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Quenching of the Ba + Cl2 chemiluminescence: Estimate of BaCl*2 radiative lifetime
Volume 20, number 5
CHEMICAL PHYSICS LETTERS
1 July 1973
QUENCHING OF THE Ba + Cl2 CHEMILUMINESCENCE: ESTIMATE OF BaCl; RADIATIVE LIFETLME David J. WREN and Michael MENZINGER Loslt Miiler Laboratories. CJni&ity of Torotrto, Toronto hf5S JAI, Oxtario. Canada Received 26 March 1973
The chemiluminescence produced by the Ba + Cl* reaction was recorded as a function of He and Nz pressure. A modified Stern-Volmer treatment of competitive electronic quenching ol‘BaCI* and BaClz emission yielded upper limits to the half pressures p 1,2(He) < 9.0 r 3 mtorr and pllZ(N2) < 1.1 f 0.2 mtorr for quenching of BaCI; by: helium and nitrogen, respectively. A lower limit of the BaClr radiative lifetime is plxed at :R > 100 cr.
When two atoms collide, the preferred processes are elastic and inelastic scattering. The probability of radiative stabilization (3) of the collision complex AB* to form a stable AB molecule (radiative twobody recombination RTBR) is usually very low, due to the fact that the collision complex dissociates (2) before it has had a chance to emit a photon: % A+B-+AB*,
(1)
k-, AB*
--, A+B,
kR AB* + AEli-Itv. In other words the complex lifetime rD = k:l i; usually much shorter than the radiative lifetime rR = kR1. The recent suggestion [l] that RTBR be a fairly prominent channel in the cherniluminescent M(Ca, Sr, Ba) -f X,(F,, Cl,, Br2) reactions [2-51 seems to be at variance with this generally accepted expeciation [6]. It implies that the decomposition lifetime of the collision complex MX; be unusually long and/or that the radiative lifetime rR = kg1 be very short. Since rR represents the time scale of mechanism (I-3) it is of great interest to obtein an estimate of this quantity. Once this infomiation is given one can ascertain whether a complex lifetime of the order of
TV, which is implied by the substantial rate? of (l-3), is in agreement with our understanding of unimolecular lifetimes of small molecules [7, S] , or whether concepts have to be invoked [3] that lie outside the customary adiabatic and equilibrium formulations (e.g., RRKM). The goal of the present study of the Ba t Cl,system, which may be regarded as a protctype for the other M f X2 reactions, is to obtain an estimate of rR_ The Stern-Volmer treatment [9] of quenching data yields the product OQ ‘R of quenching cross section and radiative lifetime. With upper limit estimates of [3Q (obtainable from the intermolecular interaction range) a lower bound of the radiative lifetime rR is then calculated. We obtain a value rR > 100 I.csec for this quantity. First results of RR&l calculations [7,8] on triatomic model molecutes make it appear unlikely that MX; collision complexes of this Iifetime are formed With a cross section approaching the experimental value?, These calculations, continuing at the time of writing, will be presented in the full report on this work. The apparatus, & diffusion pumped quartz flow system inside an oven, viewed by a spectrometer with photoelectric detection, is described elsewhere
f Preliminary crossed molecular beam experiments in this laboratory have indicated RTBR cross sections of o = 0.1 A2 for the Ba + C12 reaction. 471
Volume 20. number 5
1 July 1973
CHEMICAL PHYSICS LETTERS
[2,3,IO].A &all amount (~0.1 crn3) af Ba metal was vaporized at T = 95O'K and reacted with Cl, gas admitted upstream of the oven under production of a bright white-green flame. The pressure was monitored by a capacitance manometer (calibrated against a McLeod gauge) immediately downstream of the oven. Pressure drops were negligible at the flow rates employed here. Typical base pressures (without Cl, and quenching gas, but with the oven on) were < 3 mtorr, and the Cl2 pressure was pclZ = 0.2 mtorr. The quenching gas was admitted through a separate upstream port. All gases (Matheson: Cl, > 9870, N2 B 99.0%, He 2 99.0%) had been carefully purified. The spectrum consists of a broad and seemingly structureless continuum (RTBR) upon which the well known BaCl (C2n --t X2 C) bands are superimposed [l, 2, lo]. Admission of the quenching gas caused the flame to turn from whitish-green to pure emerald-green as an indication of the sharp decrease of the BaCl$ continuum and the survival of the green BaCl (C + X) bands. The preferential quenching of BaC!; with respect to BaCl* shows that the two emitters are formed in parallel in separate reaction channels, and that a sequential mechanism is excluded. The simple reaction scheme Ba + Cl, k-: BaCl* + Cl ,
c
1
0
1
I
I
I
I
I IO
I 15
I 20
5 p
Fig. 1. Pseudo-S tern-Volmsr test for description of Il/Iz.
L
’
(He)
[mTorr]
plot for He quenching
I
I
I
I
gas. See
I
(4)
“1Q
BaCI* fQ
+ BaCI+Q,
(5)
klR
BaCI” +
BaCl+hv;
1, = k.lR(BaCl’),
(6)
4
Ba + Cl, -+ BaCIz ,
(7)
p (N-J
[m
Torr]
klQ
BaCl; + Q + BaCl, + Q ,
(8)
Fig. 2. Pseudo-Stern-Volmer text for d-scription ofI1/12.
plot for N2 quenching
gas. See
k2R
B&t; + BaC12 + hi ;
I2 = kZR(BaC1z) ,
(9)
accounts for these observations. Since it is difficult to keep the reactant pressures constant, relative measurements were taken. The areas under the spectral features were corrected for detector response; and their ratio I, /I, was plotted in Stern-\‘olmer fashion as shown in figs. 1 ar?d 2. Mechmism (1 j(9) gives
for this ratio. This can be simplified to a linear law by realizing that kl~(Q) < kl,.The linearity of the plots (1) and (2) justifies this assumption. Quenching due to background gas (e.g., Ba, Cl,, _._)has been neglected in this scheme and the pressures in figs. 1 and 2 are above background, refetibg to He or N2 only. This was done since the composition of the background gas was uncertain and its composite quenching cross section unknown. In bur analysis, the neglected term of the t:,pe Ci gza(Qi) (where i denotes the back-
Volume 20, number 5
CHEMICAi
PHYSICS LETTERS
1 July 1973
Table 1 -----System --
PI12 a) (mtoir)
------
--_---
9.0 2 2
(A2 set)
estimate
(see)
0.484 0.968
10-s 5 X IO+
4.84 9.68
IO-4 5 x 10-s
10 50 100 200
10-S 5 x 10-4
(4.84 ?I 1) x 104
1.1 * 0.3
BaCI; + N,
--
-1
OQ
(x2)
---------
BaClr + He
TR>‘
UQTR b,
(0.99
i 0.3)
--
x :0-z
------___
a) Pressures of quenching gas Q (He, Nz) at which the Il/Iz ratio has doubled. presents an upper limit to p, ,t(Q)_ _ lz) Due to a), a lower limit. c) For a rough correction for background quenching, see test.
Due to neglect of background
1O4 5 % 10-s
quenching,
this te-
ground quencher) is included in Ic,~, and it is obvious that the experimentally determined k,, represents
survives for this time span or longer. Compared with the lifetimes of customery complexes that are long
but an upper limit to the true value. Measurements of the background quenching terms are in progress. The lower limit of the quenching cross section X radiative lifetime product evaluated from
lived on the scale of a moIecular rotation (=IO-L3 set), as inferred from fonvard-backward symmetry of angular distributions, the anomalously long BaClz Lifetime is somewhat surprising. Possible mechanisms have been suggested [3] , and further work is in progress.
is recorded in table 1. U is the mean relative velocity and N1,, the number density at pI, 2 . Since-the BaCls complex is vibrarionally and rotationally highly excited, collision cross sections are expected to be larger for this “inflated” molecule than for vibrationally unexcited BaCl$. Trial values were chosen pairwise for He and N2 to yield identical TR for both members of a pair, and are given in the table. The largest Vabe for N2) was chosen to exceed the estimated cross section fcr the BaCl;-N2 pair, and probably represents an upper Even if actually exceeds this limit to conservative estimate would the TR value still be within the quoted limits, due to the effect of neglecting the above mentioned “background” quenching. The ratios of for He and Ne, chosen to yield identical ?-R, agree well with values calculated from Selwyn and Steinfeld’s theory [ 121. Even if the extraneous quenchers (Ba, Cl?, ...) are only three times as efficient as helium, the ‘TRvalue will increase by a factor of two (see table 1). A weakly forbidden transition is indicated by the lower limit estimate TR 2 100 Psec. If the RTBR hypothesis is correct, the BaClz complex
This work has been supported by the National Research Council ac.1 by the Research Corporation.
oQ
TR
cQ
(aQ
oQ
oQ
[ 11
]
.
=
oQ
200
ii
References [II C.D. Jonah and R.N. Zare, Chem. Fhyis. Letters 9 (1971) 65.
(21 M. Men-Anger, unpublished resulis. [31 M. Menzinger and J. ‘Lheodorakopoulos, to be published. 141 C.A. Rlims, S.M. Lin and R.R. Hcrm. J. Chem. Phys. 57 (1972) 3099; SM. Lin, C.A. Minx and R.R. Herm. 5. Chem. Phys. 58. (1973) 327. [51 J.A. Habermann, KG. Anlauf, R.B. Bernstein and F.J. VanItallie, Chem. Phys. Letters 16 (1972) 442; H.J. Loesch and D.R. Hershbach, to be published. 1 T. Canington ar.d J.C. Polyani, in: MTP international review of science and technology, Vol. 9. Chemical kinetics (Butterworths, London, 1972). 1 P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York. 19i2). ‘1 D.R. Bunker, J. Chem. Pitys. 57 (1972) 332. 0. Stern and hi. Volmer, Z. Physik 20 (19 19) 183. M. Menzinger and D.J. Wren, Chem. Phys Letters 15 (1973) 431. Rcs. 3 (1970) 313. 111J.1. Steinfeld, AccountsCheti. 121I.E. Selwyn and I.[. Steinfetd.Chem. Phys. Letters 4 (1969)217.
473
CHEMICAL PHYSICS LETTERS
1 July 1973
QUENCHING OF THE Ba + Cl2 CHEMILUMINESCENCE: ESTIMATE OF BaCl; RADIATIVE LIFETLME David J. WREN and Michael MENZINGER Loslt Miiler Laboratories. CJni&ity of Torotrto, Toronto hf5S JAI, Oxtario. Canada Received 26 March 1973
The chemiluminescence produced by the Ba + Cl* reaction was recorded as a function of He and Nz pressure. A modified Stern-Volmer treatment of competitive electronic quenching ol‘BaCI* and BaClz emission yielded upper limits to the half pressures p 1,2(He) < 9.0 r 3 mtorr and pllZ(N2) < 1.1 f 0.2 mtorr for quenching of BaCI; by: helium and nitrogen, respectively. A lower limit of the BaClr radiative lifetime is plxed at :R > 100 cr.
When two atoms collide, the preferred processes are elastic and inelastic scattering. The probability of radiative stabilization (3) of the collision complex AB* to form a stable AB molecule (radiative twobody recombination RTBR) is usually very low, due to the fact that the collision complex dissociates (2) before it has had a chance to emit a photon: % A+B-+AB*,
(1)
k-, AB*
--, A+B,
kR AB* + AEli-Itv. In other words the complex lifetime rD = k:l i; usually much shorter than the radiative lifetime rR = kR1. The recent suggestion [l] that RTBR be a fairly prominent channel in the cherniluminescent M(Ca, Sr, Ba) -f X,(F,, Cl,, Br2) reactions [2-51 seems to be at variance with this generally accepted expeciation [6]. It implies that the decomposition lifetime of the collision complex MX; be unusually long and/or that the radiative lifetime rR = kg1 be very short. Since rR represents the time scale of mechanism (I-3) it is of great interest to obtein an estimate of this quantity. Once this infomiation is given one can ascertain whether a complex lifetime of the order of
TV, which is implied by the substantial rate? of (l-3), is in agreement with our understanding of unimolecular lifetimes of small molecules [7, S] , or whether concepts have to be invoked [3] that lie outside the customary adiabatic and equilibrium formulations (e.g., RRKM). The goal of the present study of the Ba t Cl,system, which may be regarded as a protctype for the other M f X2 reactions, is to obtain an estimate of rR_ The Stern-Volmer treatment [9] of quenching data yields the product OQ ‘R of quenching cross section and radiative lifetime. With upper limit estimates of [3Q (obtainable from the intermolecular interaction range) a lower bound of the radiative lifetime rR is then calculated. We obtain a value rR > 100 I.csec for this quantity. First results of RR&l calculations [7,8] on triatomic model molecutes make it appear unlikely that MX; collision complexes of this Iifetime are formed With a cross section approaching the experimental value?, These calculations, continuing at the time of writing, will be presented in the full report on this work. The apparatus, & diffusion pumped quartz flow system inside an oven, viewed by a spectrometer with photoelectric detection, is described elsewhere
f Preliminary crossed molecular beam experiments in this laboratory have indicated RTBR cross sections of o = 0.1 A2 for the Ba + C12 reaction. 471
Volume 20. number 5
1 July 1973
CHEMICAL PHYSICS LETTERS
[2,3,IO].A &all amount (~0.1 crn3) af Ba metal was vaporized at T = 95O'K and reacted with Cl, gas admitted upstream of the oven under production of a bright white-green flame. The pressure was monitored by a capacitance manometer (calibrated against a McLeod gauge) immediately downstream of the oven. Pressure drops were negligible at the flow rates employed here. Typical base pressures (without Cl, and quenching gas, but with the oven on) were < 3 mtorr, and the Cl2 pressure was pclZ = 0.2 mtorr. The quenching gas was admitted through a separate upstream port. All gases (Matheson: Cl, > 9870, N2 B 99.0%, He 2 99.0%) had been carefully purified. The spectrum consists of a broad and seemingly structureless continuum (RTBR) upon which the well known BaCl (C2n --t X2 C) bands are superimposed [l, 2, lo]. Admission of the quenching gas caused the flame to turn from whitish-green to pure emerald-green as an indication of the sharp decrease of the BaCl$ continuum and the survival of the green BaCl (C + X) bands. The preferential quenching of BaC!; with respect to BaCl* shows that the two emitters are formed in parallel in separate reaction channels, and that a sequential mechanism is excluded. The simple reaction scheme Ba + Cl, k-: BaCl* + Cl ,
c
1
0
1
I
I
I
I
I IO
I 15
I 20
5 p
Fig. 1. Pseudo-S tern-Volmsr test for description of Il/Iz.
L
’
(He)
[mTorr]
plot for He quenching
I
I
I
I
gas. See
I
(4)
“1Q
BaCI* fQ
+ BaCI+Q,
(5)
klR
BaCI” +
BaCl+hv;
1, = k.lR(BaCl’),
(6)
4
Ba + Cl, -+ BaCIz ,
(7)
p (N-J
[m
Torr]
klQ
BaCl; + Q + BaCl, + Q ,
(8)
Fig. 2. Pseudo-Stern-Volmer text for d-scription ofI1/12.
plot for N2 quenching
gas. See
k2R
B&t; + BaC12 + hi ;
I2 = kZR(BaC1z) ,
(9)
accounts for these observations. Since it is difficult to keep the reactant pressures constant, relative measurements were taken. The areas under the spectral features were corrected for detector response; and their ratio I, /I, was plotted in Stern-\‘olmer fashion as shown in figs. 1 ar?d 2. Mechmism (1 j(9) gives
for this ratio. This can be simplified to a linear law by realizing that kl~(Q) < kl,.The linearity of the plots (1) and (2) justifies this assumption. Quenching due to background gas (e.g., Ba, Cl,, _._)has been neglected in this scheme and the pressures in figs. 1 and 2 are above background, refetibg to He or N2 only. This was done since the composition of the background gas was uncertain and its composite quenching cross section unknown. In bur analysis, the neglected term of the t:,pe Ci gza(Qi) (where i denotes the back-
Volume 20, number 5
CHEMICAi
PHYSICS LETTERS
1 July 1973
Table 1 -----System --
PI12 a) (mtoir)
------
--_---
9.0 2 2
(A2 set)
estimate
(see)
0.484 0.968
10-s 5 X IO+
4.84 9.68
IO-4 5 x 10-s
10 50 100 200
10-S 5 x 10-4
(4.84 ?I 1) x 104
1.1 * 0.3
BaCI; + N,
--
-1
OQ
(x2)
---------
BaClr + He
TR>‘
UQTR b,
(0.99
i 0.3)
--
x :0-z
------___
a) Pressures of quenching gas Q (He, Nz) at which the Il/Iz ratio has doubled. presents an upper limit to p, ,t(Q)_ _ lz) Due to a), a lower limit. c) For a rough correction for background quenching, see test.
Due to neglect of background
1O4 5 % 10-s
quenching,
this te-
ground quencher) is included in Ic,~, and it is obvious that the experimentally determined k,, represents
survives for this time span or longer. Compared with the lifetimes of customery complexes that are long
but an upper limit to the true value. Measurements of the background quenching terms are in progress. The lower limit of the quenching cross section X radiative lifetime product evaluated from
lived on the scale of a moIecular rotation (=IO-L3 set), as inferred from fonvard-backward symmetry of angular distributions, the anomalously long BaClz Lifetime is somewhat surprising. Possible mechanisms have been suggested [3] , and further work is in progress.
is recorded in table 1. U is the mean relative velocity and N1,, the number density at pI, 2 . Since-the BaCls complex is vibrarionally and rotationally highly excited, collision cross sections are expected to be larger for this “inflated” molecule than for vibrationally unexcited BaCl$. Trial values were chosen pairwise for He and N2 to yield identical TR for both members of a pair, and are given in the table. The largest Vabe for N2) was chosen to exceed the estimated cross section fcr the BaCl;-N2 pair, and probably represents an upper Even if actually exceeds this limit to conservative estimate would the TR value still be within the quoted limits, due to the effect of neglecting the above mentioned “background” quenching. The ratios of for He and Ne, chosen to yield identical ?-R, agree well with values calculated from Selwyn and Steinfeld’s theory [ 121. Even if the extraneous quenchers (Ba, Cl?, ...) are only three times as efficient as helium, the ‘TRvalue will increase by a factor of two (see table 1). A weakly forbidden transition is indicated by the lower limit estimate TR 2 100 Psec. If the RTBR hypothesis is correct, the BaClz complex
This work has been supported by the National Research Council ac.1 by the Research Corporation.
oQ
TR
cQ
(aQ
oQ
oQ
[ 11
]
.
=
oQ
200
ii
References [II C.D. Jonah and R.N. Zare, Chem. Fhyis. Letters 9 (1971) 65.
(21 M. Men-Anger, unpublished resulis. [31 M. Menzinger and J. ‘Lheodorakopoulos, to be published. 141 C.A. Rlims, S.M. Lin and R.R. Hcrm. J. Chem. Phys. 57 (1972) 3099; SM. Lin, C.A. Minx and R.R. Herm. 5. Chem. Phys. 58. (1973) 327. [51 J.A. Habermann, KG. Anlauf, R.B. Bernstein and F.J. VanItallie, Chem. Phys. Letters 16 (1972) 442; H.J. Loesch and D.R. Hershbach, to be published. 1 T. Canington ar.d J.C. Polyani, in: MTP international review of science and technology, Vol. 9. Chemical kinetics (Butterworths, London, 1972). 1 P.J. Robinson and K.A. Holbrook, Unimolecular reactions (Wiley-Interscience, New York. 19i2). ‘1 D.R. Bunker, J. Chem. Pitys. 57 (1972) 332. 0. Stern and hi. Volmer, Z. Physik 20 (19 19) 183. M. Menzinger and D.J. Wren, Chem. Phys Letters 15 (1973) 431. Rcs. 3 (1970) 313. 111J.1. Steinfeld, AccountsCheti. 121I.E. Selwyn and I.[. Steinfetd.Chem. Phys. Letters 4 (1969)217.
473