Simultaneous Dissociation and Excitation of H2O
Molecule by Electron Impact.
By Goro HAYAKAWA.
(Read July 17, 1943.)
In Geissler discharge through water vapour, a system of bands in ultraviolet
is excited, which is now known to be due to hydroxyl radical.(1) The
OH bands thus excited exhibit •gabnormal rotation•h; intensity distribution
among lines in each band shows abnormally high speed of rotation of the
radical, while molecules in the discharge tube are considered to have kinetic
energy not so higher than corresponding to room temperature.
BONHOEFFER and PEARSON(2) showed experimentally that even for the
smallest discharge current, the intensity of the bands is considerable, roughly
proportional to the discharge current. Recently WAKESIMA(3) reexamined
the discharge current versus intensity relation more accurately and showed
the intensity is approximately proportional to the discharge current. He
infered from his result that OH bands are produced by simultaneous dissociation
of H2O and excitation of OH. If the dissociation of H2O and
excitation of OH occur successively as two processes, the band intensity
should be proportional to the square of discharge current.
Phenomenon in a Geissler tube, however, is very complicated; in usual
case, vapour pressure is rather high, and various kinds of collision occur
by electrons, ions and molecules. So it is desired to study the process under
more simple conditions.
A three element electron tube, made of brass and capable of
water-cooling, was used in the following experiment. Cathode is an oxide-coated
konel ribbon filament. In place of anode-plate is set a FARADAY cage. Space
between grid and plate is free from field. Light emitted in the portion near
the grid on the anode side was examined. To permit for primary collision
process alone to occur, the vapour pressure must be sufficiently low and
dissociation products removed as fast as possible. Water vapour is introduced
into the tube through a capillary tube from a reservoir containing
distilled water and is pumped out continuously. Vapour pressure in the
tube was kept at 0.003 0.006mm Hg by the control of the temperature
(1) W. WATSON, Astrophys. Journ., 60 (1924), 145.
(2) K. F. BONHOEFFER and T. G. PEARSON, Zeit. f. phys. Chem., 14 (1931), 1.O . OLDENBERG, Phys. Rev., 46 (1934), 210.
1944]
Dissociation -excitation of H2O
.
79
of the reservoir and
the pumping speed
.
Corresponding mean
free path of H2O is
7•`15mm. Near the
grid an electron may
be considered to collide
with vapour molecule
approximately only once at most. Under conditions above-mentioned band intensity measured on ƒÉ 3064 (2‡”-2II, 0-0) and ƒÉ2S11 (2‡”-2II, 1-0) versus electron
current relation is almost
linear.
BALMER
lines
are
excited
at
the
same
time
, and
also
show
their
intensities
approximately
linear
to electron current
Thus couclusion
can be made that both OH and H
spectra are excited from H2O molecule,
each by one electron impact.
The processes may be:
H2O•¨OH*(2‡”)+H(normal),
and
H2O•¨OH(normal)+H-(n>1).
Fig. 3 shows the change of the
intensities of OH bands and H
lines when colliding electron energy is varied. In lower electron energy
especially below 18 volts , it is
rather difficult to measure the
effective electron current . In such
eases, even under vapour pressure
0.003mm Hg the luminosity in the
electron beam is varied according•
to the
distance
from
the grid,
and
the
accuracy
of photometric
measurement
was
restricted
by this
reason.
The
intensity
of OH
bands
reaches
maximum
at electron
energy
a little
higher than
20 volts,
and
decreases
rather
rapidly
at
increasing voltage
.
On the other hand
the
intensity
of II lines
increases
more
slowly
from
about
Fig. 1. OH band intensity versus electron current .
(Mean of 2‡”-2II, (0-0) and (1-0).)
Fig. 2. Balmer line intensity versus
electron current.
80 Goro HAYAKAWA. [Vol. 26
20
volts,
and
reaches
maximum
at
about
a hundred
volts . Appearance
voltage
of H lines
is several
volts
higher
than
that
of OH* . The absolute
magnitudes
of the
appearance
voltages
and
their
relations
to the
abnormal
rotation
of OH
radical
are
not
concerned
in the
present
experiment
, and
will
be
discussed
after
more
accurate
measurements
of
effective
voltage
are
made.
To explain
quantitatively
the
shapes
of
the
dissociation-excitation
curves
it is necessary
to
know
more
about
unstable
excited
states
of
H2O
molecule.
But
the
observed
difference
between
both
of the
excitation
functions
may
be
understood
roughly
in
the
following
way.
The
dissociation
and
excitation
of
OH
takes
place
when the 2-quantum state of O-atom in H2O undergoes a transition by the
electron impact, while an excited H-atom is obtained when the electron
in 1-quantum state in either of two H-atoms is excited to some higher quantum
one. In the latter case the incident electron must have much higher
energy than appearance potential in order to get considerable probability
for reaching 1-quantum electron without being influenced by the outer
electrons.
I should like to express my gratitudes to professor M. KIUTI for his
interest and encouragement during the work, to Professor H. WAKESIMA
for his kind discussions, and also to Professor R. SAGANE
for his helpful
suggestions on tube technics.
Nagoya Imperial University.
(Received Nov. 22, 1943.)
Fig. 3. Dissociation-excitation funtions of H2 O molecule.
