Excitation and quenching of the oxygen bands in the nightglow

Department of Applied Mathematics and Theoretical Physics, Queen’s University,

Belfast 817 INN, U.K.

Abstract-The laboratory measurements on the rate coeEicient of the terrnoiecnlar association process 0 -I-0 + Nz + Q + N 2 are examined. It is cancluded that, owing; to the effect af rediisociation, the weakly bound ‘II, state could not have contributed appreciably. In consequence the rate of direct: entry into the X3X;, a’A9, b’;Iz, c’Z;, X3A, and A%: states during association is greater than has been supposed. The Her&erg I, Chamberlain and Her&erg II systems are severely quenched in the nightglow and O&4%:, .J3A,, c’C;) act as precursors to the Atmospheric and Infrared Atmospheric systems. It is likely that O,( Qs) also so acts.

1. ~Tff~~~~I~N

Termolecular

association 0+0-l-M-+O;“+-A4

(1)

in some instances foiiow~d by an energy transfer process (Young and Sharples$, 1963) like

Her&erg I, ~~a~b~r~ai~ and EIerzberg II systems. The first of these is the most fully studied: the value of V(‘(HzI)given is the mean of five measurements by Greer et al. (1986) at the peak at 98 km. We assume that at 98 km P’(Chm) = V(IIzII) = 0.25 fr(HzI)

(3)

because Slanger and Hue&is (1981) report that this is the relation between the total photon emission. is the main source of tke excited uxyge~ molecules that contribute to the terrestrial nightglow. The present paper is conftned to aspects ofthis source and will not consider the su~pl~m~n~ry sources of r~la~vely minor ~rn~o~anc~ that have been suggested for the Atmospheric system (cf. Witt et al., 1979 ; Krasnopolsky, 1986) and the Infrared Atmos~he~c system (cf. Llewellyn and Solheim, 1978 ; ~G~ade cs1al., 1987 ; Lopes-~oreno et al., 1988).

For reference purposes we first collect together the number densities, [], of retevant a~os~beric constituents and volume photon emission rates Y( ) that we adopt. As regards the former only [O] requires any comment. We took the mean of the seven deterrn~~at~ons made by Greer et aI, (1986) during rocket Bights. Table 1 gives the mean of three measurements of ~(Atmos) by Greer rt at. (1986) at the attitude, 95 km, ofthe peak luminosity. The value of V(ER Atmos) at the same abitude is an estimate of the contribution from oxygen association based on the work of Thomas and Young (1981) MeDade et ai. (1987) and Lopez-Moreno et raI. (19SS). Table 2 covers the

~~(5~~) = wqJm4

(61

in which T”f”l=

1

Q

so that kA is related to ka by ?C,& --(l-~)~(~~~)~

= kl,

(S)

and the factor f that arises in the case of the weakly bound “II, state takes into account that deactivation to the ground electronic state, perhaps through other electronic states %3$J+N! 875

-+ G&%A’,c,&a,B+N,

(9)

876

D. R.

BATES

TABLE 1. ATMOSPHERIC AND INFRAREDATMOSPHERIC SYSTEMS

TABLE 2. HERZBERG I, CHAMBERLAINAND HERZBERGII SYSTEMS

Altitude

95 km

Altitude

Temperature

188 K

Temperature

4.2(11) cme3 6.7(12) cme3 2.5(13) cmm3 4.3(3) cm-3 s- I 3.0(4) crne3 ss’

PI n1 Ir\r*l V(Atmos) V(IR Atmos) Note:

may be prevented

98

km 192 K

5.4(11) crnm3 4.0(12) crnm3 1.5(13) cmm3 3.1(2) cm-‘s-’ 8(l) cme3 ss’

PI n1 [Nzl

V(HzI)

V(Chm) and V(HzI1)

4.2(11) = 4.2 x 10”. by redissociation

O&J+Nz

+ O+O+Nz

(10)

as suggested by Smith (1984). Measurements on kA(S) have been made for several states but Bates (1988a) has given reasons for rejecting most of the results that have been published (and with them some claims regarding precursors). In general there has been neglect of the effect of O,(a) which is an efficient quenching agent and which can be produced copiously by heterogeneous reaction (cf. Black and Slanger, 1981) so that it may form as much as 20% of the O2 in an afterglow. Theoretical values of F(S) should not be regarded as accurate but they are the best at present available. Calculations have been carried out by Wraight (1982) and by Smith (1984). Averaging their results Krasnopolsky (1986) took F(‘II& to be 0.66 at temperatures below about 200 K. In their work Wraight and Smith used the 0,(511& potential well of depth 0.23 eV given by the ab initio quanta1 computations of Saxon and Liu (1977). The more refined ab initio quanta1 computations of Partridge et al. (1988) yield a depth D of only (0.14f0.03) eV so that F(jII,) must be less than 0.66 but judging from the slow variation with temperature (cf. Smith, 1984) not much less. It must also be greater than the value 0.34 that would ensue if the course of association were determined solely by the statistical weights of the bound states. Adopting the mean of the two extremes we took F(‘II,) to be 0.50 which cannot be seriously in error. For the other states we assumed that the relative F(S) are as Krasnopolsky (1986) inferred from the calculations of Wraight and Smith and used equation (7) to normalize them. Table 3 gives the values thus obtained.

Supposing D to be 0.23 eV Wraight (1982) judged that the redissociation process (9) is unimportant in the low temperature region of interest. The reduction of the accepted D to 0.14 eV by Partridge et al. (1988) changes the position. The numerical expression that Wraight gave forf of equation (6) contains (as he recognized) some entities that were at the time poorly determined. Letting k,(‘l$;N,) be the rate coefficient for the quenching process (9) and k, be the rate coefficient for the redissociation process (10) we have that f = k,(5& ; NJI{k,(5&

0.12

a’$, 0.07

k:, = 1.06 x 10-32(200/T)2

0.04

cm6 s-’

(12)

(Campbell and Thrush, 1967; Campbell and Gray, 1973). From the properties of O,(‘II,) the estimated value of F(‘IIJ, relation (8) and result (12) we may obtain information on k, since k, = KF(SITg)k, where K is the equilibrium O+O+N

constant

(13) for

2+02(511g)+N2.

(14)

We find that at 200 K : kD = 2.5 x 10m1’cm3 ss’

iff=

0

(15)

k, = 1.3 x 10-“cm3

iff=

1.

(16)

s-’

Quenching process (9) involves spin-change so that k,( ‘lls) should be less than the smaller of the two rate coefficients just given by several powers of ten. Since

A’3A b’C+ c’ix;, F(S)go> equations (5) and (“B) 0.03

(11)

when molecular nitrogen is the ambient gas in which the association is taking place. Measurements in this ambient gas give the association rate coefficient to be

TABLE3. STATEASSOCIATION FACTORS(AROUND200 K)

A’%;,-,

; Nz) + k,)

0.18

AT+ ” 0.06

’

‘=, 0.50

Oxygen

[O]/[NJ was less than 10m3 quenching by atomic oxygen cannot have been appreciable. It follows thatf was effectively zero in the laboratory studies and hence that k, is as in equation (15) and k, = 2.1 x 10~32(200/T)2 cm6 s- ‘.

(17)

Our analysis, which supersedes Bates (1988a), implies that the rate of direct association into states other than jIIa is about three times greater than supposed by Wraight (1982) and Krasnopolsky (1986). It also implies that the rate of direct entry into ‘II, is about 1.5 times greater than previously supposed. This is relevant in connection with the search for an ozone source in the lower thermosphere additional to the classical termolecular source (cf. Bates, 1987). At temperatures near 200 K we find that the 02(%J number density in equilibrium (14) is [O,‘II,],,

= 2.2, x 10~24exp(1506/T)[O]2

TABLE 4. CALCULATED(KLOTZ et al., 1984; KLOTZ AND FEYERIMHOFF, 1986; BATES, 1988b) TOTALTRANSITION PKOBABILlTy : ,f(U' = O)FOR ZEROTHVIBRATIONAL LEVELOF INITIAL STATE; A(NIGHTGLOW) FOR NIGHTGLOW VIBRATIONAL DISTRIBUTIONREFQRTEDBY SLANCERAND HuEsTIs(~~~~) Transition

1.0(l) 8.6(-2)

A”A ” + XT, A’jA, + a’4 A”A, + b’C;

6.9( - 1) 5.2( - 1) 8.7(-2)

8.9(-

CT;

3.0(-

1.0(O)

+ m;

b’Z+9 ---*xcz, b’Z: + a’4

1.5(l) 1)

1)

INFRARED

cm3 s-‘.

ATMOSPHERIC

SYSTEMS

The Atmospheric b’C,+-X3X; and Infrared Atmospheric u’A~ +X3X; systems differ from the other systems in that only their v’ = 0 progressions appear in the nightglow. This characteristic is due to the ease with which collisions with normal O2 molecules bring about vibrational deactivation indirectly through interchanging the electronic states (Bates, 1954). It enables the fractions of O,(b’C,+) and O,(a’A& that radiate the two band systems, R(Atmos) and R(IR Atmos), to be calculated from experimental deactivation coefficient and theoretical transition probabilities. The main deactivation processes are O@‘C,+)+Nz

+ O@A,)+N,, 2.2 x lo- ” cm3 s- ’

(20)

et al., 1976) ;

O,(b’C,+)+O

+ O,(a’A,,

X3Z,,)+0,

8.0 x 10-‘4cm3

s-’

(21)

1.5 x IO-‘* cm3 s-’

(22)

(Slanger and Black, 1979) ; O,(u’A,) + 0, --* 0,(X3X;)

and Snelling,

197 1) and

1)

8.6(-2)

1.4(-3) 1.9(-4)

a’4 -+ x%X;

Note: 1.9(-4) = 1.9 x 10m4.

crnm3

(19) 4. ATMOSPHERIC AND

A(nightglow) sm’

3.6( -

O,(u’A,)+O,+

0,(X3X,)+0,

< 1.6 x lo-l6

k, = 4.7 x 10~9(200/T)2exp(-1506/T)

I

A%C,+--t alAg A%: + b’Z,+

and that

(Findlay

A(u’ = 0) s-

‘4?z+ 1 +X%,y

(18)

(Martin

877

bands in nightglow

+ O,,

cm3 s-’

(23)

(Clark and Wayne, 1969). The rate coefficients cited refer to room temperature rather than the temperature of the 95 km level except in the case of process (22) where the slow temperature variation is known. Taking the number densities from Table 1 and the transition probabilities from Table 4 we calculate : R(Atmos)

= 0.51 and R(IR Atmos)

= 0.95(0.80). (24)

The bracketed number, which corresponds to the upper limit of the rate coefficient in equation (23), will be adopted. Fortunately the difference between it and the unbracketed number, which corresponds to neglecting process (23), is of no consequence. The potential association rate k,[O]‘[M] is the association rate that would occur if association through the ‘II, state were not appreciably hindered by redissociation. It is a useful concept. From result (24) we infer that the nightglow emission rates in the Atmospheric and Infrared systems (Table 1) require that the fractions, F(b, nightglow) and F(a, nightglow) of potential associations that lead directly or indirectly to the b’CJ and a’$ states are as in the first row of Table 5. The second row gives the corresponding fractions for direct entry into these two states (from Table 3) combined with the minor contribution from process (20). As may be seen an additional source, presumably transfer of energy from a precursor, is needed. In the case of the Atmospheric system this has also been inferred from studies of the altitude profile of the emission (Greer et al., 1981; Krasnopolsky, 1986; McDade et al., 1986). Although there have been

D. R. BATES

878

TABLE5. EXCITATIONOFATMOSPHERIC AND INFRAREDATMOSPHERICSYSTEMS

0,(A3C:

State

b-L+9

~++~-*C-+ZCor

0.10

do not provide a (Bates, 1988a). It has been shown (Bates, 1988a) that evidence that has been adduced (Ah ei al., 1986) that the Atmospheric and Infrared Atmospheric systems necessarily have different precursors is unsound. Possible precursors are one or more of the close trio O,(c’C,, A’3A,, A3ZC,+)and O,(‘II,). According to Table 3 the values of the fraction for the close trio (taken together) and for the weakly bound state are 0.28 and 0.50, respectively. Clearly either is ample. In favour of the close trio being involved there is good reason to believe (see Section 5) that the Herzberg I, Chamberlain and Herzberg II systems of the nightglow are severely quenched by irreversible processes. Note that the addition to F(a+b, nightglow) that is needed is only 0.21 while a maximum in excess* of 0.28 may be provided by transfer from the close trio of states. If 0 were the close trio’s collision partner the required contribution to F(a + b, nightglow) would be provided if the deactivation populated the b, a and X states in proportion to their statistical weights (an artless model). The rate coefficients would have to be high (Section 5). The spin allowed irreversible quenching processes with O2 or N, as the collision partners are : useful pointer to the identity of the precursor

-+O,(b’Z~)+O,(a’L\,);

(25)

-+ 20,(6’C;)

(26)

;

-+ 20,(a’A,);

(27)

-+ 0,(b’C,+)+0,(X3C;);

(28)

-+ O,(a’ A,) + 0,(X%;)

O,(c’X;)+NZ(X’Z;)

Z++X+

(34)

but this in itself does not exclude them because the rule is only valid if the collision complex is planar. Of the 15 processes listed, only the five in equations (30) and (33) do not yield one or two O2 molecules in the b and a states. Making the customary assumption that NJ, 0, and 0 are approximately equally effective as third bodies, Pn, in association it may be seen that in the lower thermosphere equation (11) is replaced by f=

l/Q +a~kd+P&&

mlkm@I,, 02) +rw,m?,

WI

;

(29)

+ 20,(X3C;); (30)

-+ Oz(b’C,f)+Nz(X’X;);

(31)

*The value 0.28 ignores that some of the quenching processes yield two of the excited molecules in question.

(35)

where k,, is the rate coefficient for O,(“I-I&+O*(x)

-+ 03+0

(36)

and k,(‘H,, 0,) and .$(‘I&, 0) are the rate coefficients for 0019)+OZ(X>

-+ O@, A’, c, b, 0,X)+%

(37)

(in which the state of the final product O1 is such as conserve spin) and

to

O~(SH,)+O-*O,(A,A’,c,b,a,X)+O.

(38)

Equation (19) gives that k, is 1.8 x lo- I2 cm3 s- ’ at 95 km. Supposing that k,, is 5.6 x IO-” cm3 s-’ (a suggested upper limit; Bates, 1987) we observe that the terms [O,]k,( slIs, 0,) and [O]k,( 51’Is,0) are equal and make k,(51’I,;02)

= 4 x lo-“cm3

s-’

f = l/3 if

O1;(A’X: or A’3A,)+02(X3C;)

Af3A.,or c’C;)+0,(X3C;)

N,(X’ZC,+). (33)

Some break the correlation rule of Shuler (1953) that

claims to the contrary these studies

O&4%:,

+

+ 0,(X%,)

0.03

(32)

or A’3A,J+N2(X’C~)

alAg

Association fraction F (see text) 0.06 0.28

Ni~tgIow requirement Contribution from direct entry and process (20)

O,(c’C,)+O,(X’C,)

--t 02(a’Ag)+N,(X’Z~);

(39) kq(TIg;O)

= 6 x lo-”

cm3 s-‘. -

We do not know whether the quenching coefficients are greater or less than in equation (39) and thus cannot predictfat 95 km. Unlessfproves to be small there might seem to be a problem as to why association does not make the Atmospheric and Infrared Atmospheric systems more intense than they are. However, we already know (Table 5) that the fraction of associations that populate the b and a states indirectly is 0.03 and 0.18 ; and if this is part of a trend in which the lower state is favoured it may well be that the corresponding fraction for the X state is 0.57 or greater [as would be required if f of equation (35) were close to unity].

Oxygen

879

bands in nightglow

TABLE 6. DEACTIVATION COEFFIC~ENTSMEASUREDINDISCHARGEFLOW

Excited molecule O,(A’Z:, O,(A’Z;,

v = l&4) u = 2)

O,(c’Z;, v = 0)

5. HERZBERG

Collision partner

Deactivation coefficient (cm’ s- ‘)

0 0, N,

l&1.5 X 10-l’ 1.3 x lo-” 9.3 x lo-l5

0 0,

I, CHAMBERLAIN

AND HERZBERG

5.9 x 1o-‘2 3 x lo-l4

II

SYSTEMS

With the objective of obtaining information on the quenching of the Herzberg I and Herzberg II systems in the nightglow, Kenner and Ogryzlo have conducted a series of discharge flow experiments at room temperature. Table 6 gives some of their results. It has been argued (Bates, 1988a) that they do not take proper account of the quenching of the Herzberg I and Herzberg II systems by collision-induced transitions of the A-A’ and A-c and of the c-A and c-A’ types, respectively. The states of the products were not determined. In the O,(A,v)-O case which presumably occurs through atom-atom interchange, the dependence of the deactivation coefficient on the vibrational quantum number u is slight. Such behaviour would not be expected when an electronic transition is involved ; on the contrary the deactivation coefficient must then be sensitive to v because this affects the energy change. A null observation that Slanger et al. (1984) made is in harmony with this : they were unable to detect any emission when they excited 02(A3C:, v = 8) by a laser. Their favoured interpretation is that the rate coefficient for 02(A3E:,

v = 8)+02

+ products

(40)

is greater than 8 x lo- ” cm3 s- ‘. Support is provided by a study that McNeal and Durana (1969) made on the emission of the Herzberg I system by an afterglow. From the range over which the intensity is quadratic in [0] Bates (1988) deduced that k, ,, the rate coefficient for Oz(A3C:,

v)+O,

+products

Reference Kenner Kenner Kenner

(1984) (1983a)

(1983b) Kenner and Ogryzlo (1983a)

Atmos), of the O,(b’Z,+) and 02(a1Ag) formed in the lower thermosphere that emit the Atmospheric and Infrared Atmospheric systems from the deactivation rates and the radiative transition probabilities. We cannot proceed similarly for R(HzI), R(Chm) and R(HzI1) because we do not know the relevant deactivation coefficients. However, we can calculate upper limits to these fractions from the photon emission rates (Table 2) and the rates of formation of 0,(A3C,+), 02(A’3A,) and O,(c’C;) by termolecular association. Using the entries in Table 3 and taking only direct entry into account we get the upper limits R(Hz1) < 4 x lo-‘,

R(Chm)

< 3 x 10-3,

R(HzI1) < 1.5 x lo-*.

(42)

Knowing the transition probability (Table 4) we may also calculate lower limits to the mean deactivation rate coefficients appropriate to the nightglow. Table 7 gives the limits obtained assuming that 0, or N, is the collision partner-those obtained assuming that 0 is the collision partner are unacceptably high. The actual deactivation rate coefficients cannot greatly exceed the limits in Table 7. For example, each would only exceed the corresponding limit by a factor of two even if O,(A), O,(c) and O,(A’) were the sole direct products of the deactivation of O,(‘II,), with the dis-

TABLE 7. LOWER LIMITS TO MEAN DEACTIVATION RATE COEFFICIENTS APPROPRIATE TO THE NIGHTGLOW OBTAINED FROM DATAINTABLE~,RESULT(~~)STATEASSOCIATIONCOEFFIC~ENTS INTABLE~ANDTRANSITIONPROBABILITIESINTABLE~

(41)

averaged over the (unknown) vibrational distribution in the afterglow is greater than 1 x lo-*’ cm3 s-l. This is at least about 10 times the corresponding rate coefficient for 0z(A3Zz, v = 2) given in Table 6. We calculated the fractions, R(Atmos) and R(IR

and Ogryzlo and Ogryzlo and Ogryzlo

Metastable AT+ Y A’3A Y C’Z,

state

Assumed collision partner 0, N* lower limit (cm’ s- ‘) 1.0 x 10-‘O 6.7 x lo-" 1.6 x lo-”

2.5 X lo-” 1.8 x lo-” 4.3 X lo-‘2

880

D. R. BATES

tribution between them proportional to F(S), and even if the deactivation coefficient with, say, O2 as the collision partner were as great as 1 x 1W ’ ’ cm3 s- ‘. It is likely that O,(b), O,(a) and O,(X) are also direct products of the deactivation. According to Slanger and Huestis (1981), the vibrational distribution n(u) normalized to unity at its maximum is 0.5 when u is 3 and 8,4 and 9,4 and 8 for the ,4, c and A’ states, respectively. Large deactivation rates are favoured by high vibrational levels being populated. Presumably intermediate complexes are formed. The severe quenching of the Herzberg I system, equation (42), explains the observed aftitude independence of the vibrational distribution in the A state (Murtagh et al., 1986). The possible deactivation processes have been catalogued (Section 3). An expe~mental check is needed. Recent work by Knickelbein et al. (1987) suggests how this might be achieved. Knickelbein et al. (1987) excited O,(b’Z+) by a laser and measured the O&‘A& produced in 0,(6’C,tE0,(X3C;) collisions via the dimal emission centred at 634 nm. A similar investigation with O,(b’C,+) replaced by O,(A%z), as in the work of Slanger et al. (1984), would be invaluable. Ideally the investigation should cover both 0,(A3C:)-0,(X3E;) and 0z(A3Cz jN,(X’C,+) collisions. Acknowledgement-I thank the U.S. Air Force for support under grant AFOSR-85-0202.

REFERENCES

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