690431)

[690521]

(690650}

[690655]

[700019]

pulse-itradiated alkaline aqueous _ solution,
RADIAT. RES. 32(3): 564-80 (1967).

Russell, J.C.; Freeman, G.R., Reactions of the
primary reducing species in the radiolysis of liquid
2-propanol, J. PHYS. CHEM. 72(3): 808-15
(1968).

Russell, J.C.; Freeman, G.R., The yields of the
primary reducing species in the’ radiolysis of liquid
ethanol, J. PHYS. CHEM. 72(3); 816-21 (1968).

Fielden, E.M.; Hart, E.J., Primary radical yields
and some rate constants in heavy water, RADIAT.
RES. 33: 426~36 (1968).

Cooper, R.; Thomas, J.K., Nanosecond-pulse
radiolysis of aniline, J. CHEM. PHYS. 48(11):
5103-9 (1968).

Schmidt, W.F.; Allen, A.O., Yield of “free ions in
irradiated liquids: determination by a clearing field,
J, PHYS. CHEM. 72(11): 3730-6 (1968).

Jha, K.N.; Freeman, G.R., I. Kinetics of reactions
of electrons during radiolysis of liquid methanol. II.
Reaction of electrons with liquid alcohols and with
water, J. CHEM. PHYS. 48(12): 5480-90 (1968).

Jacobi, W.; Jahns, A., Ionization and recombination
in dielectric fluids after y-irradiation, Molecular
Structure and Radiation Effects, Glubrecht, H.
(ed.), Georg Thieme Verlag, Stuttgart, 1968, p.25~
35.

Rajbenbach, L.A., Radiolysis of solutions of
perfluorocarbons in n-hexane, J. PHYS. CHEM.
73(2): 356-60 (1969).

Appleby, A.; Schwarz, H.A., Radical and molecular
yields in water irradiated by y rays and heavy ions,
J. PHYS. CHEM. 73: 1937-41 (1969).

Conrad, E.E.; Silverman, J., Short-lived transient
species in irradiated n-hexane, J. CHEM. PHYS
51: 450-1 (1969).

Capellos, C.; Allen, A.O., Ionization of liquids by
radiation studied by the method of pulse radiolysis.
Il. Solutions of triphenylmethyl chloride, J. PHYS.
CHEM. 73(10): 2364-8 (1969).

Jha, K.N.; Freeman, G.R., Effect of temperature on
the reactions of electrons during the y radiolysis of
liquid n-propanol, J. CHEM. PHYS. 51(7): 2839-
46 (1969).

Klassen. N.V.: Schmidt. W.F.. Ionization of liquid
argon by X-rays: W-value, CAN. J. CHEM.
41/22): 4286-8 (1969).

Theard, L.M.; Peterson, F.C.; Holroyd, R.A.
Nanosecond pulse radiolysis of liquid naphthalene
and biphenyl, J. CHEM. PHYS. 51(9): 4126-32
(1969).

Beaumond, D.; Rodgers, M.A.J., Pulse radiolysis
studies af ion association, TRANS. FARADAY
SOC. 65: 2973-80 (1969).

Kroh, J.; Hankiewicz, E., Ionic processes in the
radiolysis of liquid benzene as studied by tritium

substitution, INT. J. RADIAT. PHYS. CHEM.
1(4): 451-6 (1969).
Hecquet, M.F.; Roux, J.C.; Simonoff, G.N.,

Radiolyse de l'isopropanol et des melanges eau-
isopropanol, INT. J. RADIAT. PHYS. CHEM. 1(4):
529-40 (1969).

Capellos, C.; Allen, A.O., Ionization of liquids by
radiation studied by the method of pulse radiolysis.

14

[700047]

[700055]
[700090]
{700172}

[700173]
(700243)
[700249]
[700299]
(700349)

[700360]

[700406]

(7006501

[701042]

(710064)

[710122]

(710138}

IIL. Solutions of galvinoxyl radical, J. PHYS.

CHEM. 74(4): 840-5 (1970).
Warman, J.M.; Rzad, S.J., Relevance of the
concentration dependence of methyl radical

formation from methyl’ bromide solutions to the
distribution of ion-pair separation distances in
irradiated hydrocarbons, J. CHEM. PHYS. 52(2):
485-90 (1970).

Schmidt, W.F.; Allen, A.O., Free-ion yields in
sundry irradiated liquids, J. CHEM. PHYS. 5265):
2345-51 (1970).

Schmidt, W.F., Radiation—induced conductivity and
ion yields in neopentane at high electric fields,
RADIAT. RES. 42: 73-8 (1970).

Fletcher, J.W.; Richards, P.J.; Seddon, W.A.,
Pulse radiolysis of ethanol, CAN. J. CHEM. 48(11):
1645-50 (1970).

Head, D.A.; Walker, D.C., Yield of free-ions in the
radiolysis of formamide, a liquid of very high
dielectric constant, CAN. J. CHEM. 48(11): 1657—
63 (1970).

Barker, G.C.; Fowles, F.; Sammon, D.C.; Stringer,
B., Pulse radiolytic induced transient electrical
conductance in liquid solutions. Part 1. Technique
and the radiolysis of water, TRANS. FARADAY
SOC. 66(6): 1498-508 (1970).

Bansal, K.M.; Rzad, S.J., Electron scavenging in
the 7 radiolysis of liquid diethyl ether, J. PHYS.
CHEM. 74(15): 2888-92 (1970).

Sagert, N.H.; Reid, J.A., y-Radiolysis “of
hydrocarbons using perfluorocyclohexane as an
electron scavenger: determination of the free ion
yields, CAN. J. CHEM. 48: 2429-32 (1970).

Kroh, J.; Burzynska, E., The influence of LET on
the radiation yield of: free ions in benzene as
measured by tritium substitution, RADIOCHEM.~
RADIOANAL. LETT. 43): 119-22 (1970).

Hayon, E., Yield of ions and excited states
produced in the radiolysis of polar organic liquids,
J. CHEM. PHYS. 53(6): 2353-8 (1970).

Baxendale, J.H.; Beaumond, D.; Rodgers, M.A.J.,
Electrons, ions and excited states in the pulse
radiolysis of tetrahydrofuran, TRANS. FARADAY
SOC. 66: 1996-2003 (1970).

Horacek, K.: Freeman. G.R.. Kinetics of electron
and ion reactions during the 7 radiolysis of liquid
neopentane: dependance of charge scavenging
kinetics on hydrocarbon molecular structure, J.
CHEM. PHYS. 53(12): 4486-9 (1970).

Matsui, M.; Imamura, M.; Karasawa, T., LET
effect on the hydrogen formation in the radiolysis
of liquid tetrahydrofuran, IPCR Cyclotron Progress
Report, 1970, Vol. 4, Dec... p.126-7.

Fowles, P., Pulse radiolytic induced transient
electrical conductance in liquid solutions. Part 4.
The radiolysis of methanol, ethanol, 1-propanol
and 2—propanol, TRANS. FARADAY SOC. 67(2):
428-39 (1971).

Stoneham, T.A.; Ethridge, D.R.; Meisels, G.G.,
Precise measurements of W, the mean energy
required for ion-pair formation in gases. I.
Alkanes, J. CHEM. PHYS. 54(9): 4054-9 (1971).

Rabani, J.; Graetzel, M.; Chaudhri, S.A.; Beck, G.;
Henglein, A., The radiation chemical yield of OH”



TABLE 2. Zero-field free-ion yields in unsaturated and aromatic hydrocarbon liquids — Continued

‘Compound Temp. Method Gi Reference
K
m-Xylene 292 charge coll. 0.082 7410
p-Xylene 292 charge coll. 0.073 741110
1,2,3-Trimethy}benzene 292 charge coll. 0.12 741110
1,2,4-Trimethylbenzene 292 charge coll. 0.083 741110
1,3,5-Trimethylbenzene 293 charge coll. 0.087 741110
1,2,3,4~Tetramethylbenzene 292 charge coll. on 741110
1,2,4,5-Tetramethylbenzene 358 charge coll. 0.15 7410
Pentamethylbenzene 353 charge coll. 0.25 74110
Hexamethylbenzene 448 charge coll. 0.6 741110
Naphthalene 316 charge coll. ou aio
Anthracene 500 charge coll. ~O.1 741110
Biphenyl 373 pulse radiolysis, ion spectra <0.35 690431
“Cis-trans mixture.
Taste 3. Zero-field free-ion yields in miscellaneous non-polar liquids
Compound Temp. € Method Gi Reference
kK
Tetramethylsilane 296 1.84 charge coll. 0.74 700055
Tetramethyltin 296 2.07 charge coll. 0.62 700055
Carbon disulfide 296 2.63 charge coll. 0.314 700055
296 scav., (CaHs)xCCl 0.26 690348
‘Triethylamine 296 244 charge coll. 0.148 700055
Germanium tetrachloride 296 244 charge coll. 0.127 700055
Carbon tetrachloride 296 2.232 charge coll. 0.096 700055
296 seay., (CoHs)sCCl 0.093 690348
296 condy. 0.068" 660148
1,4~Dioxane 295 2.21 condy. 0.05, 650216
296 eondy. 0.038 660148
— scav., benzophenone ol 690521
296 charge coll. 0.046 700055
296 scav., anthracene 0.10 700360
293 2.20 charge coll. 0.08 751047
335 2.15 charge coll. 0.12 751047
374 2.08 charge coll. 0.16 791047
Carbon tetrafluoride 143 1.64 charge coll. 0.07 730095
Perfluoro—n—pentane 296 1.68 charge coll. 0.035 700055
Perfluoromethylcyclohexane 296 1.85 charge coll. 0.028 700055,
Oxygen 87 1.50 charge coll. 0.013 720034
TABLE 4. Zero-field free—ion yields in polar liquids (« > 3)
Compound Temp. € Method Gi Reference
K
Ethers:
Di-n-butyl ether = 3.1 condy. on 650216
187 4.50 charge coll. 0.21 751047
220 3.99 charge coll. 0.23, 751047
255 3.52 charge coll. 0.26 751047
293 3.14 charge coll. 0.30 751047
332 2.83 charge coll. 0.36 751047
373 2.55 charge coll. 0.42 751047
406 2.35 charge coll. 0.48 751041
436 2.20 charge coll. 0.54 751047

6


3.2. Conductivity

The specific conductance of a system is proportional to the product of the number of ions formed
and their mobility. If the mobility is known, or guessed, then a determination of the conductivity
produced by a known intensity of radiation can be used to calculate the free—ion yield. For non—polar
liquids, the method is difficult and not too reliable and has been superseded by the charge collection
method. Used with a pulse radiolysis system in aqueous and alcoholic media, this method has given
valuable results.

3.3. Charge Collection

Charge collection methods comprise steady-state and pulse methods.

In the steady-state method, the current of ions collected by the field is measured with the radiation
intensity so low and the field so high that essentially no general ion recombination occurs as the ions are
swept out by the field.

In the pulse or “clearing field” method the charge produced by a short pulse of radiation is
collected by the field and measured. In one version, the field is turned on immediately after the
radiation pulse to give a dircct mcasurcment of the zcro~ficld free~ion yicld; a ballasting condenser
takes care of the charge necessary to bring the electrodes to voltage when the field is switched on
directly after the radiation pulse. In another version, the field is maintained during the pulse and the
zero-field value obtained by extrapolation.

In all the electrical methods, the collected currents or charges must be kept very small to avoid
recombination errors, and it is difficult to avoid systematic errors in the electrical measurement systems.
Also, difficulties in measuring the energy put into the liquid may occur at these very low dose rates.

Although careful determinations of G;,; by charge collection should be accurate to +5%, values
reported by different laboratories for the same pure liquid at the same temperature unfortunately often
differ by many times this amount.

4. Tabulations of Gj

Tables 1-4 give values of Gj, for a variety of pure liquids. When values were determined by a
particular worker over a series of temperatures, only one value is usually given in the tables; values
obtained over a range of temperatures are shown graphically in figures 1—6. For temperature effects in
aromatic hydrocarbons, see [741110]. The tables give the name of the liquid, temperature of
measurement, dielectric constant of the liquid, method used, reported value of Gj, and literature
reference. The dielectric constants are omitted from the hydrocarbon tables because the values nearly
all lie about 1.8-2. When no temperature is reported, “room temperature” may be assumed. Under
“Methods” is indicated whether conductivity (condy.), charge collection (charge coll.) or scavenging
(scav., followed by the formula or name of the scavenger used) was employed.

Data on mixtures of non—polar compounds are given in [680376] and [700055], but are not
reproduced here. In mixtures of hydrocarbons, G, changes gradually with composition; but addition of a
little CCl, to neopentane results in an abrupt drop of Gj. Relative values of Gj, in mixtures of hexane
and alcohols are reported in [710464] and [731090], mixtures of benzene and alcohols in [741084],
mixtures of ethanol and water in [730143], and mixtures of alcohols and alkanes in [741035].


The term “negative ion” is however sometimes used to indicate any negatively charged entity, including
the electron.

Although attempts are made to extrapolate the free—ion yield to infinite field strength to obtain the
total ion yield, the zero-field free—ion yield is precisely determinable and has been the focus of much
recent work.

The free—ion yield depends strongly on the “radiation quality” or linear energy transfer (LET). At
high LET, the positive charges are grouped in columns, the electrons are pulled back by a large group
of positive ions instead of by just one or a few, and the escape probability becomes much lower. Moet of
the data reported here were taken with low LET or near—minimum ionization radiations such as y rays,
1-2 MeV X-rays or megavolt electron beams, and this condition may be assumed unless otherwise
indicated.

The free-ion yield is of practical significance in determining the chemical changes that occur in
liquid and amorphous systems under radiation. The scientific significance of free—ion yield
measurements lies in the fact that no adequate theory exists for the behavior of electrons in non—
crystalline condensed systems, and free—ion yields offer one type of data which may point the way to
the development of such a theory.

The purpose of this review is to assemble currently available data on zero-field free—ion yields in
pure liquids for the convenience of those who wish to use these quantities. No attempt is made here to
assess probable errors or to ascertain probable “best” values. Before this can be done, the many
differences in results reported from different laboratories will have to be resolved in the light of
independent determinations.

2. Notation and Units

The ion yield in gases is usually expressed as its reciprocal, denoted W, which is always expressed
in eV per ion pair. In liquids, the yield is usually given as G = 100/W, the number of ions of either sign
produced per 100 eV energy input. In this paper, the zero-field free—ion yield in liquids is denoted by
Gj,, while the total ion yield is Gj;. The electron is represented by e~.

3. Methods of Measurement

3.1. Scavenging

The scavenging method involves adding a substance which reacts directly with the electron (or in
some cases with the positive ion or its deprotonated successor radical) to form a chemical product which
is determined quantitatively. Examples of scavengers are anthracene, biphenyl, pyrene and the free
radical galvinoxyl, which readily add an electron to form a colored radical anion of known optical
absorptivity, and N,O, which reacts with an electron to form O~ and a molecule of nitrogen, which is
determined. There are two varieties of the method: (1) the steady-state method, in which a permanent
chemical product is determined, following an exposure to a continuous source such as y rays; (2) the
pulse method, in which the optical absorption of a transient product such as pyrene ion gives a measure
of the electron yield.

There are many sources of error. If the scavenger concentration is too high, some of the initially
recombining electrons or ions may be counted as well as the free ions; results should always be
extrapolated to zero scavenger concentration. A less obvious error occurs if some of the electrons react
with the scavenger to form other products, which are not determined; or the electron may also be
reacting with impurities, or with free radicals or other radiation products; these errors make the reported
yield too low. If, however, the determined product also is formed by the reaction of the scavenger with
other free radicals formed by the radiation, the reported yield might be too high. If the radiation dose is
too large, the product may begin to compete with the scavenger. It is generally difficult or impossible to
estimate the accuracy of a determination of a free—ion yield by the scavenger method except by
comparison with results of other methods.


[710186]

(710189)

{710215}

[710464]

1710531)

1710564]

(710638)

[719437]

{720002}

720032]

[720034]

{720105}

[720153]

[720166]

[720199]

[720232]

[720269]

as determined by conductometric pulse radiolysis,
J. PHYS. CHEM. 75(11): 1759-60 (1971).

Arai, S.; Kira, A.; Imamura, M., Pulse radiolysis
studies of acetone solutions of biphenyl and
anthracene: Formation of ions and excited singlet
state, J. CHEM. PHYS. 54(12): 5073-81 (1971).

Rodgers, M.A.J., Pulse radiolysis studies of acetone
solutions, TRANS. FARADAY SOC. 67(4): 1029-
40 (1971).

Cooper, T.K.; Walker, D.C., Free-ion yield in the
radiolysis of dimethylsulfoxide, CAN. J. CHEM.
49(13): 2248-53 (1971).

Brown, B.J.; Barker, N.T.; Sangster, D.F., The
behavior of the solvated electron in ethanol-n—
hexane mixtures, J. PHYS. CHEM. 75(23): 3639-
40 (1971).

Rabani, J.; Graetzel, M.; Chaudhri, S.A.,
Conductometric pulse radiolysis determination of
ionic yields and neutralization kinetics in liquid
ethanol, J. PHYS. CHEM. 75(25): 3893-4 (1971).

Asmus, K.-D.; Chaudhri, S.A.; Nazhat, N.B.;
Schmidt, W.F., Pulse radiolysis studies of
tetranitromethane in iso-propanol, TRANS.
FARADAY SOC. 67(9): 2607-17 (1971).

Jarnagin, R.C., Photoionization processes in organic
solids and fluids, ACC. CHEM. RES. 4(12): 420-7
as7.

Raz, B.; Jortner, J., Energy of the quasi-free
electron state in dense neon, CHEM. PHYS.
LETT. 9(3): 222-24 (1971).

Chaudhri, S.A.; Aemus, K.—D., Ion yields and ion
neutralization processes in _ pulse~irradiated
acetone, J. PHYS. CHEM. 76(1): 26-31 (1972)

Read, S.J.; Bansal, K.M., Radiolysis of

2,2,4-trimethylpentane. Kinetics of scavenging
processes, J. PHYS. CHEM. 76(17): 2374-80
(1972).

Fuochi, P.G.; Freeman, G.R., Molecular structure
effects on the free-ion yields and reaction kinetics
in the radiolysis of the  methyl-substituted
propanes and liquid argon: Electron and ion
mobilities, J. CHEM. PHYS. 56(5): 2333~41 (1972).

Nauta, H.; van Huis, C., Pulse radiolysis of
hexamethylphosphoric triamide, J. CHEM. SOC.
FARADAY TRANS. I 68(4): 647-52 (1972).

Vannikov, A.V.; Mal'tzev, E.L; Zolotarevsky, V.1.;
Rudnev, A.V., Transient charged radiolysis
products of organic liquid studied by electrical and
optical methods, INT. J. RADIAT. PHYS. CHEM.
4: 135-48 (1972).

Dodelet, J.-P.; Fuochi, P.G.; Freeman, G.R.,
Effect of electric field strength on the free ion
yields in the X-radiolysis of liquids: Influence of
molecular structure and temperature, CAN. J.
CHEM. 50(11): 1617-26 (1972).

Jha, K.N.; Freeman, G.R., Effect of pressure on the
rates of reaction of solvated electrons in liquid
ethanol, J. CHEM. PHYS. 57(4): 1408-14 (1972).

Bensasson, R.; Land, E.J., Transient species in the
pulse radiolysis of dimethyl sulphoxide, CHEM.
PHYS. LETT. 15(2): 195-8 (1972).

Dodelet, J.~P.; Freeman, G.R., Mobilities
ranges of electrons in liquids: Effect of molecular

and

15

(720404)

[720405]

[720439]

[720574]

[730059]

730095}

(730124)

[730143]

[730242]

[730292]

(730418)

(730735)

[731016]

17310541

[731069]

(731086)

structure in Cs—Cy, alkanes, CAN. J. CHEM.
50(16): 2667-79 (1972).

Schmidt, K.H., Electrical conductivity techniques
for studying the kinetics of radiation—induced
chemical reactions in aqueous solutions, INT. J.
RADIAT. PHYS. CHEM. 4(4):7439-68 (1972).

Terlecki, J.; Fiutak, J., Onsager’s recombination
theory applied to liquid alkanes, INT. J. RADIAT.
PHYS. CHEM. 4(4): 469-78 (1972).

Holroyd, R.A.; Dietrich, B.K.; Schwarz, H.A.,
Ranges of photoinjected electrons in dielectric
liquids, J. PHYS. CHEM. 76(25): 3794-800 (1972).

Rudnev, A.V.; Vannikov, A.V.; Bakh, N.A., Optical
absorption spectra and mobility of solvated
electrons in aliphatic alcohols, HIGH ENERGY
CHEM. 6(5): 416-7 (1972), Transl. from Khim.
Vys. Energ. 6(5): 473~5 (1972).

Robinson, M.G.; Freeman, G.R., X-radiolysis ion
yields and electron ranges in liquid xenon, krypton,
and argon: Effect of electric field strength, CAN.
J. CHEM. 516): 641-9 (1973).

Robinson, M.G.; Freeman, G.R., Radiolysis free ion
yields and electron ranges in polar and nonpolar
liquids: Fluoromethanes, CAN. J. CHEM. 51(2):
1010-5 (1973).

Baxendale, J.H.; Bell, C.; Wardman, P.,
Observations on solvated electrons in aliphatic
hydrocarbons at room temperature by pulse
radiolysis, J. CHEM. SOC. FARADAY TRANS. I
694): 776-86 (1973).

Roux, J.C.; Baquey, Ch.; Sutton, J., The radiolysis
of binary ethanol—water mixtures, INT. J.
RADIAT. PHYS. CHEM. 5: 309-22 (1973).

Kroh, J.; Piekarska, J.; Karolezak, S., Ionic
reactions during the radiolysis of liquid n-hexane
and 2,2,4-trimethylpentane, BULL. ACAD. POL.
SCI. SER. SCI. CHIM. 21(4): 279-83 (1973).

Jha, K.N.; Freeman, G.R., Effect of pressure on the
rates of reaction of solvated electrons and hydrogen
atoms in liquid methanol, J. AMER. CHEM. SOC.
95(18): 5891-6 (1973).

Sehested, K.; Bjerbakke, E.; Fricke, H., The
primary species yields in the “Co y-ray radiolysis
of aqueous solutions of H,SO, between pH 7 and
0.46, RADIAT. RES. 56(2): 385-99 (1973).

Konno, S.; Koba: Tonization of liquid xenon
and argon by a-particles, SCI. PAPERS INST.
PHYS. CHEM. RES. (TOKYO) 67(2): 57-62 (1973).

Cooper, T.K.; Walker, D.C.; Gillis, H.A.; Klassen,
N.V., The pulse radiolysis of dimethylsulfoxide and
binary mixtures with water, CAN. J. CHEM.
51(13): 2195-206 (1973).

Dodelet, J.-P.; Shinsaka, K.; Kortsch, U.;
Freeman, G.R., Electron ranges in liquid alkenes,
dienes, and alkynes: Range distribution function in
hydrocarbons, J. CHEM. PHYS. 595): 2376-86
ag73).

Bockrath, B.; Dorfman, L.M., Kinetics of electron
transfer from aromatic radical anions to alkyl
halides in tetrahydrofuran. Effects of sodium cation

|. PHYS. CHEM. 77(22): 2618-22 (1973).

, F.; Ward, M.D., On the molar extinction

coefficient of the solvated electron and the G(free



TABLE 1. Zero-field free-ion yields in saturated hydrocarbon liquids— Continued

Compound Temp."* — Method® Reference

kK
2,2,5,5-Tetramethylhexane 293, charge coll. 0.67 720269
2,2,6,6-Tetramethylheptane 293 charge coll. 0.47 720269
2,2,1,7-Tetramethyloctane 316 charge coll. 0.34 720269
n-Tetradecane 296 charge coll. 0.120 700055
Squalanc (CooHe:) 296 charge coll 0.119 700055

“When no temperature is given in the reference, room temperature may be assumed.
*When determinations were made at several temperatures on the same compound, only one result is given here;
the others are dieplayed in the accompanying figures.
ty under irradiation combined with measured or assumed ion mobility values
charge coll. = charge collection by clearing field,
scav. = scavenging of the negative or positive charge by the compound indicated.
*Free—ion yield in molecule/100 e’
“This octane isomer melts at 374 K.

‘TABLE 2. Zero-field free-ion yields in unsaturated and aromatic hydrocarbon liquids

‘Compound Temp. Method Ch Reference
K

Ethylene 170 charge coll 0.010 741012
Propylene 234 charge coll. 0.042 741012,
Butene-1 293 charge coll. 0.093 731054
cis-Butene-2 293° charge coll. 0.23 731054
trans~Butene-2 293 charge coll. 0.080 731054
Tsobutene 293 charge coll. 0.25, 731054
2-Methylbutene-2 292 charge coll. 0.26 731054
Hexene-1 296 charge coll. 0,062 700055

293 charge coll. 0.10 731054
Hexene-2* 296 charge coll. 0.076 700055
trans—Hexene~2 293 charge coll. 0.092 731054
cis-Hexene-3 293 charge coll. 0.13 731054
trans~Hexene-3 293 charge coll. 0.10 731054
3,3-Dimethylbutene-1 296 charge coll. 0.169 700035
2,3~Dimethylbutene-2 293 charge coll. 0.44 731054
Cyclohexene 296 charge coll. 0.15 700055

293 charge coll. 0.20 731054
Propadiene(Allene) 282 charge coll. 0.050 731054
Butadiene~1,3 269 charge coll. 0.038 731054
Pentadiene-1,4 293 charge coll. 0.067 731054
Cyclopentadiene 195, seav., NHy 0.08 652067
Hexadiene-1,5 292 charge coll. 0.066 731054
Heptadiene-1,6 292 charge coll, 0.067 731054
Octadiene~1,7 292 charge coll. 0.065 731054
Acetylene 198 charge coll. 0.02 731054
Propyne 260 charge coll. 0.17 731054
Butyne-2 293 charge coll. 0.32 731054
Hexyne-1 253 charge coll. 0.10 731054
Hexyne-2 293 charge coll. 0.19 731054
Hexyne-3 293 charge coll. 0.21 731054
Benzene 0.077 690650

— seav., tritiated water or ale. 0.098 700349

298 seav., galvinoxyl 0.055 700019

296 charge coll. 0.053 700058

292 charge coll. 0.081 T4110
Toluene 298 seav., galvinoxyl 0.051 700019

292 charge coll. 0.093 741110
tert-Butylbenzene 296 charge coll. 0.089 700055
o-Xylene 2 charge coll. 0.094 741110

5


Yields of Free lons Formed
in Liquids by Radiation

A. O. Allen

Department of Chemistry
Brookhaven National Laboratory
Upton, New York 11973

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

James A. Baker, Ill, Under Secretary
Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1976


TABLE 4. Zero-field free-ion yields in polar liquids (¢ > 3) Continued

Compound Temp. « Method Gh Reference
kK
Methylamine — 94 seav., pyrene and (C,H;); 2.2 741094
Ammonia 258 20. scav., pyrene and (CeHs); 3.3 741094
296 17. pulse radiolysis 3.2 741062
225 25 pulse radiolysis 3.0 740159
225 seav., Cu’ 2.9 740159
Ethanolamine _ 81.7 scav., anthracene 0.2 731106
Alcohols:
Neopentyl alcohol 337 8.3 seav., galvinoxyl 0.21 700019
tert~Butyl alcohol 333 18 scav., anthracene and pyrene 07 720574
296 12.3 scav., galvinoxyl 0.67 700019
Hexyl alcohol 293 13.3 scav., anthracene and pyrene Ll 720574
n—Pentyl alcohol 298 14.2 scay., galvinoxyl 0.45 700019
337 scav., galvinoxyl 0.46 700019
Tsopentyl alcohol 293 15. scay., anthracene and pyrene 0.35 720574
293 condy. 0.40 720153
n—Butyl alcohol 298 Wd scav., galvinoxyl 0.63 700019
298 scav., acetone 0.55 670004
293 seay., anthracene and pyrene 0.56 720574
293 condy. 0.58 720153
Isobutyl alcohol 293 117 seay., anthracene and pyrene 0.5 720574
Isopropyl alcohol 296 18.3 seav., anthracene 0.92 700360
— seav., (CoH). 1.0 650499
298 seav., NzO 12 680042
_ scav., NOs 17 690655
298 condy. 2.2 710064
291 scav., tetranitromethane 12 710564
293 seay., anthracene and pyrene 1.0 720574
n-Propyl alcohol —_— 20.1 scav., (CoHs)p 1.0 650499
298 scav., NzO 12 690387
298 scay., acetone 0.65 670004
298 condy. 22 710064
150-298 pulse radiolysis 1.45, 751054
Ethyl alcohol _— 24.3 scay., (CoHs)z 1.0 640113
298 scav., NzO 15 680047
296 scav., galvinoxyl 1.0 700019
294 scav., anthracene 0.98 700360
298 condy. 710064
290 condy. 710531
296 scav., acetone, nitrobenzene 17 720199
_— 7 700172
Methyl alcohol — 32.6 1 650499
298 13 670004
_ 2.0 670018
296 Ll 700360
298 2.0 680610
298 2.0 710064
— 19 731086
296 scav., nitrobenzene and acetone LT 730292
Ethylene glycol — 38. seav., (CoHs)e 1.2 650499
293 scav., anthracene and pyrene Lz 120574
Nitro compounds:
Nitrobenzene 298 34.8 scav., galvinoxyl 0.43 700019
Nitromethane 298 37.8 seav., galvinoxyl 0.31 700019
Amides:
Hexamethylphosphoramide = — 30 scav., biphenyl and anthracene 24 720105
293 condy. 12 720153
— scay., anthracene and pyrene 23 731091
Dimethylformamide 296 37. scav., anthracene 2.0 700360
Dimethylacetamide 296 38. scav., anthracene 17 700360


TABLE 4. Zero-field free-ion yields in polar liquids (€ > 3) —Continued

Compound Temp. Method Cu Reference
Di-n-propyl ether 169 5.64 charge coll. 0.23 751047
202 4.76 charge coll. 0.25, 751047
254 3.90 charge coll. 0.30 151047
293 3.43 charge coll. 0,34 751047
342 2.91 charge coll. 0.38 751047
376 2.60 charge coll. 0.44 751047
Diisopropyl ether 296 39 charge coll. 0.305 700055
Diethyl ether — 43 condy. 0.19 650216
296 charge coll. 0.35, 700055
— av., CH,Br 0.15 700249
296 + scav., anthracene 0.28 700360
195 8.0 charge coll. 0.53 751047
223 6.45 charge coll. 0.50 751047
252 5.40 charge coll. 0.49 751047
293 4.39 charge coll. 0.54 751047
328 3.70 charge coll. 0.57 751047
Dimethyl ether 165 10.7 charge coll. 0.64 751047
195 9.0 charge coll. 0.73 751047
226 14 charge coll. 0.74 751047
‘Tetrahydropyran 231 7.28 charge coll. 0.45 751047
254 6.60 charge coll. 0.48 751047
293 5.60 charge coll. 0.51 751047
‘Tetrahydrofuran 296 15 scay., pyrene, anthracene 0.68 700406
— scay., benzophenone 0.5 690521
—_— scav., maleic anydride 0.6" 701042
_ pulse radiolysis 0.39% 731069
_ seav., pyrene 0.3 741030
205 11.6 charge coll. 0.72 751047
2-Methyltetrahydrofuran 296 19 seay., anthracene 0.23 700360
Halides:
n-Butyl bromide 295 6.9 condy. 0.27 650216
n-Butyl chloride 295 1.2 condy. 0.39 650216
Fluoroform 183 21.4 charge coll. cat 730095
Methyl fluoride 183 219 charge coll. 16 730095
Difluoromethane 183 38.9 charge coll. 19 730095,
Ketones:
Methyl ethyl ketone 293 18.5 condy. 0.84 720153
Acetone 290 21 scav., anthracene LAB T0030
— scay., anthracene Ls 710186
—_ scay., stilbene and anthracene Li 710189
— condy. 12 720002
Dimethyl sulfoxide 296 46.6 scay., anthracene 1.62 700360
— seav., NzO 18 710215,
— scay., biphenyl 18 720232
— seay., anthracene and Br 12-18 731016
Esters:
Methyl formate 296 8.5 scay., anthracene 0.32 700360
1,2-Propanediol carbonate 296 64.5 seav., anthracene 2.25 700360
Nitriles:
Benzonitrile 296 25.2 scav., anthracene 1.40 700360
296 seav., trans-stilbene 14 741100
296 375 seav., anthracene 1.55 700360
296 12 scay., anthracene 0.40 700360
— 69 scav., pyrene 0.95 680227
Tributylamine 293 5. condy. 0.25 720153
Butylamine 293 5. condy. 0.27 720153
Diethylamine _ 3.6 scav., pyrene O1 741030
Ethylamine — 62 seav., pyrene and (CeHs)2 18 741094

7



TABLE 1. Zero-field free-ion yields in saturated hydrocarbon liquids

Compound Method® Gi Reference
Methane 140 charge coll. 1.13 741012
Ethane 200 charge coll. 0.158 741012
Propane 238 charge coll. 0.166 741012

233 charge coll. 0.143 720166
Cyclopropane 234 charge coll. 0.049 741012
Butane 293 charge coll. 0.225 741012
296 charge coll. 0.193 700055
Isobutane 294 charge coll. 0.31 720166
n-Pentane 296 charge coll. 0.17 731054
296 charge coll. 0.145 700055
Isopentane 296 charge coll. 0.170 700055
Neopentane 296 condy. wa 700090
296 charge coll. 0.86 700055
294 charge coll. 1.09 720269
294 scav., N,O 0.95 700650
Cyclopentane 296 charge coll. 0.155 700055
n-Hexane 296 condy. 0.10 650216
297 condy. 0.10 660149
— condy. 0.137 680821
reeale. in 720405
296 condy. 0.11 690240
296 seav., (CeH,)sCCl 0.12 690348
_— scav., CH;Br 0.12 700047
296 charge coll, 0.131 700055
298 seav., c-CoFz 0.08 700299
—_ condy. 0.13, 720408
292 charge coll. 0.18 731054
— scav., C3H,OT 0.12 730242
2-Methylpentane 296 charge coll. 0.148 700058
3-Methylpentane 296 charge coll. 0.146 700055
298 seav., -CoFy 0.17 700299
2,3-Dimethylbutane 296 charge coll. 0.192 700055
2,2—Dimethylbutane 296 charge coll. 0,304 700055
Cyclohexane 296 charge coll. 0.148 700055
— scav., CHsBr 0.13 700047
298 seay., ~CoFiz 0.14 700299
296 scav., anthracene 0.17 700360
296 scav., (CgHs)sCCl 0.13, 690348
296 scav., galvinoxyl 0.15, 700019
294 charge coll. 0.19 7381054
Methyleyclohexane 293 scav., pyrene 0,122 730124
n-Heptane 296 charge coll. 0.131 700058
2,4-Dimethylpentane 296 charge coll. 0.178 700055
2,2,3—Trimethylbutane 296 charge coll. 0.290 7200055
n—Octane 296 charge coll, 0.124 700058
2,3,4-Trimethylpentane 296 charge coll, 0.174 700055
2,2,4-Trimethylpentane 296 charge coll. 0,332 700055
— veav., CHyBr 0.36 700047
_ seav., -CeFiz 0.37 700299
296 scav., (CgHs)CCl 0.36 690348
296 seav., galvinoxyl 0.39 700019
_— scav:, CHsBr 0.13 720032
— scav., CzH,OT 0.22 730242
— scav., CHjBr 0.3 740092
2,2,3,3-Tetramethylbutane 379° charge coll. 0.80 720269
n-Nonane 296 charge coll. 0.117 700058
2,2,4,4-Tetramethylpentane 295 charge coll. 0.83 720269
2,2,3,3-Tetramethylpentane 295, charge coll. 0.42 720269
n—Decane 296 charge coll. 0.117 700055


Preface

This report is one of a series of data publications on radiation chemistry; the aim of the series
is to compile, evaluate, and present the numerical results on processes occurring in systems which
have been subjected to ionizing radiation. Various kinds of data are important in radiation chem-
istry. The quantities which were measured first were the observed radiation yields or G values (mole-
cules formed or destroyed per 100 eV). Various indirect methods based on G values have been used to
determine yields of transient species and relative rates of reactions. The spectral properties (optical,
electron spin resonance) of transients have provided a direct method for their identification, and rates
of the very fast reactions of transients which occur in irradiated systems have been measured directly
by spectroscopic methods. Conductivity and luminescence methods have also provided a means of
measuring properties of transients and their kinetics. Some reactions which occur in irradiated systems
have also been studied by other methods, such as photochemistry, electric discharge, ultrasonics, chem-
ical initiation, electron impact, etc. The emphasis in these publications is on the data of radiation
chemistry, but where other pertinent data exist, they are included.

The data of radiation chemistry are voluminous; thousands of systems have been investigated.
As a result there are certain collections, e.g. rate constants of particular types of reactions or certain
properties of transients, for which tabulations of the data are considered essential, but for which
critical assessment of each value is impossible. On the other hand, certain systems and properties
have been studied so extensively that critical examination of these data is desirable and timely.
Authors of this series of data publications have been asked to evaluate the extent to which the data
can be critically assessed, to describe their criteria for evaluation, and to designate preferred values
whenever possible.

Iv


5. Remarks on G;, Data
5.1. Hydrocarbons

The chief property determining Gf, in hydrocarbons is molecular shape. By and large, the more
nearly spherical the molecules, the bigger G?,. Despite variations in Gj, by a factor of up to 100, the
temperature coefficients are quite comparable. The effect of double and triple bonds is relatively minor,
although the difference between cis— and trans—butene-2 is surprising. Comparing results from the two
laboratories responsible for most of the data, we find that the more recent data from Freeman's
laboratory are consistently about 30% higher than the data on the same liquids by Schmidt and Allen.
Further work is required to settle this and other discrepancies.

5.2 Other Non—polar Compounds

The near-spherical tetramethides have large values of Gj, but equally spherical carbon
tetrachloride has a low value. This is attributed to the reactivity of CCl, with e-, which seems to convert
e to Cl before it emerges from the positive ion field. However, CF,, which does not ordinarily react
readily with electrons [690179], also has a very low G?,. This seems to be an outstanding anomaly.

5.3. Polar Liquids

The yield of free solvated electrons in water has been the subject of dozens of papers, and only in
recent years has agreement been obtained. The two papers cited in table 4 for G(e,,) in water contain
references to earlier work. For effects of radiation quality (LET) on the yields see [610164] and [690185].
The free—ion yield also includes some OH~ which is shown by pulse conductivity studies; therefore, Gj
is actually larger than G,~.

There is some controversy about yields of solvated electrons and free ions in methyl and ethyl
alcohols. Earlier scavenger studies gave G(e~) about 1.0, but pulse conductivity gave values near 2. In
the light of later work, it now appears that electron reaction with biphenyl and anthracene may have
been underestimated because of the short lifetime of the product ions in alcohol, while in other cases too
low a scavenger concentration was used. The two conductivity studies differ widely, however, on the
total ion pair yield; [710531] found the yield of other anions to be larger than that of solvated electrons
while [710064] found the anion yield less than 8% of the electron yield. Another disagreement involves
the yield in tetrahydrofuran.

Gj, should increase with increasing dielectric constant, which shortens the range of the Coulomb
forces. Table 4 shows a strong tendency in this direction, but with many exceptions. The yield in
ammonia, which is much larger than that in liquids of similar dielectric constant, seems connected with
the slowness of reaction of the solvated electron with the conjugate acid ion NH.

6. Total Ion Yields

Only in two polyatomic liquids has G, been determined to high enough fields to allow a reasonable
extrapolation to infinite field. Table 5 shows that the extrapolated total yield for y rays in neopentane
and CS, is close to that measured in the gas phase. In the monatomic liquids Ar, Kr and Xe, nearly
complete collection of the ions is possible, and the total ion yield in the liquid is found to be greater than
in the gas. In the solid forms of these elements, optical absorptions are found by Baldini at longer
wave-lengths than in the corresponding gas [629023] and it is believed that the resulting excited states
spontaneously ionize, in the liquid as well as the solid state [719437]. If this is true, Baldini’s absorptions
give the effective ionization potential in the liquid, 4,,. Takahashi et al. [740153] found reasons for
assuming that the ratio of W to J should be the same in the liquid and gas, and obtained the following
calculated values for W,,: Ar, 24.0; Kr, 20.1; Xe, 16.7. These calculated values agree with their
measured values within the experintental error (3 to 8%).

ll


The ranges in dielectric liquids deduced from free-ion yields agree in order of magnitude with
values obtained by photoionization of solute molecules [710638] and metal surfaces [720439]. Recent
work on electron scattering in metals has shown similar effects; the “mean free path” in the metal,
which decreases with decreasing energy at higher energies, shows a sharp upturn at electron energies
below 40 eV [749139].

The trouble with the Onsager model is that it does not conform well with physical reality, for only
about 40% of the ions are formed in isolated pairs. The remainder occur in groups, usually known as
“spurs”, which contain from 2 to 50 or more ionizations, located close together. The electrons formed by
ionization in such a group are attracted back by the collective charge of all the ions in the group,
partially shielded by the charge on all the other electrons. Obviously the Onsager formula cannot apply
to such a vague and complex situation. When the field is not too high and the escape probability from a
spur remains low, the parameter r, may be rationalized as the mean distance between the positive and
negative charges in the last pair to remain in a spur at the moment that all the other pairs have
recombined. When the field is high enough so that an appreciable number of spurs contribute more than
one free—ion pair, the use of the Onsager formula cannot be justified. Dodelet and Freeman [758020]
have proposed a model of the spur which, together with an assumed D(r), gives a reasonable fit to the
data on field effects in such diverse liquids as xenon and water. The values obtained for the parameters
GR, and r, are not however very different from those obtained (much more simply) by not taking the
spurs into account. The physical significance of such values remains doubtful as long as D(r) is taken
arbitrarily, which it must be until the process of electron slowing~down in the low energy range is better
understood.

We do not include any quoted values of the “penetration range” in our tables of data, because the
numbers have no precise quantitative significance. Nevertheless, the 7, values listed in any paper on
free—ion yields, have great qualitative meaning. It is clear that electrons of somewhat superthermal
energy must lose their momentum more slowly, in passing through a liquid, the more nearly isotropic are
the molecules of the liquid. The same behavior of thermal electrons is indicated ,by mobility
measurements of thermal electrons in hydrocarbon liquids, although quantitative correlation between Gy,
and mobility has not yet been made. A great deal of information remains to be extracted from G, data.

References
[080002] Jaffé, G., Ueber die Ionisation fluessiger Dielektrika the aliphatic alcohols, J. CHEM. PHYS. 42(2):
durch Radiumstrablen, ANN. PHYSIK. 25: 257-84 708-12 (1965).
(1908). [652067] Bonin, M.A.; Busler, W.R.; Williams, F., The
1340008} Mobler, F.L.; Taylor, L.S., Ionization of liquid polymerization of cyclopentadiene by free ions.
carbon disulphide by X-rays, J. RES. NAT. BUR. Determination of the propagation rate constant, J.
STD. 13: 659-64 (1934). AMER. CHEM. SOC. 87(2): 199-207 (1968).
1550048] Hochanadel, C.J., Proc. of the Int. Conf. Peaceful [660148] Hummel. A.: Allen, A.O.: Watson, F.H..Jr..
Uses Atomic Energy, Geneva, Aug. 8-20, 1955, Tonization of liquids by radiation. II. Dependence
Vol. 7, p.521-5, N.Y., United Nations, 1956. of the zero-field ion yield on temperature and
(610164] Allen, A.O., The radiation chemistry of water and dielectric constant, J. CHEM. PHYS. 44(9): 3431-6
aqueous solutions, D. Van Nostrand Co., Inc., 1966).
New York, 1961, p.58. [660149] Hummel, A.; Allen, A.O., Ionization of liquids by
[629023] Baldini, G., Ultraviolet absorption of solid argon, radiation. I. Methods for determination of ion
krypton, and xenon, PHYS. REV. 128(4): 1562-7 mobilities and ion yields at low voltage, J. CHEM.
(1962). PHYS. 44(9): 3426—21 (1966).
[640113] Taub, I.A.; Harter, D.A.; Sauer, M.C.; Dorfman, [660847] Ullmaier, H.A., Measurement of the ionization in
L.M., Pulse radiolysis studies. IV. The solvated liquid argon due to X-rays, PHYS. MED. BIOL.
electron in the aliphatic alcohols, J. CHEM. 11): 95-101 (1966).
PHYS. 41(4): 979-85 (1964). [670004] Basson, R.A.; van der Linde, H.J., Polarity effects
{650216] Freeman, G.R.; Fayadh, J.M., Influence of the in the radiolysis of n—alcohols, J. CHEM. SOC.
dielectric constant on the yield of free ions PT. A (1): 28-32 (1967).
produced during radiolysis of a liquid, J. CHEM. [670018] Seki, H.; Imamura, M., Radiolysis yields from
PHYS. 43(1): 86-92 (1965). y-irradiated liquid methanol containing nitrous
[650499] Sauer, M.C.,Jr.; Arai, S.; Dorfman, L.M., Pulse oxide and the effect of acid, J. PHYS. CHEM.
radiolysis studies. VII. The absorption spectra and 714): 870-75 (1967).
radiation chemical yields of the solvated electron in [670311] Fielden, E.M.; Hart, E.J., Primary radical yields in

18


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Il


Library of Congress Cataloging in Publication Data

Allen, Augustine O.
Yields of free ions formed in liquids by radiation.

(Nat. stand. ref, data, ser.; NSRDS-NBS 57)

“CODEN: NSRDAP.

Includes bibliographical references.

Supt. of Docs. No.: © 13.48:57

1, Ions. 2. Liquids, Effect of radiation on, I. Title. Il. Series:
United States. National Bureau of Standards, National standard
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CODEN: NSRDAP

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TABLE 4, Zero-field free-ion yields in polar liquids (€ > 3) —Continued

Compound Temp. « Method Gi Reference.
K
Formamide — 109. scav., N20 700173
N-Methylpropionamide 296 164, scav., anthracene 700360
Water (H,0) — 80 scav., KMnO, 670311
_ review 730418
— condy. ro
Gi, = GH") 710138
_ condy. GOK”
Gi, = GH") 700243
_ condy.
720404
20) — seav., KMn0, ZXD20) = 2.9 680061
“Much lower values were obtained with high-LET radiations.
* Absorption due to e+Na” ion pair, extrapolated to zero Na’.
*Yield includes G(anions) = 2.1, in addition to solvated electrons, as
deduced from decay kinetics; not observed by other workers.
“Yields change very little with temperature up to 523 K, C.J. Hochanadel, [550048].
oe a
0.28/- 4 i 4
b © n-PENTANE 4
2 n-HEXANE 1.0F 4
024+ 44 CYCLOHEXANE 4
L 4 o
Sey | 7
0.207 4
L METHANE 4
r 7 08 0 ETHANE
o 0.0,@ PROPANE
oer 7 4 n-BUTANE
i. vy  ISOBUTANE 7
r 7, Si @ ~— CYCLOPROPANE
ool Lo 4 PROPYLENE
eos 15 35 35 5 06+ 4
TEMPERATURE, °C
Ficure 1. G@ for n-pentane, n-hexane and cyclohexane. L 4
Open symbols from [680376]; filled symbols, [720269].
0.4- 4
L x
rN
O.2- uM a 4
o °
y 24
LL "4 ° 4
o 0° oa .
oo ° at
oly _® 1 a
100 200 300

TEMPERATURE, °K

Ficure 2. G} for low-molecular-weight saturated hydrocarbons.
Data from [741012, 720034 and 720166].



TaBLe 5. Total ion yields reported for selected liquids

Compound Temp. Radiation Source % san. Gy We Gaal Ga Reference

K obtained (eV)
Carbon disulfide Room y rays from Ra 80 3.8 26 ~1 340008
Neopentane 296 Co y rays 84 4.03 24.8 0.94 700090*
Argon 87 20-KV or S0-kV X rays 95-100 3.9 2623 ~1 660847
87 Internal conversion ~100 4.2 3.6 Ln 740029

€°. from TM"Bi

87 Ni B-rays 98 422 237 0 Ll 740153
87 Co y rays 100 45 22.5 118 690429
87 1.7 MeV X-rays 100 1.3 37 (19 730059
Krypton 148 1.7 MeV X-rays 98 13.0 17 32 730059
120 Ni B-rays a1 4.9 2.5 1.17 740153
Xenon 183 1.7 MeV X-rays ~100 13.7 13 3.0 730059
183 Po a-rays 58 17.3 1.27 730735
16 “Ni B-ravs 90 61 16.4 1.33 740153

“Gy, from 710122.

7. Theoretical Treatments

No good quantitative theory exists of the free—ion yield. Jaffé’s theory [080002] was based on a
columnar model which is unrealistic for low LET radiations and has not proved useful quantitatively.
Recent treatments start with Onsager’s formula for the probability P(r) of escape by thermal agitation of
a pair of ions separated by a distance r:

P(r) = (1 + fiB)exp(-r./7)

where r.=e?/ekT, e is electronic charge, € dielectric constant, k Boltzmann’s constant, T the absolute
temperature, and f{£) is a function of the applied field expressed as a double summation of terms again
involving € and T. The model for hydrocarbons assumes that an electron very quickly loses energy to
electronic and vibronic modes and attains a level of energy which is less than that of molecular
vibrations but still greater than thermal. It then loses energy much more slowly by elastic collisions, and
how far it travels before it is thermalized depends on how smooth a potential surface it finds, which in
turn depends on how nearly spherical the liquid molecules are. Eventually it attains thermal energy at a
distance r from the positive ion, which of course is not the same for all electrons. Some distribution
function D(r) must exist which gives the probability of the electron becoming thermalized at r, and the
escape probability is then f D(r)P(r)dr. If this model holds, the same function should determine both the
field dependence and the temperature dependence of Gz. Every worker who has in recent years
seriously considered G, has expressed his results in terms of an assumed D({r), which usually includes
two parameters, the total yield Gf, and a “mean penetration range” called b or r,. Both the temperature
coefficient of the zero-field free—ion yield and the field effect on the yield may be used to obtain the
best value of the range r, defined by an assumed D(r). Mozumder [748021] fitted experimental data on
G, for 19 hydrocarbons and other organic liquids with five different assumed forms of D(r) to obtain
values of the range r,. For any particular liquid, the five values of r, ranged over a factor of 1.6 to 1.8.
But all the calculated “mean” ranges are many times longer than anyone would have supposed prior to
the analysis of the free—ion yields. Thus, the smallest r, values in Mozumder’s results were obtained
from a “truncated power law” D(r) = (2.7/4ar,*) (r/r,y*" and ranged from 28A for 1,4—dioxane tv 133A

for neopentane. The majority of electrons formed in the ionization of liquids by a, 8, y or X-rays have
initial energies below 100 eV. Extrapolation of the usual “electron ranges” obtained by experiments on
kilovolt electrons suggests maximum ranges <I A for 100-eV electrons in hydrocarbons [660148]. The
usual “electron range” determination consists in finding how thick a foil must be inserted in an electron
beam to prevent ionization in air beyond the foil; the foil is thus only sufficient to reduce the electron
energy below the ionization threshold (15-20 eV). The extra distance traveled in liquids is required to
reduce the electron energy from a few eV to thermal levels. This distance depends strongly on the shape
of the molecules.

12