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The excitation of O2(b1Σg+) in the nightglow
Planet. Space Sci. Vol. 29, No. 4, pp. 383-389, Printed in Northern Ireland
0032~33/81/04038M7$02.00/0 0 Pergamon Press Ltd.
1981
THE EXCITATION
OF O,(b’C,+) IN THE NIGHTGLOW
R. G. H. GREER*. E. 1. LLEWELL YNt, B. I-I. SOLHElMt aad G. WITU
(Received in final form 8 October 1980) Abstract-It is proposed that the available measurements of the 02(b1Cg+-X3x=-) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.
1. INTRODUCITON
transition O,(b’Z,‘-x3&-) gives rise to both the (0,O) at 7619 A, and the (0,l) at 8645 A bands of the atmospheric system of molecular oxygen in the nightglow. Although the (0,O) band is the more intense by a factor of about 17 (Wallace and Hunten, 1968) it is totally absorbed in the lower atmosphere and can, therefore, only be studied at altitudes above 60 km with rocket-borne instrumentation. The few volumes emission profiles of the atmospheric bands that have been reported (Packer, 1961; Tarasova, 1963; Deans et al., 1976; Witt et al., 1979; Harris et al., 1980; Watanabe et al., 1980) do not all concur in the altitude of the maximum emission, but in each case it has been significantly lower than the accepted height for the oxygen green line. However, the measurement by Tarasova which shows a peak at 80 km cannot be reconciled with any known or proposed mechanism and the possibility exists that the measurement of a peak at 91 km by Deans et al. may have been influenced by aurora1 contamination. Both Packer and Witt et al. find that the (0,O) 7619 A emission peaks at 94 km and the three separate height profiles by Watanabe et al. also clearly show a peak emission altitude of 94~t 1 km. This altitude is further supported by the measurements of Harris et al. when normalized to a green line peak near 98 km. Thus, although verification is still needed, 94 km appears to be the best available estimate for the altitude of the atmospheric bands in the nightglow. It is the purpose of the present paper to suggest The
*Department of Physics, The Queens University of Belfast, Belfast BT7 lNN, N. Ireland. TInstitute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan SliN OWO, Canada. *Department of Meteorology, University of Stockholm, Stockholm, Sweden. 383
that this difference between the peak altitudes of the atmospheric bands and the green line is characteristic of an O,(b’Z,‘) excitation mechanism which may be plausibly inferred from the laboratory and observational data currently available. 2. REAPPRAISAL.
OF AVAILABLE
EVIDENCE
Recent laboratory and atmospheric measurements have suggested that a two-step mechanism is responsible for the production of the 0(‘S) green line (Slanger and Black, 1977; Thomas et al., 1979; Witt et al., 1979). The excited intermediate 02* has not been identified but requires approximately 4.2eV for transfer. Slanger and Black (1977) and Thomas et al. (1979) have adopted O,(A?Z,‘) as the excited intermediate because of the presence of the Herzberg I system in the nightglow which is observed to coexist with the green line. Barth (1964) originally suggested that the c8,- and C3AU states could be products of reaction 1 (see Table l), and Witt et al. (1979) have advanced tentative arguments which favour O,(c’S,J as the intermediate in the green line excitation. The latter authors have also attempted to resolve the longstanding problem of explaining the nightglow observations of the atmospheric bands through the direct recombination of atomic oxygen. They proposed that reactions involving vibrationally excited hydroxyl could be a significant source of O,(b’C,‘) in the nightglow but allowed that agreement with the observations, particularly in respect of the height dependence of the emission, might also be obtained through energy transfer, possibly from the green line’s intermediate. The idea that the oxygen atmospheric bands may be generated through energy transfer from a pernew. Laboratory cursor is, of course, not chemiluminescent studies of atom recombination in low pressure oxygen/nitrogen systems by Young and Sharpless (1963), Clyne et al. (1965) and
384
R. G. H. GFCEER, E. J. LLEWELLYN, B. H. SOLHEM
1. O+O+M
----+O,+M
2. O+O+M
-
3. O,(c’Z,J
+ 0,
<
O,(c’Z,-)
+M
O,(b’Z,+)
+ 0,
0,+0,
4. o&‘L,-)+O&:* o,*+o 5. O,(b’Z,+)+O-O,+O
and
G. WITT
k, = 4.7 X 10-33(~)2cm6sK’ (Campbell and Gray, 1973) k,=ak, k,, = Pk, k 3b k 4a k 4b k 4c I
k 3 = 5 x lo-l3
cm3sm1
k 4 = 3 x lo-”
cm%*
k,=Z8x10-14cm3s-1 (Slanger and Black, 1979) k 6 = 2.2 x 1O-'5 cm3sm1
6. Oz(b12,+)+M-O,+M 7. O,(C’C”J
-
0, + hv (Herzberg
8. O,(b’S,+)
-
0, + hv (atmospheric
II bands)
Young and Black (1966) have all concluded that a molecular precursor must be instrumental in exciting the b’s,+ state although, with reference to other metastable states then known to be excited, identification of the precursor was not possible. Interestingly, Young and Sharpless (1963), whilst noting the general quenching quality of O2 for- oxygen band emission, have observed an initial strong increase in the intensity of the atmospheric bands, particularly at low pressures, when 0, is added in small quantities. Young and Black concluded that this increase was due to enhanced atomic association but did not consider the possibility of collisional transfer from their “required” precursor. More recently energy transfer has been invoked by Lawton and Phelps (1978) as a possible explanation of the excitation of the b’8,’ state by low electrons. were energy Their observations rationalized by assuming that essentially all oxygen molecules excited to levels at and above 1.63 eV resulted in the formation of the b’&,’ state, thus favouring significant transfer from some higher bound level. Lawton and Phelps allowed that the intermediate state could be 02(c1CUJ in agreement with the electron impact studies of Trajmar et al. (1972), but noted that the predicted dissociation of OZ(clZ,J into two 0(3P) atoms according to the Frank-Condon principle is a difficulty with this explanation. Atmospheric excitation of the c’Z,- state has now been established by the identification of the u’ = 0 progression of the Herzberg II (clZU-- X38,-) bands in the Venus nightglow (Lawrence et al., 1977), although their appearance was peculiarly attributed to the special role of COZ in the three-
bands)
A,=10-3-2x10-2s-’ (Krupenie, 1972; Slanger, A, = 0.079 s-l (Deans et al., 1976)
1978)
body recombination of oxygen. Kenner et al. (1979) have argued that the selective appearance of the Herzberg II bands on Venus is determined neither solely, nor necessarily, by CO* and that whilst CO* may be especially effective as the third body, it is not uniquely so. Their measurements further indicated that whilst the intensity of the Herzberg I bands was proportional to [O]*, that of the Herzberg II bands went as [0] and from this Kenner et al. have concluded that O,(c’Z,J evolved by transfer from the A?&+ state. However, in adopting 0,(A3X,‘) as the excited product of reaction 1 (Table l), Kenner et al. have assumed that only the Herzberg II bands are observed in the Venus nightglow. This is now known to be incorrect as Slanger and Black (1978) have identified the IJ’= 0 progression of the relatively weak 0,(C3A,-a’A,) Chamberlain bands in the Venus spectrum. If it is assumed that 0,(A38,‘) is a precursor for both the C’S,- and C3A, states, then the requirement for the production of the 0z(A38,‘) state is incompatible with current estimates for the yield from reaction 1; depending on the corrections made for quenching the yield is between 3% and 20% (Sharp and Rees, 1970; Reed and Chandra, 1975; Gerard, 1975; Llewellyn et al., 1979). Thus there is some reason to question the validity of the transfer mechanism assumed by Kenner et al. particularly as the measurements by Slanger (1978) indicate that both the cl&and C3AU states are formed in the recombination. The Herzberg II system has not yet been positively identified in the Earth’s upper atmosphere whereas the Chamberlain bands (C3AU-a’A,) are now recognized as a permanent feature of the
The excitation of O,(b’Z,+) in the nightglow nightglow (Slanger, 1979). However, the absence of an emission is not proof that the initial state is not produced and is often an indication of strong quenching. According to Kenner et al. (1979) O,(c’X,J is quenched by 0 and 0,; the latter observation is supported by the experiment of Lawton and Phelps (1978) in which, due to the absence of atomic oxygen, quenching was necessarily by 0,. Slanger (1978) has observed that O,(c’S,-), produced through oxygen atom recombination in a system containing N2, is not quenched by Nz. The laboratory spectra also indicate the sensitivity of the excited 0, states to vibrational relaxation particularly by CO,. In the Venus nightglow only the v’= 0 bands are seen whilst in the earth’s upper atmosphere the Herzberg I and Chamberlain bands are dominated by high v’ progressions. The laboratory measurements of both Young and Black (1966) and McNeal and Durana (1969) indicate that the 0,(A3C,‘) state is not appreciably quenched in the Earth’s upper atmosphere. A comparison of the relative intensities of the Herzberg I and Chamberlain bands in laboratory and atmospheric spectra (Slanger, 1979) clearly indicates that a low quenching efficiency also applied to the C3Au state. Thus, although because of uncertainties in the quenching coefficients, we cannot dismiss them completely it would appear that neither the A%_+ nor the C3A, state is a likely precursor for the b’C,’ state in the atmosphere; an analogous situation exists for the oxygen green line where the identity of the precursor for the Barth mechanism is still in doubt. For the O,(a’A,) state the measured quenching coefficient (Clarke and Wayne, 1969) indicates that only radiative loss is significant. Thus unless there is a very large increase in the quenching coefficient with vibrational energy the a’A, state must be considered an unlikely precursor for any transfer mechanism. Undoubtedly the case for promoting the OZ(clS,J state as the excited intermediate in a transfer mechanism for the atmospheric bands is far from clear cut. However the evidence is sufficiently strong to merit the calculation of the height profile for the atmospheric O,(b’8,‘) emission from such a transfer mechanism and, by comparison, to test the extent to which it is consistent with available observations.
3. REACTION
KINJTTICS
On the basis of the reactions listed in Table 1, and for an assumed steady state, the volume emission rate, V(b), of the O,(b’8,‘-x38,-) atmospheric
385
band is given by V(b) =
&Dl*M &Pkd0J Al+ k,[O,l + k,[OlAz + kdOl+ kiMI (1)
where each term is defined according to Table 1. As the rate coefficients for reactions (3) and (4) have yet to be measured it is necessary to adopt, or estimate, values appropriate to equation (1). In their analysis of the oxygen green line in the airglow and an extension of this analysis to the aurora1 excitation, Solheim (1979), Witt et al. (1979) and Solheim and Llewellyn (1979) have estimated values appropriate to the rate constants k, and k, for the Barth mechanism and we have utilized their values in the present calculations. A value for k, of 3 X lo-” cm* sml IS . consistent both with the above works and with the corresponding value deduced by Slanger and Black (1977). It should also be noted that the value of 1 x lo-l3 cm* Sol for the rate constant k, adopted by Solheim and Llewellyn (1979) for the quenching of the green line intermediate allowed any collision partner M. However, as the recent laboratory measurements show that O,(c’Z,J, the intermediate presently favoured, is not quenched by N, we have set k, in equation (l), equal to 1 x lo-l3 [M]/[O,] or approximately 5 x lo-l3 cm3 s-l. Martin et aI. (1976) have determined the quenching rate constant for O,(b’C.,‘) by Nz to be 2.2~ lo-l5 cm3 s-l and noted that, by comparison, the quenching with 0, is negligible. This conclusion is in good agreement with earlier measurements (Noxon, 1970) and we have, therefore, adopted a value of k, equal to 2.2 x lO_” cm3 s-l. In preliminary calculations the value of k,, the rate constant for the quenching of O,(b’C,‘) by 0, was unknown but was set equal to k, since Slanger (1978) had concluded that, as with 02, the quenching efficiency with 0 was low. Recently Slanger and Black (1979) have measured this rate constant at 300 K to be 8 x lo-l4 cm3 s-l. For an atomic oxygen concentration of 101’ cm-3 this implies radiative loss equal to quenching. However, if there is an activation energy associated with atomic oxygen quenching the value of k, may be substantially reduced at temperatures corresponding to the mesopause so that this loss mechanism would be of only marginal significance @anger and Black, 1979). The fact that the effective lifetime of O,(b’Z,‘) in the atmosphere is nearly equal to the radiative lifetime implies that quenching by 0 is not a major loss process; this is in agreement with the interpretation of observations of the atmospheric bands in aurora
386
R. G. H. GREER, E. J. LLE~JSLLYN,B. H. !S~LHEIMand G. Wrrr
presented by Wallace and Chamberlain (1959). In our analysis we have, therefore, considered values and 8~ to 3 X lo-” cm3 s-l of k5 equal lo-l4 cmY1. 4. EVALUATION OF COMPUTED EMISSION PROFILES
Of crucial importance is the fact that the atmospheric band profile is seen to peak at 94 km in a number of different observations. In this respect calculations were performed for three different atomic oxygen profiles (Fig. 1) to test the sensitivity of the calculated emission to variations in this parameter. Profile A is taken from the atmospheric model of Jacchia (1977) and extrapolated below 90 km according to the calculations of Moreels et al. (1977). Profile B is that of Witt et al. (1979) and has the merit of being derived from nightglow observations of the green line made simultaneously with the atmospheric bands presented in Fig. 4. Profile C represents a direct measurement using an oxygen resonance fluorescence technique (Thomas et al., 1979). For each of the calculations the background atmosphere was assumed to be that appropriate to the mean CIRA (1972). The normalized volume emission profiles of the atmospheric bands for an excitation through direct recombination are shown in Fig. 2. The altitude of the peak emission is in each case distinctly higher than the reported measurements of 94 km and, as might be expected for a mechanism which is dependent on CO]‘, is apparently very sensitive to the atomic oxygen concentration. This behaviour
does not support a direct excitation mechanism for the atmospheric bands. In Fig. 3 the normalized emission profiles pertinent to the transfer mechanism outlined above are presented. Each profile apparently peaks at about 94 km in agreement with the observations. The uniformity in the peak emission altitudes demonstrates the reduced dependence of the emission on [O]. This is apparent from equation (1) and the adopted rate coefficients which suggest that the O,(b’C,‘) profile varies as [O][O,][M] throughout the bulk of the emitting region, when O2 is used as the transfer agent, and not as [O]‘[M]. Further the 94 km peak emission altitude is characteristic of an [O][O,][M] dependence whereas an [O]‘[M] dependence places the peak emission altitude near the oxygen atom peak. The structure exhibited on the profile C derives from the direct method of measuring the atomic oxygen concentration; such fine detail might not be apparent in the data scatter normally associated with a photometer output and could easily be dispersed in the subsequent analysis. A direct comparison between the shapes of the measured and calculated atmospheric band profiles is also possible and is presented in Fig. 4. Profile A is that obtained by Witt et al. (1979) from rocket borne photometric observations; profiles B and C are, respectively, the calculated profiles for direct excitation of O,(b’Z,+) in three body recombinations and for excitation by energy transfer from an excited intermediate. As with the previous calculations these profiles have been normalized to unity
FIG. 1. MODELATOMICOXYGENDENSITIES:A,JAC~HIA (1977):B,Wrrretal.(1979):C,THo~~s (1979).
etal.
The excitation of O,(b’Z,‘)
CURVE
in the nightglow
381
A
FIG. 2. NORMALIZED VOLUMEEMISSIONRATESFORTHEATMosPHERICBANDSFOREACHOFTHEMODBL OXYGENPROFILES,DIREcTEXCITATXON. The
volume emission rates have been normalized to unity at the altitude of the peak emission.
at their respective emission peaks. The atomic oxygen profile used in each of these calculations was that derived by Witt et al. (1979) from the oxygen green line. It is clear that the observations are in better agreement with the transfer mechanism. Whilst at the lower altitude this mechanism appears to yield excess emission it should be noted that the atomic oxygen concentration derived from the green line is here most uncertain, as the green line volume emission rate is small, so the discrepancy
-3
-4
F1c.3.
b
-+--+-d I
LOG
VOLUME
I
I
,
EMI&N
may not be significant. On the top side the observed profile is consistent with both the high and low value of k5, the rate constant for quenching of O,(b’8,‘) by 0. This is not unexpected as neither value of the rate constant implies other than a minor contribution to the loss of O,(b’Z,‘) in the atmosphere. As a further check on the proposed transfer mechanism we have attempted to estimate a value for k5 which would produce agreement between the
I
I
I
RATE
0
, I ,
lorbbxy
CURVE C , , , ,
units)
0
NORMAUZEDVOLUMEEMISSIONRATESFORTHEATMOSPHBRICBANDSFOREACHOFTHEMODEL OXYGENPROFIIESTRANSFBRMECHANISM.
The volume emission rates have been normalized to unity at the altitude of the peak emission.
R. G. H. GREER, E. J. LLE~EJ-LYN,B. H. SOLZZIMand G. Wrrr
388
8o
1
2
3
4
5
6
7
8
NORMALISED
FIG. 4.
COMPARISON
BETWEEN
OBSERVED
9
1
123L5678
10
VOLUME
EMISSION
9
10
80
RATE
(Wrrr et al., 1979) AND CALCULATED
ATMOSPHERIC
BAND
PROFILES.
(a) Transfer excitation;
(b) direct excitation in three body recombination. All volume rates have been normalized to unity at the altitude of peak emission.
observed and calculated height profile for a direct three-body excitation mechanism. The best fit consistent with a peak emission altitude of 94 km is obtained with a quenching coefficient equal to 4X lo-” cm3 s-l. This value is much larger than that measured in the laboratory and would, in contradiction of the observations, cause the effective lifetime of the atmospheric bands in the atmosphere to be significantly less than the radiative lifetime. Thus both the laboratory measurements and the atmospheric observations indicate that the oxygen atmospheric bands are not appreciably excited in the direct three-body recombination of atomic oxygen. Despite the present uncertainties in the details of the proposed transfer mechanism we may estimate the value of the product CY/~ which provides agreement between the measured and calculated overhead emissions intensity. For the Witt et al. (1979) observations this product is 0.8 which, if it is assumed that all interactions of the intermediate with 0, lead to O,(b’X,‘), would require that 80% of all recombinations yield the precursor state. In their analysis of the oxygen green line, for an assumed Barth mechanism Solheim and Llewellyn (1979) estimated that 70% of all three body recombinations yield the required precursor state. Since, as we suggest, the intermediates for the green line and the atmospheric bands are identical then the required yields are in reasonable agreement. Although it is proposed that the atmospheric bands in the airglow are excited through energy transfer it is possible to rewrite equations (1) in a
form similar to that for a direct three-body recombination; V(h) =
&MOzl k~Dl’CMlAz A, + UOI + h[CM -4 + k,[Ol+ k&f1 ’ (2)
The apparent yield of the (b’8,‘) state in the recombination is here represented by the altitude dependent term,
440J y = A,
+ k,[O]+ k,[O,]
.
This altitude dependence has been previously noted by Witt et al. (1979) in their analysis of the oxygen atmospheric bands. For the present model, Fig. 4, the apparent yield has a value of 0.24 at 94 km and 0.08 at 100 km. The latter value is close to the statistical weight of the b’C,’ state for the ground state atom recombination while the former is in reasonable agreement with the single value of 0.2 estimated by Deans et al. (1976). Applying the present conclusions to the calculation of atomic oxygen concentration from the nightglow observation by Deans et al. leads to a value of 3.5X 1Ol1 cne3 at 100 km which differs from their value of 1.7 x 1O’l cme3 but is in much closer agreement with the concentrations measured by Dickinson et al. (1980) using resonance fluorescence lamps. It is worth noting that the transfer mechanism proposed in this paper does not contradict the laboratory spectra of Slanger (1978). The emission of the atmospheric bands does not require the precursor to be vibrationally excited so that the intermediate state, assumed by us to be c’CU-,
The excitation of Oa(brZ,‘) in the nightglow could relax to the zeroth vibrational level before transferring, in agreement with the observed spectra. A vertical transition from this zeroth level would populate the v = 5 level of O,(b’Z,‘). At the mesopause it is expected that this state would undergo vibrational relaxation so giving only the 2)’= 0 bands in the airglow. However if the transfer, i.e. the vertical transition, occurred at higher altitudes then we might expect vibrational development of the bands in agreement with the observations of Gattinger and Valiance Jones (1976). 5.
CONCLUSION
In the present paper it has been proposed that the O,(blB,“) state, which is responsible for the atmospheric bands, is excited in the night~ow by energy transfer from an intermediate state, rather than directly through atomic oxygen recombination. It has been argued that laboratory and atmospheric observations do not contradict this hypothesis nor the identification of the intermediate state as 02(c*&-). Selective quenching of the latter state by molecular oxygen is the factor which characteristically causes the atmospheric bands to peak at 94 km rather than higher as with the green line. The intensities derived from the present analysis indicate that such a transfer mechanism can adequately explain the observed intensities of the atmospheric bands without resource to any other mechanism. However, the adoption of this excitation path does nothing to explain why the precursor is apparently excited with such a high efficiency in the initial recombination. Ack~wledge~en~s-his work has been supported by Grants-in-Aid from the National Research Council of Canada, The Queen’s University of Belfast and the International Meteorological Institute, University of Stockholm. REFERENCES
Barth, C. A. (1964). Am&. Geophys. 20, 182. Campbell, I. M. and Gray, C. N. (1973). Chem. Phys. I&t. 18, 607. CIRA (1972). COSPAR International Reference Atmosphere. Akademic-Verlay, Berlin. Clarke, I. D. and Wayne, R. P. (1969). Chem. Phys. Lea. 3, 405. Clyne, M. A. A., Thrush, B. A. and Wayne, R. P. (1965). J. P~o~~hern. Photobiol. 4, 9.57. Deans, A. J., Shepherd, G. G. and Evans, W. F. J. (1976). Geophys. Res. Len. 3, 441. Dickinson, P. H. G., Bain, W. C., Thomas, L., Williams, E. R., Jenkins, D. B. and Twiddy, N. D. (1980). Proc. Roy. Sot. London A369,
379.
Evans, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1972). J. geophys. Res. 77, 4899.
389
Gattinger, R. L. and Valiance Jones, A. (1976) J. geophys. Res. 81, 4789. Gerard, J. C. (1975). Planet. Space Sci. 23, 1681. Harris, F. R., Lleweflyn, E. J. and Creutzberg, F. (1980). Can. J. Phys. to be published. Haason, V., Nicholls, R. W. and Degen, V. (1970). J. Phys. (B) 3, 1192. Jacchia, L. G. (1977). SAO Special Report No. 375, Smithsonian Institution Astrophysical Observatory, Cambridge, Mass. 02138. Kenner, R. D., Ogryzlo, E. A. and Turley, S. (1979). J. P~~~~rn. 10, 199. Krupenie, P. H. (1972). J. Phys. Chem. Ref. Data. 1,423. Lawrence, G. M., Barth, C. A. and Arnabrinht. I __. V. (1977). Science.XP5,
573.
Lawton, S. A. and Phelps, A. V. (1978). J. them. Phys. 69, 1055. Llewellyn, E. J. and Solheim, B. H. (1978). Planet. Space sci. 26, 533. Llewellyn, E. J., Solheim, B. H., Stegman, J. and Witt, G. (1979). Planet. Space Sci. 27, 1507. McNeal, R. J. and Durana, S. C. (1969). J. &em. Phys. 51, 2955. Martin, L. R., Cohen, R. B. and Schatz, J. R. (1976). Chem. Phys. Len. 41, 394. Moreels, G., Megie, G., Valiance Jones, A. and Gattinger, R. L. (1977). J. atmos. rem Plays. 39, 551. Noxon, J. F. (1970). J. them. Phys. 52, 18.52. Packer, D. (1961). Ann. Geonhvs. 17. 67. Reed, E. I. and Char&a, S. (1675). j. geophys. Res. 80, 3053. Sharp, W. E. and Rees, M. H. (1970). J. geophys. Res. 75, 4894. Slanger, T. G. (1978). J. &em. Phys. 69, 4779. Slanger, T. G. (1979). C&m. Phys. Lett. 66, 344. Slanger, T. G. and Black, G. (1977). Pfanet. Space Sci. 25, 79. Slanger, T. G. and Black, G. (1978). Geophys. Res. Len. 5, 947. Slanger, T. G. and Black, G. (1979). J. them. Phys. 70, 3434. Solheim, B. H. (1979). MSc. Thesis, Dept, of Physics, University of Saskatchewan, 81. Solheim, B. H. and Llewellyn, E. 3. (1979). Planet. Space Sci. 27, 473. Tarasova, T. M. (1963). Space Research III, p. 353. North-Holland, Amsterdam. Thomas, L., Greer, R. G. H. and Dickinson, P. H. G. (1979). Planet. Soace Sci. 27. 925. Trajmar, S., Williams, W. and Kuppermann, A. (1972). J. &em. Phys. 56, 3759. Wallace, L. and Chamberlain, J. W. (1959). Planet. Space Sci. 2, 60. Wallace, L. and Hunten, D. M. (1968). J. geophys. Res. 73, 4813. Watanabe, T., Nakamura, M. and Ogawa, T. (1980). J. geophys. Res. to be published. Witt, G., Stegman, J., Solheim, B. and Llewellyn, E. J. (1979). P&met. Space Sci. 27, 341. Yau, A. W. and Shepherd, G. G. (1979). PZanet. Space Sci. 27, 481. Young, R. A. and Black, G. (1966). J. them. Phys. 44, 3741. Young, R. A. and Sharpless, R. L. (1963). J. them. Phys. 39, 1071.
0032~33/81/04038M7$02.00/0 0 Pergamon Press Ltd.
1981
THE EXCITATION
OF O,(b’C,+) IN THE NIGHTGLOW
R. G. H. GREER*. E. 1. LLEWELL YNt, B. I-I. SOLHElMt aad G. WITU
(Received in final form 8 October 1980) Abstract-It is proposed that the available measurements of the 02(b1Cg+-X3x=-) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.
1. INTRODUCITON
transition O,(b’Z,‘-x3&-) gives rise to both the (0,O) at 7619 A, and the (0,l) at 8645 A bands of the atmospheric system of molecular oxygen in the nightglow. Although the (0,O) band is the more intense by a factor of about 17 (Wallace and Hunten, 1968) it is totally absorbed in the lower atmosphere and can, therefore, only be studied at altitudes above 60 km with rocket-borne instrumentation. The few volumes emission profiles of the atmospheric bands that have been reported (Packer, 1961; Tarasova, 1963; Deans et al., 1976; Witt et al., 1979; Harris et al., 1980; Watanabe et al., 1980) do not all concur in the altitude of the maximum emission, but in each case it has been significantly lower than the accepted height for the oxygen green line. However, the measurement by Tarasova which shows a peak at 80 km cannot be reconciled with any known or proposed mechanism and the possibility exists that the measurement of a peak at 91 km by Deans et al. may have been influenced by aurora1 contamination. Both Packer and Witt et al. find that the (0,O) 7619 A emission peaks at 94 km and the three separate height profiles by Watanabe et al. also clearly show a peak emission altitude of 94~t 1 km. This altitude is further supported by the measurements of Harris et al. when normalized to a green line peak near 98 km. Thus, although verification is still needed, 94 km appears to be the best available estimate for the altitude of the atmospheric bands in the nightglow. It is the purpose of the present paper to suggest The
*Department of Physics, The Queens University of Belfast, Belfast BT7 lNN, N. Ireland. TInstitute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Saskatchewan SliN OWO, Canada. *Department of Meteorology, University of Stockholm, Stockholm, Sweden. 383
that this difference between the peak altitudes of the atmospheric bands and the green line is characteristic of an O,(b’Z,‘) excitation mechanism which may be plausibly inferred from the laboratory and observational data currently available. 2. REAPPRAISAL.
OF AVAILABLE
EVIDENCE
Recent laboratory and atmospheric measurements have suggested that a two-step mechanism is responsible for the production of the 0(‘S) green line (Slanger and Black, 1977; Thomas et al., 1979; Witt et al., 1979). The excited intermediate 02* has not been identified but requires approximately 4.2eV for transfer. Slanger and Black (1977) and Thomas et al. (1979) have adopted O,(A?Z,‘) as the excited intermediate because of the presence of the Herzberg I system in the nightglow which is observed to coexist with the green line. Barth (1964) originally suggested that the c8,- and C3AU states could be products of reaction 1 (see Table l), and Witt et al. (1979) have advanced tentative arguments which favour O,(c’S,J as the intermediate in the green line excitation. The latter authors have also attempted to resolve the longstanding problem of explaining the nightglow observations of the atmospheric bands through the direct recombination of atomic oxygen. They proposed that reactions involving vibrationally excited hydroxyl could be a significant source of O,(b’C,‘) in the nightglow but allowed that agreement with the observations, particularly in respect of the height dependence of the emission, might also be obtained through energy transfer, possibly from the green line’s intermediate. The idea that the oxygen atmospheric bands may be generated through energy transfer from a pernew. Laboratory cursor is, of course, not chemiluminescent studies of atom recombination in low pressure oxygen/nitrogen systems by Young and Sharpless (1963), Clyne et al. (1965) and
384
R. G. H. GFCEER, E. J. LLEWELLYN, B. H. SOLHEM
1. O+O+M
----+O,+M
2. O+O+M
-
3. O,(c’Z,J
+ 0,
<
O,(c’Z,-)
+M
O,(b’Z,+)
+ 0,
0,+0,
4. o&‘L,-)+O&:* o,*+o 5. O,(b’Z,+)+O-O,+O
and
G. WITT
k, = 4.7 X 10-33(~)2cm6sK’ (Campbell and Gray, 1973) k,=ak, k,, = Pk, k 3b k 4a k 4b k 4c I
k 3 = 5 x lo-l3
cm3sm1
k 4 = 3 x lo-”
cm%*
k,=Z8x10-14cm3s-1 (Slanger and Black, 1979) k 6 = 2.2 x 1O-'5 cm3sm1
6. Oz(b12,+)+M-O,+M 7. O,(C’C”J
-
0, + hv (Herzberg
8. O,(b’S,+)
-
0, + hv (atmospheric
II bands)
Young and Black (1966) have all concluded that a molecular precursor must be instrumental in exciting the b’s,+ state although, with reference to other metastable states then known to be excited, identification of the precursor was not possible. Interestingly, Young and Sharpless (1963), whilst noting the general quenching quality of O2 for- oxygen band emission, have observed an initial strong increase in the intensity of the atmospheric bands, particularly at low pressures, when 0, is added in small quantities. Young and Black concluded that this increase was due to enhanced atomic association but did not consider the possibility of collisional transfer from their “required” precursor. More recently energy transfer has been invoked by Lawton and Phelps (1978) as a possible explanation of the excitation of the b’8,’ state by low electrons. were energy Their observations rationalized by assuming that essentially all oxygen molecules excited to levels at and above 1.63 eV resulted in the formation of the b’&,’ state, thus favouring significant transfer from some higher bound level. Lawton and Phelps allowed that the intermediate state could be 02(c1CUJ in agreement with the electron impact studies of Trajmar et al. (1972), but noted that the predicted dissociation of OZ(clZ,J into two 0(3P) atoms according to the Frank-Condon principle is a difficulty with this explanation. Atmospheric excitation of the c’Z,- state has now been established by the identification of the u’ = 0 progression of the Herzberg II (clZU-- X38,-) bands in the Venus nightglow (Lawrence et al., 1977), although their appearance was peculiarly attributed to the special role of COZ in the three-
bands)
A,=10-3-2x10-2s-’ (Krupenie, 1972; Slanger, A, = 0.079 s-l (Deans et al., 1976)
1978)
body recombination of oxygen. Kenner et al. (1979) have argued that the selective appearance of the Herzberg II bands on Venus is determined neither solely, nor necessarily, by CO* and that whilst CO* may be especially effective as the third body, it is not uniquely so. Their measurements further indicated that whilst the intensity of the Herzberg I bands was proportional to [O]*, that of the Herzberg II bands went as [0] and from this Kenner et al. have concluded that O,(c’Z,J evolved by transfer from the A?&+ state. However, in adopting 0,(A3X,‘) as the excited product of reaction 1 (Table l), Kenner et al. have assumed that only the Herzberg II bands are observed in the Venus nightglow. This is now known to be incorrect as Slanger and Black (1978) have identified the IJ’= 0 progression of the relatively weak 0,(C3A,-a’A,) Chamberlain bands in the Venus spectrum. If it is assumed that 0,(A38,‘) is a precursor for both the C’S,- and C3A, states, then the requirement for the production of the 0z(A38,‘) state is incompatible with current estimates for the yield from reaction 1; depending on the corrections made for quenching the yield is between 3% and 20% (Sharp and Rees, 1970; Reed and Chandra, 1975; Gerard, 1975; Llewellyn et al., 1979). Thus there is some reason to question the validity of the transfer mechanism assumed by Kenner et al. particularly as the measurements by Slanger (1978) indicate that both the cl&and C3AU states are formed in the recombination. The Herzberg II system has not yet been positively identified in the Earth’s upper atmosphere whereas the Chamberlain bands (C3AU-a’A,) are now recognized as a permanent feature of the
The excitation of O,(b’Z,+) in the nightglow nightglow (Slanger, 1979). However, the absence of an emission is not proof that the initial state is not produced and is often an indication of strong quenching. According to Kenner et al. (1979) O,(c’X,J is quenched by 0 and 0,; the latter observation is supported by the experiment of Lawton and Phelps (1978) in which, due to the absence of atomic oxygen, quenching was necessarily by 0,. Slanger (1978) has observed that O,(c’S,-), produced through oxygen atom recombination in a system containing N2, is not quenched by Nz. The laboratory spectra also indicate the sensitivity of the excited 0, states to vibrational relaxation particularly by CO,. In the Venus nightglow only the v’= 0 bands are seen whilst in the earth’s upper atmosphere the Herzberg I and Chamberlain bands are dominated by high v’ progressions. The laboratory measurements of both Young and Black (1966) and McNeal and Durana (1969) indicate that the 0,(A3C,‘) state is not appreciably quenched in the Earth’s upper atmosphere. A comparison of the relative intensities of the Herzberg I and Chamberlain bands in laboratory and atmospheric spectra (Slanger, 1979) clearly indicates that a low quenching efficiency also applied to the C3Au state. Thus, although because of uncertainties in the quenching coefficients, we cannot dismiss them completely it would appear that neither the A%_+ nor the C3A, state is a likely precursor for the b’C,’ state in the atmosphere; an analogous situation exists for the oxygen green line where the identity of the precursor for the Barth mechanism is still in doubt. For the O,(a’A,) state the measured quenching coefficient (Clarke and Wayne, 1969) indicates that only radiative loss is significant. Thus unless there is a very large increase in the quenching coefficient with vibrational energy the a’A, state must be considered an unlikely precursor for any transfer mechanism. Undoubtedly the case for promoting the OZ(clS,J state as the excited intermediate in a transfer mechanism for the atmospheric bands is far from clear cut. However the evidence is sufficiently strong to merit the calculation of the height profile for the atmospheric O,(b’8,‘) emission from such a transfer mechanism and, by comparison, to test the extent to which it is consistent with available observations.
3. REACTION
KINJTTICS
On the basis of the reactions listed in Table 1, and for an assumed steady state, the volume emission rate, V(b), of the O,(b’8,‘-x38,-) atmospheric
385
band is given by V(b) =
&Dl*M &Pkd0J Al+ k,[O,l + k,[OlAz + kdOl+ kiMI (1)
where each term is defined according to Table 1. As the rate coefficients for reactions (3) and (4) have yet to be measured it is necessary to adopt, or estimate, values appropriate to equation (1). In their analysis of the oxygen green line in the airglow and an extension of this analysis to the aurora1 excitation, Solheim (1979), Witt et al. (1979) and Solheim and Llewellyn (1979) have estimated values appropriate to the rate constants k, and k, for the Barth mechanism and we have utilized their values in the present calculations. A value for k, of 3 X lo-” cm* sml IS . consistent both with the above works and with the corresponding value deduced by Slanger and Black (1977). It should also be noted that the value of 1 x lo-l3 cm* Sol for the rate constant k, adopted by Solheim and Llewellyn (1979) for the quenching of the green line intermediate allowed any collision partner M. However, as the recent laboratory measurements show that O,(c’Z,J, the intermediate presently favoured, is not quenched by N, we have set k, in equation (l), equal to 1 x lo-l3 [M]/[O,] or approximately 5 x lo-l3 cm3 s-l. Martin et aI. (1976) have determined the quenching rate constant for O,(b’C.,‘) by Nz to be 2.2~ lo-l5 cm3 s-l and noted that, by comparison, the quenching with 0, is negligible. This conclusion is in good agreement with earlier measurements (Noxon, 1970) and we have, therefore, adopted a value of k, equal to 2.2 x lO_” cm3 s-l. In preliminary calculations the value of k,, the rate constant for the quenching of O,(b’C,‘) by 0, was unknown but was set equal to k, since Slanger (1978) had concluded that, as with 02, the quenching efficiency with 0 was low. Recently Slanger and Black (1979) have measured this rate constant at 300 K to be 8 x lo-l4 cm3 s-l. For an atomic oxygen concentration of 101’ cm-3 this implies radiative loss equal to quenching. However, if there is an activation energy associated with atomic oxygen quenching the value of k, may be substantially reduced at temperatures corresponding to the mesopause so that this loss mechanism would be of only marginal significance @anger and Black, 1979). The fact that the effective lifetime of O,(b’Z,‘) in the atmosphere is nearly equal to the radiative lifetime implies that quenching by 0 is not a major loss process; this is in agreement with the interpretation of observations of the atmospheric bands in aurora
386
R. G. H. GREER, E. J. LLE~JSLLYN,B. H. !S~LHEIMand G. Wrrr
presented by Wallace and Chamberlain (1959). In our analysis we have, therefore, considered values and 8~ to 3 X lo-” cm3 s-l of k5 equal lo-l4 cmY1. 4. EVALUATION OF COMPUTED EMISSION PROFILES
Of crucial importance is the fact that the atmospheric band profile is seen to peak at 94 km in a number of different observations. In this respect calculations were performed for three different atomic oxygen profiles (Fig. 1) to test the sensitivity of the calculated emission to variations in this parameter. Profile A is taken from the atmospheric model of Jacchia (1977) and extrapolated below 90 km according to the calculations of Moreels et al. (1977). Profile B is that of Witt et al. (1979) and has the merit of being derived from nightglow observations of the green line made simultaneously with the atmospheric bands presented in Fig. 4. Profile C represents a direct measurement using an oxygen resonance fluorescence technique (Thomas et al., 1979). For each of the calculations the background atmosphere was assumed to be that appropriate to the mean CIRA (1972). The normalized volume emission profiles of the atmospheric bands for an excitation through direct recombination are shown in Fig. 2. The altitude of the peak emission is in each case distinctly higher than the reported measurements of 94 km and, as might be expected for a mechanism which is dependent on CO]‘, is apparently very sensitive to the atomic oxygen concentration. This behaviour
does not support a direct excitation mechanism for the atmospheric bands. In Fig. 3 the normalized emission profiles pertinent to the transfer mechanism outlined above are presented. Each profile apparently peaks at about 94 km in agreement with the observations. The uniformity in the peak emission altitudes demonstrates the reduced dependence of the emission on [O]. This is apparent from equation (1) and the adopted rate coefficients which suggest that the O,(b’C,‘) profile varies as [O][O,][M] throughout the bulk of the emitting region, when O2 is used as the transfer agent, and not as [O]‘[M]. Further the 94 km peak emission altitude is characteristic of an [O][O,][M] dependence whereas an [O]‘[M] dependence places the peak emission altitude near the oxygen atom peak. The structure exhibited on the profile C derives from the direct method of measuring the atomic oxygen concentration; such fine detail might not be apparent in the data scatter normally associated with a photometer output and could easily be dispersed in the subsequent analysis. A direct comparison between the shapes of the measured and calculated atmospheric band profiles is also possible and is presented in Fig. 4. Profile A is that obtained by Witt et al. (1979) from rocket borne photometric observations; profiles B and C are, respectively, the calculated profiles for direct excitation of O,(b’Z,+) in three body recombinations and for excitation by energy transfer from an excited intermediate. As with the previous calculations these profiles have been normalized to unity
FIG. 1. MODELATOMICOXYGENDENSITIES:A,JAC~HIA (1977):B,Wrrretal.(1979):C,THo~~s (1979).
etal.
The excitation of O,(b’Z,‘)
CURVE
in the nightglow
381
A
FIG. 2. NORMALIZED VOLUMEEMISSIONRATESFORTHEATMosPHERICBANDSFOREACHOFTHEMODBL OXYGENPROFILES,DIREcTEXCITATXON. The
volume emission rates have been normalized to unity at the altitude of the peak emission.
at their respective emission peaks. The atomic oxygen profile used in each of these calculations was that derived by Witt et al. (1979) from the oxygen green line. It is clear that the observations are in better agreement with the transfer mechanism. Whilst at the lower altitude this mechanism appears to yield excess emission it should be noted that the atomic oxygen concentration derived from the green line is here most uncertain, as the green line volume emission rate is small, so the discrepancy
-3
-4
F1c.3.
b
-+--+-d I
LOG
VOLUME
I
I
,
EMI&N
may not be significant. On the top side the observed profile is consistent with both the high and low value of k5, the rate constant for quenching of O,(b’8,‘) by 0. This is not unexpected as neither value of the rate constant implies other than a minor contribution to the loss of O,(b’Z,‘) in the atmosphere. As a further check on the proposed transfer mechanism we have attempted to estimate a value for k5 which would produce agreement between the
I
I
I
RATE
0
, I ,
lorbbxy
CURVE C , , , ,
units)
0
NORMAUZEDVOLUMEEMISSIONRATESFORTHEATMOSPHBRICBANDSFOREACHOFTHEMODEL OXYGENPROFIIESTRANSFBRMECHANISM.
The volume emission rates have been normalized to unity at the altitude of the peak emission.
R. G. H. GREER, E. J. LLE~EJ-LYN,B. H. SOLZZIMand G. Wrrr
388
8o
1
2
3
4
5
6
7
8
NORMALISED
FIG. 4.
COMPARISON
BETWEEN
OBSERVED
9
1
123L5678
10
VOLUME
EMISSION
9
10
80
RATE
(Wrrr et al., 1979) AND CALCULATED
ATMOSPHERIC
BAND
PROFILES.
(a) Transfer excitation;
(b) direct excitation in three body recombination. All volume rates have been normalized to unity at the altitude of peak emission.
observed and calculated height profile for a direct three-body excitation mechanism. The best fit consistent with a peak emission altitude of 94 km is obtained with a quenching coefficient equal to 4X lo-” cm3 s-l. This value is much larger than that measured in the laboratory and would, in contradiction of the observations, cause the effective lifetime of the atmospheric bands in the atmosphere to be significantly less than the radiative lifetime. Thus both the laboratory measurements and the atmospheric observations indicate that the oxygen atmospheric bands are not appreciably excited in the direct three-body recombination of atomic oxygen. Despite the present uncertainties in the details of the proposed transfer mechanism we may estimate the value of the product CY/~ which provides agreement between the measured and calculated overhead emissions intensity. For the Witt et al. (1979) observations this product is 0.8 which, if it is assumed that all interactions of the intermediate with 0, lead to O,(b’X,‘), would require that 80% of all recombinations yield the precursor state. In their analysis of the oxygen green line, for an assumed Barth mechanism Solheim and Llewellyn (1979) estimated that 70% of all three body recombinations yield the required precursor state. Since, as we suggest, the intermediates for the green line and the atmospheric bands are identical then the required yields are in reasonable agreement. Although it is proposed that the atmospheric bands in the airglow are excited through energy transfer it is possible to rewrite equations (1) in a
form similar to that for a direct three-body recombination; V(h) =
&MOzl k~Dl’CMlAz A, + UOI + h[CM -4 + k,[Ol+ k&f1 ’ (2)
The apparent yield of the (b’8,‘) state in the recombination is here represented by the altitude dependent term,
440J y = A,
+ k,[O]+ k,[O,]
.
This altitude dependence has been previously noted by Witt et al. (1979) in their analysis of the oxygen atmospheric bands. For the present model, Fig. 4, the apparent yield has a value of 0.24 at 94 km and 0.08 at 100 km. The latter value is close to the statistical weight of the b’C,’ state for the ground state atom recombination while the former is in reasonable agreement with the single value of 0.2 estimated by Deans et al. (1976). Applying the present conclusions to the calculation of atomic oxygen concentration from the nightglow observation by Deans et al. leads to a value of 3.5X 1Ol1 cne3 at 100 km which differs from their value of 1.7 x 1O’l cme3 but is in much closer agreement with the concentrations measured by Dickinson et al. (1980) using resonance fluorescence lamps. It is worth noting that the transfer mechanism proposed in this paper does not contradict the laboratory spectra of Slanger (1978). The emission of the atmospheric bands does not require the precursor to be vibrationally excited so that the intermediate state, assumed by us to be c’CU-,
The excitation of Oa(brZ,‘) in the nightglow could relax to the zeroth vibrational level before transferring, in agreement with the observed spectra. A vertical transition from this zeroth level would populate the v = 5 level of O,(b’Z,‘). At the mesopause it is expected that this state would undergo vibrational relaxation so giving only the 2)’= 0 bands in the airglow. However if the transfer, i.e. the vertical transition, occurred at higher altitudes then we might expect vibrational development of the bands in agreement with the observations of Gattinger and Valiance Jones (1976). 5.
CONCLUSION
In the present paper it has been proposed that the O,(blB,“) state, which is responsible for the atmospheric bands, is excited in the night~ow by energy transfer from an intermediate state, rather than directly through atomic oxygen recombination. It has been argued that laboratory and atmospheric observations do not contradict this hypothesis nor the identification of the intermediate state as 02(c*&-). Selective quenching of the latter state by molecular oxygen is the factor which characteristically causes the atmospheric bands to peak at 94 km rather than higher as with the green line. The intensities derived from the present analysis indicate that such a transfer mechanism can adequately explain the observed intensities of the atmospheric bands without resource to any other mechanism. However, the adoption of this excitation path does nothing to explain why the precursor is apparently excited with such a high efficiency in the initial recombination. Ack~wledge~en~s-his work has been supported by Grants-in-Aid from the National Research Council of Canada, The Queen’s University of Belfast and the International Meteorological Institute, University of Stockholm. REFERENCES
Barth, C. A. (1964). Am&. Geophys. 20, 182. Campbell, I. M. and Gray, C. N. (1973). Chem. Phys. I&t. 18, 607. CIRA (1972). COSPAR International Reference Atmosphere. Akademic-Verlay, Berlin. Clarke, I. D. and Wayne, R. P. (1969). Chem. Phys. Lea. 3, 405. Clyne, M. A. A., Thrush, B. A. and Wayne, R. P. (1965). J. P~o~~hern. Photobiol. 4, 9.57. Deans, A. J., Shepherd, G. G. and Evans, W. F. J. (1976). Geophys. Res. Len. 3, 441. Dickinson, P. H. G., Bain, W. C., Thomas, L., Williams, E. R., Jenkins, D. B. and Twiddy, N. D. (1980). Proc. Roy. Sot. London A369,
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Evans, W. F. J., Llewellyn, E. J. and Valiance Jones, A. (1972). J. geophys. Res. 77, 4899.
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