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Comparison of reactivity of Ar(3P0) and Ar(3P2) metastable states. Rate constants for quenching by Kr and CO
CHEMICAL PHYSICS LETTERS
Volume 80, number I
COMPARlSON
OF REACTIVITY
RATE CONSTANTS
OF Ar(3Po)
FOR QUENCHING
AND Ar(3P2)
15 May 1981
METASTABLE
STATES.
BY Kr AND CO
Michael F. GOLDE and Robert A. POLETTI Department Received
of Chemistry. 5 November
1980;
University
of Pittsburgh.
in final form 10
Artsburgh.
February
Pennsylvania
15260,
L’SA
1981
A sunple method 1s described for studying the reactions of metastable Ar(3Po) and ArCPz) charge-flow system. CO and Kr quench these states with rate constants in the ratio ko (CO)/k, ko(Kr) = 18 & 2.
1. Introduction
Studies of the reactions of metastable inert-gas atoms have given a wealth of information concerning their rates and mechanisms [ 1,2]. In recent investigations (e.g. see refs. [3-S] ), the reactions of the two helium metastable species, 2s 1 So and 2s SSI , have been studied independently but much less is known concerning the relative mechanisms for reaction of the 3P and 3P2 spin-orbit components of the lowest ?zpg (II + 1) s1 excited states of the heavier inert gases. These states have average excitation energies which range from 16.6 eV for Ne to 8 _8 eV for Xe, but the separations of the 3Po and 3P2 states are much smaller, respectively 777,1410,5219 and 9129 cm-I for Ne, Ar, Kr and Xe, the 3P. states lying above 3P, _ Velazco et al. [6] have m_easured total quenching rate constants of Ar(3Po) and Ar(3P2) by a large number of reagents, finding two general classes of behavior: (i) the majority of reagents which quench both states at about the collision rate, and (ii) a few atomic and diatomic species, which quench more slowly, with differences in rate constants of up to a factor of ten between the two states. No such body of data exists for the products of the quenching processes. Recently, Derouard et al_ [7] reported that ArCPo) and Ar(3P-,) can be separated in a discharge-flow system via laser excitation of either metastable to a higher state and used the technique to study the reactions of the individual states with N,. These states have also been separated by Gersh and Muschlitz [S] _using an inhomogeneous magnetic field. 18
atoms separately in a dis(CO) = 8 + 1 and ka (Kr)/
In the present paper, we describe a simple technique to achieve this separation and, in the following paper, present results showing large differences in the products of the reactions of Ar(3 PO) and Ar(3 P2) with some chlorine- and brominecontaining compounds. The technique relies on the different rates of quenching of Ar(3P2) and Ar(3PO) by Kr; we derive ic, (Kr)/ko (Kr) = 18 2 2, larger than previously measured [6]. We also fmd, for quenching by CO, /co (CO)/ k, (CO) = 8 t 1, in agreement with a previous measurement [6] _
2. Experimental
The discharge-flow apparatus, used for study of emission from the reactions of argon metastable atoms, Ar*,with bromine-containing compounds, RRr, [9] was modified to allow addition of gases between the dc discharge and the main reagent inlet (see fig. 1). Typically, with an Ar flow rate of 300 ~01 s-l at a total pressure of l-l .5 Torr, Kr, added through the new inlet at a flow rate of up to 10 mol s-l, interacted with the Ar* atoms over a time period of 1 S-2 ms, quenching the Ar(3P2) atom appreciably but the Ar(SPo) atom only weakly. Excess halogencontaining reagent, added through the concentric inlet tube, reacted with the remaining Ar(3P2, SPo) atoms within the 2-3 cm long observation region. In complementary studies, Kr was replaced by a CO/Ar mixture. Only very small flows, less than 0.3 mol s-l of CO, were
Volume 80, number 1
CHEMICAL PHYSICS LETTERS PUMP REACTION
PRE-
REACTION
-26O’i/l~A
Fig. 1. Experimental reaction vesseL The argon inlet tube has at the observation regionid. 1.0 an;the monochromator field of view at the flame has a rectangular aoss section 3 X 1 cm* _ required to quench Ar(3 PO), leaving Ar (3 Pz) little affected. The experimental data consisted of emission spectra from ArCPO, 3P2) + RX reaction products (X = Cl, Br) as a function of Kr or CO flow, including spectra at high flows, at which essentially only Ar(3Po) or Ar(3P2) atoms respectively remained_ Atomic lines were always close to triangular in shape and peak heights were used as a measure of the intensity. Monochromator slit widths were usually 500 pm, 300 m being used where necessary to ensure complete resolution of close-lying atomic lines. For the weakest lines, particularly Br(4P,,,-2Pp/2) at 158.3 nm, care was taken to subtract weak background emission, which was usually due to scattered light from the monochromator, arising from stronger spectral features_ For Ar* + CF2Br2, for instance, this background was always less than 4% of the peak intensity. Emission from the pre-reactions, Ar* + Kr or CO, was usually weak in comparison; where necessary, particularly at high CO Bow rates, corrections were made for such background emission.
3. Results Addition of Kr between the discharge and reagent inlet has been found to cause marked changes in the observed emission spectra of the products of several reactions: (i) Ar* + HBr. The inrensity of the Br(4p45s 4P5122e 2) line rises relative to that of the_ArBr continuum. ( ii) Ar* + HCl, DCI. The intensity of the ArCI(B-X)
15 May 1981
continuum rises markedly relative to that of HCl (DCl) emission (a complex banded system extending from = 135 nm to at least 185 mn and which appears to correspond to the B lZ+-X lIZ+ transitions of these molecules [lo,1 11). (iii) Ar* + OcS_ The vibrational distrrbution in the CO(A IfI) product state shifts to higher levels. (iv) Ar* + Cl,, CF,Br, CF,Br,, CH,Br,, CHBr,. The relative intensities of the five Cl or Br lines, np4(tz + 1) s1 2.4Ps2p”J, alter very markedly_ (v) Ar* + Cl,, CC14. The structure of the ArCl spectrum alters in shape, with the appearance of a new continuum_ Many of these data are discussed in the following paper. All the data are consistent with the presence in the Ar discharge products of a minor excited species, with higher energy than the dominant Ar(3P2) species and which is quenced less rapidly by Kr than is Ar(3P& Excited states of Kr produced by energy transfer from Ar’ [12] can be ruled out, firstly because no metastable states with sufficient energy are known, but primarily because this species yields ArCl* in its reactions with HCI, Cl2 and Ccl,. The resonant states, Ar(1*3Pl), were ruled out by examining the variation of emission intensity with reagent (i.e. RCL or RBr) flow rate; for reaction with resonant states, it is expected that the emission intensity from the observation region will increase monotonically with RX concentration over the range used, (O-3-3) X 10r4 cmW3, whereas for reactions of metastable states, the signal should saturate at low RX concentrations, which quench the metastabie state completely within the observation zone. The latter behavior was found, identifying the species involved as Ar(3Po), which has been found, under experimental conditions similar to those used here, to have a concentration 5--10% of that of Ar(3P2) [13]. For a quantitative analysis of the experimental system, emission intensities from the reactions of Ar* with Cl, and some brominated methanes, especially CF,Br, and CHBr, , were studied as a function of added pre-reagent Kr or CO_ The channels monitored were: Ar* + Cl, + ArCl* + Cl,
(1)
+ Ar + Cl + cl* @+PJ),
(2)
Ar* + RBr + Ar + R + Br* @4PJ).
(3) 19
15 May 1981
CHEMICAL PHYSICS LETTERS
Volume 80, number 1
Although the pre-reactions, Ar* + Kr or CO, yield long-lived product states, Kr(3PZ) (energy: 99 eV) and CO(asII, u<4) (energy < ‘7.0 eV) [14], the states of Cl and Br studied are energetically inaccessible to mterfering reactions of Kr* or CO* with the halogencontaming reagent. Thus, in these experiments, the exclmer and atom:c emissions were used as tracers for their precursors, ArCP,,) and Ar(3P2). Representative data, showing the dependence of emission intensities on Kr concentration at constant RX concentration, appear in figs. 2 and 3, where I and I0 are, respectively, the intensities in the presence and absence of Kr. For Ar* + Cl,, fig. 2, the ArCl continuum (monitored at 151 nm) is quenched rapidly at low wr] , reflecting quenching of Its principal precursor, Ar(3P2), while the Cl(2P3,2) state, whose formation is exothermic only for the reaction of Ar(3Po), is quenched much more slowly_ At high [Kr], Ar(3Po) is the dominant metastable and the decay plots for the two emission features become parallel_ It should be noted that the radiative liferimes of the emitting states, Cl* and ArCl*, are less than 1 ps, so that direct quenching by Kr is unimportant _ For Ar* + CF,Br,, the analogous plots of Br line intensity as a function of Kr concentration, fig. 3, &splay double exponential behavior for both lines shown, with a common slope at high [Kr] . Clearly, the fast 3P2 component 1s quite small for the 4P,12-2P$2 line, although the formation of Br(4P1/7) by reaction of Ar( 3P2) is exothemic. All the other-fines studied with these and the other reagents showed analogous double exponential decays.
I
I
I
20
0 16”‘[Kr]
I
cme3
Fig_ 3. Dependence on Kr concentration of atomic emission intensities generated by the reactions Ar(3Po,2) + CT2Br2. Lines are calculated fits.
Such data were analyzed as follows. In the absence of Kr, the intensity of a given spectral feature is given by:
&g+p2.
(4)
where 8 is the intensity due to reaction of Ar(3PJ). The ratios G/p and &‘/p are designated ‘~0 and a2 respectively; these quantities of course depend on the relative concentrations of Ar(3P2) and Ar(3P0) and thus on discharge and argon flow conditions. In the presence of Kr, the intensity is reduced to: I=4
exp(-k0[Kr]7)+e
exp(-k2[Kr]
T),
(5)
where kJ are the rate constants for quenching of Ar(3PJ) by Kr and T is the effective flow time, typically l-2 ms, between the Kr inlet and the reagent, RX, inlet point. These equations yield a\
-3I
0
I
5 lO-‘4 [Krl
I/p
ArCP*(l5lnm) .A*_ 1
IO
+(l 15
cmw3
Fig. 2. Dependence on Kr concentration of atomic and extimer emission intensities generated by the reactions Ar(3Po,$ + Cl2. Lines are caiculated fits.
20
=%
exp(-kOIKr]
r)
-aO)exp(-k2[Kr]+
(6)
Fits to the data were sought, with cyo, k2/ko and k2r as variable parameters. For a given reagent, for which several Br or Cl lines were monitored, a0 was allowed to differ for each emission
feature,
but common
values
of k2T and k2/ko were used. Calculated lines are in-
Volume 80, number 1
CHEMICAL
PHYSICS
-+.__&rCQ*(l52nm) -QA7_
Pn
~-4__
-
CQ”(4P,,2)
Fig. 4. Dependence on CO concentration of atomic and extimer emission intensities generated by the reactions A@Po,*) + Cl2 _ Lines are calculated fits.
eluded in figs. 2 and 3. In fig.2, k2r = 6.4 X lo-l5 cm3, ka/ko = 16 and “0 = 0.882 for the Cl(2P3/22P$2) line and cro = 0.080 for the ArCl* continuum at 151 run. Because of the extreme values of oo, the linear portion of the plot for Cl(2P3/2) yields kov- directly, while the rapid early decay of the ArCl continuum determines k2T, so that combined fits to the two features are very sensitive to changes in any of the parameters_ By testing the effects of such changes, we conclude that good fits are possrble for k2/ko = 17 ? 1, and o. ~0.86 + 0.02 and 0.08 + 0.01 for Cl*(2P3/2) and ArCl respectively_ Analogous fits to the data of fig_ 3 yielded k2/ko = 18 +- 2, cru (Br(4P,,2)) = 0.50 f 0.03 and oo(Br(4Ps,2)) = 0.033 f 0.003. The effect of CO addition on the intensity of spectral features in the Ar* + Cl2 reaction is shown in fig.4. The behavior of the CleP3/2) and ArCl emissions is complementary to that in fig. 2, because CO quenches Ar(3Po) more rapidly than it does Ar(3P2)_ The analysis of these data was carried out independently of those of fig_2 and yieided k. (CO)/k2(CO) = 8 F 1, with o. = 0.88 a 0.01 and 0.09 t 0.02 for Cl(2P3/2) and ArCl* respectively.
4. Discussion The principal features of this study are, firstly, a technique for measuring relative quenching rate constants for ArcPo) and Ar(3P2) and, secondly, a method for observing product channels in the separate reac-
LETT.ERS
,
15 May 1981
tions of Ar(3Po) and Ar(3P2) (see the following paper for details). Thermal rate constants for quenching of A~CPJ) by Kr and CO have been measured by two methods: the discharge-flow technique [13,6], in which the metastable atoms were monitored in absorption, and a static system [ 15 ,161, in which Ar was excited by high-energy electrons and the excited states monitored either in absorption or by N2 tracer emission. For quenching by CO, the two methods have reached agreement, yielding k. = 1.3 X 10-10 cm3 s- I and k2 = (1.6 + 0.2) X lo-11 cm3 s-l, and a ratio ko/ k2 = 8 -1 f 1.5, coinciding with the value obtained in this study. We have estimated T using a plug-flow approximation and assuming instantaneous mixing of the reagents and obtain absolute values of k. and k2 in agreement with the above values. There is less satisfactory agreement concerning the Kr quenching data. For Ar(3P2) + Kr, Velazco et al. [6] report two separate determinations within 5% of 5.8 X lOmE cm3 s-l, while Bourene and Ie Calve 1151 obtained 1.4 X lO_ll cm3 s-l, using the tracer technique_ For Ar(3Po) + Kr, the discharge-flow technique 1131 yielded 2.3 X lo-= cm3 s-l. The discharge -flow experiments thus predict k,/k,, = 2.5 and, even using the higher value of k2, the ratio is still well below that measured here. Using our estimate of r, we calculate k2 (Kr) = 4 X lo-l2 cm3 s-l, favoring the lower of the previous values. The present determination of k2/ko depends primarily on our assignment of Ar(3Po) as the long-lived reactive species. The arguments for this, given above, are supported by the excellent agreement cf the CO results with the literature data and also by the fact that the independent analyses of the Kr and CO data yielded consistent values of cyo for each of the spectral features studied. We thus prefer the lower published value of k,, 5 23X lo-l2 cm3 s-1, and derive k, (Kr) = 3 _4 X lo-13 cm3 s-1, the uncertainty in k. being dominated by that in k,. It is not clear why the discharge-flow measurement should be in error; however, this rate constant was the smallest measured 113 1, thus presumably requiring large Kr flows and probably the detection of small amounts of decay. It is of interest to consider why ko and k2 differ so greatly for quenching by Kr and CO. For Kr, near-resonant energy transfer is favored [12], with dominant transfer from ArcP2) to Kr(4p55p [3/2] 2), with an energy mismatch of only 20 cm-l_ No such fortuitonu 21
Volume
80, number
1
CHEMICAL
PHYSICS
energy resonance occurs for Ar(3 Po) + Kr. The bond energy of CO is 1139 eV, rather close to the excitation energies of Ar(3P2) and Ar(3P,-,), 11.55 and 11.72 eV respectively_ Recent measurements [17] have shown that dissociation of CO is the major quenching channel and it is possible that a small energy barrier in the Ar(3P2) + CO reaction is the cause of the low rate constant_
Acknowledgement We acknowledge the Donors of the Petroleum Research Fund, administered by the American Chemical Society, the Research Corporation and the Air Force Office of Sclentlfic Research for support of this research.
References [l]
[2]
22
M.F. Golde, Specialists Periodical Reports, Gas kinetics and energy transfer, Vol. 2 (Chem. Sot., London, 1976) p_ 121. D.L. King and D-W. Setser, Ann. Rev. Phys. Chem. 27 (1976) 407.
LETTERS
15 May 1981
2. Physik 131 H. Hotop, A. Niehaus and A.L. Schmeltekopf, 229 (1969) 1. R.F. Stebbings, [41 J-P. Riola, J-S. Howard, R-D_ Rundeland J. Phys. B7 (1974) 376. t51 R.hl. Jordan and P-E. Siska, J. Chem. Phys. 69 (1978) 4634. [61 J.E. VeIazco, J-H. Kolts and D.W. Setser, J. Chem. Phys. 69 (1978) 4357. 171 J. Derouard, T.D. Nguyen and N. Sadeghi, J. Chem. Phys. 72 (1980) 6698. WI M-E_ Gersh and E-E. Muschlitz. J. Chem. Phys_ 59 (1973) 3538. 191 M-F. Golde and A. Kvaran, J. Chem. Phys 72 (1980) 434. 1101 J.K. Jacques and R-T_ Barrow, Proc. Phys. Sot. (London) 73 (1959) 538. A-E. Douglas and F-R. Greening, Can. J. Phys. 57 (1979) [ill 1650. 1121 L-G. Piper, D-W. Setser and M.A.A. Clyne, J. Chem. Phys. 63 (1975) 5018. r131 L.C. Piper, J.E. VeIazco and D-W. Setser, J. Chem. Phys. 59 (1973) 3323. t141 D.H. Stedman and D-W. Setser, 3. Chem. Phys. 52 (1970) 3957. 1151 M. Bou&me and J. le CalvB, J. Chem_ Phys. 58 (1973) 1452. [161 M. Bouri%e, 0. Dutuit and J. le Calvt?, J. Chem. Phys. 63 (1975) 1668. 1171 J. Balamuta and M-F. Golde, unpublished.
Volume 80, number I
COMPARlSON
OF REACTIVITY
RATE CONSTANTS
OF Ar(3Po)
FOR QUENCHING
AND Ar(3P2)
15 May 1981
METASTABLE
STATES.
BY Kr AND CO
Michael F. GOLDE and Robert A. POLETTI Department Received
of Chemistry. 5 November
1980;
University
of Pittsburgh.
in final form 10
Artsburgh.
February
Pennsylvania
15260,
L’SA
1981
A sunple method 1s described for studying the reactions of metastable Ar(3Po) and ArCPz) charge-flow system. CO and Kr quench these states with rate constants in the ratio ko (CO)/k, ko(Kr) = 18 & 2.
1. Introduction
Studies of the reactions of metastable inert-gas atoms have given a wealth of information concerning their rates and mechanisms [ 1,2]. In recent investigations (e.g. see refs. [3-S] ), the reactions of the two helium metastable species, 2s 1 So and 2s SSI , have been studied independently but much less is known concerning the relative mechanisms for reaction of the 3P and 3P2 spin-orbit components of the lowest ?zpg (II + 1) s1 excited states of the heavier inert gases. These states have average excitation energies which range from 16.6 eV for Ne to 8 _8 eV for Xe, but the separations of the 3Po and 3P2 states are much smaller, respectively 777,1410,5219 and 9129 cm-I for Ne, Ar, Kr and Xe, the 3P. states lying above 3P, _ Velazco et al. [6] have m_easured total quenching rate constants of Ar(3Po) and Ar(3P2) by a large number of reagents, finding two general classes of behavior: (i) the majority of reagents which quench both states at about the collision rate, and (ii) a few atomic and diatomic species, which quench more slowly, with differences in rate constants of up to a factor of ten between the two states. No such body of data exists for the products of the quenching processes. Recently, Derouard et al_ [7] reported that ArCPo) and Ar(3P-,) can be separated in a discharge-flow system via laser excitation of either metastable to a higher state and used the technique to study the reactions of the individual states with N,. These states have also been separated by Gersh and Muschlitz [S] _using an inhomogeneous magnetic field. 18
atoms separately in a dis(CO) = 8 + 1 and ka (Kr)/
In the present paper, we describe a simple technique to achieve this separation and, in the following paper, present results showing large differences in the products of the reactions of Ar(3 PO) and Ar(3 P2) with some chlorine- and brominecontaining compounds. The technique relies on the different rates of quenching of Ar(3P2) and Ar(3PO) by Kr; we derive ic, (Kr)/ko (Kr) = 18 2 2, larger than previously measured [6]. We also fmd, for quenching by CO, /co (CO)/ k, (CO) = 8 t 1, in agreement with a previous measurement [6] _
2. Experimental
The discharge-flow apparatus, used for study of emission from the reactions of argon metastable atoms, Ar*,with bromine-containing compounds, RRr, [9] was modified to allow addition of gases between the dc discharge and the main reagent inlet (see fig. 1). Typically, with an Ar flow rate of 300 ~01 s-l at a total pressure of l-l .5 Torr, Kr, added through the new inlet at a flow rate of up to 10 mol s-l, interacted with the Ar* atoms over a time period of 1 S-2 ms, quenching the Ar(3P2) atom appreciably but the Ar(SPo) atom only weakly. Excess halogencontaining reagent, added through the concentric inlet tube, reacted with the remaining Ar(3P2, SPo) atoms within the 2-3 cm long observation region. In complementary studies, Kr was replaced by a CO/Ar mixture. Only very small flows, less than 0.3 mol s-l of CO, were
Volume 80, number 1
CHEMICAL PHYSICS LETTERS PUMP REACTION
PRE-
REACTION
-26O’i/l~A
Fig. 1. Experimental reaction vesseL The argon inlet tube has at the observation regionid. 1.0 an;the monochromator field of view at the flame has a rectangular aoss section 3 X 1 cm* _ required to quench Ar(3 PO), leaving Ar (3 Pz) little affected. The experimental data consisted of emission spectra from ArCPO, 3P2) + RX reaction products (X = Cl, Br) as a function of Kr or CO flow, including spectra at high flows, at which essentially only Ar(3Po) or Ar(3P2) atoms respectively remained_ Atomic lines were always close to triangular in shape and peak heights were used as a measure of the intensity. Monochromator slit widths were usually 500 pm, 300 m being used where necessary to ensure complete resolution of close-lying atomic lines. For the weakest lines, particularly Br(4P,,,-2Pp/2) at 158.3 nm, care was taken to subtract weak background emission, which was usually due to scattered light from the monochromator, arising from stronger spectral features_ For Ar* + CF2Br2, for instance, this background was always less than 4% of the peak intensity. Emission from the pre-reactions, Ar* + Kr or CO, was usually weak in comparison; where necessary, particularly at high CO Bow rates, corrections were made for such background emission.
3. Results Addition of Kr between the discharge and reagent inlet has been found to cause marked changes in the observed emission spectra of the products of several reactions: (i) Ar* + HBr. The inrensity of the Br(4p45s 4P5122e 2) line rises relative to that of the_ArBr continuum. ( ii) Ar* + HCl, DCI. The intensity of the ArCI(B-X)
15 May 1981
continuum rises markedly relative to that of HCl (DCl) emission (a complex banded system extending from = 135 nm to at least 185 mn and which appears to correspond to the B lZ+-X lIZ+ transitions of these molecules [lo,1 11). (iii) Ar* + OcS_ The vibrational distrrbution in the CO(A IfI) product state shifts to higher levels. (iv) Ar* + Cl,, CF,Br, CF,Br,, CH,Br,, CHBr,. The relative intensities of the five Cl or Br lines, np4(tz + 1) s1 2.4Ps2p”J, alter very markedly_ (v) Ar* + Cl,, CC14. The structure of the ArCl spectrum alters in shape, with the appearance of a new continuum_ Many of these data are discussed in the following paper. All the data are consistent with the presence in the Ar discharge products of a minor excited species, with higher energy than the dominant Ar(3P2) species and which is quenced less rapidly by Kr than is Ar(3P& Excited states of Kr produced by energy transfer from Ar’ [12] can be ruled out, firstly because no metastable states with sufficient energy are known, but primarily because this species yields ArCl* in its reactions with HCI, Cl2 and Ccl,. The resonant states, Ar(1*3Pl), were ruled out by examining the variation of emission intensity with reagent (i.e. RCL or RBr) flow rate; for reaction with resonant states, it is expected that the emission intensity from the observation region will increase monotonically with RX concentration over the range used, (O-3-3) X 10r4 cmW3, whereas for reactions of metastable states, the signal should saturate at low RX concentrations, which quench the metastabie state completely within the observation zone. The latter behavior was found, identifying the species involved as Ar(3Po), which has been found, under experimental conditions similar to those used here, to have a concentration 5--10% of that of Ar(3P2) [13]. For a quantitative analysis of the experimental system, emission intensities from the reactions of Ar* with Cl, and some brominated methanes, especially CF,Br, and CHBr, , were studied as a function of added pre-reagent Kr or CO_ The channels monitored were: Ar* + Cl, + ArCl* + Cl,
(1)
+ Ar + Cl + cl* @+PJ),
(2)
Ar* + RBr + Ar + R + Br* @4PJ).
(3) 19
15 May 1981
CHEMICAL PHYSICS LETTERS
Volume 80, number 1
Although the pre-reactions, Ar* + Kr or CO, yield long-lived product states, Kr(3PZ) (energy: 99 eV) and CO(asII, u<4) (energy < ‘7.0 eV) [14], the states of Cl and Br studied are energetically inaccessible to mterfering reactions of Kr* or CO* with the halogencontaming reagent. Thus, in these experiments, the exclmer and atom:c emissions were used as tracers for their precursors, ArCP,,) and Ar(3P2). Representative data, showing the dependence of emission intensities on Kr concentration at constant RX concentration, appear in figs. 2 and 3, where I and I0 are, respectively, the intensities in the presence and absence of Kr. For Ar* + Cl,, fig. 2, the ArCl continuum (monitored at 151 nm) is quenched rapidly at low wr] , reflecting quenching of Its principal precursor, Ar(3P2), while the Cl(2P3,2) state, whose formation is exothermic only for the reaction of Ar(3Po), is quenched much more slowly_ At high [Kr], Ar(3Po) is the dominant metastable and the decay plots for the two emission features become parallel_ It should be noted that the radiative liferimes of the emitting states, Cl* and ArCl*, are less than 1 ps, so that direct quenching by Kr is unimportant _ For Ar* + CF,Br,, the analogous plots of Br line intensity as a function of Kr concentration, fig. 3, &splay double exponential behavior for both lines shown, with a common slope at high [Kr] . Clearly, the fast 3P2 component 1s quite small for the 4P,12-2P$2 line, although the formation of Br(4P1/7) by reaction of Ar( 3P2) is exothemic. All the other-fines studied with these and the other reagents showed analogous double exponential decays.
I
I
I
20
0 16”‘[Kr]
I
cme3
Fig_ 3. Dependence on Kr concentration of atomic emission intensities generated by the reactions Ar(3Po,2) + CT2Br2. Lines are calculated fits.
Such data were analyzed as follows. In the absence of Kr, the intensity of a given spectral feature is given by:
&g+p2.
(4)
where 8 is the intensity due to reaction of Ar(3PJ). The ratios G/p and &‘/p are designated ‘~0 and a2 respectively; these quantities of course depend on the relative concentrations of Ar(3P2) and Ar(3P0) and thus on discharge and argon flow conditions. In the presence of Kr, the intensity is reduced to: I=4
exp(-k0[Kr]7)+e
exp(-k2[Kr]
T),
(5)
where kJ are the rate constants for quenching of Ar(3PJ) by Kr and T is the effective flow time, typically l-2 ms, between the Kr inlet and the reagent, RX, inlet point. These equations yield a\
-3I
0
I
5 lO-‘4 [Krl
I/p
ArCP*(l5lnm) .A*_ 1
IO
+(l 15
cmw3
Fig. 2. Dependence on Kr concentration of atomic and extimer emission intensities generated by the reactions Ar(3Po,$ + Cl2. Lines are caiculated fits.
20
=%
exp(-kOIKr]
r)
-aO)exp(-k2[Kr]+
(6)
Fits to the data were sought, with cyo, k2/ko and k2r as variable parameters. For a given reagent, for which several Br or Cl lines were monitored, a0 was allowed to differ for each emission
feature,
but common
values
of k2T and k2/ko were used. Calculated lines are in-
Volume 80, number 1
CHEMICAL
PHYSICS
-+.__&rCQ*(l52nm) -QA7_
Pn
~-4__
-
CQ”(4P,,2)
Fig. 4. Dependence on CO concentration of atomic and extimer emission intensities generated by the reactions A@Po,*) + Cl2 _ Lines are calculated fits.
eluded in figs. 2 and 3. In fig.2, k2r = 6.4 X lo-l5 cm3, ka/ko = 16 and “0 = 0.882 for the Cl(2P3/22P$2) line and cro = 0.080 for the ArCl* continuum at 151 run. Because of the extreme values of oo, the linear portion of the plot for Cl(2P3/2) yields kov- directly, while the rapid early decay of the ArCl continuum determines k2T, so that combined fits to the two features are very sensitive to changes in any of the parameters_ By testing the effects of such changes, we conclude that good fits are possrble for k2/ko = 17 ? 1, and o. ~0.86 + 0.02 and 0.08 + 0.01 for Cl*(2P3/2) and ArCl respectively_ Analogous fits to the data of fig_ 3 yielded k2/ko = 18 +- 2, cru (Br(4P,,2)) = 0.50 f 0.03 and oo(Br(4Ps,2)) = 0.033 f 0.003. The effect of CO addition on the intensity of spectral features in the Ar* + Cl2 reaction is shown in fig.4. The behavior of the CleP3/2) and ArCl emissions is complementary to that in fig. 2, because CO quenches Ar(3Po) more rapidly than it does Ar(3P2)_ The analysis of these data was carried out independently of those of fig_2 and yieided k. (CO)/k2(CO) = 8 F 1, with o. = 0.88 a 0.01 and 0.09 t 0.02 for Cl(2P3/2) and ArCl* respectively.
4. Discussion The principal features of this study are, firstly, a technique for measuring relative quenching rate constants for ArcPo) and Ar(3P2) and, secondly, a method for observing product channels in the separate reac-
LETT.ERS
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15 May 1981
tions of Ar(3Po) and Ar(3P2) (see the following paper for details). Thermal rate constants for quenching of A~CPJ) by Kr and CO have been measured by two methods: the discharge-flow technique [13,6], in which the metastable atoms were monitored in absorption, and a static system [ 15 ,161, in which Ar was excited by high-energy electrons and the excited states monitored either in absorption or by N2 tracer emission. For quenching by CO, the two methods have reached agreement, yielding k. = 1.3 X 10-10 cm3 s- I and k2 = (1.6 + 0.2) X lo-11 cm3 s-l, and a ratio ko/ k2 = 8 -1 f 1.5, coinciding with the value obtained in this study. We have estimated T using a plug-flow approximation and assuming instantaneous mixing of the reagents and obtain absolute values of k. and k2 in agreement with the above values. There is less satisfactory agreement concerning the Kr quenching data. For Ar(3P2) + Kr, Velazco et al. [6] report two separate determinations within 5% of 5.8 X lOmE cm3 s-l, while Bourene and Ie Calve 1151 obtained 1.4 X lO_ll cm3 s-l, using the tracer technique_ For Ar(3Po) + Kr, the discharge-flow technique 1131 yielded 2.3 X lo-= cm3 s-l. The discharge -flow experiments thus predict k,/k,, = 2.5 and, even using the higher value of k2, the ratio is still well below that measured here. Using our estimate of r, we calculate k2 (Kr) = 4 X lo-l2 cm3 s-l, favoring the lower of the previous values. The present determination of k2/ko depends primarily on our assignment of Ar(3Po) as the long-lived reactive species. The arguments for this, given above, are supported by the excellent agreement cf the CO results with the literature data and also by the fact that the independent analyses of the Kr and CO data yielded consistent values of cyo for each of the spectral features studied. We thus prefer the lower published value of k,, 5 23X lo-l2 cm3 s-1, and derive k, (Kr) = 3 _4 X lo-13 cm3 s-1, the uncertainty in k. being dominated by that in k,. It is not clear why the discharge-flow measurement should be in error; however, this rate constant was the smallest measured 113 1, thus presumably requiring large Kr flows and probably the detection of small amounts of decay. It is of interest to consider why ko and k2 differ so greatly for quenching by Kr and CO. For Kr, near-resonant energy transfer is favored [12], with dominant transfer from ArcP2) to Kr(4p55p [3/2] 2), with an energy mismatch of only 20 cm-l_ No such fortuitonu 21
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energy resonance occurs for Ar(3 Po) + Kr. The bond energy of CO is 1139 eV, rather close to the excitation energies of Ar(3P2) and Ar(3P,-,), 11.55 and 11.72 eV respectively_ Recent measurements [17] have shown that dissociation of CO is the major quenching channel and it is possible that a small energy barrier in the Ar(3P2) + CO reaction is the cause of the low rate constant_
Acknowledgement We acknowledge the Donors of the Petroleum Research Fund, administered by the American Chemical Society, the Research Corporation and the Air Force Office of Sclentlfic Research for support of this research.
References [l]
[2]
22
M.F. Golde, Specialists Periodical Reports, Gas kinetics and energy transfer, Vol. 2 (Chem. Sot., London, 1976) p_ 121. D.L. King and D-W. Setser, Ann. Rev. Phys. Chem. 27 (1976) 407.
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2. Physik 131 H. Hotop, A. Niehaus and A.L. Schmeltekopf, 229 (1969) 1. R.F. Stebbings, [41 J-P. Riola, J-S. Howard, R-D_ Rundeland J. Phys. B7 (1974) 376. t51 R.hl. Jordan and P-E. Siska, J. Chem. Phys. 69 (1978) 4634. [61 J.E. VeIazco, J-H. Kolts and D.W. Setser, J. Chem. Phys. 69 (1978) 4357. 171 J. Derouard, T.D. Nguyen and N. Sadeghi, J. Chem. Phys. 72 (1980) 6698. WI M-E_ Gersh and E-E. Muschlitz. J. Chem. Phys_ 59 (1973) 3538. 191 M-F. Golde and A. Kvaran, J. Chem. Phys 72 (1980) 434. 1101 J.K. Jacques and R-T_ Barrow, Proc. Phys. Sot. (London) 73 (1959) 538. A-E. Douglas and F-R. Greening, Can. J. Phys. 57 (1979) [ill 1650. 1121 L-G. Piper, D-W. Setser and M.A.A. Clyne, J. Chem. Phys. 63 (1975) 5018. r131 L.C. Piper, J.E. VeIazco and D-W. Setser, J. Chem. Phys. 59 (1973) 3323. t141 D.H. Stedman and D-W. Setser, 3. Chem. Phys. 52 (1970) 3957. 1151 M. Bou&me and J. le CalvB, J. Chem_ Phys. 58 (1973) 1452. [161 M. Bouri%e, 0. Dutuit and J. le Calvt?, J. Chem. Phys. 63 (1975) 1668. 1171 J. Balamuta and M-F. Golde, unpublished.