- Email: [email protected]
Efficient chemical production of metastable alkaline earth atoms
Volume 43. number 1
CHEMICAL PHYSICS LETTERS
EFFICIENT CHEMICAL PRODUCTION
OF METASTABLE
ALKALINE
1 October 1976
EARTH ATOMS *
D.J. BENARD, W.D. SLAFER and PM. LEE Physics Department and Quantum Jnstitute. L’niuersity of California. Santa Barbara, Califomia 93106. USA Received 3 hfay 1976
The production of metastablc Mg; Ca and Sr atoms ~3s found to occur in fast flow low pressure wtdyzd NzO-CO flames. Despite 3n efficient production mechanism, the final density of cxcitcd atoms was limited by quenching IO less than 2% of the initial metal atom density.
1. Introduction The reaction of N20 with CO to yield N2 and CO2 as products is highly cxothermic but does not proceed rapidly at low temperatures [ 11 except on surfaces or unless catalyzed by alkali metal atoms as initially postulated by Fenimore [2) Na + N,O --*NaO + N, + 24.5 kcal, NaO+CO+Na+
+61.9 kcal.
[3 ] of this chain reaction scheme was later obtained in a chemical laser application where the Na catalyzed combustion of N20 and CO was used to excite the 10.8~ N20 laser [4] by energy transfer. Due to the weakness of the alkali metal oxide bond energy, there is little exothermicity and only weak chemiluminescence produced in the initial (oxidation) step of the chain reaction. The overall reaction however, is accompanied by intense Na Dline emission that-is effciently excited in the later step. ‘Ihe bond energies of the alkaline earth oxides are conversely very large and the oxidations of Mg, Ca, Sr or Ba by N20 are all highiy exothermic reactions which have been noted for their chemiluminescent efficiency (5.6). These reactions have received special interest recently as potential visible or near infrared chemical lasers. Although no visible or electronic transition lasers Confirmation
*Work zupportcd in part by the Air Force Oflice of Scicntific Research Grant No. AFOSR-76-2954.
have yet been demonstrated, it has been shown that energy can be transferred from the reaction products to yield CO, lasers [7]. We have conducted experiments on alkafine earth atom catalyzed combustion of N20 and CO in a low pressure fast flow reactor with special attention to possible electronic excitations of the metal atoms, as seen in the Na catalyzed experiments. In general one wouId expect no such production of excited atoms resulting from the reduction of an alkaline earth metal oxide by CO due to a lack of exothe,micity. Considering the reaction MgO + CO + CO, + Mg+ + 36 kcal, one finds that the available energy falls far short of the 63 kcal/mole required to populate the lowest excited states of atomic Mg. These states are a triplet manifold which decays to the ground state by a forbidden (jPt --, *So) transition at 457.1 nm with a radiative lifetime [8] of 4.5 ms. The two remaining components of the triplet (3Po and 3P2) act as reservoir states which are equilibrated to the 3Pt state but show no appreciable radiative decay, thus the emission is representative of only one third of the total metastable population 191. Such states are very sensitive to quenching which may occur either by passive species (CO) which convert electronic energy into diatomic vibration or by active species (N20) which consume the excited species in highly exothermic chemicaf reactions. 69
Volume 43. number
1
CHEMICAL
2 Experimental 7he alkali metal atoms were produced by vaporization from the solid in a resistively heated alumina crucible [lo]. He carrier gas was employed to flush the crucible and sweep the metal vapors ‘through a heated s;a;nless steel nozzle into the throat of a radial gas in*Gttor of !/2 inch diameter, where premixed N20 and CO were admitted through high velocity jets. The reaction zone was situated at the center of a 2” six-way ABC cross and baffles were used to restrain the flow to a limited cross sectional area. The exhaust products were pumped off by means of a 3 10 cfm Kinney vacuum pump.
AU gases were admitted
1 October
PHYSICS LETTERS
to the reactor
through calibrated flowmeters and mean flow velocities were calculated from the measured flows and the measured pressure at the center of the reactor by USC of a Mctiod gauge. The metal atom flow was obtained from the mass evaporation rate which was found to be quite uniform and reproducible with use of the heated crucible and nozzle upon which there was negligible condensation of the metal. By visual observation of the Mg + N,O flame in the absence of CO one could determine from the spatial profile of the chemiluminescence that mixing was essentially complete within a distance of =2cm from the radial injector. At a typical flow velocity of =2OO m/s the resultant mixing time would be on the order of 100 Jls. Emission spectra were taken with a 0.4 meter McRrcrson 218 monochromator with slit widths of 100~ and a 1200 lines/mn grating blazed at 500 run. Detection was accomplished by USCof an RCA 6199 photomultiplior tube. The spectra shown in figs. 1a and 1b are not corrected for instrumental response but are essentially flat over the wavelength region shown. Calibrated measurements of the Mg 457.1 nm emission were made by comparison with a standard lamp.
1976
the N,O supply was lessth8n.optimum for maximum emission from the Mg + NtO flame the addition of CO. caused the.emission to change from white to deep blue. There was no evidence of any direct reaction of Mg with CO in the absence of N20. In the initial segments of the tests, Na impurities gave the characteristic D-line emission in the mixed N,O-CO flow until the Mg atom flow was established which eliminated the Na impurity. Spectra of the chemiluminescence produced in the wavelength range 440-540 nm are shown in figs. 1a and I b which detail the gross changes that occur upon the addition of CO to the Mg + N,O flame. The white emission which arises from the oxidation of Mg in N20 is due to a broad, featureless, unresolvable continuum that extends from ~650 to 35Orun. Fig. la also shows the MgO (B 1Z+ +XtC+,Au=O)bandnearSOOnm which is weakly excited in the Mg + N,O flame. The prominent spectral features in fig. lb which account for the green and blue emissions observed upon addition 1
1
I
(0)
M-
I
1
3. Results The addition of CO to the Mg + N,O flame resulted in a dramatic and visual change in the observed emission. With M excess supply of N,O the flame appeared green to the eye whereas without CO present in the r&ctor’the emission was white. On the other hand, if 7n
WAVELENGTH
- nm
Fig. 1. Emission spectra obtained atoms entrained
from the combustion of Mg in a He carrier flow upon mixing with (a) an
equivalent tlow of N20 and (b) a premixed flow of N20 and Co in a 1 : 2 ratio. Total press&
g 0.5 ton.
Volume 43, number 1
1 October 1976
CHEMICALPHYSICS LETL-ERS
of CO are respectively the MgO bands and the forbidden Mg atom (3Ft + *So) transition at 457.1 nm. The strong excitation of the MgO bands in the Mgcatalyzed N,OCO flame also makes resolvable the Au = 21 bands as we!!. The continuum emission was also suppressed upon the addition of CO. Upon optimization of the Mg atom emission it was found that the intensity of 457.1 nm radiation was consistent titb a volume concentration of 5 X !O**/cm3 in the flame. This result was obtained when the mass evaporation rate was set to 10-3g/s and the remaining t!ows were chosen to yield a stoichiometric ratio N,O/CO/Mg = 1/2/lwith the He carrier flow adjusted to yield a total pressure near 0.5 torr. One could visually observe the formation of a blue fringe on the trailing edge of the green flame. Observa!ion of the flame through interfercncc filters disclosed that the metastable atom emission was uniformly distributed throughout the flame. From the width of the fringe (= 1 mm) one can conclude that the metastables are strongly quenched and a maximum quenching lifetime of Z=5~ is estimated based on the known flow velocity. Therefore the rate of metastable production within the flame is at least 10!8/cm3 s. This value integrated over the volume of the flame compares favorably with the total inlet flow of Mg atoms. Thus although the conversion efficiency (rate of production of mctastables divided by the rate of Mg atoms consumed) appears to be = 1OO%, the steady state concentration of Mg a:oms in the 3P manifold is limited to about 1.5% of al! Mg bearing species in the flame. The solid products generated in the reaction zone were collected upstream on the surface of a cone in the pumpoff line. The composition of the product varied from pure MgO to a uniform mixture of Mg and MgO powder as the mole fraction of CO was varied from zero to increasingly higher values. A genera! lengthening of the flame similar to that noted in a Na atom catalyzed chain reaction was observed upon addition of N30 and CO broth in excess of the Mg supplied by evaporation. Similar results were also obtained in Ca and Sr catalyzed flames upon addition of CO. The white (continuum) emtssion characteristic of the oxidation in N20 was replaced by a red or orange chemiluminescence depending on the N,O/CO stoichiometry. The red emissions were the equivalent 3Pt + lSn transitions in Ca and Sr at 657.3 nm and 6893 nm
respectively. The production of Ca and Sr metastabtes was less efficient than in the Mg catalyzed ff antes, but due to the shortened radiative lifetimes of the heavier a!krdine eart!is the atomic emission was of equal or greater intensity. The orange emission were identified in each case as belonging to the band spectra of the meta! oxide. In contradistinction to the cases of the Mg. Ca and Sr catalysts, there was no equivalent effect upon the addition of CO’ro a Ba + N,O flame. Prior to the addition of CO, the flame emission was resolvable into a complex series of overlapping BaO (A t C+ + X t 9) bands which upon CO addition showed only an overall a 50% quenching (CO/N20 = 2) and a genera! sharp erring of the band heads, indicative of a decreased vibrational-rotational temperature. No B~I(~P, -+ tSO) emission was observed and there was no other evidence suggestive of any change in chemistry upon the addition of CO to the reactor.
4. Discussion In order to actount for the strong atomic emission at 457 I nm, an efficient production mechanism is required; which therefore cannot involve endothermic reactions. Hence if the basic chemistry involves a two step chain reaction scheme similar to Fenimore’s ana!. ysis one must of necessity postulate the existence of some active species which can carry over a portion of the energy released in the initial oxidation to augment the reducing step which lacks sufficient exothermfcity. To efficiently carry out this function the active species must be relatively long lived and be efficient!y produced in the reaction of ground state Mg with N20. Spectroscopically the evidencesuggests two possibi!ities which are the MgO (B t Z’) state and the species which gives rise to the continuum emission. The former can be ruled out on the basis of its short (22 ns) radiative lifetime [I I], whereas the later alternative would be consistent with the observed quenching of the continuum upon the addition of CO. The genera! smoothness of the continuum implies that the active species is polyatomic, therefore we propose a two step reaction mecharksm involving the initial formation of a long-lived polyatomic excited complex which either dissociates to ground state MgO and N2, producirig the continuum
emission,
or is reduced by CO 71
Volume 43. number ! to yield N,, CO, and mctastable ing to the scheme Mg+N,O+
Mg: ON; +MgO+N,
CHEMICAL PHYSICS LElTERS
Mg products, accord+hv,
Mg : ON; + CO + N, + CO, + Mg(3P). The overall reaction does not conserve spin, therefore we further propose that the polyatomic complex Mg : ONi is formed in a triplet state in violation of spin conservation in ‘the initial reaction. The radiative dissociation of the complex then involves a forbidden triplet-smglet transition which accounts for its lifetime and also for the production of triplet products in the latter steps of the ch&il reaction. We attribute our inability to make the Ba metastables and the less efficient production of Ca and Sr metastables to the generally decreased validity of spin conservation in heavier molecules and hence a shortened complex lifctimc or the non-existence of a reaction compiex altogether. We do note, however, that Jonah and Zare [ 121 as well as Wren and Menzinger [ 131 have observed continuum emission in molecular beam chemiluminescence experiments involving the oxidation of Ba in Clz. They also find related evidence of a long-lived polyatomic comp!ex that stabilizes itself by a radiative three body rccombination. It is also interesting that our results implX,&at the N,O acts effectively as an 0(3P) donor rather than O(lD) as expected from correlation arguments [ 141. This assumption would however explain thE reluctance of CO to be oxidized by N,O directly. At this time we have not determined the mechanism which is responsible for the strong and seiective excitation of the MgO bands that occurs in the reaction chain. We can rule out energy transfer from the metastable to the metal oxide at least in the Ca and Sr catalyzed flames since such reactions would be endothermic. The production of rnetastables in the Mg
72
1 October 1976
catalyzed flame is efficient enough, however, to allow substantial subsequent oxidation of the excited atoms by N,O. Treating the oxidant as an O(3P) donor one finds that in a reaction with Ms(~P) the product correlates [I 51 at lowest energy lo MgO (B * C’), hence the process may account for the observed selective excitation of the MgO band system. To positively identify all mechanisms will however require further measurements and experiments which are now in progress. References 1 I ] C.E.H. Bawn, Trans. Faraday Sot. 3 1 (1936) 46 1. 121 C.P. Fenimore and R. Kelso. J. Am. Chcm. Sot. 72 (1950) 5045. 131 R.E. Walker et al.. IEEE J. Quantum Electron QE-9 (1973) 197. [4J D.J. Benard, R.C. Bcnzon and R.E. Walker, Appt. Phys. Lcttcrs 25 (1973) 82. (5 ] G.A. Capclle. H.P. Broida and.R.W. Field, J. Chcm. Phys. 62 (1975) 3131. 161 C.R. Jones and H.P. Broida, J. Chcm. Phys. 59 (1973) 6677. 171 D.J. Benard, Chcm. Phys. Letters 35 (1975) 167. 181 P.9. Furcinitti. J.J. Wright and L.C. Balling. Phyr Rev. Al2 (1975) 12. J.J. Wright. J.F. Dawson and L.C. Balling. Phys. Rev. A9 (1974) 83. J.B. West. R.S. Bradford, J.D. Eversole and C.R. Jones, Rev. Sci. Instr. 46 (1975) 164. P. Domaille. MgO (B ’ x* + X ’ x*) Radiative Lifetime Measurement by Pulxd Dye Laser Induced Fluorescence, UCSB Quantum Institute. private communication. C.D. Jonah and R.N. Zare. Chem. Phys. Letters 20 (1973) 471. D.J. Wren and M. Menzinger, Chcm. Phys. Letters 27 (1974) 572. D. Husain and J.R. Wiescnfcld, J. Chem. Phys. 62 (1975) 2010. J. Schamps and II. Lefcbvrc-Brian, J. Chcm. Phys. 61 (1974) 1652.
CHEMICAL PHYSICS LETTERS
EFFICIENT CHEMICAL PRODUCTION
OF METASTABLE
ALKALINE
1 October 1976
EARTH ATOMS *
D.J. BENARD, W.D. SLAFER and PM. LEE Physics Department and Quantum Jnstitute. L’niuersity of California. Santa Barbara, Califomia 93106. USA Received 3 hfay 1976
The production of metastablc Mg; Ca and Sr atoms ~3s found to occur in fast flow low pressure wtdyzd NzO-CO flames. Despite 3n efficient production mechanism, the final density of cxcitcd atoms was limited by quenching IO less than 2% of the initial metal atom density.
1. Introduction The reaction of N20 with CO to yield N2 and CO2 as products is highly cxothermic but does not proceed rapidly at low temperatures [ 11 except on surfaces or unless catalyzed by alkali metal atoms as initially postulated by Fenimore [2) Na + N,O --*NaO + N, + 24.5 kcal, NaO+CO+Na+
+61.9 kcal.
[3 ] of this chain reaction scheme was later obtained in a chemical laser application where the Na catalyzed combustion of N20 and CO was used to excite the 10.8~ N20 laser [4] by energy transfer. Due to the weakness of the alkali metal oxide bond energy, there is little exothermicity and only weak chemiluminescence produced in the initial (oxidation) step of the chain reaction. The overall reaction however, is accompanied by intense Na Dline emission that-is effciently excited in the later step. ‘Ihe bond energies of the alkaline earth oxides are conversely very large and the oxidations of Mg, Ca, Sr or Ba by N20 are all highiy exothermic reactions which have been noted for their chemiluminescent efficiency (5.6). These reactions have received special interest recently as potential visible or near infrared chemical lasers. Although no visible or electronic transition lasers Confirmation
*Work zupportcd in part by the Air Force Oflice of Scicntific Research Grant No. AFOSR-76-2954.
have yet been demonstrated, it has been shown that energy can be transferred from the reaction products to yield CO, lasers [7]. We have conducted experiments on alkafine earth atom catalyzed combustion of N20 and CO in a low pressure fast flow reactor with special attention to possible electronic excitations of the metal atoms, as seen in the Na catalyzed experiments. In general one wouId expect no such production of excited atoms resulting from the reduction of an alkaline earth metal oxide by CO due to a lack of exothe,micity. Considering the reaction MgO + CO + CO, + Mg+ + 36 kcal, one finds that the available energy falls far short of the 63 kcal/mole required to populate the lowest excited states of atomic Mg. These states are a triplet manifold which decays to the ground state by a forbidden (jPt --, *So) transition at 457.1 nm with a radiative lifetime [8] of 4.5 ms. The two remaining components of the triplet (3Po and 3P2) act as reservoir states which are equilibrated to the 3Pt state but show no appreciable radiative decay, thus the emission is representative of only one third of the total metastable population 191. Such states are very sensitive to quenching which may occur either by passive species (CO) which convert electronic energy into diatomic vibration or by active species (N20) which consume the excited species in highly exothermic chemicaf reactions. 69
Volume 43. number
1
CHEMICAL
2 Experimental 7he alkali metal atoms were produced by vaporization from the solid in a resistively heated alumina crucible [lo]. He carrier gas was employed to flush the crucible and sweep the metal vapors ‘through a heated s;a;nless steel nozzle into the throat of a radial gas in*Gttor of !/2 inch diameter, where premixed N20 and CO were admitted through high velocity jets. The reaction zone was situated at the center of a 2” six-way ABC cross and baffles were used to restrain the flow to a limited cross sectional area. The exhaust products were pumped off by means of a 3 10 cfm Kinney vacuum pump.
AU gases were admitted
1 October
PHYSICS LETTERS
to the reactor
through calibrated flowmeters and mean flow velocities were calculated from the measured flows and the measured pressure at the center of the reactor by USC of a Mctiod gauge. The metal atom flow was obtained from the mass evaporation rate which was found to be quite uniform and reproducible with use of the heated crucible and nozzle upon which there was negligible condensation of the metal. By visual observation of the Mg + N,O flame in the absence of CO one could determine from the spatial profile of the chemiluminescence that mixing was essentially complete within a distance of =2cm from the radial injector. At a typical flow velocity of =2OO m/s the resultant mixing time would be on the order of 100 Jls. Emission spectra were taken with a 0.4 meter McRrcrson 218 monochromator with slit widths of 100~ and a 1200 lines/mn grating blazed at 500 run. Detection was accomplished by USCof an RCA 6199 photomultiplior tube. The spectra shown in figs. 1a and 1b are not corrected for instrumental response but are essentially flat over the wavelength region shown. Calibrated measurements of the Mg 457.1 nm emission were made by comparison with a standard lamp.
1976
the N,O supply was lessth8n.optimum for maximum emission from the Mg + NtO flame the addition of CO. caused the.emission to change from white to deep blue. There was no evidence of any direct reaction of Mg with CO in the absence of N20. In the initial segments of the tests, Na impurities gave the characteristic D-line emission in the mixed N,O-CO flow until the Mg atom flow was established which eliminated the Na impurity. Spectra of the chemiluminescence produced in the wavelength range 440-540 nm are shown in figs. 1a and I b which detail the gross changes that occur upon the addition of CO to the Mg + N,O flame. The white emission which arises from the oxidation of Mg in N20 is due to a broad, featureless, unresolvable continuum that extends from ~650 to 35Orun. Fig. la also shows the MgO (B 1Z+ +XtC+,Au=O)bandnearSOOnm which is weakly excited in the Mg + N,O flame. The prominent spectral features in fig. lb which account for the green and blue emissions observed upon addition 1
1
I
(0)
M-
I
1
3. Results The addition of CO to the Mg + N,O flame resulted in a dramatic and visual change in the observed emission. With M excess supply of N,O the flame appeared green to the eye whereas without CO present in the r&ctor’the emission was white. On the other hand, if 7n
WAVELENGTH
- nm
Fig. 1. Emission spectra obtained atoms entrained
from the combustion of Mg in a He carrier flow upon mixing with (a) an
equivalent tlow of N20 and (b) a premixed flow of N20 and Co in a 1 : 2 ratio. Total press&
g 0.5 ton.
Volume 43, number 1
1 October 1976
CHEMICALPHYSICS LETL-ERS
of CO are respectively the MgO bands and the forbidden Mg atom (3Ft + *So) transition at 457.1 nm. The strong excitation of the MgO bands in the Mgcatalyzed N,OCO flame also makes resolvable the Au = 21 bands as we!!. The continuum emission was also suppressed upon the addition of CO. Upon optimization of the Mg atom emission it was found that the intensity of 457.1 nm radiation was consistent titb a volume concentration of 5 X !O**/cm3 in the flame. This result was obtained when the mass evaporation rate was set to 10-3g/s and the remaining t!ows were chosen to yield a stoichiometric ratio N,O/CO/Mg = 1/2/lwith the He carrier flow adjusted to yield a total pressure near 0.5 torr. One could visually observe the formation of a blue fringe on the trailing edge of the green flame. Observa!ion of the flame through interfercncc filters disclosed that the metastable atom emission was uniformly distributed throughout the flame. From the width of the fringe (= 1 mm) one can conclude that the metastables are strongly quenched and a maximum quenching lifetime of Z=5~ is estimated based on the known flow velocity. Therefore the rate of metastable production within the flame is at least 10!8/cm3 s. This value integrated over the volume of the flame compares favorably with the total inlet flow of Mg atoms. Thus although the conversion efficiency (rate of production of mctastables divided by the rate of Mg atoms consumed) appears to be = 1OO%, the steady state concentration of Mg a:oms in the 3P manifold is limited to about 1.5% of al! Mg bearing species in the flame. The solid products generated in the reaction zone were collected upstream on the surface of a cone in the pumpoff line. The composition of the product varied from pure MgO to a uniform mixture of Mg and MgO powder as the mole fraction of CO was varied from zero to increasingly higher values. A genera! lengthening of the flame similar to that noted in a Na atom catalyzed chain reaction was observed upon addition of N30 and CO broth in excess of the Mg supplied by evaporation. Similar results were also obtained in Ca and Sr catalyzed flames upon addition of CO. The white (continuum) emtssion characteristic of the oxidation in N20 was replaced by a red or orange chemiluminescence depending on the N,O/CO stoichiometry. The red emissions were the equivalent 3Pt + lSn transitions in Ca and Sr at 657.3 nm and 6893 nm
respectively. The production of Ca and Sr metastabtes was less efficient than in the Mg catalyzed ff antes, but due to the shortened radiative lifetimes of the heavier a!krdine eart!is the atomic emission was of equal or greater intensity. The orange emission were identified in each case as belonging to the band spectra of the meta! oxide. In contradistinction to the cases of the Mg. Ca and Sr catalysts, there was no equivalent effect upon the addition of CO’ro a Ba + N,O flame. Prior to the addition of CO, the flame emission was resolvable into a complex series of overlapping BaO (A t C+ + X t 9) bands which upon CO addition showed only an overall a 50% quenching (CO/N20 = 2) and a genera! sharp erring of the band heads, indicative of a decreased vibrational-rotational temperature. No B~I(~P, -+ tSO) emission was observed and there was no other evidence suggestive of any change in chemistry upon the addition of CO to the reactor.
4. Discussion In order to actount for the strong atomic emission at 457 I nm, an efficient production mechanism is required; which therefore cannot involve endothermic reactions. Hence if the basic chemistry involves a two step chain reaction scheme similar to Fenimore’s ana!. ysis one must of necessity postulate the existence of some active species which can carry over a portion of the energy released in the initial oxidation to augment the reducing step which lacks sufficient exothermfcity. To efficiently carry out this function the active species must be relatively long lived and be efficient!y produced in the reaction of ground state Mg with N20. Spectroscopically the evidencesuggests two possibi!ities which are the MgO (B t Z’) state and the species which gives rise to the continuum emission. The former can be ruled out on the basis of its short (22 ns) radiative lifetime [I I], whereas the later alternative would be consistent with the observed quenching of the continuum upon the addition of CO. The genera! smoothness of the continuum implies that the active species is polyatomic, therefore we propose a two step reaction mecharksm involving the initial formation of a long-lived polyatomic excited complex which either dissociates to ground state MgO and N2, producirig the continuum
emission,
or is reduced by CO 71
Volume 43. number ! to yield N,, CO, and mctastable ing to the scheme Mg+N,O+
Mg: ON; +MgO+N,
CHEMICAL PHYSICS LElTERS
Mg products, accord+hv,
Mg : ON; + CO + N, + CO, + Mg(3P). The overall reaction does not conserve spin, therefore we further propose that the polyatomic complex Mg : ONi is formed in a triplet state in violation of spin conservation in ‘the initial reaction. The radiative dissociation of the complex then involves a forbidden triplet-smglet transition which accounts for its lifetime and also for the production of triplet products in the latter steps of the ch&il reaction. We attribute our inability to make the Ba metastables and the less efficient production of Ca and Sr metastables to the generally decreased validity of spin conservation in heavier molecules and hence a shortened complex lifctimc or the non-existence of a reaction compiex altogether. We do note, however, that Jonah and Zare [ 121 as well as Wren and Menzinger [ 131 have observed continuum emission in molecular beam chemiluminescence experiments involving the oxidation of Ba in Clz. They also find related evidence of a long-lived polyatomic comp!ex that stabilizes itself by a radiative three body rccombination. It is also interesting that our results implX,&at the N,O acts effectively as an 0(3P) donor rather than O(lD) as expected from correlation arguments [ 141. This assumption would however explain thE reluctance of CO to be oxidized by N,O directly. At this time we have not determined the mechanism which is responsible for the strong and seiective excitation of the MgO bands that occurs in the reaction chain. We can rule out energy transfer from the metastable to the metal oxide at least in the Ca and Sr catalyzed flames since such reactions would be endothermic. The production of rnetastables in the Mg
72
1 October 1976
catalyzed flame is efficient enough, however, to allow substantial subsequent oxidation of the excited atoms by N,O. Treating the oxidant as an O(3P) donor one finds that in a reaction with Ms(~P) the product correlates [I 51 at lowest energy lo MgO (B * C’), hence the process may account for the observed selective excitation of the MgO band system. To positively identify all mechanisms will however require further measurements and experiments which are now in progress. References 1 I ] C.E.H. Bawn, Trans. Faraday Sot. 3 1 (1936) 46 1. 121 C.P. Fenimore and R. Kelso. J. Am. Chcm. Sot. 72 (1950) 5045. 131 R.E. Walker et al.. IEEE J. Quantum Electron QE-9 (1973) 197. [4J D.J. Benard, R.C. Bcnzon and R.E. Walker, Appt. Phys. Lcttcrs 25 (1973) 82. (5 ] G.A. Capclle. H.P. Broida and.R.W. Field, J. Chcm. Phys. 62 (1975) 3131. 161 C.R. Jones and H.P. Broida, J. Chcm. Phys. 59 (1973) 6677. 171 D.J. Benard, Chcm. Phys. Letters 35 (1975) 167. 181 P.9. Furcinitti. J.J. Wright and L.C. Balling. Phyr Rev. Al2 (1975) 12. J.J. Wright. J.F. Dawson and L.C. Balling. Phys. Rev. A9 (1974) 83. J.B. West. R.S. Bradford, J.D. Eversole and C.R. Jones, Rev. Sci. Instr. 46 (1975) 164. P. Domaille. MgO (B ’ x* + X ’ x*) Radiative Lifetime Measurement by Pulxd Dye Laser Induced Fluorescence, UCSB Quantum Institute. private communication. C.D. Jonah and R.N. Zare. Chem. Phys. Letters 20 (1973) 471. D.J. Wren and M. Menzinger, Chcm. Phys. Letters 27 (1974) 572. D. Husain and J.R. Wiescnfcld, J. Chem. Phys. 62 (1975) 2010. J. Schamps and II. Lefcbvrc-Brian, J. Chcm. Phys. 61 (1974) 1652.