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Mechanism of production of electronically excited BaO in the reaction of Ba vapor with O2
1 December
CHEMICAL PHYSICS LETTERS
Volume 17, number 3
1972
MECHANISM OF PRODUCTION OF ELECTRONICALLY EXCITED I330 IN THE.REACTlON OF Ba VAPOR WITH O,# R.H. OBENAUF;CJ. HSU a_ndII;R;PALhfEE_ Fuel Scierrce Sebtion, Department of Material Sciences, Tire Pennsylvania State University, University Park, Pennsylvania 16802, USA Received 7-7 September 1972 Revised manuscript received 10 November
1972
The reaction Ba + 02 wzs studied by a diffusion flame technique. Emission from the A - X system of BaO was found to be unaffected by the presence of atomic oxygen bu: greatly affected by addition of inert third body. Resuits are explained by the incorporation of the complex BaOz into the mechanism.
The reaction
of barium
meta!
vapor
with
molecular
oxygen has recently been investigated in several laboratories [l-3]. The reaction is accompanied by emission from the A(lZ) --f X(lZ) system of BaO. Sakurai et al. [3] have proposed that the mechanism of excitation of the BaO involves the formation of grd’und-state BaO via the reaction, Ba + O2 + BaO(X’Z) t 0, and the subsequent excitation of BaO by energy transfer from the recombination of atomic oxygen: BaO(XIx)
+ 20 + BaO(A’C) + 0, .
(1)
If the species Ba02 exists in the gas phase, an alternative route for BaO excitation is possible. BaO, has been postulated as a stable species [3,4] and has been observed in a low-temperature matrix [5], although high temperature studies (T> 1365°K) of the Ba-0, system [6] revealed no evidence for it. However, recent moiecular beam experiments [ I] have been interpreted
in terms
of a BaOZ collision
cornpiex
which perhaps may be regarded as a molecule of appreciable stabirity. As part
of a continuing
investigation
of the produc-
BaO(A1C) via the atomic oxygen mechanism [reaction (I)] is unlikely; however, the results can be explained by invoking the formation of BaO, as an intermediate. Experiments were conducted using a cylindrical Pyrex diffusion flame apparatus with a quartz window at one end. Barium metal (Fisher 99.4%) was vaporized from an alumina crucible heated to about 1100°K. 0, was introduced through a Pyrex nozzle located ap proximately 5 cm above the crucible, while argon could be added either through the nozzle or through a side arm at the end of the reactor. 0, and Ar flows were measured by monitoring the pressure drop from a known volume with V&dyne model DP7 differential pressure transducers. The total pressure, typically 0.2 torr, was measured through a separate sidearm using a McLeod gauge. Oxygen atoms were generated using a microwave discharge (2.5 GHz, Raytheon 125W, equipped with an Evenson cavity) through a 5% mixture of molecular oxygen in argon. Typical 0, flows were 1-2 X 10m7 mole/set. Although atom concentrations were never directly measured, the presence of atoms was confirmed
by the observation
of the air afterglow
in
species in low-pressure diffusion flames !7--9] j we are currently examining
the reactor upon addition Coectra were rccnrded L=_-___ ..__ - __--_-__
emission from flames of Ba vapor burning in 02, N20, and N02. Our observations imply that production of
F/6.3 plane-grating 0.75 m spectrograph using Kodak 103a-F and 103a-0 plates. Slit widths and exposure times were 100 ~1and 1-2 hours, respectively. Emission spectra were obtained covering wave-
tion
of electronicaI!y
# Work,supported
excited
by the National Science Foundation
of NO,. With the aid_ nf a Jarrp!!--b_sh
455
Votume 17, number 3
1 December 1972
CHEMICAL iPHYSICS LETTERS
Fig. 1. Observed emission spectrum from the rc3ction of molecu!ar oxygen with b&um vapor: (A), sty - xtr: system of&& (B), unidentified bands. fengths from 3000 to 6 100 ,& for the reaction of barium vapor with oxygen. In cases where the oxidant was molecular oxygen ‘and in cases where a mixture of molecular and atomic oxygen was used, identical spectra were recorded from the diffuse yellow-green flame. (A typicalspectrum over the range 4000 to 5200 tii is shown in fig, I.) More than 50 bands befonging te the A + X system of BaO were identified over the region 4350 to 6050 A in the present study:.
$ All of the bands reported in ref. (11 were observed. In addition, seven unidentified bands, degraded to the red and of lower intensity, were observed from 4063 to 4343 A. In experiments on the reaction of Ba with NzO and NOz, we have observed these seven bands and ten additional bands extending down to 3450 X. AI1 of the following bands have been observed with NzO as the oxidant, the first seven with oxygen and the frost ten with NO2: 4344, 4273,4197,4129,4064,4001,3942,3884,3831,3~76, 3723, 3674,3625, 3.580,3X35, 3492, and 3450 a. Wavelengths were measured in air to an uncertainty of I A. Although it is obvious from inspection of the plates that the 17 bands are related, we are unable to assign rhem to sequences or progressions involving any known states of
B&l or to any impurity. A number of the bands have aJso been seen by Kraus and Broida [lOI in the reaction of Ba with 02. The bands seem to extend to the red beyond .___ _ .
435U A but we cant?ot be certain beCaUse of the strength of the overlapping A -, X system in that region A rotational analysis would greatly help in the identification of these bands; their rotational structure is unresolved on our plates
..-
_&Ithou& a faint yeflow emission was visible throughout the reactor when 9a reacted with 0, in the absence of carrier gas, no emission could be recorded, even at exposure times greater thm four hours, unless an inert gas was introduced at flows in excess of the reactant flow. The argument for Ba02 as an intermediate relies mainly on this observation. (This phero menon did not occur in the reaction of Ba with N,O or NO,. UnIike the reaction with 02, both of these reactions are capable energeticaiiy of producing electron.icaHy excited BaO in a single elementary step.) The direct formation of BaO, Ba(‘S) + 0,(X3ZC)
4
BaO(XfE) + OC3P) ,
(2)
while allowed, is exothennic
by only about 15 kcal (taking Do(BaO) = 133 kcal [12]) and therefore cannot be the source of A-state excitation. Reaction (I) is 118 kcaI exothermic; however, if O-atoms were the exciting species, an enhancement of the emission or change jn the intensity distribution should have been observed when the.microwave discharge was
used. This was not the case. Sakurai et al. 137 in a For comparison, we injected fmely pulverirted BaClz into a fuel-lean 02-CH4 flame. The brilliant green flame showed strong emission from the A -+ X system and also from the Parkinson system [ 111 of BaO fS -f X). the latter extending to wavelengths shorter than 3000 A. None of the aforementioned unidentified bands was observed.
Volume
17, number
CHEMICAL
3
PHYSICS
similar experiment observed a decrease in BaO fluorescence when using an oxygen discharge. Analogous arguments may be presented for the direct (spin for,c D...?CA 1 P\ cm,..-.--..-A r+-*L:A>,-\ “,UUC,,, C^--&:-l”l,,l~L,“ll “I DiiU\.B-~5) 1lUlll &lUUllU blrilC barium and oxygen atoms. The reaction Ba2 f 0, + 2BaO may also be neglected as Ba2 has never been observed, no doubt because &(Ba,) is very small. (Consider the sequence: Be, (Do = 16 + ? kcal [12]), Mg2 (DO = 7.2 + ? kcal and [121>, f&,Sr2,
Ba2. The latter three dissociation are not known, but sureiy &@a2 j < 3 kcai.j Otir results may be explained by the following reaction sequence:
energies
Ba(‘S) + 0,(X3C) [BaO,]
t
+ BaO(X’C)
@a021 f M 2 BaOi
[BaO,]
,
(3)
+ 0(3p))
(4)
f M ,
(5)
BaOi+Ba+BaO’+BaO(X’Z),
(6)
where daggers indicate vibrational excitation and the asterisk represents electronic excitation. Reaction (5) accounts for our observation that an added inert gas
greatly enhances the emission. The BaOz complex, if not coIlisional!y stabilized, will dissociate into barium and oxygen or into an oxygen atom and a groundstate barium
oxide molecule.
MoIecuIar
oxygen
may
act as a stabilizing collision partner as we11 as argon*. If the complex is stabilized, it may be reactivated [reaction (-5)] or it may react further with a Ba atom [reaction (6)]. If one assumes the O-O bond in BaO, to be more or less peroxidic, i.e., to have a strength of about
50 kcal, the exothennicity
of reaction
(6)
becomes about 83 kcal. If all of this energy occasionally goes into excitation of one of the product BaO molecules, one should observe emission down to a short wavelength limit of 3450 A, which is beyond the limit (3942 A) that we observed with 0,. However, this does not include the probability that the Ba02 is excited. In a similar experiment, Parker and Broida 1133, operating at about one torr with helium carrier, studied the emission from the Ba-O2 reaction * Wewere, however, unable to observe this effect with 02, aa we could not sustain a flame of Ba burning in pure 02 for more than fifteen minutes at higher 02 flows, and thus a spectrum was unattainable. This phenomenon is apparently
caused by formation of solid BaO within the alumina crucible,
inhibiting
barium
vaporization.
LETTERS
1
December 1972
and observed bands of the Parkinson (B-X) system at wavelen&s down to 2900 A. According to our suggested mechanism, this requires that the BaO, in reaction (6j carry a range of excitations up to a ievei iess than 34 kcal below the dissociation energy of the BaO-0 bond. If the bond is weak, then little if any excitation is required; but if it is strong+-, high excitation is necessary (and expected). These considerations point up the need for definitive thermochemical data on gaseous Ba02, which seems likely to be playing 2 significant role in the chemiluminescent reaction between Ba and 0,. * Dr. R.N. Zare of Olumbia University, in a private communication, expresses rhe opinion that Boo2 may be very stable, with D(Ba-02) perhaps even exceeding D(Ba-0) in BaO. If this is so, then D(Ba-0) is also large and our pronneprl mrrhanicm rap_p.nt. vieI. RxO in the A qtnte u~tlez.~ --_ ._. -.__ ____ - _._____, , ---r-l- _.__--. _._“... as we indicated in reaction (6), the participating BaOz is highly excited vibrationally [or is stabilized in an excited electronic state of suitable energy, reaction (511. This suggests that energy transfer from excited Ba02 should also be considered as a possible means of exciting BaO. We rhink that reaction (6) is likely to be more efficient than energy transfer; however, the question quite clearly merits further study.
References [I] A. Schultz. H.W. Cruse and R.N. Zare, J. Chem. Phys. 57 (1972) 1354. [ 71 C. Batalli-Cosmovici and K.W. Michel, Chem. Phys. Letters 11 (1971) 24.5. [3] K. Snkurai, J.E. Johnson and H.P. Broida, J. Chem. Phys. 52 (1970) 1625. [4] hf. Ackerrnann and R.J. Thorn, in: Progress in ceramic science, ed. J.E. Burke (Pergamon Press, London, 1961) p. 42. [5] S. Abramowitz and N. Acquista, J. Res. Nat]. Bur. Std. 75A (1970) 23. [6] R.J. Newbury, G.W. Barton Jr. and A.W. Searcy, J. Chem. Phys. 48 (1968) 793. 171 H.B. Palmer and D.W. Naegeli, I. Mol. Spectry. 28 (1968) 417. [8] CM Pathak and H.B. Palmer, J. X101.Spectry. 33 (1970)
137. [9] H.B. Palmer, J. Chem. Phys. 54 (1971 j 3244. [IO] J. Kraus and H.P. Broida, unpublished results. [ 111 W.H. Parkinson, Proc. Phys. Sot. (London) 78 (1961) 705. [12] A.G. tiaydon, Dissociation energies (Chapman and Hall, London, 1968). [ 131 R.W. Parker and H.P. Broida. unpublished results
457
CHEMICAL PHYSICS LETTERS
Volume 17, number 3
1972
MECHANISM OF PRODUCTION OF ELECTRONICALLY EXCITED I330 IN THE.REACTlON OF Ba VAPOR WITH O,# R.H. OBENAUF;CJ. HSU a_ndII;R;PALhfEE_ Fuel Scierrce Sebtion, Department of Material Sciences, Tire Pennsylvania State University, University Park, Pennsylvania 16802, USA Received 7-7 September 1972 Revised manuscript received 10 November
1972
The reaction Ba + 02 wzs studied by a diffusion flame technique. Emission from the A - X system of BaO was found to be unaffected by the presence of atomic oxygen bu: greatly affected by addition of inert third body. Resuits are explained by the incorporation of the complex BaOz into the mechanism.
The reaction
of barium
meta!
vapor
with
molecular
oxygen has recently been investigated in several laboratories [l-3]. The reaction is accompanied by emission from the A(lZ) --f X(lZ) system of BaO. Sakurai et al. [3] have proposed that the mechanism of excitation of the BaO involves the formation of grd’und-state BaO via the reaction, Ba + O2 + BaO(X’Z) t 0, and the subsequent excitation of BaO by energy transfer from the recombination of atomic oxygen: BaO(XIx)
+ 20 + BaO(A’C) + 0, .
(1)
If the species Ba02 exists in the gas phase, an alternative route for BaO excitation is possible. BaO, has been postulated as a stable species [3,4] and has been observed in a low-temperature matrix [5], although high temperature studies (T> 1365°K) of the Ba-0, system [6] revealed no evidence for it. However, recent moiecular beam experiments [ I] have been interpreted
in terms
of a BaOZ collision
cornpiex
which perhaps may be regarded as a molecule of appreciable stabirity. As part
of a continuing
investigation
of the produc-
BaO(A1C) via the atomic oxygen mechanism [reaction (I)] is unlikely; however, the results can be explained by invoking the formation of BaO, as an intermediate. Experiments were conducted using a cylindrical Pyrex diffusion flame apparatus with a quartz window at one end. Barium metal (Fisher 99.4%) was vaporized from an alumina crucible heated to about 1100°K. 0, was introduced through a Pyrex nozzle located ap proximately 5 cm above the crucible, while argon could be added either through the nozzle or through a side arm at the end of the reactor. 0, and Ar flows were measured by monitoring the pressure drop from a known volume with V&dyne model DP7 differential pressure transducers. The total pressure, typically 0.2 torr, was measured through a separate sidearm using a McLeod gauge. Oxygen atoms were generated using a microwave discharge (2.5 GHz, Raytheon 125W, equipped with an Evenson cavity) through a 5% mixture of molecular oxygen in argon. Typical 0, flows were 1-2 X 10m7 mole/set. Although atom concentrations were never directly measured, the presence of atoms was confirmed
by the observation
of the air afterglow
in
species in low-pressure diffusion flames !7--9] j we are currently examining
the reactor upon addition Coectra were rccnrded L=_-___ ..__ - __--_-__
emission from flames of Ba vapor burning in 02, N20, and N02. Our observations imply that production of
F/6.3 plane-grating 0.75 m spectrograph using Kodak 103a-F and 103a-0 plates. Slit widths and exposure times were 100 ~1and 1-2 hours, respectively. Emission spectra were obtained covering wave-
tion
of electronicaI!y
# Work,supported
excited
by the National Science Foundation
of NO,. With the aid_ nf a Jarrp!!--b_sh
455
Votume 17, number 3
1 December 1972
CHEMICAL iPHYSICS LETTERS
Fig. 1. Observed emission spectrum from the rc3ction of molecu!ar oxygen with b&um vapor: (A), sty - xtr: system of&& (B), unidentified bands. fengths from 3000 to 6 100 ,& for the reaction of barium vapor with oxygen. In cases where the oxidant was molecular oxygen ‘and in cases where a mixture of molecular and atomic oxygen was used, identical spectra were recorded from the diffuse yellow-green flame. (A typicalspectrum over the range 4000 to 5200 tii is shown in fig, I.) More than 50 bands befonging te the A + X system of BaO were identified over the region 4350 to 6050 A in the present study:.
$ All of the bands reported in ref. (11 were observed. In addition, seven unidentified bands, degraded to the red and of lower intensity, were observed from 4063 to 4343 A. In experiments on the reaction of Ba with NzO and NOz, we have observed these seven bands and ten additional bands extending down to 3450 X. AI1 of the following bands have been observed with NzO as the oxidant, the first seven with oxygen and the frost ten with NO2: 4344, 4273,4197,4129,4064,4001,3942,3884,3831,3~76, 3723, 3674,3625, 3.580,3X35, 3492, and 3450 a. Wavelengths were measured in air to an uncertainty of I A. Although it is obvious from inspection of the plates that the 17 bands are related, we are unable to assign rhem to sequences or progressions involving any known states of
B&l or to any impurity. A number of the bands have aJso been seen by Kraus and Broida [lOI in the reaction of Ba with 02. The bands seem to extend to the red beyond .___ _ .
435U A but we cant?ot be certain beCaUse of the strength of the overlapping A -, X system in that region A rotational analysis would greatly help in the identification of these bands; their rotational structure is unresolved on our plates
..-
_&Ithou& a faint yeflow emission was visible throughout the reactor when 9a reacted with 0, in the absence of carrier gas, no emission could be recorded, even at exposure times greater thm four hours, unless an inert gas was introduced at flows in excess of the reactant flow. The argument for Ba02 as an intermediate relies mainly on this observation. (This phero menon did not occur in the reaction of Ba with N,O or NO,. UnIike the reaction with 02, both of these reactions are capable energeticaiiy of producing electron.icaHy excited BaO in a single elementary step.) The direct formation of BaO, Ba(‘S) + 0,(X3ZC)
4
BaO(XfE) + OC3P) ,
(2)
while allowed, is exothennic
by only about 15 kcal (taking Do(BaO) = 133 kcal [12]) and therefore cannot be the source of A-state excitation. Reaction (I) is 118 kcaI exothermic; however, if O-atoms were the exciting species, an enhancement of the emission or change jn the intensity distribution should have been observed when the.microwave discharge was
used. This was not the case. Sakurai et al. 137 in a For comparison, we injected fmely pulverirted BaClz into a fuel-lean 02-CH4 flame. The brilliant green flame showed strong emission from the A -+ X system and also from the Parkinson system [ 111 of BaO fS -f X). the latter extending to wavelengths shorter than 3000 A. None of the aforementioned unidentified bands was observed.
Volume
17, number
CHEMICAL
3
PHYSICS
similar experiment observed a decrease in BaO fluorescence when using an oxygen discharge. Analogous arguments may be presented for the direct (spin for,c D...?CA 1 P\ cm,..-.--..-A r+-*L:A>,-\ “,UUC,,, C^--&:-l”l,,l~L,“ll “I DiiU\.B-~5) 1lUlll &lUUllU blrilC barium and oxygen atoms. The reaction Ba2 f 0, + 2BaO may also be neglected as Ba2 has never been observed, no doubt because &(Ba,) is very small. (Consider the sequence: Be, (Do = 16 + ? kcal [12]), Mg2 (DO = 7.2 + ? kcal and [121>, f&,Sr2,
Ba2. The latter three dissociation are not known, but sureiy &@a2 j < 3 kcai.j Otir results may be explained by the following reaction sequence:
energies
Ba(‘S) + 0,(X3C) [BaO,]
t
+ BaO(X’C)
@a021 f M 2 BaOi
[BaO,]
,
(3)
+ 0(3p))
(4)
f M ,
(5)
BaOi+Ba+BaO’+BaO(X’Z),
(6)
where daggers indicate vibrational excitation and the asterisk represents electronic excitation. Reaction (5) accounts for our observation that an added inert gas
greatly enhances the emission. The BaOz complex, if not coIlisional!y stabilized, will dissociate into barium and oxygen or into an oxygen atom and a groundstate barium
oxide molecule.
MoIecuIar
oxygen
may
act as a stabilizing collision partner as we11 as argon*. If the complex is stabilized, it may be reactivated [reaction (-5)] or it may react further with a Ba atom [reaction (6)]. If one assumes the O-O bond in BaO, to be more or less peroxidic, i.e., to have a strength of about
50 kcal, the exothennicity
of reaction
(6)
becomes about 83 kcal. If all of this energy occasionally goes into excitation of one of the product BaO molecules, one should observe emission down to a short wavelength limit of 3450 A, which is beyond the limit (3942 A) that we observed with 0,. However, this does not include the probability that the Ba02 is excited. In a similar experiment, Parker and Broida 1133, operating at about one torr with helium carrier, studied the emission from the Ba-O2 reaction * Wewere, however, unable to observe this effect with 02, aa we could not sustain a flame of Ba burning in pure 02 for more than fifteen minutes at higher 02 flows, and thus a spectrum was unattainable. This phenomenon is apparently
caused by formation of solid BaO within the alumina crucible,
inhibiting
barium
vaporization.
LETTERS
1
December 1972
and observed bands of the Parkinson (B-X) system at wavelen&s down to 2900 A. According to our suggested mechanism, this requires that the BaO, in reaction (6j carry a range of excitations up to a ievei iess than 34 kcal below the dissociation energy of the BaO-0 bond. If the bond is weak, then little if any excitation is required; but if it is strong+-, high excitation is necessary (and expected). These considerations point up the need for definitive thermochemical data on gaseous Ba02, which seems likely to be playing 2 significant role in the chemiluminescent reaction between Ba and 0,. * Dr. R.N. Zare of Olumbia University, in a private communication, expresses rhe opinion that Boo2 may be very stable, with D(Ba-02) perhaps even exceeding D(Ba-0) in BaO. If this is so, then D(Ba-0) is also large and our pronneprl mrrhanicm rap_p.nt. vieI. RxO in the A qtnte u~tlez.~ --_ ._. -.__ ____ - _._____, , ---r-l- _.__--. _._“... as we indicated in reaction (6), the participating BaOz is highly excited vibrationally [or is stabilized in an excited electronic state of suitable energy, reaction (511. This suggests that energy transfer from excited Ba02 should also be considered as a possible means of exciting BaO. We rhink that reaction (6) is likely to be more efficient than energy transfer; however, the question quite clearly merits further study.
References [I] A. Schultz. H.W. Cruse and R.N. Zare, J. Chem. Phys. 57 (1972) 1354. [ 71 C. Batalli-Cosmovici and K.W. Michel, Chem. Phys. Letters 11 (1971) 24.5. [3] K. Snkurai, J.E. Johnson and H.P. Broida, J. Chem. Phys. 52 (1970) 1625. [4] hf. Ackerrnann and R.J. Thorn, in: Progress in ceramic science, ed. J.E. Burke (Pergamon Press, London, 1961) p. 42. [5] S. Abramowitz and N. Acquista, J. Res. Nat]. Bur. Std. 75A (1970) 23. [6] R.J. Newbury, G.W. Barton Jr. and A.W. Searcy, J. Chem. Phys. 48 (1968) 793. 171 H.B. Palmer and D.W. Naegeli, I. Mol. Spectry. 28 (1968) 417. [8] CM Pathak and H.B. Palmer, J. X101.Spectry. 33 (1970)
137. [9] H.B. Palmer, J. Chem. Phys. 54 (1971 j 3244. [IO] J. Kraus and H.P. Broida, unpublished results. [ 111 W.H. Parkinson, Proc. Phys. Sot. (London) 78 (1961) 705. [12] A.G. tiaydon, Dissociation energies (Chapman and Hall, London, 1968). [ 131 R.W. Parker and H.P. Broida. unpublished results
457