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The rate of reaction of Ge(3P0) atoms with N2O
CHEMICAL PHYSICS LETTERS
Volume 49, number 3
THE RATE
OF REACTION
Peter M. SWEARENGEN, Air Force Weapons Laboratory.
OF Ge(3Po)
1 August
1977
ATOMS WITH N,O
Steven J. DAVIS,
Steven G. HADLEY
Kirtland Air Force Base, New hfexico 871 I 7, USA
and Thomas M. NIEMCZYK Department of Chemistry, University of New Mexico. Albuquerque,
New Mexico 87131, USA
Received 30 March 1977
The gas phase reaction
of CLZ(~P~) f N20
has been studied
in a flow tube system.
A hollow
cathode
discharge
in the
flow system produced the ground state Ge atoms for the reaction, and the disappearance of the atoms was followed by A rate constant atomic absorption spectroscopy_ The presence of GeO(a 3 S’) was confirmed by emission spectroscopy. (6.7 f 2.5) X IO-l3 cm3 molecule-’ se1 was determined for this reaction at 300 I-L
of
Experimental
l_ Introduction
2.
A second order rate constant for reaction of a specific atomic state of germanium with nitrous oxide is reported here for the first time. Other workers have reported rate constants for the reaction of specific spin orbit states of germanium atoms with a number of different oxidizers [I] _ There were several reasons for studying this particular system. First, a triplet excited state product molecule, GeO, is formed in a chemical reaction that has sufficient exothermicity to form the excited triplet state product directly. Secondly, this triplet state (a 3Zf) should be metastable which is desirable in a chemically pumped laser system. In addition, the difference in equilibrium internuclear separations between the ground state and this metastable electronic state may provide the basis for developing a population inversion between vibronic levels of the a 3x+ and X 1 Z states of GeO. The presence of electronically excited GeO in our system was confirmed by comparison of the emission spectrum of the reaction of Ge + N20 with results published previously [2,3] _We anticipate measurements of the rate of production of a 3t;c GeO molecules in the near future.
In this study, a flow tube system with a hollow cathode discharge produced ground state germanium atoms (4 3Po) in concentrations that could be monitored by absorption of atomic resonance radiation. The removal of the Ge atoms was studied in the reaction:
Ge(4 3Po) + N20 + products _ The nature of the products will be reported in a sub- -. sequent communication. A gas phase flow reactor similar to the one described in a previous paper was used to obtain the kinetic data [3] _ For our studies, the observation region of the glass flow tube was replaced with a two
inch diameter aluminum
tube with quartz windows.
A schematic drawing of the system we used is shown in fig. 1. Ground state Ge atoms were produced by flowing GeH4 gas entrained in Ar through a hollow cathode discharge that dissipated about 450 W. Care was taken to maintain this condition in every experiment_ The flow system upstream and downstream of the hollow cathode was assembled from one inch i.d. Pyrex tubing. Three inches downstream from the 571
Volume 49, number 3
CHEMZCAL PHYSICS
3. F&h I.! Q(T
SO”RCE
Fig. 1. Block diagram of apparatus.
1 August 1977
LEFERS
and discussion
From the Beer-Lambert law, [Gel is proportional to ln(I&), the absorbance (defined as A). In the work reported here, we have injected mixtures of less than 0.1% germane in argon into the hollow cathode discharge. If it is assumed that the dissociation efficiency of the cathode system was not affected by the germane concentration, then the absorbance should be a linear function of injected germane. Measurements were made of the absorbance as a fiurction of germane injection pressures and adherence to the Beer-Lambert law under the conditions of this experiment was demonstrated as shown in fig. 2. All the data used to obtain the rate constant were taken at absorbances of 0.4 or less. Thus, we believe that the Beer-Lambert absorption law is valid under the conditions of our experimen
The rate constant for the reaction between Ge(4 3Pc and N,O can be calculated from a measurement of the decrease in the Ge(4 3Po) atom population as a function of distance in the flow direction, using the known fiow velocity in our apparatus. Typical data for atoms
cathode, the flow turned 90 degrees and then expanded into the two inch diameter observation tube. Ten inches beyond the turning point, N,Q was added through a spoked injector nozzle. Observation windows on either side of the aluminum flow tube extended for 18 inches of the 24-inch vessel. The optical arrangement consisted of a Westinghouse hollow cathode Ge lamp which produced the Ge resonance line at 265.1 nm corresponding to the 4 3Po + 5 3P, transition, a quartz collimating lens, a light chopper, a beam aperture, a focusing lens, and a 0.3 meter model 218 GCA MacPherson monochromator. An EMI 9785QB photomultiplier tube was used with a PAR 128A lock-in amplifier for the atomic absorption
this decrease in Ge(4 3Po) concentration at an N20 of 5.6 X 1014/cm3 is shown in fig. 3. A kinetic analysis of our experiment shows that
concentration
In@,,
/A,,)
= [(x~---x~)/uI !kz
fY201+ k)2
where A,, and A,, are the absorbancies. positions x1 and x2 in the flow direction
In(I,,lI), at respectively;
measurements. The complete optical system rested on steel platfbrm that was in turn mounted on the carriage of a lathe bed. Measurements could thus be made in the length of the flow tube in metered increments, as shown in fig. 1. _ A temperature of 300 K was assumed in all calculations that required a specific temperature. This value was arrived at after conside.ration of the discussion of Ferguson et al. [4], on the technique of flowing afterglow measurements_ The velocity in the flow tube at the gas reaction site was calculated to be 4.95 X lo3 cm/s, assuming plug flow conditions. Pressure measurea
ments were taken using a Pennwalt Corporation Wallace and Tieman
512
series 300 absolute pressure gauge.
A
o-o:’ GERUAHE
X iG12
cmm3
Fig. 2. Absorbance by Gel4 3Po) as a function of germane pressure injected into the hollow cathode discharge.
CHEMICAL PHYSICS LETTERS
Volume 49, number 3
1 August 1977
Table 1 k = 265.1 nm resonance line, P = 1.3 X 10:’ fer gas -K2W20l+K
Ka (lo-r3
(10’4 cnl-3
(103 s-l)
molecule-’
2.38 2.97 4.18 5.09 5.59 6.18 7.08 8.34 9.30 12.00 13.50
1.24 1.37 1.36 1.52 1.57 1.67 1.66 1.74 1.82 1.76 2.16
3.78 7.41 5.02 7.27 7.51 8.41 7.20 7.07 7.10 5.08 7.48
IN201
Rn A
TIME.
AFTER
ADDITION
OF N20.
(lO-3
SEC)
K2 = (6.7 = 2.5) X lo-l3
Fig. 3. Typical data for absorbance by Ge(4 3Poj versus time.
For this run, N20 concentration in rhe flowtube was 5.6 x 1U14 molecules/cm3.
_
i-c&a
ro*
2.00 [
1.80 -
o_ 25 SF +
1.60 -
l-l
cm-j
Ar as buf-
cm3 s-r)
mean 6.67 cm3 molecule-’ s-r
u
is the gas velocity, k2 the second order rate constant for the disappearance of Ce(4 3P,) atoms with N,O, and k is the rate constant for the disappearance of these atoms in other processes. Possible effects due to axial diffusion were checked for and found to be negligible within the precision of this experiment. For all of our experimental conditions the N,O concentration was greater than ten times that of the GeH4, thus one can assume [N20] is constant. The functional dependence of (k2 [N20] + k) on [N20] for all data is shown in fig. 4. All data for the given N20 concentrations are shown in table I_ A least squares analysis of the obtained data yielded a rate constant of 6.7 X lo-l3 cm3 molecule-l s-l with a standard deviation of 0.6 X 10-13_ Our estimate of the systematic error is about 35%, and we report a rate constant of
?I
5
CJ
(6.7 t 2.5) X IO-t3 cm3 molecule-L
1.40 -
s-l .
55
References
G-G
2.0
4.0
6.0
c’ $0
. x
8.6 1014
10.0 12.0
14.0
cu-3
-I
Fig. 4. (kz [NaO] f k).as a function of NaO concentration for data collected.
all
r11 A. Brown and D. Husain, Can. J. Chem. 54 (1976) 4. PI G. Hager, R. Harris and S.G. Hadley, J. Chem. Phys. 63
(1975) 2810. 131 G. Hager, L.E. Wilson and SC. Hadley, Chem. Phys. Letters 27 (1974) 439. 141 E.E. Ferguson, F.C. Fehsenfeld and A.L. Schmeltekopf, Advan. At. Mol. Phys. 5 (1969)1.
573
Volume 49, number 3
THE RATE
OF REACTION
Peter M. SWEARENGEN, Air Force Weapons Laboratory.
OF Ge(3Po)
1 August
1977
ATOMS WITH N,O
Steven J. DAVIS,
Steven G. HADLEY
Kirtland Air Force Base, New hfexico 871 I 7, USA
and Thomas M. NIEMCZYK Department of Chemistry, University of New Mexico. Albuquerque,
New Mexico 87131, USA
Received 30 March 1977
The gas phase reaction
of CLZ(~P~) f N20
has been studied
in a flow tube system.
A hollow
cathode
discharge
in the
flow system produced the ground state Ge atoms for the reaction, and the disappearance of the atoms was followed by A rate constant atomic absorption spectroscopy_ The presence of GeO(a 3 S’) was confirmed by emission spectroscopy. (6.7 f 2.5) X IO-l3 cm3 molecule-’ se1 was determined for this reaction at 300 I-L
of
Experimental
l_ Introduction
2.
A second order rate constant for reaction of a specific atomic state of germanium with nitrous oxide is reported here for the first time. Other workers have reported rate constants for the reaction of specific spin orbit states of germanium atoms with a number of different oxidizers [I] _ There were several reasons for studying this particular system. First, a triplet excited state product molecule, GeO, is formed in a chemical reaction that has sufficient exothermicity to form the excited triplet state product directly. Secondly, this triplet state (a 3Zf) should be metastable which is desirable in a chemically pumped laser system. In addition, the difference in equilibrium internuclear separations between the ground state and this metastable electronic state may provide the basis for developing a population inversion between vibronic levels of the a 3x+ and X 1 Z states of GeO. The presence of electronically excited GeO in our system was confirmed by comparison of the emission spectrum of the reaction of Ge + N20 with results published previously [2,3] _We anticipate measurements of the rate of production of a 3t;c GeO molecules in the near future.
In this study, a flow tube system with a hollow cathode discharge produced ground state germanium atoms (4 3Po) in concentrations that could be monitored by absorption of atomic resonance radiation. The removal of the Ge atoms was studied in the reaction:
Ge(4 3Po) + N20 + products _ The nature of the products will be reported in a sub- -. sequent communication. A gas phase flow reactor similar to the one described in a previous paper was used to obtain the kinetic data [3] _ For our studies, the observation region of the glass flow tube was replaced with a two
inch diameter aluminum
tube with quartz windows.
A schematic drawing of the system we used is shown in fig. 1. Ground state Ge atoms were produced by flowing GeH4 gas entrained in Ar through a hollow cathode discharge that dissipated about 450 W. Care was taken to maintain this condition in every experiment_ The flow system upstream and downstream of the hollow cathode was assembled from one inch i.d. Pyrex tubing. Three inches downstream from the 571
Volume 49, number 3
CHEMZCAL PHYSICS
3. F&h I.! Q(T
SO”RCE
Fig. 1. Block diagram of apparatus.
1 August 1977
LEFERS
and discussion
From the Beer-Lambert law, [Gel is proportional to ln(I&), the absorbance (defined as A). In the work reported here, we have injected mixtures of less than 0.1% germane in argon into the hollow cathode discharge. If it is assumed that the dissociation efficiency of the cathode system was not affected by the germane concentration, then the absorbance should be a linear function of injected germane. Measurements were made of the absorbance as a fiurction of germane injection pressures and adherence to the Beer-Lambert law under the conditions of this experiment was demonstrated as shown in fig. 2. All the data used to obtain the rate constant were taken at absorbances of 0.4 or less. Thus, we believe that the Beer-Lambert absorption law is valid under the conditions of our experimen
The rate constant for the reaction between Ge(4 3Pc and N,O can be calculated from a measurement of the decrease in the Ge(4 3Po) atom population as a function of distance in the flow direction, using the known fiow velocity in our apparatus. Typical data for atoms
cathode, the flow turned 90 degrees and then expanded into the two inch diameter observation tube. Ten inches beyond the turning point, N,Q was added through a spoked injector nozzle. Observation windows on either side of the aluminum flow tube extended for 18 inches of the 24-inch vessel. The optical arrangement consisted of a Westinghouse hollow cathode Ge lamp which produced the Ge resonance line at 265.1 nm corresponding to the 4 3Po + 5 3P, transition, a quartz collimating lens, a light chopper, a beam aperture, a focusing lens, and a 0.3 meter model 218 GCA MacPherson monochromator. An EMI 9785QB photomultiplier tube was used with a PAR 128A lock-in amplifier for the atomic absorption
this decrease in Ge(4 3Po) concentration at an N20 of 5.6 X 1014/cm3 is shown in fig. 3. A kinetic analysis of our experiment shows that
concentration
In@,,
/A,,)
= [(x~---x~)/uI !kz
fY201+ k)2
where A,, and A,, are the absorbancies. positions x1 and x2 in the flow direction
In(I,,lI), at respectively;
measurements. The complete optical system rested on steel platfbrm that was in turn mounted on the carriage of a lathe bed. Measurements could thus be made in the length of the flow tube in metered increments, as shown in fig. 1. _ A temperature of 300 K was assumed in all calculations that required a specific temperature. This value was arrived at after conside.ration of the discussion of Ferguson et al. [4], on the technique of flowing afterglow measurements_ The velocity in the flow tube at the gas reaction site was calculated to be 4.95 X lo3 cm/s, assuming plug flow conditions. Pressure measurea
ments were taken using a Pennwalt Corporation Wallace and Tieman
512
series 300 absolute pressure gauge.
A
o-o:’ GERUAHE
X iG12
cmm3
Fig. 2. Absorbance by Gel4 3Po) as a function of germane pressure injected into the hollow cathode discharge.
CHEMICAL PHYSICS LETTERS
Volume 49, number 3
1 August 1977
Table 1 k = 265.1 nm resonance line, P = 1.3 X 10:’ fer gas -K2W20l+K
Ka (lo-r3
(10’4 cnl-3
(103 s-l)
molecule-’
2.38 2.97 4.18 5.09 5.59 6.18 7.08 8.34 9.30 12.00 13.50
1.24 1.37 1.36 1.52 1.57 1.67 1.66 1.74 1.82 1.76 2.16
3.78 7.41 5.02 7.27 7.51 8.41 7.20 7.07 7.10 5.08 7.48
IN201
Rn A
TIME.
AFTER
ADDITION
OF N20.
(lO-3
SEC)
K2 = (6.7 = 2.5) X lo-l3
Fig. 3. Typical data for absorbance by Ge(4 3Poj versus time.
For this run, N20 concentration in rhe flowtube was 5.6 x 1U14 molecules/cm3.
_
i-c&a
ro*
2.00 [
1.80 -
o_ 25 SF +
1.60 -
l-l
cm-j
Ar as buf-
cm3 s-r)
mean 6.67 cm3 molecule-’ s-r
u
is the gas velocity, k2 the second order rate constant for the disappearance of Ce(4 3P,) atoms with N,O, and k is the rate constant for the disappearance of these atoms in other processes. Possible effects due to axial diffusion were checked for and found to be negligible within the precision of this experiment. For all of our experimental conditions the N,O concentration was greater than ten times that of the GeH4, thus one can assume [N20] is constant. The functional dependence of (k2 [N20] + k) on [N20] for all data is shown in fig. 4. All data for the given N20 concentrations are shown in table I_ A least squares analysis of the obtained data yielded a rate constant of 6.7 X lo-l3 cm3 molecule-l s-l with a standard deviation of 0.6 X 10-13_ Our estimate of the systematic error is about 35%, and we report a rate constant of
?I
5
CJ
(6.7 t 2.5) X IO-t3 cm3 molecule-L
1.40 -
s-l .
55
References
G-G
2.0
4.0
6.0
c’ $0
. x
8.6 1014
10.0 12.0
14.0
cu-3
-I
Fig. 4. (kz [NaO] f k).as a function of NaO concentration for data collected.
all
r11 A. Brown and D. Husain, Can. J. Chem. 54 (1976) 4. PI G. Hager, R. Harris and S.G. Hadley, J. Chem. Phys. 63
(1975) 2810. 131 G. Hager, L.E. Wilson and SC. Hadley, Chem. Phys. Letters 27 (1974) 439. 141 E.E. Ferguson, F.C. Fehsenfeld and A.L. Schmeltekopf, Advan. At. Mol. Phys. 5 (1969)1.
573