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On the proposals of Chapman and of Barth for O(1S) formation in the upper atmosphere
Planet. Space Sci., Vol. 27. pp. 717-718.
Pcrgamoo
Press Ltd.,
1979. Printed in NW
RESEARCH ON THJZ PROPOSALS
Ireland
NOTE
OF CHAPMAN AND OF BARTH IN THE UPPER ATMOSPHERE
FOR 0(‘S)
FORMATION
(Received 13 iVouember 1978) Akatrad-It is argued that Chapman’s process is too slow to explain the green line emission in the nightglow but that Barth’s two-stage mechanism is fast enough. The key to the dilTerence in their efficiencies is that the life-time of Chapman’s intermediate [0,] complex is very much shorter than that of Barth’s intermediate [0,] complex because of its greater internal energy. Consequently the chance of the representative mass point passing from the initial surface to the required final surface is very much less in the former case than the latter where 0(‘S) is one of the products in a quite considerable fraction of the decays of the complex.
Opinion on how the 0(‘S) atoms yielding the green line, 5577 A, of the nightglow are formed has, over the years (cf. Bates, 1978a), been divided between the proposal of Chapman (1931) and that of Barth (1962, 1964). Recently there has been a marked shift of support from the former towards the latter (Slanger and Black, 1977; Bates, 1978b; Witt, Stegman, Solheim and Llewellyn, 1979; Thomas, Greer and Dickinson, 1979). The object of this note is to compare the chemical kinetics of these alternatives. The proposal of Chapman (1931) may be written [0(3P) + o(sP)l* + O(3P)A
0,(X,
2)< 4) + O(lS),
(1)
in which the asterisk indicates that the pair of oxygen atoms within the square brackets approach on a potential surface belonging to a suitable electronic state and in which the reactants on the left may be regarded as leading to an intermediate [O,] complex. Erroneous laboratory measurements were reported which seemingly gave the rate coefficient -y,-, averaged over all approaches to be several powers of ten too low to account for the nightglow emission. These erroneous measurements st&ulated Barth (1962, 1964) to put forward hi two-stage mechanism: 0(3P)+0(3P)+M-02*fM,
(2)
the asterisk indicating the same electronic state as in (l), with the molecule possessing suflicient energy for (2) to be followed by o**+o(sP)~o,(x,
u c4)+0(%),
which must compete with
M not here including an 0(3P) O,*+O(‘P)%Oc+O(‘D
(4) atom, with or
3P)
-Y,B,a4l yB
(7)
={B3+B3n(0)+B,n(M)+%’
The green line photon emission rate I(5577) is less than the excitation rate due to (1) and (3) because of deactivation in collisions with normal oxygen atoms which, according to Olson (1973), proceeds mainly through 0(‘S) + 0(3P) + O(‘D) + O(‘D) + 0.255 eV.
(5)
& = 5.0
X
lo-”
exp (-610/RT)
cm3 s-l,
(9)
which is in good accord with a single measurement at 300 K by Lorents and Hue& (1975). Paucity of simultaneously observed n(0) and I(5577) profiles make the value which ‘yc must have if Chapman’s process is to account for the green line emission, uncertain. However on allowing for deactivation, with ps as given by (9) there seems general agreement that -yc would have to be at least around 1 X 1O-33 cm6 s-i at 200 K. The laboratory work of Campbell and Thrush (1967) has shown that at 200K the total association coefficient yr,, for 0(3P) + O(“P) + N, + O,(or O,*) + N,
[O(3P) + O(3P)J+ + O(‘P) + O,*(or 0,) + 0(3P),
and with *s O,*-O,+hu,
(8)
Measurements by Slanger and Black (1976a) give the rate coefficient for the process to be
(10)
is only 1 X 1O-32 cm6 s-l; and the effectiveness of an N, molecule as a third body cannOt differ greatly from that of an 0 atom. Now in (1) the approach must be made along a suitable potential surface. More important the lifetime of the [O,] complex towards decomposition on this surface is extremely brief and consequently the probability that the representative mass point passes from the initial surface to the final surface (representing a different spin state) in an encounter must be small. It is difficult to believe that yc of (1) is not far less than the rate coefficient yrl of
(3)
e, O,*+M-O,+M,
the plane symbol 0, being, throughout this note, used for a molecule possessing insufficient energy to allow the formation of an O(rS) atom. Barth’s mechanism gives an 0(‘S) production rate of Anne, where
(11)
which is itself likely to be less than y,c; and it is hence
(6) 717
718
Reseat¬e
diit to believe that yo is not well below the value required to explain the nightglow. There is no evidence to the contrary. The behaviour of the representative mass point in the case of 0,*-O(3P) collisions is completely different. Because the energy of the [Od complex formed is lower than in Chapman’s process its lifetime on the initial potential surface must, as is characteristic of unimolecular decomposition, be much longer. Hence the chance of reaction path (3) being followed, despite a change of spin state being entailed, will be high provided the competing exothermic reaction paths are less probable. This proviso may well be met if 0,” represents 0,(A3 &,+), O&r C-) or 0,(C3L-) as suggested by Slanger and Black (1977): thus stabilization of Oa*, in the sense of making it incapable of yielding 0(‘S), would require a change of potential surface, the transition zones for which may be relatively inaccessible to the representative mass point; and again both specified states are metastable. The laboratory work of Slanger and Black (1976b) which was carried out under conditions such that Ban(M)~~&+Bs~n(O)+%.
(12)
is very unlikely to give an 0(‘S) atom directly becamte in a simple approach of the trio followed by one atom receding from the pair which become bound, there is little chance of the representative mass point making the neeessary traversal of the small tmmition zone on the multidimensional potential surface. However the formation of an ox* molecule in the same collision, or with an Ns molecule as the third body, is straightfonvard and quite likely ot occur. Furthermore in subsequent collisions with O(‘P) atoms a fair fraction of those formed yield 0(‘S) atoms through an [O;] complex sufficiently long-lived to allow traversals of the transition zone to be not uucommon, the process concerned (3) being a serious rival to the other procemes (4), (5) and (6) by which the Oa+ molecules may be deactivated. D. R. Bates Department of Applied Mathetnat& and Theore.ricolPhysics, Queen’s University, Belfast, Northern Ireland.
so that (7) reduces to ,@B =
%&@.,P
(13)
indicates that process (3) is indeed fairly rapid. Shmger and Black (1976b) fouud yB = 1.4 x 10esc exp (- 13OO/RT)cm6 s-r,
(14)
sothatat2OOK &/&=
5.5 x lo-=/y,>5.5.
(15)
With the aid of earlier data of Young and Black (1966) they also deduced that at 300 K 8$8,‘33. Results (15) and (16) provide strong evidence that in Barth’s process (3) the representative mass point passes quite readily from the initial to the 8nal surface. They are in striking contrast to the closely corresponding inference that (17) Y&II = 1. A slightly diflerent way of describing the conclusions on Chapman’s and Barth’s proposals may be helpful. The three-body collision (1,11)
laQmlENcEs
Barth, C. A. (1962). J. geophys. I&s. 67, 1628. Barth, C. A. (1964). A& Geephys. 28, 182. Bates, D. R. (1978a). planet. Spoor Sci. 26,897. Bates, D. R. (1978b). Q. JZ.R. Asrr. See. 19, 310. Campbell, I. M. and Thrush, B. A. (1%7). J’tuc. R. Sot. A296,222. Chapman, S. (1931). Rot. R. Sot. X32,353. Lorents, D. C. and Hue&s, D. L. (1975). Lam Snccrmscopy -(E&s.S. I-Iaro& J. C.. Pebay-Peyroula, W. Hiinsch and S. E. Harris) p. 100. Springer, Berlin. Olson, R. E. (1973). Chetu. whys. Len. 19, 137. Slanger, T. G. and Black, G. (1976a). J. Chem. Phys. 64, 3763. Slanger, T. G. and Black, G. (1976b). J. Cftem. Phys. 64, 3767. Slanger, T. G. and Black, G. (1977). Planet. !3pace Sci. 25, 79. Thomas, L. Greer, R. G. H. and Dickinson, P. H. G. (1979). Pkmet. Spuce Sci. 27,341. Win, G., Stegman, J.. Solheim, B. and Uewellyn, E. G. (1979). Pkanet.specc sci. 27,341. Young, A. and Black, J. (1966). J. Chcm. Phys. 44,374l.
Pcrgamoo
Press Ltd.,
1979. Printed in NW
RESEARCH ON THJZ PROPOSALS
Ireland
NOTE
OF CHAPMAN AND OF BARTH IN THE UPPER ATMOSPHERE
FOR 0(‘S)
FORMATION
(Received 13 iVouember 1978) Akatrad-It is argued that Chapman’s process is too slow to explain the green line emission in the nightglow but that Barth’s two-stage mechanism is fast enough. The key to the dilTerence in their efficiencies is that the life-time of Chapman’s intermediate [0,] complex is very much shorter than that of Barth’s intermediate [0,] complex because of its greater internal energy. Consequently the chance of the representative mass point passing from the initial surface to the required final surface is very much less in the former case than the latter where 0(‘S) is one of the products in a quite considerable fraction of the decays of the complex.
Opinion on how the 0(‘S) atoms yielding the green line, 5577 A, of the nightglow are formed has, over the years (cf. Bates, 1978a), been divided between the proposal of Chapman (1931) and that of Barth (1962, 1964). Recently there has been a marked shift of support from the former towards the latter (Slanger and Black, 1977; Bates, 1978b; Witt, Stegman, Solheim and Llewellyn, 1979; Thomas, Greer and Dickinson, 1979). The object of this note is to compare the chemical kinetics of these alternatives. The proposal of Chapman (1931) may be written [0(3P) + o(sP)l* + O(3P)A
0,(X,
2)< 4) + O(lS),
(1)
in which the asterisk indicates that the pair of oxygen atoms within the square brackets approach on a potential surface belonging to a suitable electronic state and in which the reactants on the left may be regarded as leading to an intermediate [O,] complex. Erroneous laboratory measurements were reported which seemingly gave the rate coefficient -y,-, averaged over all approaches to be several powers of ten too low to account for the nightglow emission. These erroneous measurements st&ulated Barth (1962, 1964) to put forward hi two-stage mechanism: 0(3P)+0(3P)+M-02*fM,
(2)
the asterisk indicating the same electronic state as in (l), with the molecule possessing suflicient energy for (2) to be followed by o**+o(sP)~o,(x,
u c4)+0(%),
which must compete with
M not here including an 0(3P) O,*+O(‘P)%Oc+O(‘D
(4) atom, with or
3P)
-Y,B,a4l yB
(7)
={B3+B3n(0)+B,n(M)+%’
The green line photon emission rate I(5577) is less than the excitation rate due to (1) and (3) because of deactivation in collisions with normal oxygen atoms which, according to Olson (1973), proceeds mainly through 0(‘S) + 0(3P) + O(‘D) + O(‘D) + 0.255 eV.
(5)
& = 5.0
X
lo-”
exp (-610/RT)
cm3 s-l,
(9)
which is in good accord with a single measurement at 300 K by Lorents and Hue& (1975). Paucity of simultaneously observed n(0) and I(5577) profiles make the value which ‘yc must have if Chapman’s process is to account for the green line emission, uncertain. However on allowing for deactivation, with ps as given by (9) there seems general agreement that -yc would have to be at least around 1 X 1O-33 cm6 s-i at 200 K. The laboratory work of Campbell and Thrush (1967) has shown that at 200K the total association coefficient yr,, for 0(3P) + O(“P) + N, + O,(or O,*) + N,
[O(3P) + O(3P)J+ + O(‘P) + O,*(or 0,) + 0(3P),
and with *s O,*-O,+hu,
(8)
Measurements by Slanger and Black (1976a) give the rate coefficient for the process to be
(10)
is only 1 X 1O-32 cm6 s-l; and the effectiveness of an N, molecule as a third body cannOt differ greatly from that of an 0 atom. Now in (1) the approach must be made along a suitable potential surface. More important the lifetime of the [O,] complex towards decomposition on this surface is extremely brief and consequently the probability that the representative mass point passes from the initial surface to the final surface (representing a different spin state) in an encounter must be small. It is difficult to believe that yc of (1) is not far less than the rate coefficient yrl of
(3)
e, O,*+M-O,+M,
the plane symbol 0, being, throughout this note, used for a molecule possessing insufficient energy to allow the formation of an O(rS) atom. Barth’s mechanism gives an 0(‘S) production rate of Anne, where
(11)
which is itself likely to be less than y,c; and it is hence
(6) 717
718
Reseat¬e
diit to believe that yo is not well below the value required to explain the nightglow. There is no evidence to the contrary. The behaviour of the representative mass point in the case of 0,*-O(3P) collisions is completely different. Because the energy of the [Od complex formed is lower than in Chapman’s process its lifetime on the initial potential surface must, as is characteristic of unimolecular decomposition, be much longer. Hence the chance of reaction path (3) being followed, despite a change of spin state being entailed, will be high provided the competing exothermic reaction paths are less probable. This proviso may well be met if 0,” represents 0,(A3 &,+), O&r C-) or 0,(C3L-) as suggested by Slanger and Black (1977): thus stabilization of Oa*, in the sense of making it incapable of yielding 0(‘S), would require a change of potential surface, the transition zones for which may be relatively inaccessible to the representative mass point; and again both specified states are metastable. The laboratory work of Slanger and Black (1976b) which was carried out under conditions such that Ban(M)~~&+Bs~n(O)+%.
(12)
is very unlikely to give an 0(‘S) atom directly becamte in a simple approach of the trio followed by one atom receding from the pair which become bound, there is little chance of the representative mass point making the neeessary traversal of the small tmmition zone on the multidimensional potential surface. However the formation of an ox* molecule in the same collision, or with an Ns molecule as the third body, is straightfonvard and quite likely ot occur. Furthermore in subsequent collisions with O(‘P) atoms a fair fraction of those formed yield 0(‘S) atoms through an [O;] complex sufficiently long-lived to allow traversals of the transition zone to be not uucommon, the process concerned (3) being a serious rival to the other procemes (4), (5) and (6) by which the Oa+ molecules may be deactivated. D. R. Bates Department of Applied Mathetnat& and Theore.ricolPhysics, Queen’s University, Belfast, Northern Ireland.
so that (7) reduces to ,@B =
%&@.,P
(13)
indicates that process (3) is indeed fairly rapid. Shmger and Black (1976b) fouud yB = 1.4 x 10esc exp (- 13OO/RT)cm6 s-r,
(14)
sothatat2OOK &/&=
5.5 x lo-=/y,>5.5.
(15)
With the aid of earlier data of Young and Black (1966) they also deduced that at 300 K 8$8,‘33. Results (15) and (16) provide strong evidence that in Barth’s process (3) the representative mass point passes quite readily from the initial to the 8nal surface. They are in striking contrast to the closely corresponding inference that (17) Y&II = 1. A slightly diflerent way of describing the conclusions on Chapman’s and Barth’s proposals may be helpful. The three-body collision (1,11)
laQmlENcEs
Barth, C. A. (1962). J. geophys. I&s. 67, 1628. Barth, C. A. (1964). A& Geephys. 28, 182. Bates, D. R. (1978a). planet. Spoor Sci. 26,897. Bates, D. R. (1978b). Q. JZ.R. Asrr. See. 19, 310. Campbell, I. M. and Thrush, B. A. (1%7). J’tuc. R. Sot. A296,222. Chapman, S. (1931). Rot. R. Sot. X32,353. Lorents, D. C. and Hue&s, D. L. (1975). Lam Snccrmscopy -(E&s.S. I-Iaro& J. C.. Pebay-Peyroula, W. Hiinsch and S. E. Harris) p. 100. Springer, Berlin. Olson, R. E. (1973). Chetu. whys. Len. 19, 137. Slanger, T. G. and Black, G. (1976a). J. Chem. Phys. 64, 3763. Slanger, T. G. and Black, G. (1976b). J. Cftem. Phys. 64, 3767. Slanger, T. G. and Black, G. (1977). Planet. !3pace Sci. 25, 79. Thomas, L. Greer, R. G. H. and Dickinson, P. H. G. (1979). Pkmet. Spuce Sci. 27,341. Win, G., Stegman, J.. Solheim, B. and Uewellyn, E. G. (1979). Pkanet.specc sci. 27,341. Young, A. and Black, J. (1966). J. Chcm. Phys. 44,374l.