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Absolute rates for the reaction of o-atoms with vibrationally excited HCl-molecules
A B S O L U T E RATES FOR T H E REACTION O F O-ATOMS W I T H VIBRATIONALLY E X C I T E D H C I - M O L E C U L E S M. KNEBA AND J. WOLFRUM
Max-Planck-Institut fiir Strdmungsforschung, 3400---Giittingen, W. Germany
The reaction between ground state oxygen atoms and vibrationally excited HCI molecules was studied directly in a fast discharge-flow reactor using a chemical HC1 laser for vibrational excitation of HCI by optical pumping. Time-resolved infrared fluorescence, nozzle-beam mass spectroscopy a n d atomic line resonance absorption were used for quantitative detection of the reactants and products. For HC1 (v = 1) the measured rate constants for the elementary steps O(~P) + HCI(v = 1)--~ OH(v = 0) + CI(zP3/2)
(1')
a"
OH(v = 0) + C1(2p~/2) b
3
--~O(P)+HCI(v=0) were: k~, = (6.0 + 0.1)'10 H, kl, ~ = (3.7 + 1.5)'101~ kva. = (1.6 + 0.9).109, k t , b = (5.6 -+ 0.3)'1011 (cm z mo1-1 sec - l ) at 298 K. For reaction ( l ' a ) an Arrhenius activation energy of (6 -4- 2) kJ mol -~ was determined. The rate for the processes O(aP) + HCl(v = 2)--~ OH(v = 1, 0) + CI(2Pa/2) b
(1")
O(3p) + HCI(v = 1, 0)
was found to be k 1- = (3.8 __+0.6). 10 lz at 298 K. The measurements show that the contribution of HCI(v = 1, 2) molecules to the thermal reaction is small.
Introduction
section upon the relative translational energy of the reactants, is required. Experimental investigations of reactions of state of selected molecules are especially interesting for such internal degrees of freedom for which a n o n e q u i l i b r i u m population of excited states can be maintained for some time in the reacting mixture, so that an e n h a n c e d chemical reactivity can compete with the energy transfer rates. While rotational excitation will relax to equilibrium within a few collisions, longer lifetimes can be expected for vibrational degrees of freedom. Nonequilibrium populations in vibrational levels are generated in many exothermic elementary steps taking place during a combustion process. Observation of the primary energy distribution of such reactions and application of the principle of microscopic reversibility allows the determination of spe-
Most measurements of the energy dependence of chemical reaction rates have been carried out u n d e r conditions where the rate of reaction is slow compared to that of collisional energy transfer. The Arrhenius parameters obtained in this way, however, contain no direct information on how the various degrees of freedom of the reacting molecules contribute to overcoming the potential energy barrier of the chemical reaction. For a better understanding of the exact temperature behaviour of chemical reaction rates over the extended temperature range covered by combustion processes, detailed information on the role of internal degrees of freedom in the reaction, as well as on the accurate dependence of the reactive cross 497
498
KINETICS
cific rate constants in the reverse endothermic direction. More direct measurements of such specific rate constants, also applicable to thermoneutral and exothermic reactions, are possible using the high photon flux of infrared lasers for specific vibrational excitation by optical pumping. The important parameters which can be extracted from such experiments are the absolute rates for the reactive and inelastic pathways and their variation with temperature. From the temperature dependence of vibrationally enhanced reaction rates information can be obtained as to whether the vibrational excitation not only reduces the energy threshold but also changes the reactive cross section. In the experiments, the oxidation of a simple diatom~c molecule (HC1) by oxygen atoms is used as a model system for elementary combustion reactions.
a thermostated (200-400 K) flow reactor. Laser pulses from an electrically excited chemical HC1 laser were used to populate higher (v - 1) vibrational levels of HCI. The decay of the HCI(v) in the presence or absence of the atoms or radicals is monitored as a function of time, using a liquid nitrogen cooled InSb photovoltaic detector (Texas Instr.). The detector signal is amplified and averaged over typically 100 laser shots on a Datalab DL 102 signal averager. The signal to noise of all fluorescence signals was >10 for PHct > 10 m tort. The exponential decay times of the infrared fluorescence were determined by comparison Of the decay oscillograms with the decay signal generated by a calibrated RC-unit. The v = 2 ~ 1 fluorescence is separated by a cold gas HCI filter in front of the InSb detector. The fraction of laser excited HCI molecules, determined by comparing the intensities of the v = 1 --~ 0 and v = 2 ~ 1 fluorescence [1], varied between 5" 10-~ and 10 -~ depending on the laser intensity.
II. Experimental The experiments were performed using discharge flow systems in three different arrangements shown in Fig. la-c.
Laser-induced Infrared Fluorescence As illustrated in Fig. la HCI molecules in the vibrational ground state are mixed with oxygen atoms from two parallel microwave discharges in
He,O2(H2)
Time-resolved Mass Spectroscopy For the direct observation of the reaction products from the vibrationally excited reactants, the thermostated (200-500 K) flow tube (Fig. lb) is coupled to a quadrupole mass spectrometer (Extranuclear 270-9) by a molecular beam sampling system. During the experiment the intensity of a certain m/epeak is followed as function of time starting with the onset of the laser pulse. A signal is obtained
He,O2(H2)
NO -j j.~A~ so P PM /
FILTER " J V
i,~
MIRROR
I
PUMP HCI-LASER
PUMP FIG. la. Schematic of the discharge-flow system with infrared-fluorescence detection of laser excited HCl(v) molecules.
ABSOLUTE RATES FOR T H E REACTION OF O-ATOMS
499
He, 0 2 MWD
IR-DETECTOR
_--THERMOSTATE
HCt{ v = O ) ~
HCI
NO 2
PUMPS
~___{
t
t
L-
~
i
HCl, He
THERMOSTATED FLOW REACTOR
FLUORESCENCE CELL
20 kV
,I
i
T
I
i
:
MASS FILTER
0.2~F
CuSOt.- SOLUTION
,
o
# PUMP
H Cl- LASER
"-Itl
/j
H2/Cl 2
FIe. lb. Schematic of the arrangement for time-resolved mass spectrometric analysis of the HCI(v) reactions.
during the transit time of the volume excited by the laser pulse (10 mm diam., 150 mm long) as it passes the sampling nozzle. With a time resolution of 10 ~s the detection limit for H ~ Cl(m/e = 36} is 5.10 '1 molecules cm -a. The laser used here consists of a plexiglass tube of 2 m active length with CaF~ Brewster's angle windows. Laser action is initiated by a transverse electrical discharge [20 J] switched by a thyratron over 1600 tungsten pin electrodes (4 rows equally spaced) electrolytically coupled by a CuSO 4 solution in a flowing Ha/C12 mixture. The optical cavity is formed by a 3 m-radius gold coated mirror and a Ge fiat (50% transmission) 2, 5 m apart. With a 10:1 H a / C l a mixture (total pressure 70 torr) the laser delivers 10 mJ/cm 2 at a repetition rate of one Hz. At the entrance window to the flow tube (distance to the Ge fiat 3 m) the laser beam has a nearly rectangular profile of 25 • 25 mm power half width.
Vacuum-- Ultraviolet Absorption The experimental set up for observation of atoms produced in the reaction of the laser vibrationally excited molecules is shown in Fig, lc. Atomic resonance emission is obtained from a microwave discharge in a mixture of He with traces of C12 molecules. Infrared laser emission and atomic resonance radiation were passed coaxially through the discharge flow reactor, The line intensity was monitored by a solar blind photomultiplier (EMR) behind the exit slit of a 0,3 m vacuum monochromator (McPherson). The absorption signals were accumulated by a signal averager (Datalab 102), The discharge flow systems were coated with metaphosphoric acid to minimize surface-catalyzed atom recombination. The He flow was measured with a calibrated capillary flow meter. Other gas flows were determined by measuring the pressure
500
KINETICS
He,O2 (Hz)
~
INFRARED
H ,Cl
;
/,-
.
.
.
.
.
.
.
.
.
.
.
.
.
.
RESONANCE
LAMP
,.^ _ r a _
OEreCr0R
r
.
.
.
.
.
.
.
.
7~I .
.
.
.
.
.
.
.
.
Plu 2
.
.
.
.
MWD ._
I11.;I
.
"1~, ~ V
.
PUMP
~
I ~
,
MONOCHROMATOR AVERAGER
H 2 , CI 2
SCOPE
HCl - L ASER Fie. lc. Schematic of the atomic resonance absorption arrangement for detection of chlorine atoms formed in reactions of HCI (v).
increase in a calibrated volume. The total pressure in the flow tube (typically around 3 torr) was measured with a calibrated Wallace and Tierman vacuum gauge. The flow velocity in the flow tubes varied between 5 and 20 m/s. Gases with the best commercial purity were used, He (99,9996%), O~, H 2 (99,999%) from Messer-Griesheim, HC1 (99,99%) and NO~ (99,5%) from Matheson.
ExperimentalResultsandDiscussion
for overcoming the potential energy barrier of reaction (1). When HC1 (v = 1) molecules are generated in the flow system by absorption of the laser pulse, the decay of HCI (v = 1) is significantly accelerated in the presence of oxygen atoms. Semilog plots of the HC1 (v = 1 ~ 0) infrared fluorescence intensity were linear over almost two decades. As shown in Fig. 2 the subtracted decay constants (with and without atomic oxygen) vary linearly with the atom concentration and give a value for ~z
Ground state oxygen atoms react relatively slowly with thermal HCI at room temperature. The rate of the reaction
O(aP) + HCI(v = 1)--* OH + C1 b
(1')
3
O ( P ) + HCI(v = 0)
of k l, = (6.0 + 0.I). 1011 cm 3 mol-I s-1 at 298 K O(3P) + HCI---~ OH + C1 n H~ = 3,3 kJ m o l - I
(1)
has been measured directly by several methods up to temperatures of 720 K. As shown in Table I, there is relatively good agreement among different investigations on the rate at room temperature, while the reported Arrhenius activation energies range from 18, 7 to 31,4 kJ m o l - 1. Since the first vibrational level of HC1 corresponds to 34,3 kJ mol-l, the single quantum excitation should deliver enough energy
In the experiments described here, the second vibrational level in HCI could be directly excited by the absorption of two photons from the HC1 laser via the subsequent steps HC1 (v = 0) h ~-~ HC1 (v = 1) h~'~-I
HCl
(v = 2)
For low HC1 concentrations, the vibrational energy exchange is slow compared to the removal of HCI(v = 2) by O(3P) atoms. The rate for the processes
501
ABSOLUTE RATES FOR T H E REACTION O F O-ATOMS TABLE I Reported thermal rate coefficients for reaction (1)
ReL
Author
[2]
Balakhnin, Egorov, Intezarova [3] Wong, Belles [41 Brown, Smith [51 Singleton, Cvetanovic [6] Hack, Mex, Wagner [7] Ravishankara, Smith, Watson, Davis
Year
Temperature range (K)
1971
295-371
5.3
18.7
1971 1975 1976 1977 1977
365-628 293-440 298-554 293-718 350-454
0.76 0.7 1.4 0.84 0.95
29.4 24.7 25 26.5 31.4
(10 s
k 129s mol - l s - l )
cm 3
E~ (kJ mol - l )
a
HCI(v = 2) + O(~P) ~ O H ( v = 1, 0) + C1
(1")
b
HCI(v = 1, 0) + O(~P)
0.01) were employed. The measured rate k v, is in good agreement with a recent investigation by Macdonald and Moore [9] using the direct v = 0 --9 2 excitation with an optical parametric oscillator.
was determined to be k v, --- (3.8 + 0.6)'10 ~2 cm 3 mol -~ s -~ at 298 K. Mass Spectrometric Determination o f k~,o
The measured rate constant k ~, is in good agreement with values reported b y Smith et al. [8] and Moore et al. [91. The first measurement of k l, in this laboratory [10] and similar experiments by Karny et al. [111 gave a higher value for k~,. However, these experiments have been carried out with higher HCI(v = 0, 1, 2) concentrations. Thus, the high k , o / k 1, ratio allows effective removal of vibrational quanta via the v = 2 state, which results in a decrease of the v = 1 decay time. Compared to [10,11] in the present experiments m u c h lower initial HC1 concentrations (10 -9 mol em -~) and excitation ratios ([HCI(v = 1)] 0 / [ H C I ( v = 0)] ,, <-
As shown in Fig. 3 the consumption of HC1 due to the enhanced reaction rate of vibrationally excited HC1 molecules could be measured directly by timeresolved beam sampling mass spectroscopy. With the [O] o/[HC1] o ratios used in the experiments (s. Table II) the hydroxyl radicals formed in the reactions (l'a) and (l"a) are removed by the step O(3P) + O H ~
02 + H
so that further consumption of HC1 by O H + HC1---> H~O + C1
,2
y
(2)
k 2 = 1013" cm 3 mol -~ s -~ at 298 K
(3)
k 3 = 10 l~~ cm3 mol - l s -~ a t 2 9 8 K [6,7] HCllv=2;'
10 ' A -7 o "
/ 0
8
can be neglected. During the reaction time hydrogen atoms formed in step (2) contribute also less than 10% to the measured HCI consumption by the step
/
H + H C I " * H 2 + CI
6
(4)
k 4 = 10 l~176 cm 3 mol - l s -1 at 298 K. The measured HC1 consumption z~ [HCI] is given by the relation
2
0 ~
~
. . . . . 2 4 6 O - ATOM- CONCENTRATION (10"9mol/cm3}
8
FIG. 2. Observed HCI (v) decay rates as a function o f the oxygen atom concentration: [He] = 1.9" 1 0 - 7 tool cm -a, [HC1] = 1, 5' 10 -9 mol cm -a, [HCI(v = 1)lo: [HCI(v = 0)]o - 10-2, T = 298 K.
a [HCII = [01 o {k,,o [HCl(v = 1)1 o 9r(v = 1)'(1 - e -'/'(v=l)) + kva 9 [HCI(v = 2)] o" "r(v = 2)" (1 - e-'/'(~=z))} (5) where contributions from higher vibrational states (v -> 3) are neglected due to their low population.
502
KINETICS TABLE II Mass spectrometric results Experimental conditions for the consumption of HCI(A [HCI]) in Reaction (l'a) PH~ = 2.7-5.2 Torr, [O] o = 3.7- 1 0 - 9 mol cm -3, x(v = 1) = 260 #s, T = 298 K kl, a
[HCI(v = 1)] o (10 -9 tool cm -a)
[HCI(o = 0)] ~
0.99 1.25 2.33 1.11 2.22 1.11 0.84 1.2 0.57 0.8
10.2 12.9 23.7 23.7 23.7 23.7 8.7 12.0 5.9 8.2
A [HCI] exp. 0.108 0.062 0.135 0.079 0.113 0.056 0.032 0.109 0.033 0.044
A direct measurement of k ~., = (9 • 7)" 10" cm = mol -~ s -~ using an optical parametric oscillator is reported by Macdonald and Moore [9]. By observing the OH(v = 0, 1) radicals formed in the reactions (1, l'a, l"a) and kinetic modeling of the reactive and inelastic channels of the O(3p) + HCI(v = 0, 1, 2) system Butler et al. [12] obtained a higher rate constant ratio of k ~.=/k. = 2.13.104. From this one obtains k~., = 1.9"10 d cm 3 mol -~ s -~ using k~ given in ref. [6]. Table II gives the values for k ~,, obtained from formula (5) using the two reported values for k v,. This results in a rate constant of k~,, = (3.7 • 1.5).10 ~9 cm = mol -~ s -~ at298 K.
m/e
(10 ~9 cm 3 tool - t s - ' ) [or. [9] cot. [121
: 36
11.3 5.2 6.1 7.6 5.3 5.2 4.4 9.6 6.0 5.7
7,7 3,6 4,1 5,6 3,7 4,1 2,6 6,2 4,2 4,4
3,7 1,9 1,8 3,3 2,0 2,8 0,6 2,5 2,2 2,9
Similar measurements of k ~,, have been carried out at 354 K giving a rate constant k t,, = (5,4 • 1,5)" 10 ~9 cm 3 mol -t s -~ from which an Arrhenius activation energy of E~ " = (6 + 2) kJ mol-* was calculated.
Measurement of CI(+ p,/=) Formation by Resonance Absorption The chlorine atoms formed on the reactive pathways can populate two electronic states with an energy difference of 10.5 kJ mol -~ much below the HC1 vibrational quanta. As demonstrated in Fig. 4 both spin-orbit states are populated after laser vibrational excitation. Absolute calibration of the
- LASER + LASER
N
[ct [+P,,+ >] [ct
I
I
I
0
5
10
LASER
[ms]
FIG. 3. Oscillograms showing mass spectrometric measurement of the consumption of HCI in reaction (lX). Concentrations in tool cm-3: [He] = 2,5.10 -7, [HCI]o = 1.3.10 -s, [HCI(v = 1)] o = 9.9"10 -~~ [O]o = 3.7"10 -~, A [ H ~ C I ] = 8.2-10 -H, x(v = 1) = 260 5:50 Its, T = 298 K.
1]
.. 0 / LASER
10
20 [ms]
FiG. 4. Oscillograms showing the formation of CI(2Pz/=) and CI(=PI/2) atoms in reaction (1 ~) Concentrations in tool era-a: [He[ = 1.7" 10 -7, [HC1], --- 2.6"10 -9, [HCI(v = 1)] o --- 3.8"10 -~9, l O l o --6-1o -~~ A [CI(2Pa/2)I = 10 - ~ , A [CI(~P~/z)I = 8-10 -la r ( v - - 1) = 700/xs, T = 2 9 8 K .
ABSOLUTE RATES FOR T H E REACTION O F O-ATOMS
503
TABLE III Atomic resonance line absorption results Experimental conditions: PH~ = 2.7 -- 2.9 Torr, Po = 0.01 Torr, r(v = 1) = 700/~s, T - 298 K
kt,a. [HCI(v --- 0)] o
2.6 1.2 1.5 2.9 12.0
[O] o [HCI(v = I)1 o (10 -9 mol cm -3) 0.6 0.6 0.6 0.6 0.13
A [C12P~/21 (10 -t3 mol cm -~)
0.46 0.~ 0.28 0.~ 2.4
8.0 3.8 4.3 5.2 5.8
absorption signals was obtained by production of known amounts of C1 atoms in the discharge-flow system. The population of the ~Pt/2 level exceeds the thermal population by a factor of 5. As shown in Fig. 4 the CI(2Pt/2) atoms are rapidly deactivated by collisions with HCI. The measured and corrected (for contribution from v = 2) values for 0(3P) + HCI(v = 1 ) ~ OH(v =
O)
+ Cl(2Pt/2)
(10 ~ cm ~ tool -t s -t) exp. cor. [9] 4.1 4.1 3.7 2.3 2.1
t KI
~T
I01A'I,
2.1 2.1 1.9 1.2 1.0
O
HCI (v=l}~Cl' ~ CI*HCr Iv=t,0} o
tO1] !
O ~'~,
j~ ~'~.~
O~k (T) =
k{u
fHCl(
~ [ H C I {v)] - - k ( v : l .
1
101'
(l'a')
are shown in Table III giving a rate constant
~,N
"~
.cv-- ct. Hcr~,0~
HCU~-01
to I
"~'~NO~. / k (v,O. t"1
k v , . = (1,6 + 0,9)" 10Ocm 3 t o o l - ' s - ' at 298 K. I/T
(101 K -l)
Fie. 5. Contribution from HCI(v = 0, 1) to the thermal rate of reaction (5). Data from Ref. [14].
Conclusions The rate of the chemical reaction of O(3P) atoms with HCI increases strongly at room temperature after single quantum vibrational excitation. However, the rate enhancement is much less than the factor exp (E(v = 1)/RT). Since the Arrhenius preexponential factor is not changed significantly by vibrational excitation, the contribution of HCI(v = 1) molecules to the thermal reaction is small for most temperatures of interest. At 200 K thermal excited HCl(v = 1) molecules contribute less than 10-3% and at 2000 K about 10% to the total consumption of HCI by O(3P) atoms. This is also true for HCI(v = 2) molecules. As shown by quasiclassical trajectory calculations [13], the remaining thermal activation energies for HCI(v = 1, 2) are very similar. Despite an increase by a factor around 10" at room temperature, the contribution from HCI(v = 2) in the thermal reaction is below 10% up to 2000 K. However, this finding must not necessarily be valid for other HC1 hydrogen atom abstraction reactions. In similar experiments on the reaction aTCl + H35CI(v = 1 ) ~ H37CI(v = 0, 1) + ~C1
(5')
we have observed [14] that the chemical reaction dominates over vibrational relaxation. Here the contribution from HCI(v = 1) will induce a substantial curvature in the Arrhenius graph as shown in Fig. 5.
Acknowledgment The authors wish to thank Prof. Dr. H. Gg. Wagner for his continuous interest and many helpful discussions. The financial support of the Deutsche Forschungsgcmeinsehaft is gratefully acknowledged.
REFERENCES [1] ARNOLDI,D. ANDWOLFRUM,J.: Ber. Bunsenges. Phys. Chem. 80, 899. (1976) [2] BALAKHNIN,V. P., EGOBOV,B. I., INTEZABOVA,E. I.: Kinetics Catalysis I~, 258 (1971) [3] WONG, E. L., BELLES, F. E.: NASA Technical Note D-6495 (1971)
504
KINETICS
[4] BROWN,R.D.H., SMITH, I.W.M.: Int. J. Chem. Kinet. 2, 301 (1975) [5] SINGLETON,D. L., CVETAbdOVIC,R.].: 12th Informal Conference on Photochemistry F 8 (1976) [6] HACK,W., MET, G., WACNER,H. GC.: Ber. Bunsenges. Phys. Chemie 81, 677 (1977) [7] RAVlSHANKARA,A. R., SMITH, G., WATSON, R. T., AND DAVIES, D. D.: J. Phys. Chem. 81, 2220 (1977) [8] BROWN,R.D.H., GLASS,G. P. AND SMITH, I.W.M.: Chem. Phys. Lett. 32, 517 (1975) [9] McDONALD,R. G., AND MOORE, C. B.: J. Chem.
Phys. 68, 513 (1978) [10] ARNOLD1, D., AND WOLFRUM, J.: Chem. Phys. Lett. 35, 104) (1975) [11] KARNV,Z., KATZ,B. ANDSZ6KE, A.: Chem. Phys. Lett. 35, 100 (1975) [12] BUTLER,]. E., l'luoc~Ns, J. W., LIN, M. C., AND SMITh, G. K.: Chem. Phys. Lett. 58, 216 (1978) [13] BROWN, R. D. H., AND SMITH, I. W. W.: Int. J. of Chem. Kin., 10, 1 (1978) [14] KNERA, M. AND WOLFRVM,J.: J. Phys. Chem. 83, 69 (1979)
COMMENTS F. Kaufman, Univ. of Pittsburgh, USA. Your correlation diagram shows the states of HOC1 as intermediates between reactant O + HC1 and product OH + C1 even though the reaction seems to be a simple H-transfer step, at least for O ('~P), for which HOC1 has the wrong geometry. Could you comment? Authors" Reply. The approach of the reactants O(aP) and HCI on a triplet surface followed by a non-adiabatic transition to the singlet HOCI surface as an intermediate complex has been discussed as the possible origin of the potential energy barrier in reaction (1). However, the fact that this crossing point appears necessarily to be lower than the saddle
point of the lowest triplet surface is of course an artifact of the single coordinate correlation diagram. The experimental results on C1 + OH [1] and the observed formation of OH(v = 1) from O(aP) + HCI(v = 2) [2] indicate that the reaction occurs predominantly adiabatically on a triplet surface.
REFERENCES [1] BLACKWELL,B. A., POLANYI,J. C. ANDSLOAN,J. J.: Chem. Phys. 24, 25(1977). [2] BUTLER, 1" E., HUO~;ENS, J. W., LIN, M. C. ANO SMITh, G. K.: Chem. Phys. Lett. 58, 216 (1978)
Max-Planck-Institut fiir Strdmungsforschung, 3400---Giittingen, W. Germany
The reaction between ground state oxygen atoms and vibrationally excited HCI molecules was studied directly in a fast discharge-flow reactor using a chemical HC1 laser for vibrational excitation of HCI by optical pumping. Time-resolved infrared fluorescence, nozzle-beam mass spectroscopy a n d atomic line resonance absorption were used for quantitative detection of the reactants and products. For HC1 (v = 1) the measured rate constants for the elementary steps O(~P) + HCI(v = 1)--~ OH(v = 0) + CI(zP3/2)
(1')
a"
OH(v = 0) + C1(2p~/2) b
3
--~O(P)+HCI(v=0) were: k~, = (6.0 + 0.1)'10 H, kl, ~ = (3.7 + 1.5)'101~ kva. = (1.6 + 0.9).109, k t , b = (5.6 -+ 0.3)'1011 (cm z mo1-1 sec - l ) at 298 K. For reaction ( l ' a ) an Arrhenius activation energy of (6 -4- 2) kJ mol -~ was determined. The rate for the processes O(aP) + HCl(v = 2)--~ OH(v = 1, 0) + CI(2Pa/2) b
(1")
O(3p) + HCI(v = 1, 0)
was found to be k 1- = (3.8 __+0.6). 10 lz at 298 K. The measurements show that the contribution of HCI(v = 1, 2) molecules to the thermal reaction is small.
Introduction
section upon the relative translational energy of the reactants, is required. Experimental investigations of reactions of state of selected molecules are especially interesting for such internal degrees of freedom for which a n o n e q u i l i b r i u m population of excited states can be maintained for some time in the reacting mixture, so that an e n h a n c e d chemical reactivity can compete with the energy transfer rates. While rotational excitation will relax to equilibrium within a few collisions, longer lifetimes can be expected for vibrational degrees of freedom. Nonequilibrium populations in vibrational levels are generated in many exothermic elementary steps taking place during a combustion process. Observation of the primary energy distribution of such reactions and application of the principle of microscopic reversibility allows the determination of spe-
Most measurements of the energy dependence of chemical reaction rates have been carried out u n d e r conditions where the rate of reaction is slow compared to that of collisional energy transfer. The Arrhenius parameters obtained in this way, however, contain no direct information on how the various degrees of freedom of the reacting molecules contribute to overcoming the potential energy barrier of the chemical reaction. For a better understanding of the exact temperature behaviour of chemical reaction rates over the extended temperature range covered by combustion processes, detailed information on the role of internal degrees of freedom in the reaction, as well as on the accurate dependence of the reactive cross 497
498
KINETICS
cific rate constants in the reverse endothermic direction. More direct measurements of such specific rate constants, also applicable to thermoneutral and exothermic reactions, are possible using the high photon flux of infrared lasers for specific vibrational excitation by optical pumping. The important parameters which can be extracted from such experiments are the absolute rates for the reactive and inelastic pathways and their variation with temperature. From the temperature dependence of vibrationally enhanced reaction rates information can be obtained as to whether the vibrational excitation not only reduces the energy threshold but also changes the reactive cross section. In the experiments, the oxidation of a simple diatom~c molecule (HC1) by oxygen atoms is used as a model system for elementary combustion reactions.
a thermostated (200-400 K) flow reactor. Laser pulses from an electrically excited chemical HC1 laser were used to populate higher (v - 1) vibrational levels of HCI. The decay of the HCI(v) in the presence or absence of the atoms or radicals is monitored as a function of time, using a liquid nitrogen cooled InSb photovoltaic detector (Texas Instr.). The detector signal is amplified and averaged over typically 100 laser shots on a Datalab DL 102 signal averager. The signal to noise of all fluorescence signals was >10 for PHct > 10 m tort. The exponential decay times of the infrared fluorescence were determined by comparison Of the decay oscillograms with the decay signal generated by a calibrated RC-unit. The v = 2 ~ 1 fluorescence is separated by a cold gas HCI filter in front of the InSb detector. The fraction of laser excited HCI molecules, determined by comparing the intensities of the v = 1 --~ 0 and v = 2 ~ 1 fluorescence [1], varied between 5" 10-~ and 10 -~ depending on the laser intensity.
II. Experimental The experiments were performed using discharge flow systems in three different arrangements shown in Fig. la-c.
Laser-induced Infrared Fluorescence As illustrated in Fig. la HCI molecules in the vibrational ground state are mixed with oxygen atoms from two parallel microwave discharges in
He,O2(H2)
Time-resolved Mass Spectroscopy For the direct observation of the reaction products from the vibrationally excited reactants, the thermostated (200-500 K) flow tube (Fig. lb) is coupled to a quadrupole mass spectrometer (Extranuclear 270-9) by a molecular beam sampling system. During the experiment the intensity of a certain m/epeak is followed as function of time starting with the onset of the laser pulse. A signal is obtained
He,O2(H2)
NO -j j.~A~ so P PM /
FILTER " J V
i,~
MIRROR
I
PUMP HCI-LASER
PUMP FIG. la. Schematic of the discharge-flow system with infrared-fluorescence detection of laser excited HCl(v) molecules.
ABSOLUTE RATES FOR T H E REACTION OF O-ATOMS
499
He, 0 2 MWD
IR-DETECTOR
_--THERMOSTATE
HCt{ v = O ) ~
HCI
NO 2
PUMPS
~___{
t
t
L-
~
i
HCl, He
THERMOSTATED FLOW REACTOR
FLUORESCENCE CELL
20 kV
,I
i
T
I
i
:
MASS FILTER
0.2~F
CuSOt.- SOLUTION
,
o
# PUMP
H Cl- LASER
"-Itl
/j
H2/Cl 2
FIe. lb. Schematic of the arrangement for time-resolved mass spectrometric analysis of the HCI(v) reactions.
during the transit time of the volume excited by the laser pulse (10 mm diam., 150 mm long) as it passes the sampling nozzle. With a time resolution of 10 ~s the detection limit for H ~ Cl(m/e = 36} is 5.10 '1 molecules cm -a. The laser used here consists of a plexiglass tube of 2 m active length with CaF~ Brewster's angle windows. Laser action is initiated by a transverse electrical discharge [20 J] switched by a thyratron over 1600 tungsten pin electrodes (4 rows equally spaced) electrolytically coupled by a CuSO 4 solution in a flowing Ha/C12 mixture. The optical cavity is formed by a 3 m-radius gold coated mirror and a Ge fiat (50% transmission) 2, 5 m apart. With a 10:1 H a / C l a mixture (total pressure 70 torr) the laser delivers 10 mJ/cm 2 at a repetition rate of one Hz. At the entrance window to the flow tube (distance to the Ge fiat 3 m) the laser beam has a nearly rectangular profile of 25 • 25 mm power half width.
Vacuum-- Ultraviolet Absorption The experimental set up for observation of atoms produced in the reaction of the laser vibrationally excited molecules is shown in Fig, lc. Atomic resonance emission is obtained from a microwave discharge in a mixture of He with traces of C12 molecules. Infrared laser emission and atomic resonance radiation were passed coaxially through the discharge flow reactor, The line intensity was monitored by a solar blind photomultiplier (EMR) behind the exit slit of a 0,3 m vacuum monochromator (McPherson). The absorption signals were accumulated by a signal averager (Datalab 102), The discharge flow systems were coated with metaphosphoric acid to minimize surface-catalyzed atom recombination. The He flow was measured with a calibrated capillary flow meter. Other gas flows were determined by measuring the pressure
500
KINETICS
He,O2 (Hz)
~
INFRARED
H ,Cl
;
/,-
.
.
.
.
.
.
.
.
.
.
.
.
.
.
RESONANCE
LAMP
,.^ _ r a _
OEreCr0R
r
.
.
.
.
.
.
.
.
7~I .
.
.
.
.
.
.
.
.
Plu 2
.
.
.
.
MWD ._
I11.;I
.
"1~, ~ V
.
PUMP
~
I ~
,
MONOCHROMATOR AVERAGER
H 2 , CI 2
SCOPE
HCl - L ASER Fie. lc. Schematic of the atomic resonance absorption arrangement for detection of chlorine atoms formed in reactions of HCI (v).
increase in a calibrated volume. The total pressure in the flow tube (typically around 3 torr) was measured with a calibrated Wallace and Tierman vacuum gauge. The flow velocity in the flow tubes varied between 5 and 20 m/s. Gases with the best commercial purity were used, He (99,9996%), O~, H 2 (99,999%) from Messer-Griesheim, HC1 (99,99%) and NO~ (99,5%) from Matheson.
ExperimentalResultsandDiscussion
for overcoming the potential energy barrier of reaction (1). When HC1 (v = 1) molecules are generated in the flow system by absorption of the laser pulse, the decay of HCI (v = 1) is significantly accelerated in the presence of oxygen atoms. Semilog plots of the HC1 (v = 1 ~ 0) infrared fluorescence intensity were linear over almost two decades. As shown in Fig. 2 the subtracted decay constants (with and without atomic oxygen) vary linearly with the atom concentration and give a value for ~z
Ground state oxygen atoms react relatively slowly with thermal HCI at room temperature. The rate of the reaction
O(aP) + HCI(v = 1)--* OH + C1 b
(1')
3
O ( P ) + HCI(v = 0)
of k l, = (6.0 + 0.I). 1011 cm 3 mol-I s-1 at 298 K O(3P) + HCI---~ OH + C1 n H~ = 3,3 kJ m o l - I
(1)
has been measured directly by several methods up to temperatures of 720 K. As shown in Table I, there is relatively good agreement among different investigations on the rate at room temperature, while the reported Arrhenius activation energies range from 18, 7 to 31,4 kJ m o l - 1. Since the first vibrational level of HC1 corresponds to 34,3 kJ mol-l, the single quantum excitation should deliver enough energy
In the experiments described here, the second vibrational level in HCI could be directly excited by the absorption of two photons from the HC1 laser via the subsequent steps HC1 (v = 0) h ~-~ HC1 (v = 1) h~'~-I
HCl
(v = 2)
For low HC1 concentrations, the vibrational energy exchange is slow compared to the removal of HCI(v = 2) by O(3P) atoms. The rate for the processes
501
ABSOLUTE RATES FOR T H E REACTION O F O-ATOMS TABLE I Reported thermal rate coefficients for reaction (1)
ReL
Author
[2]
Balakhnin, Egorov, Intezarova [3] Wong, Belles [41 Brown, Smith [51 Singleton, Cvetanovic [6] Hack, Mex, Wagner [7] Ravishankara, Smith, Watson, Davis
Year
Temperature range (K)
1971
295-371
5.3
18.7
1971 1975 1976 1977 1977
365-628 293-440 298-554 293-718 350-454
0.76 0.7 1.4 0.84 0.95
29.4 24.7 25 26.5 31.4
(10 s
k 129s mol - l s - l )
cm 3
E~ (kJ mol - l )
a
HCI(v = 2) + O(~P) ~ O H ( v = 1, 0) + C1
(1")
b
HCI(v = 1, 0) + O(~P)
0.01) were employed. The measured rate k v, is in good agreement with a recent investigation by Macdonald and Moore [9] using the direct v = 0 --9 2 excitation with an optical parametric oscillator.
was determined to be k v, --- (3.8 + 0.6)'10 ~2 cm 3 mol -~ s -~ at 298 K. Mass Spectrometric Determination o f k~,o
The measured rate constant k ~, is in good agreement with values reported b y Smith et al. [8] and Moore et al. [91. The first measurement of k l, in this laboratory [10] and similar experiments by Karny et al. [111 gave a higher value for k~,. However, these experiments have been carried out with higher HCI(v = 0, 1, 2) concentrations. Thus, the high k , o / k 1, ratio allows effective removal of vibrational quanta via the v = 2 state, which results in a decrease of the v = 1 decay time. Compared to [10,11] in the present experiments m u c h lower initial HC1 concentrations (10 -9 mol em -~) and excitation ratios ([HCI(v = 1)] 0 / [ H C I ( v = 0)] ,, <-
As shown in Fig. 3 the consumption of HC1 due to the enhanced reaction rate of vibrationally excited HC1 molecules could be measured directly by timeresolved beam sampling mass spectroscopy. With the [O] o/[HC1] o ratios used in the experiments (s. Table II) the hydroxyl radicals formed in the reactions (l'a) and (l"a) are removed by the step O(3P) + O H ~
02 + H
so that further consumption of HC1 by O H + HC1---> H~O + C1
,2
y
(2)
k 2 = 1013" cm 3 mol -~ s -~ at 298 K
(3)
k 3 = 10 l~~ cm3 mol - l s -~ a t 2 9 8 K [6,7] HCllv=2;'
10 ' A -7 o "
/ 0
8
can be neglected. During the reaction time hydrogen atoms formed in step (2) contribute also less than 10% to the measured HCI consumption by the step
/
H + H C I " * H 2 + CI
6
(4)
k 4 = 10 l~176 cm 3 mol - l s -1 at 298 K. The measured HC1 consumption z~ [HCI] is given by the relation
2
0 ~
~
. . . . . 2 4 6 O - ATOM- CONCENTRATION (10"9mol/cm3}
8
FIG. 2. Observed HCI (v) decay rates as a function o f the oxygen atom concentration: [He] = 1.9" 1 0 - 7 tool cm -a, [HC1] = 1, 5' 10 -9 mol cm -a, [HCI(v = 1)lo: [HCI(v = 0)]o - 10-2, T = 298 K.
a [HCII = [01 o {k,,o [HCl(v = 1)1 o 9r(v = 1)'(1 - e -'/'(v=l)) + kva 9 [HCI(v = 2)] o" "r(v = 2)" (1 - e-'/'(~=z))} (5) where contributions from higher vibrational states (v -> 3) are neglected due to their low population.
502
KINETICS TABLE II Mass spectrometric results Experimental conditions for the consumption of HCI(A [HCI]) in Reaction (l'a) PH~ = 2.7-5.2 Torr, [O] o = 3.7- 1 0 - 9 mol cm -3, x(v = 1) = 260 #s, T = 298 K kl, a
[HCI(v = 1)] o (10 -9 tool cm -a)
[HCI(o = 0)] ~
0.99 1.25 2.33 1.11 2.22 1.11 0.84 1.2 0.57 0.8
10.2 12.9 23.7 23.7 23.7 23.7 8.7 12.0 5.9 8.2
A [HCI] exp. 0.108 0.062 0.135 0.079 0.113 0.056 0.032 0.109 0.033 0.044
A direct measurement of k ~., = (9 • 7)" 10" cm = mol -~ s -~ using an optical parametric oscillator is reported by Macdonald and Moore [9]. By observing the OH(v = 0, 1) radicals formed in the reactions (1, l'a, l"a) and kinetic modeling of the reactive and inelastic channels of the O(3p) + HCI(v = 0, 1, 2) system Butler et al. [12] obtained a higher rate constant ratio of k ~.=/k. = 2.13.104. From this one obtains k~., = 1.9"10 d cm 3 mol -~ s -~ using k~ given in ref. [6]. Table II gives the values for k ~,, obtained from formula (5) using the two reported values for k v,. This results in a rate constant of k~,, = (3.7 • 1.5).10 ~9 cm = mol -~ s -~ at298 K.
m/e
(10 ~9 cm 3 tool - t s - ' ) [or. [9] cot. [121
: 36
11.3 5.2 6.1 7.6 5.3 5.2 4.4 9.6 6.0 5.7
7,7 3,6 4,1 5,6 3,7 4,1 2,6 6,2 4,2 4,4
3,7 1,9 1,8 3,3 2,0 2,8 0,6 2,5 2,2 2,9
Similar measurements of k ~,, have been carried out at 354 K giving a rate constant k t,, = (5,4 • 1,5)" 10 ~9 cm 3 mol -t s -~ from which an Arrhenius activation energy of E~ " = (6 + 2) kJ mol-* was calculated.
Measurement of CI(+ p,/=) Formation by Resonance Absorption The chlorine atoms formed on the reactive pathways can populate two electronic states with an energy difference of 10.5 kJ mol -~ much below the HC1 vibrational quanta. As demonstrated in Fig. 4 both spin-orbit states are populated after laser vibrational excitation. Absolute calibration of the
- LASER + LASER
N
[ct [+P,,+ >] [ct
I
I
I
0
5
10
LASER
[ms]
FIG. 3. Oscillograms showing mass spectrometric measurement of the consumption of HCI in reaction (lX). Concentrations in tool cm-3: [He] = 2,5.10 -7, [HCI]o = 1.3.10 -s, [HCI(v = 1)] o = 9.9"10 -~~ [O]o = 3.7"10 -~, A [ H ~ C I ] = 8.2-10 -H, x(v = 1) = 260 5:50 Its, T = 298 K.
1]
.. 0 / LASER
10
20 [ms]
FiG. 4. Oscillograms showing the formation of CI(2Pz/=) and CI(=PI/2) atoms in reaction (1 ~) Concentrations in tool era-a: [He[ = 1.7" 10 -7, [HC1], --- 2.6"10 -9, [HCI(v = 1)] o --- 3.8"10 -~9, l O l o --6-1o -~~ A [CI(2Pa/2)I = 10 - ~ , A [CI(~P~/z)I = 8-10 -la r ( v - - 1) = 700/xs, T = 2 9 8 K .
ABSOLUTE RATES FOR T H E REACTION O F O-ATOMS
503
TABLE III Atomic resonance line absorption results Experimental conditions: PH~ = 2.7 -- 2.9 Torr, Po = 0.01 Torr, r(v = 1) = 700/~s, T - 298 K
kt,a. [HCI(v --- 0)] o
2.6 1.2 1.5 2.9 12.0
[O] o [HCI(v = I)1 o (10 -9 mol cm -3) 0.6 0.6 0.6 0.6 0.13
A [C12P~/21 (10 -t3 mol cm -~)
0.46 0.~ 0.28 0.~ 2.4
8.0 3.8 4.3 5.2 5.8
absorption signals was obtained by production of known amounts of C1 atoms in the discharge-flow system. The population of the ~Pt/2 level exceeds the thermal population by a factor of 5. As shown in Fig. 4 the CI(2Pt/2) atoms are rapidly deactivated by collisions with HCI. The measured and corrected (for contribution from v = 2) values for 0(3P) + HCI(v = 1 ) ~ OH(v =
O)
+ Cl(2Pt/2)
(10 ~ cm ~ tool -t s -t) exp. cor. [9] 4.1 4.1 3.7 2.3 2.1
t KI
~T
I01A'I,
2.1 2.1 1.9 1.2 1.0
O
HCI (v=l}~Cl' ~ CI*HCr Iv=t,0} o
tO1] !
O ~'~,
j~ ~'~.~
O~k (T) =
k{u
fHCl(
~ [ H C I {v)] - - k ( v : l .
1
101'
(l'a')
are shown in Table III giving a rate constant
~,N
"~
.cv-- ct. Hcr~,0~
HCU~-01
to I
"~'~NO~. / k (v,O. t"1
k v , . = (1,6 + 0,9)" 10Ocm 3 t o o l - ' s - ' at 298 K. I/T
(101 K -l)
Fie. 5. Contribution from HCI(v = 0, 1) to the thermal rate of reaction (5). Data from Ref. [14].
Conclusions The rate of the chemical reaction of O(3P) atoms with HCI increases strongly at room temperature after single quantum vibrational excitation. However, the rate enhancement is much less than the factor exp (E(v = 1)/RT). Since the Arrhenius preexponential factor is not changed significantly by vibrational excitation, the contribution of HCI(v = 1) molecules to the thermal reaction is small for most temperatures of interest. At 200 K thermal excited HCl(v = 1) molecules contribute less than 10-3% and at 2000 K about 10% to the total consumption of HCI by O(3P) atoms. This is also true for HCI(v = 2) molecules. As shown by quasiclassical trajectory calculations [13], the remaining thermal activation energies for HCI(v = 1, 2) are very similar. Despite an increase by a factor around 10" at room temperature, the contribution from HCI(v = 2) in the thermal reaction is below 10% up to 2000 K. However, this finding must not necessarily be valid for other HC1 hydrogen atom abstraction reactions. In similar experiments on the reaction aTCl + H35CI(v = 1 ) ~ H37CI(v = 0, 1) + ~C1
(5')
we have observed [14] that the chemical reaction dominates over vibrational relaxation. Here the contribution from HCI(v = 1) will induce a substantial curvature in the Arrhenius graph as shown in Fig. 5.
Acknowledgment The authors wish to thank Prof. Dr. H. Gg. Wagner for his continuous interest and many helpful discussions. The financial support of the Deutsche Forschungsgcmeinsehaft is gratefully acknowledged.
REFERENCES [1] ARNOLDI,D. ANDWOLFRUM,J.: Ber. Bunsenges. Phys. Chem. 80, 899. (1976) [2] BALAKHNIN,V. P., EGOBOV,B. I., INTEZABOVA,E. I.: Kinetics Catalysis I~, 258 (1971) [3] WONG, E. L., BELLES, F. E.: NASA Technical Note D-6495 (1971)
504
KINETICS
[4] BROWN,R.D.H., SMITH, I.W.M.: Int. J. Chem. Kinet. 2, 301 (1975) [5] SINGLETON,D. L., CVETAbdOVIC,R.].: 12th Informal Conference on Photochemistry F 8 (1976) [6] HACK,W., MET, G., WACNER,H. GC.: Ber. Bunsenges. Phys. Chemie 81, 677 (1977) [7] RAVlSHANKARA,A. R., SMITH, G., WATSON, R. T., AND DAVIES, D. D.: J. Phys. Chem. 81, 2220 (1977) [8] BROWN,R.D.H., GLASS,G. P. AND SMITH, I.W.M.: Chem. Phys. Lett. 32, 517 (1975) [9] McDONALD,R. G., AND MOORE, C. B.: J. Chem.
Phys. 68, 513 (1978) [10] ARNOLD1, D., AND WOLFRUM, J.: Chem. Phys. Lett. 35, 104) (1975) [11] KARNV,Z., KATZ,B. ANDSZ6KE, A.: Chem. Phys. Lett. 35, 100 (1975) [12] BUTLER,]. E., l'luoc~Ns, J. W., LIN, M. C., AND SMITh, G. K.: Chem. Phys. Lett. 58, 216 (1978) [13] BROWN, R. D. H., AND SMITH, I. W. W.: Int. J. of Chem. Kin., 10, 1 (1978) [14] KNERA, M. AND WOLFRVM,J.: J. Phys. Chem. 83, 69 (1979)
COMMENTS F. Kaufman, Univ. of Pittsburgh, USA. Your correlation diagram shows the states of HOC1 as intermediates between reactant O + HC1 and product OH + C1 even though the reaction seems to be a simple H-transfer step, at least for O ('~P), for which HOC1 has the wrong geometry. Could you comment? Authors" Reply. The approach of the reactants O(aP) and HCI on a triplet surface followed by a non-adiabatic transition to the singlet HOCI surface as an intermediate complex has been discussed as the possible origin of the potential energy barrier in reaction (1). However, the fact that this crossing point appears necessarily to be lower than the saddle
point of the lowest triplet surface is of course an artifact of the single coordinate correlation diagram. The experimental results on C1 + OH [1] and the observed formation of OH(v = 1) from O(aP) + HCI(v = 2) [2] indicate that the reaction occurs predominantly adiabatically on a triplet surface.
REFERENCES [1] BLACKWELL,B. A., POLANYI,J. C. ANDSLOAN,J. J.: Chem. Phys. 24, 25(1977). [2] BUTLER, 1" E., HUO~;ENS, J. W., LIN, M. C. ANO SMITh, G. K.: Chem. Phys. Lett. 58, 216 (1978)