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Saturation of the a(6, 0) transition in the v2 band of NH3 by irradiation with a tea CO2 laser
200
SATURATION
OF THE a(6, 0) TRANSITION
IN THE vz BAND OF NH3 BY IRRADIATION
WITH A TEA CO2 LASER
P. John and
B. G. Gowenlock, Department Edinburgh
of Chemistry, EH14 4AS
R. G. Harrison Department Edinburgh
R. Leo
Heriot-Watt
University,
Riccarton,
Currie,
(Gt. Britain)
and S. Butcher
of Physics, EH14
Heriot-Watt
University,
Riccarton,
Currie,
4AS (Gt. Britain)
C, Steel Department
of Chemistry,
Experimental
Brandeis
the absorption
coefficients
(0.1-l J pulse-', half maximum
NaCl beam
followed
in NH
photon
3
for these
occur
fig.
1.
(~5% of major
optical
1.6cm pathlength
doses
system
reactions,
A typical
torr.
peak)
a 200ns width
secondary
and transmitted incorporating
cell and dual detection
at
pulses
intensities
front system
and rear of
and aR(6,l)
absorption line centres of the v2 fundamental 1 -1 -1 Line and 1075.8&m respectively. -1 -1 measured and 10.7 MHz torr (FWHM) of 8.9 MHz torr
at 1076.01cm
coefficient
doses
conditions,
by the a(6,l)
ensure
transition
that contribution
does not exceed
pressure.
greater
by loglO Do/Dt, where
transmitted
induced
drag detectors.
12% at the highest
defined
(U.S.A.)
f1075.99cm'-1) of a CO2 TEA
lo-200
of the incident
lines2 , under non-saturation
At incident
range
laser
from the CO2 TEA laser exhibited
by low intensity
coefficients
to the absorption circa
in the pressure
pulse
Measurement
splitters,
broadening
of NH3 at the R16 line
out in a conventional
The aR(6,O) mode
5Hz)
duration.
calibrated
Massachusetts
sensitised
to the study of NH3
laser have been measured
of longer
Waltham,
Study
As a prerequisite
was carried
University,
than 00.1
J cm
-2
the optical
Do and Dt are the corrected
respectively
The linear dependence
exhibits
saturation
of the optical
density,
incident
as demonstrated
density
on
absorbed
and in dose,
201
1.5
1.0
/torr
l0g,&?
‘t 0.5
0
VT7
w
-iT
w
v ,
0.5
10
1
*
1.0
1.5
IO/J cm-* Figure
1.
Dependence laser
of
the optical
IO,
dose,
NH3 pressures; 80 torr;
fig. a/ cm
A,
density
at 1075.99cm
-1
50 torr;
acquired
in the present
[i)
study,
and that published linear
extrapolation
saturation procedure related
Do
where
and McGee
-
the optical dose,
=1.6cm;
that measured 2
demonstrate
dose
density
coefficient,
dependence
measured
under
is a satisfactory may be empirically
-&-
+ 2.303
and L is the optical
of a
at Brandeis that;
thus,
B ~.~
A and B are constants
length
sbsorption
density,
to zero absorbed
and, moreover,
‘;j;=
of the
of the pressure
of the optical
to the absorbed
path
l , 150 torr; A, 100 torr; Cl,
as in fig. 3. with by Chang
conditions,
1% 10
Optical
on the incident
V, 10 torr.
2, allows measurement, by extrapolation, -1 , at low incident doses. Comparison
University3
.
, 200 torr;
I
of NH3
pathlength.
202
I
I
1
l&J Figure
2.
Variation I
of
A,
the present at aacb other
(iii)
cmm2
the optical density of NH3 with absorbed -1 . Optical path length = 1.6cm.
80 torr;
n,
computed
data,
200
tori-; 0,
150
torr;
a,
100
torr;
V, 50 torr.
as a mean
is within pressure, 2,3 .
of at least
the combined
five
determinations
experinaental
error
of
the
studies
verification relaxation is obtained
that
rapid
processes from
inter
play
and/or
an integral
the average
number
intramolecular part
- A%/(1
A1
= 1.648
-I-BD) 4 x 10 AT/vCo2P
R-R
and V-V
in the absorption
of quanta
<-IlP,since,
where
dose,
at 1075.99cm
abs' NH3 pressures;
(ii)
0.6
0.5
0.4
0.3
0.2
0.1
absorbed
per
process molecule,
203
OA
100
50
Pressure
Figure
Comparison
3.
of NH
data
and
related
treatment
dose
a, for the a(6,O)
and
theoretical
work;
of the absorption
transition
A, ref.
2;
study,
vide
saturation
phenomena
infra, en,
, ref.
of the
3.
incident
laser
The experimental
demonstrates = Al/B,
max in terms
in the v2 band
0
the frequency vco* P the pressure of NH3.
a maximum value,
reaches
of-the
dependences
temperature;
D the absorbed
asymptotically
tot-r
l , present
3'
T is the translational pulse;
/
of the pressure
coefficient,
200
150
that
which
of a simple
exceed
two level
0.5.
A
system
is thus precluded. Whereas absorbed
the linear
dose
molecular vibrational
in fig.
mechanism,
relationship
between
2 may be predicted described
and rotational
below,
relaxation
from
the optical steady
incorporating processes,
density
state rapid
versus
solutions
to a
intermolecular,
the pressure
dependence
of
204
the gradients
cannot
be
80 reconciled.
NH3(v2=0,
306,
K=O)
+ hv(1075.99cm-1)
1 *Z+-l3(v2=1,
NH3(v2=1,
J=7,
K=O)
+ M +NH3(v2-1,
J=m,K==n)
NH3(v2-1,
J=m,
K=n)
+ M
NH3 b2=0,
J-l'.
J-7, K=O)
+M
5
K=nl)
The set of coupled (8) have
been
solved
Kutta
procedure.
prior
to irradiation,
from
pumped
integrated
Whilst data
levels
of the
agreement
is satisfactory
pressures.
The
over
between
of the Runge-
and s(7,O)
states, calculated
sublevels
populations been
constants
reactions
of the
performed
encompassing efficiency
were
loglODo/Dt,
obtained have
H.M.
Mould,
W.C.
Price
model
T.Y.
Chang
of this fact with
and G.R. Wilkinson,
respect
to multi-photon
Spectrochem_Acta,E,313
3.
C. Steel,
4.
S. Gill,
Appl.Phys.Lett.,E,526(1976)
counnunication.
Proc.Camb.Phil.Soc.,47,96.
from
at lower
be discussed.
and J.D. McGee, private
the of
and the experimental
'a disparity-occurs
(1959). 2.
employ-
been
References 1.
of the
pathlength.
the theoretical pressures,
have
equations(l)4
distribution
(collisional
of en> and thence the optical
at high
will
rate
of the reverse
implications
absorption-processes
dependent
pulse
2) to 40 MHztorr-1
Values
representing
(J,K) rotational
a laser
relaxation
+ M
modification
a Boltzmann
of the time
during
The rates
balancing.
calculated
energy
+ Pi
K=O)
of the a(6,O)
from
torr-'(ref.
unity).
detailed
Gill's
obtained
levels
5~6,
equations
using
populations
of intermolecular
8.9 MHz
NH3(u2=0,
were
Calculations
and terminal
approx.
+
differential
Initial
in NH3'.
ing a range range
+M
Id)
J-l,
NH3(v2fo,
numerically
the established
v2 band
y
SATURATION
OF THE a(6, 0) TRANSITION
IN THE vz BAND OF NH3 BY IRRADIATION
WITH A TEA CO2 LASER
P. John and
B. G. Gowenlock, Department Edinburgh
of Chemistry, EH14 4AS
R. G. Harrison Department Edinburgh
R. Leo
Heriot-Watt
University,
Riccarton,
Currie,
(Gt. Britain)
and S. Butcher
of Physics, EH14
Heriot-Watt
University,
Riccarton,
Currie,
4AS (Gt. Britain)
C, Steel Department
of Chemistry,
Experimental
Brandeis
the absorption
coefficients
(0.1-l J pulse-', half maximum
NaCl beam
followed
in NH
photon
3
for these
occur
fig.
1.
(~5% of major
optical
1.6cm pathlength
doses
system
reactions,
A typical
torr.
peak)
a 200ns width
secondary
and transmitted incorporating
cell and dual detection
at
pulses
intensities
front system
and rear of
and aR(6,l)
absorption line centres of the v2 fundamental 1 -1 -1 Line and 1075.8&m respectively. -1 -1 measured and 10.7 MHz torr (FWHM) of 8.9 MHz torr
at 1076.01cm
coefficient
doses
conditions,
by the a(6,l)
ensure
transition
that contribution
does not exceed
pressure.
greater
by loglO Do/Dt, where
transmitted
induced
drag detectors.
12% at the highest
defined
(U.S.A.)
f1075.99cm'-1) of a CO2 TEA
lo-200
of the incident
lines2 , under non-saturation
At incident
range
laser
from the CO2 TEA laser exhibited
by low intensity
coefficients
to the absorption circa
in the pressure
pulse
Measurement
splitters,
broadening
of NH3 at the R16 line
out in a conventional
The aR(6,O) mode
5Hz)
duration.
calibrated
Massachusetts
sensitised
to the study of NH3
laser have been measured
of longer
Waltham,
Study
As a prerequisite
was carried
University,
than 00.1
J cm
-2
the optical
Do and Dt are the corrected
respectively
The linear dependence
exhibits
saturation
of the optical
density,
incident
as demonstrated
density
on
absorbed
and in dose,
201
1.5
1.0
/torr
l0g,&?
‘t 0.5
0
VT7
w
-iT
w
v ,
0.5
10
1
*
1.0
1.5
IO/J cm-* Figure
1.
Dependence laser
of
the optical
IO,
dose,
NH3 pressures; 80 torr;
fig. a/ cm
A,
density
at 1075.99cm
-1
50 torr;
acquired
in the present
[i)
study,
and that published linear
extrapolation
saturation procedure related
Do
where
and McGee
-
the optical dose,
=1.6cm;
that measured 2
demonstrate
dose
density
coefficient,
dependence
measured
under
is a satisfactory may be empirically
-&-
+ 2.303
and L is the optical
of a
at Brandeis that;
thus,
B ~.~
A and B are constants
length
sbsorption
density,
to zero absorbed
and, moreover,
‘;j;=
of the
of the pressure
of the optical
to the absorbed
path
l , 150 torr; A, 100 torr; Cl,
as in fig. 3. with by Chang
conditions,
1% 10
Optical
on the incident
V, 10 torr.
2, allows measurement, by extrapolation, -1 , at low incident doses. Comparison
University3
.
, 200 torr;
I
of NH3
pathlength.
202
I
I
1
l&J Figure
2.
Variation I
of
A,
the present at aacb other
(iii)
cmm2
the optical density of NH3 with absorbed -1 . Optical path length = 1.6cm.
80 torr;
n,
computed
data,
200
tori-; 0,
150
torr;
a,
100
torr;
V, 50 torr.
as a mean
is within pressure, 2,3 .
of at least
the combined
five
determinations
experinaental
error
of
the
studies
verification relaxation is obtained
that
rapid
processes from
inter
play
and/or
an integral
the average
number
intramolecular part
- A%/(1
A1
= 1.648
-I-BD) 4 x 10 AT/vCo2P
R-R
and V-V
in the absorption
of quanta
<-IlP,since,
where
dose,
at 1075.99cm
abs' NH3 pressures;
(ii)
0.6
0.5
0.4
0.3
0.2
0.1
absorbed
per
process molecule,
203
OA
100
50
Pressure
Figure
Comparison
3.
of NH
data
and
related
treatment
dose
a, for the a(6,O)
and
theoretical
work;
of the absorption
transition
A, ref.
2;
study,
vide
saturation
phenomena
infra, en,
, ref.
of the
3.
incident
laser
The experimental
demonstrates = Al/B,
max in terms
in the v2 band
0
the frequency vco* P the pressure of NH3.
a maximum value,
reaches
of-the
dependences
temperature;
D the absorbed
asymptotically
tot-r
l , present
3'
T is the translational pulse;
/
of the pressure
coefficient,
200
150
that
which
of a simple
exceed
two level
0.5.
A
system
is thus precluded. Whereas absorbed
the linear
dose
molecular vibrational
in fig.
mechanism,
relationship
between
2 may be predicted described
and rotational
below,
relaxation
from
the optical steady
incorporating processes,
density
state rapid
versus
solutions
to a
intermolecular,
the pressure
dependence
of
204
the gradients
cannot
be
80 reconciled.
NH3(v2=0,
306,
K=O)
+ hv(1075.99cm-1)
1 *Z+-l3(v2=1,
NH3(v2=1,
J=7,
K=O)
+ M +NH3(v2-1,
J=m,K==n)
NH3(v2-1,
J=m,
K=n)
+ M
NH3 b2=0,
J-l'.
J-7, K=O)
+M
5
K=nl)
The set of coupled (8) have
been
solved
Kutta
procedure.
prior
to irradiation,
from
pumped
integrated
Whilst data
levels
of the
agreement
is satisfactory
pressures.
The
over
between
of the Runge-
and s(7,O)
states, calculated
sublevels
populations been
constants
reactions
of the
performed
encompassing efficiency
were
loglODo/Dt,
obtained have
H.M.
Mould,
W.C.
Price
model
T.Y.
Chang
of this fact with
and G.R. Wilkinson,
respect
to multi-photon
Spectrochem_Acta,E,313
3.
C. Steel,
4.
S. Gill,
Appl.Phys.Lett.,E,526(1976)
counnunication.
Proc.Camb.Phil.Soc.,47,96.
from
at lower
be discussed.
and J.D. McGee, private
the of
and the experimental
'a disparity-occurs
(1959). 2.
employ-
been
References 1.
of the
pathlength.
the theoretical pressures,
have
equations(l)4
distribution
(collisional
of en> and thence the optical
at high
will
rate
of the reverse
implications
absorption-processes
dependent
pulse
2) to 40 MHztorr-1
Values
representing
(J,K) rotational
a laser
relaxation
+ M
modification
a Boltzmann
of the time
during
The rates
balancing.
calculated
energy
+ Pi
K=O)
of the a(6,O)
from
torr-'(ref.
unity).
detailed
Gill's
obtained
levels
5~6,
equations
using
populations
of intermolecular
8.9 MHz
NH3(u2=0,
were
Calculations
and terminal
approx.
+
differential
Initial
in NH3'.
ing a range range
+M
Id)
J-l,
NH3(v2fo,
numerically
the established
v2 band
y