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Chemiluminescence from the gas phase reaction of atomic carbon with lead oxide
Volume 59, number 1
cHEMILumNEs
CHEMICAL
PHYSICS
LETTERS
1 November
1978
CENCE FROM THE GAS PHASE REACTION
OF ATOMIC CARBON WITH LEAD OXIDE U.C. SRIDHARAN, Depar&zent
T.G. DIGIUSEPPE, D.L. McFADDENz
of Gaemishy,
Boston Cdiege,
and P. DAVIDOVITS
Chestnut Hi& Massachusetts 0216 7. U!Z4
Received 25 July 1978 The gas phase reaction C + PbO -CO f Pb was studied. The relative population distriiution of the electronically excited Pb produced in this reaction was determined. The total cross section for the reaction and the cross section for the production of electronically excited states are estimated to be 20 and 5 A’ respectively.
1_ Introduction The reactions of carbon atoms with metal oxides, that is, C + MO + CO + M are highly exoergic. In most cases the energy is sufficient to produce higbly excited metal atoms. The reaction C f NaO + CO + Na* has been proposed by Luria et al. [ 1] as a possible mechanism of light emission in their flame experiments. However, prior to the present work this class of reactions has not been subject to a specific study. In this paper we present studies of chemiluminescence produced in the reaction CtPbO-*CO+Pb*.
(1)
The carbon atoms were produced in a flowing microwave discharge of 1% CO in helium 133. The PbO molecules were produced in a quartz lined oven heated to about 1050 K and were entrained in a helium flow which carried them into the reaction zone. The PbO flow could be intercepted by an electrically opemted flag. The reaction occurs in the region where the two flows merge which is about 5 cm from each source. A disk shaped source of light is visible in this region. This chemihuninescence is detected by a monochromator-photomultiplier combination which was calibrated to provide absolute intensity measurements. The production of carbon atoms in the micrbwave discharge was monitored by the absorption of
With the reactants in the ground state the exoergicity of this reaction is about 7.2 eV. Light emission from iead atoms excited up to this limit was observed. The analysis of the chemiluminescence indicates that population inversion is obtained among several electronically excited states of lead atoms.
2. Experimental The experiments were performed in a flow apparatus shown schematically in fig. 1. The details of this apparatus have been described in a previous publication [2] z Alfked P. Sloan Foundation Fellow.
Fig. 1. Schematicof apparatus.The optics for light collection andabsorptionmeasurements are along the axis perpendicniar tothisdiagram. 43
Volume 59, number 1
CHEMICAL
PHYSICS
1 November 1978
LE’ITERS
2479 A carbon line (2p2 ‘So + 3s ‘Pi) produced by an electrodeless discharge lamp containing 5 X 10m2 torr of CO in 20 torr helium 131. The density of carbon was computed from the weighed deposit of carbon on a glass slide which intercepted the flow_ From this measurement the density of carbon atoms in the reaction zone was estimated to be about IOrr cme3. The density of PbO in the reaction zone obtained from the deposition experiment was about lOi2 cm-3_
o 6d 3Do I 6d3D02 Q
6d3+
The density of ground state lead produced by the chemical reaction was monitored by the absorption of Pb resonance light also produced by an electrodeless microwave discharge lamp containing solid Iead, iodine and about 15 torr argon [4] _ The density of le. d in the reaction zone under typical reaction condiions was about I-25 X 1010 cme3. The background pressure in the reaction chamber was about 1.4 torr which was due mostly to the helium carrier gas that flows into the reaction chamber. The tlow rate of helium into the chamber through the oven and the microwave discharge were 1700 SCCM and 2775 SCCM respectiveIy. In add&on to the desired reactants, C and PbO, the reaction zone contains several other species which complicate the experiment_ We have perfo_med a mumber of tests which indicate that the main source of ground state and excited state lead in the reaction zone is the primary reaction C + PbO --t CO + Pb. These tests are described in the appendix.
7d3C$o
-
09S3P0 I
06s3P0 1
Fig_ 2. Normalized steady state relative population distri-
bution.
state and are plotted as a function of the state enerm. The accuracy of the transition probability data used here was reported to be about 150%. The results in fig. 2 show that while the population of the excited states tends to decrease with energy, the distribution is non-Boltznrann and population inversion exists between several states. The inversions between the p state and the s and d states
3. Results and discussion
are worth noting. Radiative transitions from the p states to the ground states are electricdipole forbidden-Hence, the 8p 3D2 state has a longer radiative lifetime than the lower lying s and d states, and this
The intensities of 68 atomic lead lines were recorded in each _mn_Light from levels as high as 7.3 eV was observed_ The relative number of photons/s were calculated from the average of four independent runs
is a favorable condition for steady state inversion. Table 1 lists the inversion condition between the 8p 3& state and the lower d and s states. The populations in higher p states could not be calculated because the transition probabilities are not availabbfe.
which were consistent among each other within about 15%. The steady state popularion density of an excited state can be calculated from the emitted light intensity data if the Einstein A coefficient for that transition is known. in the case of lead, transition probabilities are not available for all the energy levels. The rehtive population densities using the available probability data [S] are shown in fig. 2. These densities are divided by the degeneracy factor for the
The measurements in this expei- _ f tow an order of magnitude estimate of th x ..Js~ _ ction for the C + PbO reaction. A steady state solution of the rate equation shows that the density of lead [Pb] , in the reaction region is given by [Pb]
= [C] lpbO] -o/TD -
Here ii is the average relative velocity between carbon atoms and PbO, CTis the average reaction cross section
CHEMICAL
Volume 59, number 1
PHYSICS
LETTERS
1 November
1978
Table 1 Population inversions observed in the experiment Transition
8p3Di
6d 3F: 6d 3Dp 6d 3D: 7s 3P: 8s 3P’z 7s ‘P’i’
-
Wavelength Olm)
1.78 1.54 l-70 266 3.07 3.99
between the two species and n, is the rate of diiappearance of lead atoms from the reaction region. In the estimation of the cross section the only unknown is 7~. It seems reasonable to assume that the prime mechanism for the disappearance of lead is diffusion. A simple calculation for our experimental conditions then yields a value Of lo3 s- ’ for 7~. ‘Lhe total reaction cross section is then in turn estimated to be 20 A2. From the light intensity measurements we further estimate that the cross sectron for the production of electronically excited states is about 5 A’.
was supported
upper level
lower level
==22
15 5.3 El3 0.97 2.9 12
intensities
Population ratio upper to lower level 9.7 2.6 1.7 26 12 27
of various
lead lines were in good
agree-
ment with those obtained in the experiments with heated lead oxide. Using a microwave discharge of CO as a source of carbon atoms complicates the analysis of the experimental results. In addition to carbon atoms the discharge flow contains other species which could react and directly or indirectly also produce excited Iead atoms. The foliowing are the processes which are most likely to be responsible for such unwanted re2c tionn (a) The reaction CO+PbO+COz+Pb
Acknowledgement This work
Life time (ns)
in part by grants from
NSF and AFSOR.
followed by excitation of Pb with the energetic species, M* present in the discharge. That is Pb+M*+Pb*+M.
Appendix: A discussion of reaction mechanisms In connection with the reacticn mechanism we shall first discuss the PbO source. It is known [6] that the vapor above solid PbO contains PbO molecules as well as an appreciable fraction of polymers (PbO), _ Two observations suggest that the reaction with the monomer is the main source of the observed chemiluminescence. (1) The semi-log plot of the intensity of the lead emission versus the reciprocal of the oven temperature was linear and the heat of sublimation calculated from tie slope was within 10% of the accepted value for ‘&e monomer IS]_ (2) PbO gas was prepared by passing N20 over heated lead and when this flow was merged with the flow from the CO/He discharge lead emission was observed- The relative
(b) Production of the ground state lead atoms and subsequent excitation by colJ.ision with the energetic species in the discharge. That is M*+PbOAM+Pb+O and M*+Pb+M+Pb*. (c) Reactions with excited species formed in the discharge: e.g., CO* + PbO + CO2 + Pb*. ‘ihe following observations indicate that above processes do not play a significant role in our experiment. Merging the two flows without the microwave discharge does not produce any observable reaction. 45
Volume 59. numixr 1
CaMcAL
PEiYSicS LETTERS
Neither chemiluminescence nor ground state lead is detected. This indicates that the first step of the suggested process (a) is negligiiIe under our experimental conditions. Wnen the flow from a pure He discharge was merged with the PbO molecules a small amount of emission mostly from the low lying excited levels of lead was observed_ Absorption of resonance radiation
by ground state lead atoms was barely detectable (less than one percent)_ The addition of one percent CO into the discharge increased the emitted light by about a factor of 100. J3igherexcited levels of Pb were seen in conformity with the exoergicity of reaction (I). The density of ground state lead increases by about an order of magnitude. The experiment with pure helium discharge was repeated with lead instead of PbO in the oven. While the lead emission from the He discharge remained almost the same as in the PbO experiments, introduction of CO caused only a factor of 2 increase in the emitted intensity, and only th= low lying excited lead atoms were seen. These observations show that wl+ie process (b) does occur, its role in our experiment is not significant.
Reaction type (c) cannot be ruled out as conchusively as one would like. The following observations, however, suggest rhat it is not a significant process in our experiment. The most likely sou_rceof CO* in reaction (c) is the metastable state of this molectzle such as CO {a 3KI) which has a sufficiently long lifetime to reach the reaction zone- However, the
46
I November 1978
resuits of Taylor and Setser 173 suggest that under our experimental conditions the mefastable molecules would be quenched by CO molecules in a period shorter than the flight time to the reaction zone. Furthermore, the lead chemiluminescence observed in our experiment was fcund to vary linearly with the measured carbon atom density- We were not equipped to monitor the triplet CO states, however, it would be a coincidence if the density of the CO* and that of the carbon atoms were linearly related. As noted earlier, the lead atoms produced in this system were excited to various levels in conformity with the exoergicity of reaction (1). This also would be an unexpected coincidence.
References Bf. Luria, DJ. Eckstrom and S-W. Benson, J. Chem. Phys- 64 (1976) 3103_ Dl UC. Stidharan, D.L. McFadden and P. Davidovits, J. Chem. Phys. 65 (1976) 5373. Dl E-Y-Y_ Lam, P. Gasper and A9. Wolf, J. Phys. Chem. 75 (1971) 445. 141 P-T. Cunningham and JX. Lintz, J. Opt. Sot. Am. 57 (1967) 1000. [51 CK Cork and W.R. Bozman, Natl. Bur. Std. Monograph 53 (1962). 161 J. Drowart, R_ Colin and G. Exsteen, Trans. Faraday Sot. 61 (1965) 1376. I71 G.W. Taylor and D.W. Setser, Chem. Phys. Letters 8 (1971) 51_
[II
cHEMILumNEs
CHEMICAL
PHYSICS
LETTERS
1 November
1978
CENCE FROM THE GAS PHASE REACTION
OF ATOMIC CARBON WITH LEAD OXIDE U.C. SRIDHARAN, Depar&zent
T.G. DIGIUSEPPE, D.L. McFADDENz
of Gaemishy,
Boston Cdiege,
and P. DAVIDOVITS
Chestnut Hi& Massachusetts 0216 7. U!Z4
Received 25 July 1978 The gas phase reaction C + PbO -CO f Pb was studied. The relative population distriiution of the electronically excited Pb produced in this reaction was determined. The total cross section for the reaction and the cross section for the production of electronically excited states are estimated to be 20 and 5 A’ respectively.
1_ Introduction The reactions of carbon atoms with metal oxides, that is, C + MO + CO + M are highly exoergic. In most cases the energy is sufficient to produce higbly excited metal atoms. The reaction C f NaO + CO + Na* has been proposed by Luria et al. [ 1] as a possible mechanism of light emission in their flame experiments. However, prior to the present work this class of reactions has not been subject to a specific study. In this paper we present studies of chemiluminescence produced in the reaction CtPbO-*CO+Pb*.
(1)
The carbon atoms were produced in a flowing microwave discharge of 1% CO in helium 133. The PbO molecules were produced in a quartz lined oven heated to about 1050 K and were entrained in a helium flow which carried them into the reaction zone. The PbO flow could be intercepted by an electrically opemted flag. The reaction occurs in the region where the two flows merge which is about 5 cm from each source. A disk shaped source of light is visible in this region. This chemihuninescence is detected by a monochromator-photomultiplier combination which was calibrated to provide absolute intensity measurements. The production of carbon atoms in the micrbwave discharge was monitored by the absorption of
With the reactants in the ground state the exoergicity of this reaction is about 7.2 eV. Light emission from iead atoms excited up to this limit was observed. The analysis of the chemiluminescence indicates that population inversion is obtained among several electronically excited states of lead atoms.
2. Experimental The experiments were performed in a flow apparatus shown schematically in fig. 1. The details of this apparatus have been described in a previous publication [2] z Alfked P. Sloan Foundation Fellow.
Fig. 1. Schematicof apparatus.The optics for light collection andabsorptionmeasurements are along the axis perpendicniar tothisdiagram. 43
Volume 59, number 1
CHEMICAL
PHYSICS
1 November 1978
LE’ITERS
2479 A carbon line (2p2 ‘So + 3s ‘Pi) produced by an electrodeless discharge lamp containing 5 X 10m2 torr of CO in 20 torr helium 131. The density of carbon was computed from the weighed deposit of carbon on a glass slide which intercepted the flow_ From this measurement the density of carbon atoms in the reaction zone was estimated to be about IOrr cme3. The density of PbO in the reaction zone obtained from the deposition experiment was about lOi2 cm-3_
o 6d 3Do I 6d3D02 Q
6d3+
The density of ground state lead produced by the chemical reaction was monitored by the absorption of Pb resonance light also produced by an electrodeless microwave discharge lamp containing solid Iead, iodine and about 15 torr argon [4] _ The density of le. d in the reaction zone under typical reaction condiions was about I-25 X 1010 cme3. The background pressure in the reaction chamber was about 1.4 torr which was due mostly to the helium carrier gas that flows into the reaction chamber. The tlow rate of helium into the chamber through the oven and the microwave discharge were 1700 SCCM and 2775 SCCM respectiveIy. In add&on to the desired reactants, C and PbO, the reaction zone contains several other species which complicate the experiment_ We have perfo_med a mumber of tests which indicate that the main source of ground state and excited state lead in the reaction zone is the primary reaction C + PbO --t CO + Pb. These tests are described in the appendix.
7d3C$o
-
09S3P0 I
06s3P0 1
Fig_ 2. Normalized steady state relative population distri-
bution.
state and are plotted as a function of the state enerm. The accuracy of the transition probability data used here was reported to be about 150%. The results in fig. 2 show that while the population of the excited states tends to decrease with energy, the distribution is non-Boltznrann and population inversion exists between several states. The inversions between the p state and the s and d states
3. Results and discussion
are worth noting. Radiative transitions from the p states to the ground states are electricdipole forbidden-Hence, the 8p 3D2 state has a longer radiative lifetime than the lower lying s and d states, and this
The intensities of 68 atomic lead lines were recorded in each _mn_Light from levels as high as 7.3 eV was observed_ The relative number of photons/s were calculated from the average of four independent runs
is a favorable condition for steady state inversion. Table 1 lists the inversion condition between the 8p 3& state and the lower d and s states. The populations in higher p states could not be calculated because the transition probabilities are not availabbfe.
which were consistent among each other within about 15%. The steady state popularion density of an excited state can be calculated from the emitted light intensity data if the Einstein A coefficient for that transition is known. in the case of lead, transition probabilities are not available for all the energy levels. The rehtive population densities using the available probability data [S] are shown in fig. 2. These densities are divided by the degeneracy factor for the
The measurements in this expei- _ f tow an order of magnitude estimate of th x ..Js~ _ ction for the C + PbO reaction. A steady state solution of the rate equation shows that the density of lead [Pb] , in the reaction region is given by [Pb]
= [C] lpbO] -o/TD -
Here ii is the average relative velocity between carbon atoms and PbO, CTis the average reaction cross section
CHEMICAL
Volume 59, number 1
PHYSICS
LETTERS
1 November
1978
Table 1 Population inversions observed in the experiment Transition
8p3Di
6d 3F: 6d 3Dp 6d 3D: 7s 3P: 8s 3P’z 7s ‘P’i’
-
Wavelength Olm)
1.78 1.54 l-70 266 3.07 3.99
between the two species and n, is the rate of diiappearance of lead atoms from the reaction region. In the estimation of the cross section the only unknown is 7~. It seems reasonable to assume that the prime mechanism for the disappearance of lead is diffusion. A simple calculation for our experimental conditions then yields a value Of lo3 s- ’ for 7~. ‘Lhe total reaction cross section is then in turn estimated to be 20 A2. From the light intensity measurements we further estimate that the cross sectron for the production of electronically excited states is about 5 A’.
was supported
upper level
lower level
==22
15 5.3 El3 0.97 2.9 12
intensities
Population ratio upper to lower level 9.7 2.6 1.7 26 12 27
of various
lead lines were in good
agree-
ment with those obtained in the experiments with heated lead oxide. Using a microwave discharge of CO as a source of carbon atoms complicates the analysis of the experimental results. In addition to carbon atoms the discharge flow contains other species which could react and directly or indirectly also produce excited Iead atoms. The foliowing are the processes which are most likely to be responsible for such unwanted re2c tionn (a) The reaction CO+PbO+COz+Pb
Acknowledgement This work
Life time (ns)
in part by grants from
NSF and AFSOR.
followed by excitation of Pb with the energetic species, M* present in the discharge. That is Pb+M*+Pb*+M.
Appendix: A discussion of reaction mechanisms In connection with the reacticn mechanism we shall first discuss the PbO source. It is known [6] that the vapor above solid PbO contains PbO molecules as well as an appreciable fraction of polymers (PbO), _ Two observations suggest that the reaction with the monomer is the main source of the observed chemiluminescence. (1) The semi-log plot of the intensity of the lead emission versus the reciprocal of the oven temperature was linear and the heat of sublimation calculated from tie slope was within 10% of the accepted value for ‘&e monomer IS]_ (2) PbO gas was prepared by passing N20 over heated lead and when this flow was merged with the flow from the CO/He discharge lead emission was observed- The relative
(b) Production of the ground state lead atoms and subsequent excitation by colJ.ision with the energetic species in the discharge. That is M*+PbOAM+Pb+O and M*+Pb+M+Pb*. (c) Reactions with excited species formed in the discharge: e.g., CO* + PbO + CO2 + Pb*. ‘ihe following observations indicate that above processes do not play a significant role in our experiment. Merging the two flows without the microwave discharge does not produce any observable reaction. 45
Volume 59. numixr 1
CaMcAL
PEiYSicS LETTERS
Neither chemiluminescence nor ground state lead is detected. This indicates that the first step of the suggested process (a) is negligiiIe under our experimental conditions. Wnen the flow from a pure He discharge was merged with the PbO molecules a small amount of emission mostly from the low lying excited levels of lead was observed_ Absorption of resonance radiation
by ground state lead atoms was barely detectable (less than one percent)_ The addition of one percent CO into the discharge increased the emitted light by about a factor of 100. J3igherexcited levels of Pb were seen in conformity with the exoergicity of reaction (I). The density of ground state lead increases by about an order of magnitude. The experiment with pure helium discharge was repeated with lead instead of PbO in the oven. While the lead emission from the He discharge remained almost the same as in the PbO experiments, introduction of CO caused only a factor of 2 increase in the emitted intensity, and only th= low lying excited lead atoms were seen. These observations show that wl+ie process (b) does occur, its role in our experiment is not significant.
Reaction type (c) cannot be ruled out as conchusively as one would like. The following observations, however, suggest rhat it is not a significant process in our experiment. The most likely sou_rceof CO* in reaction (c) is the metastable state of this molectzle such as CO {a 3KI) which has a sufficiently long lifetime to reach the reaction zone- However, the
46
I November 1978
resuits of Taylor and Setser 173 suggest that under our experimental conditions the mefastable molecules would be quenched by CO molecules in a period shorter than the flight time to the reaction zone. Furthermore, the lead chemiluminescence observed in our experiment was fcund to vary linearly with the measured carbon atom density- We were not equipped to monitor the triplet CO states, however, it would be a coincidence if the density of the CO* and that of the carbon atoms were linearly related. As noted earlier, the lead atoms produced in this system were excited to various levels in conformity with the exoergicity of reaction (1). This also would be an unexpected coincidence.
References Bf. Luria, DJ. Eckstrom and S-W. Benson, J. Chem. Phys- 64 (1976) 3103_ Dl UC. Stidharan, D.L. McFadden and P. Davidovits, J. Chem. Phys. 65 (1976) 5373. Dl E-Y-Y_ Lam, P. Gasper and A9. Wolf, J. Phys. Chem. 75 (1971) 445. 141 P-T. Cunningham and JX. Lintz, J. Opt. Sot. Am. 57 (1967) 1000. [51 CK Cork and W.R. Bozman, Natl. Bur. Std. Monograph 53 (1962). 161 J. Drowart, R_ Colin and G. Exsteen, Trans. Faraday Sot. 61 (1965) 1376. I71 G.W. Taylor and D.W. Setser, Chem. Phys. Letters 8 (1971) 51_
[II