Pramana
-
J.
Phys., Vol. 36, No. 4, April 1991, pp. 423-427. © Printed in India.
Observation
of
two-photon induced photoemission optogalvanic effect
P R SASI
KUMAR,
G PADMAJA, A V
RA
VI
KUMAR,
V P N
NAMPOORI
and C P G VALLABHAN
Laser Division,
Department
of
Physics, Cochin University
or
Science
and
Technology, Cocbin
682022, India
MS received
13
November
1990;
revised
19
December 1990
AMtracL Observation
of
laser induced
two-photon
pbotoemission optogalvanic
(TPPOG)
effect from tungsten electrode in a discharge cell using 564 run radiation
obtained
from a
pulsed dye laser
is
described.
The
magnitude
or
the
POG
signal is studied
as
a function
of
laser energy under various discharge parameters.
Competition
between
one-pboton
and
two-pboton processes
has
been observed wben nitrogen
gas
is used in
the
discharge cell.
Keywords. Optogalvanic effect; two-photon absorption; photoemission.
PACS No. 33-80
1.
Introduction
Optogalvanic spectroscopy (OGS)
is
a technique where a glow discharge
is
perturbed
by electromagnetic radiation
due
to resonant energy transfer between the radiation
and
atoms, ions,
or
molecules (Green et
a11976;
Smith
and
Schenck
1978~
The
energy
absorbed by the species alters the plasma charge density and produces a measurable
impedance change. Ambient fluctuations in discharge current usuaUy determine the
minimum detectable
OG
signal level.
OG
effect can also
be
produced by injecting
electrons into the discharge via photoelectric emission from the electrode surface by
laser irradiation.
The
photoelectrons emitted from the cathode may excite
or
ionize
some of the atoms
on
its way
to
the anode in the discharge
and
produce more electrons
by collisions.
The
effective
quantum
efficiency in the plasma discharge
is
at
least
(Vjd)l/Z
(V
is the applied voltage and d the inter--electrode separation) times larger
than that in vacuum due to the increase in the coJlisionally produced secondary
electrons (Downey
et
aI1988). This photoemission optogalvanic
(POG)
phenomenon
is
a
non
resonant process and since the magnitude
of
the signal depends
on
the surface
composition
of
the electrode material, the method is useful as an in situ analytical
tool for plasma electrode surface characterization using various plasma gases and
different electrodes (Downey
et al
1988).
In this paper
we
report for the first time,
the observation
of
two-photon induced photoemission optogalvanic
(TPPOG)
signal
using pulsed dye laser.
1.
Experimental set up
The
output
radiation from a pulsed dye laser
(Quanta
Ray PDL-2) pumped by the
second harmonic
of
a Q-switched
Nd:YAG
laser
(Quanta
Ray
DCR-II)
was used
as
423
154
424
P R
Sasi
Kumar et al
NdYAG
laser
Trigger
wave\ergth
scan
drive
(RO
Dye
laser
R
F"1pft I
(I).
Schematic diagram
of
the experimental set up.
\
V
'/
H.V
Ficure 1(b). Typical
eRO
trace oCthe
POG
signal (vertical scale (}I V/div. horizontal scale
5 ms/div) in Nz
at
a laser energy
0(2
mJ-.
gas pressure
0(7
ton
and
discharge
current
of
12
mA.
the
e)l~itation
source. The discharge cell was made of glass having
an
inner diameter
of 1
cm
with tungsten electrodes at an inter-electrode separation of -- ] cm. Provision
for
continuous
flow
of the sample gas into the cell
at
rotary vacuum has also been
provided. The schematic diagram
of
the experimental set up
is
shown in figure 1
(a).
The discharge was produced
by
dc excitation
in
the cell with a ballast resistance in
series across the tungsten electrodes. The discharge parameters were adjusted so as
to
get
a steady discharge. The discharge was produced in gases like N
1
,
Ar,
N0
2
etc
at
various pressures and discharge currents. Pulsed dye laser beam was focussed
on
10 the cathode using a lens. The
POG
signal taken out via a capacitor can be displayed
on
a CRO. A typical
CRO
trace
of
the
POG
signal obtained with
10
ns pulses
at
564
nm
radiation from the dye laser
is
shown in figure 1
(b).
The
signal amplitude was
measured
by
varying laser energy, discharge current
and
gas pressure.
155
Optogalvanic effect
425
3.
Results
and
discussion
The variation of
POG
signal at
564
nm
as a function of laser energy for different
discharge currents
at
5 torr and 7 torr
ofN
2 gas pressures
is
studied. The work function
of the electrode material (tungsten)
is
4·3eV (Robert 1974) which approximately
coincides with twice the photon energy at 564 nm. Two-photon process has been
confirmed from the log-log plot (figure
2)
of laser energy
(1)
versus signal amplitude
(l\ V) which has a slope of - 2 implying that
AV
_12.
At
low pressure,
(5
torr) the
slope
is
less than 2 indicating the presence
of
one-photon (OP) process along with
two-photon (TP) process. The presence of one-photon process can
be
explained
as
due to the
optog~lvanic
(OG) signal arising from the excitation of
N2
molecules
by
the laser radiation
at
564nm wavelength.
It
should
be
noted that 564nm emission
coincides with 5,0 band of the first positive system (wavelength region
700-500
nm)
and
3,4
band of the Gaydoo's Green system (wavelength region 640-SOOnm) in the
spectrum of
N2
molecules (Gaydon and Pearse 1965). The presence
of
OP
and
TP
processes becomes more obvious when
we
make a least square fitting of the signal
amplitude
l\
V as a quadratic function of laser energy 1
as,
l\V
=
AI
+ BI2
Figure 3 shows the above curve fitted to experimental observations.
Variations of
A and B coefficients with discharge current for the two processes are
shown in figure
4.
At
a pressure of 5 torr, the A-coefficient
is
negative
at
low currents.
This behaviour
is
similar to
OG
signal dependence on current where similar change
of sign
is
observed (Van Veldhuizen
et
al
1984).
As
the discharge current increases,
the B-coefficient decreases gradually showing that the
OP
process becomes more
predominant at higher current (figure 4(a». This fact
is
also revealed in the variation
of slope in the log-log plot.
When the pressure
is
increased to 7 torr, the A-coefficient
is
always found to
be
negative, the magnitude of which increases with current (figure
4(b)~
It
has been
shown that an increase
in
gas
pre~sure
leads to a more negative
OG
signal (Erez
et
al
t 979). However the B-coefficient
is
not so sensitive to current as in the case of
discharge at 5 torr.
5
Torr
7
Torr
1·6
1.6
>
1·2
_1.2
<l
S
~
·8
Cl
.8
0
0
-J
-J
·4
0
·4
.6
·8
·3
·4
·6
·8
Log
I
Log
I
Figure
2-
Log-log plot of laser energy
vs
POG
signal
amplitude at different discharge
currents.
155
426
PR
Sas; Kumar et al
5
Torr.
11.8
mA
7 Torr,
16mA
1.6
3.2
1·2
2·4
~
.s
~
>
1.6
>
<:J
<J
·4
.8
0
6
10
0
2
6
10
J
(mJ)
J
(mJ)
F"tpre
3.
Curve for
AY
=
AI
+
Bll
at
S tOrT
and
7
ton,
I
is
the laser energy
per
pulse.
Points correspond
to
experimental observations.
f
,28
A
50
B
40
30
20
20
10
..
5!
0
)(
- 10
-20
10
-30
-40
11
12
13
14
15
16
17
6
18
19
11
12
13
14
15
16
17 18
19
mA
mA
F
....
"(a).
Variation
of
coefI'lcients
A [in V/J]
and
B [in
V/JI]
with discharge
current
at
StOrT.
-
80
-
90
-100
<
-110
-120
-
HO
'-------:-::--------:-":"-----:-!
14
15
16 17
mA
56
55
m
~
54
a:l
53
52
51
14
15
16
17
mA
F"!pn
4("). Variation
of
coefficients A [in V/J] and B [in V/J
I
]
with discharge current
at
7torr.
157
Optogalvanic effect
427
In conclusion,
we
have observed two-photon induced photoemission optogalvanic
effect using tungsten electrode in N
2 gas discharge.
Acknowledgements
Authors are thankful to DST for financial assistance. Two
of
the authors
(GP
and
A VRK) are grateful
to
CSIR for research
feUowships.
References
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ud
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