THERMAL ElECTRON ATTACHMENT TO 0 AS STUDIED BY
A MICROWAVE CAVITY TECHNIQUE COM~INED WITH PULSE
RADIOLYSIS

X-40 Y. Hatano, H. Shimamori,*)y, Nagahara, M. Yasuda
andY. Kokaku
Dept. of Chern., Tokyo Institute of Technology
Meguro-ku, Tokyo
The low-energy electron attachment to 0 2 has extensively beenstudied using an electron swarm or a beam technique.
Its mechanism
has been considered to be a three-body process which proceeds in
two stages as was first proposed by Bloch and Bradbury (B-B), thatis initial electron capt~#e of 0? resulting in the formation_Qf avibr~t~onal~y excited 0 2
and suosequent stabilization of 0 2 bycolllSlon Wlth another 0 2 or with a foreign molecule M.

---+ 02 -*(l)020 -* ~e(2)+0220 -* +0---+
(3)
+
02
02
2
2 *0 M---+ 0 2M+( 4)+2The details of the B-B mechanism, however, have ngt been fullyunderstood. On the autoionization lifetime of 0 2 - which is kno~£oto be in the excited vibrational level of v'=4, the ~l~er of 10sec has been suggested theoretically, while about 10
sec hasbeen proposed using an electron swarm method. Fu#thermore, themechanism of the collisional stabilization of o2 - by third bodies
has not been clarified,
In this study a microwave cavity method combined with pulse
radiolysis has been used to observe the disappearance of 1~4~mal
electron directly with fast response by attachment to 0 2 .
The experimental apparatus is shown in Fig. 1. Microwave powerfrom a klystron(9V54) is transmitted in an X-band waveguide via avariable attenuator, an isolator and a magic tee to a brass cylindrical microwave cavity(loaded Q=5000) which has a resonant
frequency of 9536 MHz and is operated in the TE 011 mode.
Incident
microwave power to the cavity is about 50 pwatt.
For a vacuum
seal a 0.3 mm mica foil with epoxy cement is inserted into a partof the waveguide. Electrons are produced by irradiation of a nsec
X-ray pulse from a 0.1 mm tungsten foil attached to the pulser
window of a Febetron 706. For uniform X-ray expoJ-,l:~e the cavity
is located 50 em away from the pulser window. Since the dose isabout 20 mR per pulse, the electron cgnce~3ration in the cavity
is estimated to be of the order of 10 em
for the gas pressuresused. In order to observe exclusively the electron attachment
process, we made experiments at relatively high pressures and thetime-scale was restricted to within about 100 ~sec, in which theelectron loss by diffusion to the cavity wall could entirely beneglected. The effect of electron-ion recombination could also
be neglected because of the very low dose of X-rays used.
Various compounds such as rare gases, diatomic molecules,hydrocarbons and alcohols have been used as third bodies. For
almost all third bodies trLe electron attacrunent obsys the :0--3mechanism.
In the case of N?, C2 H4 and co 2 a~~ third bodj es,
howe-,·er, s:Jme multi-body processes have an Lm_portant cm'ltribution
to the attacbment mechanism in addition to the simple B-B mechanism,
e+227

The rate constant of irJitial electron capture has been found to be( 4. 8 ± 0. 6) xJo-llcm3 s-·1. TLree- bcdy rate constar,_ts have beenobtained for various third codies (Table 1). The difference inthese values is correlated with the smal:Lest vibrational energy
level spacing of tJ.r:jrd-body moltc:c11les and is ascribed to thedifference in the efficiency of collisio,r:.al stabilization of 02-*,
whic:h is classified into tvro cases depending on whether thevibrational relaxation is effected by V-T or V-V transitiohs.
Conventional theories have failed to explain the magnitudes of
V-T transition probabilities obtained here. The lifetime ofOr*(v'= 4) l:cas been esUmateC. to be (1.0 ± 0.3)XlQ-10 s.
Table 1. Tt_ree-body rate constants
at about 300 K(Xl0-30 cm6 s-1)
c0EFG

Osc.lnput

Fig.l. Scb.ematic representation of the appratus"
A, Febetron 706
B, Ool mm tu.ngsten foilr<"• microwave cavity
D, 0.3 mm mica
E, isolator
F, attem;:.ator
G, klystron~~'magic teeI, crystal diodeJ, gas cell

MkMHeNeAr

0.033 ± 0.003
0.023 ± 0.003
0.05 ± 0.01
0.05 ± O.OJ.
0.085 ± 0.005
0.48 ± 0.03
0.140 ± 0.005
0.085 ± 0.005
± 0.2
2o3
0.34 ± 0.01
± 0.1
1.7± 0.5
2.7± 0.2
3-35± 0.4
7.9± Oo7
8.0± 0.48.1± 2llJ.8
± 0.2
3-4

KrXe
H2D2
N2gn4
C2H6C2H4C3H8n-C4H10n-C5n12
neo-C5H12n-C6Hl4
CH30H
C2E60H02*) Present address: Radj_ation Laboratory, Ur:.iv. of ~otre Dame,Notre Dame, Indian9. 46556, UBA.
::;_) H. Sh~mamor~ andY. Hatano, Chern. Pbys. Le~ter.s, ]§_, 242 (1976).
2) H. Shlmamorl and Y. Hatano, Chem. Ph;vs., l.o:, 439 (1976).
3) H. Shimamori andY. Hatano, Chern. Phys., 21, 187 (197'/).
4) H. Shimamori andY. 1-Iata:v:,o, 26th IUPAC Congress, Tokyo, 1977.

228