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Formation of C2(e3πg, C1πg) in the photolysis of CO(X1ϵg+) at 193 nm with an ArF laser
Volume 81, number 3
FORMATION OF C2(e 3IIg, C
CHEMICAL PHYSICS LETTERS
liig) IN THE PHOTOLYSIS OF CO(X 1 ~ ; )
1 August 1981
AT 193 nm
WITH AN ArF LASER W. HACK and W. LANGEL Max-Planck-Institut ffir StrOmungsforschung, D-3400 GiSttingen, FR G
Received 27 May 1981 ; in final for m 29 .!une 1981
+ was irradiated with an ArF laser (193 nm) at room temperature at pressures of 2-20 mbar. C(2 1 D) atoms CO(X 1 Eg) were formedby two-photon absorption and observed by the 247.8 nm C(3 1 PO._~ 2 1 S) fluorescence. Fluorescence of C2(e 3Fig ~ a 3rI u, C 1Pig~ A 1 Fig and d 3IIg ~ a 3Hg) was detected. The emission time dependence led to the conclusion that C~ is formed by recombination of electronically excited carbon atoms.
1. Introduction Only very few small molecules (e.g. C302 [1 ]) are useful as "clean" C-atom sources, which do not dissociate into a variety of different reactive radicals in addition to C atoms during photolysis. One o f them is CO, which is a readily available starting material for the photochemical separation of 12C and 13C, as was demonstrated in ref. [2]. On the other hand, CO has the strongest known bond. Thus it cannot be dissociated by only one quantum unless radiation with very short wavelength is used. Up to now, CO was only photolyzed by weak far and vacuumUV sources [2,3]. Since the electronic states are higher in energy than the photon energy of most of the common laser systems and the density o f states is very small, it had not previously been possible to pump the CO molecule into a dissociative state by multiphoton absorption. In those experiments fluorescence from highly excited states was seen rather than photochemical reaction [4]. Only very recent work has shown that C atoms were formed in an isotope-selective process, when CO was irradiated by an ArF laser [5]. The atoms appear in the electronically excited 2 1D state. The ArF line is strongly absorbed by the electronically excited C atoms (3 1po ~ 2 1D) at 193.1 nm. From these atoms, the fluorescence C(3 1po -+ 2 1S) can be observed at 247.8 nm.
In this work, we wanted to observe the removal of electronically excited carbon atoms. For this purpose, photodissociation of CO at 193 nm seems to be a useful method.
2. Experimental The apparatus was similar to that described in ref. [6] except for the present use of a high-transmission monochromator. A home-made.excimer laser wa~ u~ed on the ArF line (193 nm, 1 0 - 3 0 mJ/pulse). The time dependence of the laser pulse is characterized by its fwhm of 15 us. CO was irradiated flowing vertically through a nearly cubic teflon cell at pressures of 1 . 9 - 1 9 mbar at room temperature. It has the highest commercially available purity (99.97%, Messer Griesheim) and was used without further purification. The laser beam passed horizontally through two suprasil windows (distance 70 mm) and was reflected and focused to the center of the cell by a concave aluminium-coated mirror (r = 300 ram) behind the outlet window. The absorption of the laser line by CO was too weak to be detected directly. Fluorescence was observed along the second horizontal axis. It was dispersed by an Oriel 7240 double monochromator ( f = 125 mm, resolution 2 nm) and monitored by an RCA 4831 photomultiplier. A
0 0 0 9 - 2 6 1 4 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
387
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
C( 31 Po--21 S)
1.5
C2 (C~lrg ~ Aqr~) 3
I [ a.u.]
1 August 1981
v'
vj 3 2 I
1.0
o
0.5 C2 (d 3 Trg~a31ru ) (342..,676 nm)
0 190
250 4
350 3510
450 40
550 h [nm] 35 ns lifetime
Fig. 1. Luminescence spectrum of 1.9 mbar CO after excitation in the focused beam of an ArF laser. Tektronix R 7912 transient recorder coupled to a DEC PDP 11/04 minicomputer was used to measure pulse height and decay time. The characteristic decay times of the fluorescence were evaluated by convoluting the excitation function with a single exponential decay fimetion and fitting the recorded fluorescence signal. Since the excitation and decay times were of the same order of magnitude, a convolution rather than a simple logarithmic analysis of the signals had to be applied.
measured lifetime of C(3 1 sO), are given in table 1. The 3 1po state is populated by the absorption 3 1po +. 2 1D of carbon atoms [5]. The emission o f carbon atoms is proof of previous dissociation of CO. The energy of at least two ArF laser photons is needed to break the CO bond. Rhodes et at. [5] concluded from the dependence of emission at 247.8 nm on the laser power, that it is indeed a two-photon process. In our experiment, the C(3 1po) fluorescence was shorter than the laser pulse. Therefore we could only evaluate a reasonable lifetime of C(3 1po) by using the square of the laser pulse as the excitation
3. Results and discussion The luminescence spectrum observed in this work at a CO pressure of 1.9 mbar is shown in fig. 1. A very similar spectrum was observed at 19 mbar. Besides the intensive fluorescence of C(3 1 po _+ 2 1 S) produced by a two-photon dissociation of CO and the laser excitation of C(21D), the emission of three systems of electronically excited C2 is observed. No fluorescence from any state of the CO molecule is seen. Lifetimes of two electronic states of C 2 (C 1 IIg, d 3 IIg) have been measured. These results, together with the 388
Table 1 Fluorescence decay times after excitation of CO by an ArF laser State
W a v e l e n g t h Pressure (nm) (mbar)
C(3 1SO)
248
11
C2(C 1rig) C2(d 3rig) C2(d 3Fig)
387.4 465.8 510
3.3 11.7 11.5
Decay time (ns) 2 10 40 35
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
function for the convolution. The necessity of using the square of the laser pulse rather than the pulse itself implies, that the production of C atoms depends on the square of the laser intensity and affirms that the atoms are formed in a two-photon process. The absorption spectrum of CO at half the wavelength of the laser line (96.7 nm) shows sharp lines without any broadening or diffuseness [7]. This indicates that the energy levels, which correspond to two laser photons (2 × 52200 cm -1) belong to non-predissociated singlet states. Therefore, a triplet-state intermediate is supposed to be involved in the formation of the C atoms [5]: 3CO(Eex c = 104500 cm -1) -+C(23p)+o(23p),
AH = 15100cm -1,
-+ C(2 1D) + 0 ( 2 3p),
AH = 3600 cm -1 .
The emission spectrum given in fig. 1 shows, in addition to the fluorescence of C atoms at 247•8 nm, a molecular spectrum. This can be assigned to the emission of higher excited states of C 2. Between 240 and 340 nm, the Fox-Herzberg system C2(e 311g _.~ a 3IIg) and between 340 and 410 nm the Deslandresd'Azambuja system C2(C 1 IIg -+ A 1ilu) is found. The fluorescefice at 466 and 510 nm belongs to the Swan bands C2(d 3IIg ~ a 3Ilg) (7"0 = 136 ns [8,9]). In the e 3Ilg state, emission from the vibrational levels u' = 0 - 3 and in the C I rlg state, emission from v' = 0 - 5 can be assigned. High vibrational levels in these states cannot be seen since they are perturbed • by other electronic states (C 1 IIg v ! > 6 by C I 1 IIg, E 311g o' > 4 by d 3IIg) [9]. C2(C 111g) has a lifetime of 10 ns under our experimental conditions. For C2(d 3Ilg), a lifetime of 37 +- 5 ns is observed (table 1). Since C2 does not absorb the laser radiation [9], C~ has to be produced in a chemical reaction. The time dependence of the emission of C~ shows that it is formed by particles with lifetimes of a few nanoseconds. Two reaction channels can be proposed to form C2 during photolysis of CO: CO* + CO(*)
(1) -+ C2(e 3IIg(o' = 0 - 5 ) , C 111g(u' = 0 - 3 ) ) + O 2 . Using the dissociation energies in refs. [8,9] for 02, CO and C2, one finds that the sum of the excita-
1 August 1981
tion energy of the CO* molecules must reach up to 130000 cm -1 . Even if CO* is primarily produced in a non-emitting state, collisions, necessary to cause chemical reactions, should populate fluorescing electronic states [8]. Thus the lack of CO fluorescence indicates that reaction (1) is unimportant• Since CO* cannot form C~, free carbon atoms r must be involved in its formation• The reaction of excited carbon atoms with ground-state CO can be excluded by simple energy arguments, but not the reaction with electronically excited CO. Highly excited states of CO, however, populated by two quanta, are excluded by the arguments given above• One laser photon has only enough energy to pump CO into its a 311 state. Since this electronic transition is very weak and, moreover, the laser line does not coincide with absorption lines of the transition CO a 3 FI +- X 1 ~+ [81, CO(a 311) is not produced by absorption. Therefore, the recombination of C atoms is the only reasonable channel for C~ production. This can occur by radiative recombination C* + C(*) -+ C2 + hv,
(2a)
or by recombination with a collision partner, e.g. CO C* + C (*) + CO -+ C~ + CO,
(2b)
which can as well proceed with C20 as an intermediate
[101. The time dependence of the C~ fluorescence shows that at least one of the C atoms in reaction (2) must be in one of the excited states 2 1D or 3 1po. The ground-state carbon atoms and the excited-state C(2 1 SO) have lifetimes of the order of milliseconds under our experimental conditions [1]. If the C~ were produced in reactions of only that long-lived species, its fluorescence would be seen over a much longer time than the few nanoseconds observed. Therefore C(2 tD) or C(3 1po) must be involved in at least one step of the recombination reaction (2). The C(2 1D) atoms are rapidly transferred to the C(3 t po) state, the absorption of the laser line by them is strongly saturated [5]. Thus they have a very short lifetime in the system discussed here, in spite of their long free lifetime [11]. The formation of C2(e 3 Fig and C 111g) and its time dependence can be explained by the reactions of C(2 1D) with itself and with C(2 3p). As the potential energy diagram of C2 shows, C2(e 3IIg) correlates with 389
CHEMICAL PHYSICS LETTERS
Volume 81, number 3 '
l II 1 E (I03 crn-I]
I
i
I
I
E'3E~
1 August 1981
Acknowledgement
I
C(~S)
The authors wish to thank Professor Dr. H.Gg. Wagner for his continuous interest in their work.
60
References
~o
Fill\ L-L:"/77~e% I LIIw //~/-c,~g
2o F- It,IV/LA?: O
I /\"/y~,:,~r,, I- 1%z/~
0.I
I
l
I
0.2 0.3 r [nm]
t
l
0./.
Fig. 2. Potential energy curves of the C2 molecule (from ref. [91). C(2 3p) + C(2 1D) and C2(C 1jig) with C(2 1D) + C(2 1D) (fig. 2 from ref. [9]). The unperturbed vibrational levels of the C2(e 31-Ig) and C2(C 111g) states could be identified directly in the emission spectrum (fig. 1). The perturbation at higher vibrational energies in e 3Ilg leads to population of d 3IIg and appearance of the Swan bands (d 3IIg -+ a 3IIu)in the spectrum. The direct observation of lower and the indirect observation of higher vibrational levels leads to the conclusion that in both states a random distribution of vibrational energy is generated by the recombination.
390
[1] D. Husain and L.J. Kirsch, Trans. Faraday Soc. 67 (1971) 2886, 3166. [2] G. Liuti, S. Dondes and P. Harteck, J. Chem. Phys. 42 (1965) 4052. [3] A.C. Vikis, J. Chem. Phys. 69 (1978) 697,703; Chem. Phys. Letters. 57 (1978) 522. [4] R.A. Bernheim, C. Kittrell and D.K. Veirs, Chem. Phys. Letters 51 (1977) 325; A.C. Provorov, B.P. Stoieheff and S. Wallace, J. Chem. Phys. 67 (1977) 593. [5] J. Bokor, J. Zavelovich and C.K. Rhodes, J. Chem. Phys. 72 (1980) 965. [6] W. Hack and W. Langel, Nuovo Cimento, to be published. [7] M. Ogana and S. Ogawa, J. Mol. Spectry. 41 (1972) 393. [81 S.N. Suchard, Spectroscopic constants for selected heteronuelear diatomie molecules (IFI/Plenum Press, New York, 1975). [9] S.N. Suchard and J.E. Melzer, Spectroscopic constants for selected homonu¢lear diatomic molecules (IFI/Plennm Press, New York, 1976). [10] D. Husain and L.J. Kirseh, Trans. Faraday Soc. 67 (1971) 2025. [11] R.J. Donovan, Progr. Reaction Kinetics 10 (1979) 253.
FORMATION OF C2(e 3IIg, C
CHEMICAL PHYSICS LETTERS
liig) IN THE PHOTOLYSIS OF CO(X 1 ~ ; )
1 August 1981
AT 193 nm
WITH AN ArF LASER W. HACK and W. LANGEL Max-Planck-Institut ffir StrOmungsforschung, D-3400 GiSttingen, FR G
Received 27 May 1981 ; in final for m 29 .!une 1981
+ was irradiated with an ArF laser (193 nm) at room temperature at pressures of 2-20 mbar. C(2 1 D) atoms CO(X 1 Eg) were formedby two-photon absorption and observed by the 247.8 nm C(3 1 PO._~ 2 1 S) fluorescence. Fluorescence of C2(e 3Fig ~ a 3rI u, C 1Pig~ A 1 Fig and d 3IIg ~ a 3Hg) was detected. The emission time dependence led to the conclusion that C~ is formed by recombination of electronically excited carbon atoms.
1. Introduction Only very few small molecules (e.g. C302 [1 ]) are useful as "clean" C-atom sources, which do not dissociate into a variety of different reactive radicals in addition to C atoms during photolysis. One o f them is CO, which is a readily available starting material for the photochemical separation of 12C and 13C, as was demonstrated in ref. [2]. On the other hand, CO has the strongest known bond. Thus it cannot be dissociated by only one quantum unless radiation with very short wavelength is used. Up to now, CO was only photolyzed by weak far and vacuumUV sources [2,3]. Since the electronic states are higher in energy than the photon energy of most of the common laser systems and the density o f states is very small, it had not previously been possible to pump the CO molecule into a dissociative state by multiphoton absorption. In those experiments fluorescence from highly excited states was seen rather than photochemical reaction [4]. Only very recent work has shown that C atoms were formed in an isotope-selective process, when CO was irradiated by an ArF laser [5]. The atoms appear in the electronically excited 2 1D state. The ArF line is strongly absorbed by the electronically excited C atoms (3 1po ~ 2 1D) at 193.1 nm. From these atoms, the fluorescence C(3 1po -+ 2 1S) can be observed at 247.8 nm.
In this work, we wanted to observe the removal of electronically excited carbon atoms. For this purpose, photodissociation of CO at 193 nm seems to be a useful method.
2. Experimental The apparatus was similar to that described in ref. [6] except for the present use of a high-transmission monochromator. A home-made.excimer laser wa~ u~ed on the ArF line (193 nm, 1 0 - 3 0 mJ/pulse). The time dependence of the laser pulse is characterized by its fwhm of 15 us. CO was irradiated flowing vertically through a nearly cubic teflon cell at pressures of 1 . 9 - 1 9 mbar at room temperature. It has the highest commercially available purity (99.97%, Messer Griesheim) and was used without further purification. The laser beam passed horizontally through two suprasil windows (distance 70 mm) and was reflected and focused to the center of the cell by a concave aluminium-coated mirror (r = 300 ram) behind the outlet window. The absorption of the laser line by CO was too weak to be detected directly. Fluorescence was observed along the second horizontal axis. It was dispersed by an Oriel 7240 double monochromator ( f = 125 mm, resolution 2 nm) and monitored by an RCA 4831 photomultiplier. A
0 0 0 9 - 2 6 1 4 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company
387
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
C( 31 Po--21 S)
1.5
C2 (C~lrg ~ Aqr~) 3
I [ a.u.]
1 August 1981
v'
vj 3 2 I
1.0
o
0.5 C2 (d 3 Trg~a31ru ) (342..,676 nm)
0 190
250 4
350 3510
450 40
550 h [nm] 35 ns lifetime
Fig. 1. Luminescence spectrum of 1.9 mbar CO after excitation in the focused beam of an ArF laser. Tektronix R 7912 transient recorder coupled to a DEC PDP 11/04 minicomputer was used to measure pulse height and decay time. The characteristic decay times of the fluorescence were evaluated by convoluting the excitation function with a single exponential decay fimetion and fitting the recorded fluorescence signal. Since the excitation and decay times were of the same order of magnitude, a convolution rather than a simple logarithmic analysis of the signals had to be applied.
measured lifetime of C(3 1 sO), are given in table 1. The 3 1po state is populated by the absorption 3 1po +. 2 1D of carbon atoms [5]. The emission o f carbon atoms is proof of previous dissociation of CO. The energy of at least two ArF laser photons is needed to break the CO bond. Rhodes et at. [5] concluded from the dependence of emission at 247.8 nm on the laser power, that it is indeed a two-photon process. In our experiment, the C(3 1po) fluorescence was shorter than the laser pulse. Therefore we could only evaluate a reasonable lifetime of C(3 1po) by using the square of the laser pulse as the excitation
3. Results and discussion The luminescence spectrum observed in this work at a CO pressure of 1.9 mbar is shown in fig. 1. A very similar spectrum was observed at 19 mbar. Besides the intensive fluorescence of C(3 1 po _+ 2 1 S) produced by a two-photon dissociation of CO and the laser excitation of C(21D), the emission of three systems of electronically excited C2 is observed. No fluorescence from any state of the CO molecule is seen. Lifetimes of two electronic states of C 2 (C 1 IIg, d 3 IIg) have been measured. These results, together with the 388
Table 1 Fluorescence decay times after excitation of CO by an ArF laser State
W a v e l e n g t h Pressure (nm) (mbar)
C(3 1SO)
248
11
C2(C 1rig) C2(d 3rig) C2(d 3Fig)
387.4 465.8 510
3.3 11.7 11.5
Decay time (ns) 2 10 40 35
Volume 81, number 3
CHEMICAL PHYSICS LETTERS
function for the convolution. The necessity of using the square of the laser pulse rather than the pulse itself implies, that the production of C atoms depends on the square of the laser intensity and affirms that the atoms are formed in a two-photon process. The absorption spectrum of CO at half the wavelength of the laser line (96.7 nm) shows sharp lines without any broadening or diffuseness [7]. This indicates that the energy levels, which correspond to two laser photons (2 × 52200 cm -1) belong to non-predissociated singlet states. Therefore, a triplet-state intermediate is supposed to be involved in the formation of the C atoms [5]: 3CO(Eex c = 104500 cm -1) -+C(23p)+o(23p),
AH = 15100cm -1,
-+ C(2 1D) + 0 ( 2 3p),
AH = 3600 cm -1 .
The emission spectrum given in fig. 1 shows, in addition to the fluorescence of C atoms at 247•8 nm, a molecular spectrum. This can be assigned to the emission of higher excited states of C 2. Between 240 and 340 nm, the Fox-Herzberg system C2(e 311g _.~ a 3IIg) and between 340 and 410 nm the Deslandresd'Azambuja system C2(C 1 IIg -+ A 1ilu) is found. The fluorescefice at 466 and 510 nm belongs to the Swan bands C2(d 3IIg ~ a 3Ilg) (7"0 = 136 ns [8,9]). In the e 3Ilg state, emission from the vibrational levels u' = 0 - 3 and in the C I rlg state, emission from v' = 0 - 5 can be assigned. High vibrational levels in these states cannot be seen since they are perturbed • by other electronic states (C 1 IIg v ! > 6 by C I 1 IIg, E 311g o' > 4 by d 3IIg) [9]. C2(C 111g) has a lifetime of 10 ns under our experimental conditions. For C2(d 3Ilg), a lifetime of 37 +- 5 ns is observed (table 1). Since C2 does not absorb the laser radiation [9], C~ has to be produced in a chemical reaction. The time dependence of the emission of C~ shows that it is formed by particles with lifetimes of a few nanoseconds. Two reaction channels can be proposed to form C2 during photolysis of CO: CO* + CO(*)
(1) -+ C2(e 3IIg(o' = 0 - 5 ) , C 111g(u' = 0 - 3 ) ) + O 2 . Using the dissociation energies in refs. [8,9] for 02, CO and C2, one finds that the sum of the excita-
1 August 1981
tion energy of the CO* molecules must reach up to 130000 cm -1 . Even if CO* is primarily produced in a non-emitting state, collisions, necessary to cause chemical reactions, should populate fluorescing electronic states [8]. Thus the lack of CO fluorescence indicates that reaction (1) is unimportant• Since CO* cannot form C~, free carbon atoms r must be involved in its formation• The reaction of excited carbon atoms with ground-state CO can be excluded by simple energy arguments, but not the reaction with electronically excited CO. Highly excited states of CO, however, populated by two quanta, are excluded by the arguments given above• One laser photon has only enough energy to pump CO into its a 311 state. Since this electronic transition is very weak and, moreover, the laser line does not coincide with absorption lines of the transition CO a 3 FI +- X 1 ~+ [81, CO(a 311) is not produced by absorption. Therefore, the recombination of C atoms is the only reasonable channel for C~ production. This can occur by radiative recombination C* + C(*) -+ C2 + hv,
(2a)
or by recombination with a collision partner, e.g. CO C* + C (*) + CO -+ C~ + CO,
(2b)
which can as well proceed with C20 as an intermediate
[101. The time dependence of the C~ fluorescence shows that at least one of the C atoms in reaction (2) must be in one of the excited states 2 1D or 3 1po. The ground-state carbon atoms and the excited-state C(2 1 SO) have lifetimes of the order of milliseconds under our experimental conditions [1]. If the C~ were produced in reactions of only that long-lived species, its fluorescence would be seen over a much longer time than the few nanoseconds observed. Therefore C(2 tD) or C(3 1po) must be involved in at least one step of the recombination reaction (2). The C(2 1D) atoms are rapidly transferred to the C(3 t po) state, the absorption of the laser line by them is strongly saturated [5]. Thus they have a very short lifetime in the system discussed here, in spite of their long free lifetime [11]. The formation of C2(e 3 Fig and C 111g) and its time dependence can be explained by the reactions of C(2 1D) with itself and with C(2 3p). As the potential energy diagram of C2 shows, C2(e 3IIg) correlates with 389
CHEMICAL PHYSICS LETTERS
Volume 81, number 3 '
l II 1 E (I03 crn-I]
I
i
I
I
E'3E~
1 August 1981
Acknowledgement
I
C(~S)
The authors wish to thank Professor Dr. H.Gg. Wagner for his continuous interest in their work.
60
References
~o
Fill\ L-L:"/77~e% I LIIw //~/-c,~g
2o F- It,IV/LA?: O
I /\"/y~,:,~r,, I- 1%z/~
0.I
I
l
I
0.2 0.3 r [nm]
t
l
0./.
Fig. 2. Potential energy curves of the C2 molecule (from ref. [91). C(2 3p) + C(2 1D) and C2(C 1jig) with C(2 1D) + C(2 1D) (fig. 2 from ref. [9]). The unperturbed vibrational levels of the C2(e 31-Ig) and C2(C 111g) states could be identified directly in the emission spectrum (fig. 1). The perturbation at higher vibrational energies in e 3Ilg leads to population of d 3IIg and appearance of the Swan bands (d 3IIg -+ a 3IIu)in the spectrum. The direct observation of lower and the indirect observation of higher vibrational levels leads to the conclusion that in both states a random distribution of vibrational energy is generated by the recombination.
390
[1] D. Husain and L.J. Kirsch, Trans. Faraday Soc. 67 (1971) 2886, 3166. [2] G. Liuti, S. Dondes and P. Harteck, J. Chem. Phys. 42 (1965) 4052. [3] A.C. Vikis, J. Chem. Phys. 69 (1978) 697,703; Chem. Phys. Letters. 57 (1978) 522. [4] R.A. Bernheim, C. Kittrell and D.K. Veirs, Chem. Phys. Letters 51 (1977) 325; A.C. Provorov, B.P. Stoieheff and S. Wallace, J. Chem. Phys. 67 (1977) 593. [5] J. Bokor, J. Zavelovich and C.K. Rhodes, J. Chem. Phys. 72 (1980) 965. [6] W. Hack and W. Langel, Nuovo Cimento, to be published. [7] M. Ogana and S. Ogawa, J. Mol. Spectry. 41 (1972) 393. [81 S.N. Suchard, Spectroscopic constants for selected heteronuelear diatomie molecules (IFI/Plenum Press, New York, 1975). [9] S.N. Suchard and J.E. Melzer, Spectroscopic constants for selected homonu¢lear diatomic molecules (IFI/Plennm Press, New York, 1976). [10] D. Husain and L.J. Kirseh, Trans. Faraday Soc. 67 (1971) 2025. [11] R.J. Donovan, Progr. Reaction Kinetics 10 (1979) 253.