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The reaction of O2(1Δg) with atomic nitrogen and with atomic oxygen
Volume 3, number 6
June 1969
CHEiKICAL PHYSICS LETTERS
THE
REACTION
OF AND
O&hg)
WITH
WITH
ATOMIC
ATOMIC
NITROGEN
OXYGEN
I. D. CLARK and R. P. WAYNE P?zysicaZ Chemistry Laboratory, South Parks Road, Oxfmd OX1 3QZ. UK Received 9 May 1969
Atomic N and 0 quench 02(b ) with room temperature rate constants of 2.8 f 2.0 x 10B16 and c 1 3 x lo-16cm3molec-lsec-1 8 espectlvely; NO cannot. therefore, be formed efficiently m the atmospheric D-region by reactlou of N with 02(hg).
Considerable concentrations of 02(‘Ag) exist in the upper atmosphere; recent laboratory determmations [l-3] of the rates of physical quenching of 02(lAg) by atmospheric gases are consistent with those derived [4] from observations of the variation of [02(lAg)] m the atmosphere with altitude between about 30 km and 80 km. At higher altitudes, however, the relatively high abundancies of atomic nitrogen and oxygen suggest the possibility that these species could contribute to the decay of 02(lAg). Further, the reaction of atomic r-itrogen with 02(lAg) could lead to the formation of NO N + 02(lAg)
-NO
+ 0
(1)
and it has been postulated [5] that reaction (1) produces significant concentrations of NO in the D-reaon of the atmosphere. In this paper we report measurements of the rate constants for the deactivation of 02(‘Ag) by N and by 0.
Q + o,&,) 2
products .
[Q = N or 0]
(2)
The rate constant for (1) must be equal to or less than the overall rate constant for deactivation by N atoms. A modified form of the diluted-discharge flow apparatus [3], shown in fig. 1, was used to measure the rate of reaction (2) with Q = N and Q = 0. The high sensitivity for 02(lAg) of the photoionization detector permitted measurements to be made with [02(1ag)] << [Q]. A radiofrequency discharge in an oxygen-helium mixture was used to produce 02(lAg) in an inlet tube
from which gases entered the main flow tube, and the relative [02(lAg)] were measured at a point 116 cm downstream from the inlet pomt by measurmg the ion current (48 V collector potential across Pt electrodes) consequent upon exposure to Ar resonance radiation. The AR resonance lines at 11.70, 11.61 eV ionize 02(lAg) but not ground state 02, N2, He, N or 0. A Kr resonance lamp (10.64, 10.10 eV) may be used to detect lower ionization potential impurities such as NO. Atomic nitrogen is introduced into the main tube 6 cm upstream from the 02(lA,) iniet: a set of capillaries with controlling taps i% used
Fig. 1. Apparatus.
405
Volume 3, number 6
CHEMICAL
PHYSfCS
to divert varying amounts of the nitrogen flow through a microwave discharge tube. Under the experimental co&&ions employed (fO2] from rf discharge tube only a few percent of the total concentration) it can be shown [3] that the products of the He-02 discharge cause a negligible decay of 02(lAg) in the main tube, and that, apart from the small contribution from NO (formed from the trace N2 impurity in the 02), the ionization current, i, produced by the Ar lamp is proportional to [02(lAg)]. The concentration of 02(lAg) at the point of mixing with the N-N2 flow was estimated to be 1 - 5 x lo4 Torr by comparison of the relative [02(‘Ag)] produced by the rf discharge with that produced by a 100 watt microwave discharge which is known[6] to form 5 - 10% of 03(‘Ag). Since the N or 0 atom concentrations at this point were found by NO titration to be 0.5 - 3 x 10-2 Torr, it is apparent that the condition [02(lAg)] CC [Q] is well satisfied. At any distance x downstream from the inlet point, [02(1Ag)], in the presence of atoms, Q, is given by (3) where k, IS the first order decay constant for 02(lAg) in the absence of atoms and v the linear flow velocity. It is therefore apparent that for ion currents measured at the pomt x = 116 cm ln i/zQ kQ = (4) 116 I/u 6
[Q]dx
where z is the 02(IAg) ion current m the absence of atoms and iQ is the current measured when Q is present. The dependence of [Q] on x, required to evalu&t.e the integral in (4), was determined for 0 atoms by monitoring the mtensity of the Ot NO afterglow, and for N atoms by NO titration. The decay of [N] was first order, and due mainly to reaction with ground state 02
LETTERS
June 1969
main flow t&e. Slow flow velocities (22 cm s-1) were required in order to measure i/i& accurately. At these velocities, the atom concentrations in the photoionization region were sufficiently small that the background electrometer current produced by atom recombination on the cathode was only a small fraction of the photoionization current. No ion current was produced by the Kr lamp when N atoms were present, thus indicating that all the NO produced in reactions (1) and (5) is rapidly removed by reaction (6). The values of ln i/Q measured for four different 0 and N atom concentrations are shown in fig. 2. Twice the standard deviation of the points given in fig. 2 for 0 atoms yields kg c 1.3 x lo-16 cm3 molec-lsec-l
.
Quenching of 02(lAg) by 0 or 03 is therefore clearly negligible in the N atom experiments. The slope of the line formed by the N atom points in fig. 2, combined with the uncertainty 111 [N]o yields kN = 2.8 f 2.0 X lo-l5
cm3 molec-lsec-l
.
Both valnes for kQ refer to a temperature of 297 * 20K. The value for kN must represent an upper hmit for kl, since there appear to be no processes (such as O(lD) + 02, or 0 + 0 + M) which cnuld efficiently regenerate 02(lAg) under the experimental conditions employed [7]. The rate constant for quenching of 02(lAg) by 0 atoms is consistent with the upper limit (< 10-15cm3molec-Isec-1) given by Evans et al. 141, although it 1s considerably smaller than the rate constant (2 X 10-14cm3molec-1sec-I) s.tggested earlier by Valiance Jones and Gattmger [8]. Our upper limit for kl at T = 3CWK IS about 100 times smaller than that require11 by Hunten and McElroy [5] to ezplain, if reacticln (1) is the
N+02(3~;)*NO+0
(5) at low atom concentration, although second or der atom recombination processes (N + N + N2 and N + C + N2) contribute to the decay at higher N atom concentrations. 0 atoms could be produced in the reaction N+NO-N2+0
(6!
by titrating the N atom flow before it reached the 406
Fig.
2. Values of in ‘/iQat four different atom concentlXLtiOIlB.
Volume 3, number 6
CHEMICAL PHYSICS LETTERS
major source of NO, the NO concentrations in the D-region. The corresponding reaction of N atoms with ground state oxygen (reaction 5) possesses an activation energy of about ‘7 kcal mole-l [Q]; Hunten and McElroy suggest that the energy of excitation of O#A ) (= 21 kcal mole-l) could partially o&come the activation barrier, so that kl could take the required value of 3 X lo-l3 cm3 molec-lsec-1 at T = 200%. We have made preliminary measurements of kl at T = 195oK which suggest that the rate 1s not more than a few times slower at this temperature than at T = 29PK. Thus, although the activation energy for the quenchmg
June 1969
reaction does, indeed, appear to be smzll, the pre-exponential factor for (1) must be considerably less than that for (5). The rate constants determined 631 ud present work) for the quenching of O&A,) by several atomic and molcLufar species are summarised in table 1. It might be speculated that, since neither of the monatomic species Ar or 0 is a good PhVsicat deactivator of Oz(lA,), the relatively high efficiency of quenching by N argues for a spc ,ific chemical quenchmg process such as reaction (1). REFERENCES
Table 1 Rate constants for quenching of 02(lAg). Quenching species
02
Rate constant (cm3 molec-lsec-1)
2.4 = 0.2 x 10-18
N2
< 1.1 x 10-19
CO2 H20
3.9 = 0.8 x 10-18 1.6 -I 0.5 x lo-l7
Ar
c 2.1 x 10-19
0
6 1.3 x lo-16
N
2.8 * 2.0 x lo-15
[l] I. D. Clark and R. P. Wayne, Chem. Phys. Letters 3 (1969) 93. [2] F. D. Fmdlay, C. J. Fortin and D. R. SnelJing, to be publlshed. [3] I. D. Clark and R. P. Wayne, Proc. Roy. Sac. A, submitted. [4J W. F. J. Pvans, D. M. Hunten, E. J. Llewellyn and A. Valiance Jones, J. Geophys. Res. 73 (1968) 2885. [5] D. M. Hunten and RI. B. McElroy, d. Geophys. Res. 73 (1968) 2421. [6] S. J Arnbld and E. A. Ogryzlo, Can. J. Pbys. 45 (1967) 2053. [7] R P-Wayne, Advan Photochem., to be published. [8] A. Vallance Jones and R. L. Gattmger, Planet. Space Sci. 11 (1963) 961. [9] M. A. A. Clyne and 8. A. Thrxsh, Proc. Roy. Sot. A 261 (1961) 259.
407
June 1969
CHEiKICAL PHYSICS LETTERS
THE
REACTION
OF AND
O&hg)
WITH
WITH
ATOMIC
ATOMIC
NITROGEN
OXYGEN
I. D. CLARK and R. P. WAYNE P?zysicaZ Chemistry Laboratory, South Parks Road, Oxfmd OX1 3QZ. UK Received 9 May 1969
Atomic N and 0 quench 02(b ) with room temperature rate constants of 2.8 f 2.0 x 10B16 and c 1 3 x lo-16cm3molec-lsec-1 8 espectlvely; NO cannot. therefore, be formed efficiently m the atmospheric D-region by reactlou of N with 02(hg).
Considerable concentrations of 02(‘Ag) exist in the upper atmosphere; recent laboratory determmations [l-3] of the rates of physical quenching of 02(lAg) by atmospheric gases are consistent with those derived [4] from observations of the variation of [02(lAg)] m the atmosphere with altitude between about 30 km and 80 km. At higher altitudes, however, the relatively high abundancies of atomic nitrogen and oxygen suggest the possibility that these species could contribute to the decay of 02(lAg). Further, the reaction of atomic r-itrogen with 02(lAg) could lead to the formation of NO N + 02(lAg)
-NO
+ 0
(1)
and it has been postulated [5] that reaction (1) produces significant concentrations of NO in the D-reaon of the atmosphere. In this paper we report measurements of the rate constants for the deactivation of 02(‘Ag) by N and by 0.
Q + o,&,) 2
products .
[Q = N or 0]
(2)
The rate constant for (1) must be equal to or less than the overall rate constant for deactivation by N atoms. A modified form of the diluted-discharge flow apparatus [3], shown in fig. 1, was used to measure the rate of reaction (2) with Q = N and Q = 0. The high sensitivity for 02(lAg) of the photoionization detector permitted measurements to be made with [02(1ag)] << [Q]. A radiofrequency discharge in an oxygen-helium mixture was used to produce 02(lAg) in an inlet tube
from which gases entered the main flow tube, and the relative [02(lAg)] were measured at a point 116 cm downstream from the inlet pomt by measurmg the ion current (48 V collector potential across Pt electrodes) consequent upon exposure to Ar resonance radiation. The AR resonance lines at 11.70, 11.61 eV ionize 02(lAg) but not ground state 02, N2, He, N or 0. A Kr resonance lamp (10.64, 10.10 eV) may be used to detect lower ionization potential impurities such as NO. Atomic nitrogen is introduced into the main tube 6 cm upstream from the 02(lA,) iniet: a set of capillaries with controlling taps i% used
Fig. 1. Apparatus.
405
Volume 3, number 6
CHEMICAL
PHYSfCS
to divert varying amounts of the nitrogen flow through a microwave discharge tube. Under the experimental co&&ions employed (fO2] from rf discharge tube only a few percent of the total concentration) it can be shown [3] that the products of the He-02 discharge cause a negligible decay of 02(lAg) in the main tube, and that, apart from the small contribution from NO (formed from the trace N2 impurity in the 02), the ionization current, i, produced by the Ar lamp is proportional to [02(lAg)]. The concentration of 02(lAg) at the point of mixing with the N-N2 flow was estimated to be 1 - 5 x lo4 Torr by comparison of the relative [02(‘Ag)] produced by the rf discharge with that produced by a 100 watt microwave discharge which is known[6] to form 5 - 10% of 03(‘Ag). Since the N or 0 atom concentrations at this point were found by NO titration to be 0.5 - 3 x 10-2 Torr, it is apparent that the condition [02(lAg)] CC [Q] is well satisfied. At any distance x downstream from the inlet point, [02(1Ag)], in the presence of atoms, Q, is given by (3) where k, IS the first order decay constant for 02(lAg) in the absence of atoms and v the linear flow velocity. It is therefore apparent that for ion currents measured at the pomt x = 116 cm ln i/zQ kQ = (4) 116 I/u 6
[Q]dx
where z is the 02(IAg) ion current m the absence of atoms and iQ is the current measured when Q is present. The dependence of [Q] on x, required to evalu&t.e the integral in (4), was determined for 0 atoms by monitoring the mtensity of the Ot NO afterglow, and for N atoms by NO titration. The decay of [N] was first order, and due mainly to reaction with ground state 02
LETTERS
June 1969
main flow t&e. Slow flow velocities (22 cm s-1) were required in order to measure i/i& accurately. At these velocities, the atom concentrations in the photoionization region were sufficiently small that the background electrometer current produced by atom recombination on the cathode was only a small fraction of the photoionization current. No ion current was produced by the Kr lamp when N atoms were present, thus indicating that all the NO produced in reactions (1) and (5) is rapidly removed by reaction (6). The values of ln i/Q measured for four different 0 and N atom concentrations are shown in fig. 2. Twice the standard deviation of the points given in fig. 2 for 0 atoms yields kg c 1.3 x lo-16 cm3 molec-lsec-l
.
Quenching of 02(lAg) by 0 or 03 is therefore clearly negligible in the N atom experiments. The slope of the line formed by the N atom points in fig. 2, combined with the uncertainty 111 [N]o yields kN = 2.8 f 2.0 X lo-l5
cm3 molec-lsec-l
.
Both valnes for kQ refer to a temperature of 297 * 20K. The value for kN must represent an upper hmit for kl, since there appear to be no processes (such as O(lD) + 02, or 0 + 0 + M) which cnuld efficiently regenerate 02(lAg) under the experimental conditions employed [7]. The rate constant for quenching of 02(lAg) by 0 atoms is consistent with the upper limit (< 10-15cm3molec-Isec-1) given by Evans et al. 141, although it 1s considerably smaller than the rate constant (2 X 10-14cm3molec-1sec-I) s.tggested earlier by Valiance Jones and Gattmger [8]. Our upper limit for kl at T = 3CWK IS about 100 times smaller than that require11 by Hunten and McElroy [5] to ezplain, if reacticln (1) is the
N+02(3~;)*NO+0
(5) at low atom concentration, although second or der atom recombination processes (N + N + N2 and N + C + N2) contribute to the decay at higher N atom concentrations. 0 atoms could be produced in the reaction N+NO-N2+0
(6!
by titrating the N atom flow before it reached the 406
Fig.
2. Values of in ‘/iQat four different atom concentlXLtiOIlB.
Volume 3, number 6
CHEMICAL PHYSICS LETTERS
major source of NO, the NO concentrations in the D-region. The corresponding reaction of N atoms with ground state oxygen (reaction 5) possesses an activation energy of about ‘7 kcal mole-l [Q]; Hunten and McElroy suggest that the energy of excitation of O#A ) (= 21 kcal mole-l) could partially o&come the activation barrier, so that kl could take the required value of 3 X lo-l3 cm3 molec-lsec-1 at T = 200%. We have made preliminary measurements of kl at T = 195oK which suggest that the rate 1s not more than a few times slower at this temperature than at T = 29PK. Thus, although the activation energy for the quenchmg
June 1969
reaction does, indeed, appear to be smzll, the pre-exponential factor for (1) must be considerably less than that for (5). The rate constants determined 631 ud present work) for the quenching of O&A,) by several atomic and molcLufar species are summarised in table 1. It might be speculated that, since neither of the monatomic species Ar or 0 is a good PhVsicat deactivator of Oz(lA,), the relatively high efficiency of quenching by N argues for a spc ,ific chemical quenchmg process such as reaction (1). REFERENCES
Table 1 Rate constants for quenching of 02(lAg). Quenching species
02
Rate constant (cm3 molec-lsec-1)
2.4 = 0.2 x 10-18
N2
< 1.1 x 10-19
CO2 H20
3.9 = 0.8 x 10-18 1.6 -I 0.5 x lo-l7
Ar
c 2.1 x 10-19
0
6 1.3 x lo-16
N
2.8 * 2.0 x lo-15
[l] I. D. Clark and R. P. Wayne, Chem. Phys. Letters 3 (1969) 93. [2] F. D. Fmdlay, C. J. Fortin and D. R. SnelJing, to be publlshed. [3] I. D. Clark and R. P. Wayne, Proc. Roy. Sac. A, submitted. [4J W. F. J. Pvans, D. M. Hunten, E. J. Llewellyn and A. Valiance Jones, J. Geophys. Res. 73 (1968) 2885. [5] D. M. Hunten and RI. B. McElroy, d. Geophys. Res. 73 (1968) 2421. [6] S. J Arnbld and E. A. Ogryzlo, Can. J. Pbys. 45 (1967) 2053. [7] R P-Wayne, Advan Photochem., to be published. [8] A. Vallance Jones and R. L. Gattmger, Planet. Space Sci. 11 (1963) 961. [9] M. A. A. Clyne and 8. A. Thrxsh, Proc. Roy. Sot. A 261 (1961) 259.
407