Lifetime studies of NO(A 2Σ+, υ′ = 4), NO(B 2Π, υ′ = 9) and NO(D2 Σ+, υ′ = 0)

Chemical Physics 52 (1980) 399-404 ©North-Holland Publishing Company

LIFETIME STUDIES OF NO(A 2Z+, o' = 4), NO(B 2II, v' = 9) AND NO(D 2 ~+, v' = 0) T. HIKIDA, S. YAGI and Y. MORI

Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo, Japan Received 13 May 1980 Lifetimes of NO(A 2 2+, v' = 4), NO(B 2 I1, v' = 9) and NO(D 2Z+, v' = 0) have been studied using a single photon counting technique combined with nanosecond flash excitation. The zero-pressure lifetimes and quenching rate constants have been obtained. These values and our earlier reports show that both NO(B 2II, o' = 9) and NO(C 2II, v' ~ 0) predissociate with large quantum yields of 0.9 or more. On the contrary, NO(A 2E+, v' = 4) and NO(D 2 N+, v' = 0) undergo very slow predissociation, though these levels are more energetic than NO(C 211,v' = 0). Possible dissociation mechanisms are discussed. 1. Introduction In previous papers, we have reported that the weak 13(v' = 9) band fluorescence of NO was excited by the 184.9 nm mercury resonance line from a conventional low pressure mercury lamp [ 1 ] and quenched by collisions with He, H2, CO, CO2, CF4 and N2 as well as NO [2], though the lifetime is supposed to be very short due to the presence of predissociation. The relative values of quenching rate constants for these molecules have been determined from static intensity measurements of the 13band fluorescence. In order to determine absolute quenching rate constants the lifetime of NO(B Zll, v' = 9) has to be known. The lifetime was estimated from an approximated value of the 13(v' = 9) fluorescence quantum yield [3] and a radiative lifetime of the (v' = 0) band fluorescence [4]. Therefore, the obtained quenching rate constants of NO(B 2 II, v' = 9) involve some ambiguities of the fluorescence quantum yield and the radiative lifetime of NO(B Zll, v' = 9). The observation of the 3' (v' = 4) emission bands has been supposed to be difficult since the 3' (v' = 4) absorption seriously superposes with the e (v' = 0) absorption band [5]. The observation of the 3` (v' = 4) band fluorescence [1 ] induced by the 184.9 nm excitation of the mixture of NO and He is, therefore, an important experimental result for the investigation of NO(A 2£+, v' = 4).

In this paper we describe direct lifetime measurements of NO(B 211, v' = 9), NO(A 2~+, v,= 4) and NO(D 22+, v' = 0). The fluorescence from NO(B 2II, v' = 9) was obtained by the excitation of NO at 184.9 nm using a pulsed low pressure mercury lamp [6,7] and the decay of the fluorescence intensity was monitored. The decay rate constant of NO(A 2~+, v' = 4) was obtained by monitoring the fluorescence intensity from the gas mixture of NO and He at various pressures excited with the pulsed low pressure mercury lamp. This fluorescence has been assigned to predominantly consist of the 7 (v' = 4) emission [ 1 ]. The lifetime of NO(A 2 E+, v' = 4) was also examined by exciting NO with undispersed output from a N2 light pulser [8] which excited NO into various excited levels including A 2~+ (v' = 4) and D 2Z+ (v' = 0), and by monitoring the superposed fluorescence decay at a wavelength corresponding to the 7 (v' = 4) and e (v' = 0) bands. The biexponential fluorescence decay profiles gave the decay rate constants of NO(A 2E+, v' = 4) and NO(D 2~+, v' = 0). The former is found to be consistent with that obtained by the 184.9 nm excitation of NO and He mixtures, and the latter is in agreement with the reported values [ 9 - i 1 ].

2. Experimental The decay profile of the NO fluorescence was measured by a technique of single photon counting

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T. Hikida et al. / L i f e t i m e s o f N O ( A 2 2 +, v' = 4), N O ( B 2II, v' = 9), N O ( D 2 2+, v' = O)

and delayed coincidence. Pulsed light at 184.9 nm from a pulsed low pressure mercury lamp directly excited NO fluorescence from the level B 211, v,= 9. A typical pulse width (fwhm) of the output from the light pulser at 184.9 nm was 10 ns and its repetition rate was about 5 kHz with operating voltage of 8 - 1 0 kV. The NO fluorescence was dispersed through a 1.0 m monochromator with a 0.8 nm band-pass at 213.7 nm corresponding to the ~3(9,4) fluorescence band of NO [ 1 ]. This band was rather strong and expected to be free from interference of Hg lines or other spurious emissions. The NO pressure was varied from 0.4 to 2.2 Torr. When the mixture of NO and He at a pressure higher than 20 Torr was irradiated at 184.9 nm strong fluorescence bands were observed and the emitter was assigned to NO(A 2E+, v' = 4) [1]. The decay profile of this 3'(4, n) band fluorescence was obtained by irradiating the mixture of NO (1.0 Torr) and He (up to 500 Torr) with the pulsed low pressure mercury lamp which emits the 184.9 nm resonance line and by monitoring the 7(4,3) fluorescence intensity time profile at 209.0 nm with a 0.8 nm band pass through the monochromator. The lifetime of NO(A 22;+, v' = 4) was also examined by the direct excitation of NO with the undispersed light output of a N2 light pulser. The free running type N2 light pulser with its quasicontinuous output extending from 180 nm to visible region excited NO into various levels including A 22;+, v' = 4 and D 22;+, v' = 0. The fluorescence spectrum thus excited consisted of many emission bands but the 7(4,3) and e(0,3) bands at 209.0 nm were isolated and suffered little interference from other band fluorescence. The fluorescence decay profile observed through the 1.0 m monochromator at 209.0 nm with the band pass of 0.8 nm should be a superposition of two different decay profiles of the 7(4,3) and e(0,3) bands. The output of the N2 light pulser was not strong enough for a monochromatic observation of the fluorescence decay excited by monochromatic light through another monochromator. The analysis of the deconvoluted fluorescence decay profile easily gave the lifetimes of NO(A 2Z+, v' = 4) and NO(D 2N+, v' = 0). The N2 light pulser was operated at 10 kV, 1 atm of N2 and a repetition rate of about 5 kHz. The pressure of NO was varied from 0.3 to 2.3 Torr. Commercially available highest purity NO in a

glass bottle was used without further purification. Helium from a steel cylinder was used after purification by passing through a silica gel trap cooled at liquid nitrogen temperature. All the observed time profiles were deconvoluted according to the least-squares method reported by Ware et al. [14] with the corresponding time profile of the excitation light pulse at 187.2 nm (the N2 light pulser) or at 184.9 nm (the Hg light pulser). The former corresponds to the wavelength of the 7(4,0) and e(0,0) absorption bands, and the latter corresponds to the wavelength of the/3(9,0) absorption band. The profiles of these excitation light pulses were measured through the monochromator filled with N2 gas by reflecting the excitation light using an aluminum reflector placed at the position of the reaction cell containing NO without any change in geometries of the excitation and detection systems.

3. Results and discussion

Fig. 1 illustrates (a) the 184.9 nm excitation pulse profile and (b) a typical decay profile of the fluorescence from NO(B 2II, v' = 9) observed at 213.7 nm (0.8 nm band width) when NO at 1.6 Tort was excited by the pulsed 184.9 nm resonance line. The insert of fig. 1 shows that the deconvoluted decay function of the profile (b) by the excitation pulse (a) is in accordance with a first order law. The decay rate of NO(B 211, v ' = 9) in the presence of NO at 1.6 Torr thus found is 1.0 X 108 s -1 . The line fitted to the profile (b) is a convoluted function of the excitation pulse (a) with a value of 1.0 × 108 s -1. Fig. 2 shows a Stern Vohner i:lot of the decay rate obtained from experimental results such as shown in fig. 1. The extrapolated zero-pressure decay rate is (7.2 +-0.3) × 107 s -l, and the apparent bimolecular quenching rate constant for the ground state NO is (3.0 -+0.2) × 101 l M -1 s -1 which is obtained from the slope of the linear plot. The zero-pressure decay rate and the emission quantum yield [3] of NO(B 211, v' = 9) yield the radiative decay rate constant. The radiative decay rate constant is found to be (1.0 +- 0.3) × 106 s -1, and is somewhat larger than reported values for NO(B 2II, v' = 0) [4, 10,15]. This probably indicates the presence of slight perturbations due to other excited levels [16].

T. tIikida et al. / L i f e t i m e s o f N O { A 2 ,~+, v' = 4), N O ( B 217, v' = 9}, N O [ D 2 2 +, u' = O)

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Fig. 1. (a) Excitation light pulse observed at 184.9 nm. (b) Time resolved fluorescence of NO(B 211, u' = 9) observed at 213.7 nm when NO at 1.6 Tort was excited by the pulsed 184.9 nm ttg resonance line (a). The line fitted to the curve is the convoluted function of the excitation profile (a) with a decay rate constant of 1.0 X 108 s-1 . (c) The insert shows the deconvoluted decay function which is in accordance with a first order law. The slope of the linear plot is 1.0 X 108 s-I .

The quenching half-pressure o f NO is f o u n d to be 4.4 Torr f r o m fig. 2, while 1.7 Torr was r e p o r t e d by the previous steady state m e a s u r e m e n t [2]. This discrepancy may be explained by the following high resolution fluorescence spectra of/3 (v' = 9) emission bands. Rotational structures o f the/3 (v' = 9) fluorescence bands induced by the 184.9 n m m e r c u r y resonance line were investigated at various NO pressures. F o r the most intense/3(9,4) fluorescence band at 214 nm,

401

the fluorescence spectra were f o u n d to consist o f m a n y rotational lines even at the lowest N O pressure studied, 70 m T o r r , at which pressure collisional relaxation should have little effect on the emission spectrum. Analysis o f partially resolved rotational structures o f the emission spectra (0.04 n m resolution) using the rotational constants [ 17] shows that the absorption o f " b r o a d " 184.9 n m resonance line by NO is p r e d o m i n a n t l y due to the Pz(10.5), P2(5.5), P2(4.5), R l ( 1 5 . 5 ) , R2(10.5), R2(9.5), Q 1 ( 1 2 . 5 ) a n d Q2(7.5) rotational lines. The Q branch absorptions are e x p e c t e d to be very weak but located near the center o f the resonance line where the light is m o s t strong. T h e y c o n t r i b u t e to the excitation o f NO fluorescence. The allowe d P and R branch absorption lines as far as 5 cm -l apart f r o m the center o f the resonance line, can induce the NO fluorescence w i t h appreciable intensities. Fig. 3 shows the variation o f the emission intensity o f rotational lines o f NO(B 2 II, v' = 9) induced by the 184.9 nm resonance line versus the pressure o f NO. The emission lines p u m p e d by the Q b r a n c h absorption lines are weak at low NO pressures and increase their intensities gradually with the pressure. The intensities o f emission lines due to P and R branch absorption, which are d o m i n a n t emission lines o f the N O fluorescence at low pressures, also increase w i t h the pressure o f NO m o r e rapidly and reach m a x i m u m

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Fig. 2. Stern-Volmer plot of the first order decay rate constant of NO(B 211, v' = 9) versus pressure of NO. The slope is (3.0 +- 0.2) X l0 ll M-l s-1 and the intercept is (7.2 -+ 0.3) X 107 s-1 .

Fig. 3. Fluorescence intensity of rotational lines of NO(B 21~, v' = 9) excited by the 184.9 nm Hg resonance line versus pressure of NO. The fluorescence intensity induced by the P and R branch absorption lines (o) is the sum of the FI(10.5), F1(15.5), F2(4.5), and F2(9.5) emission intensity measured at 213.95 nm. The fluorescence intensity induced by the Q branch absorption lines (e) is the sum of the F1(12.5 ) and F2(7.5) emission intensity measured at 214.10 nm.

402

T. Hikida et al. / Lifetimes o f NO{A 2 E ÷, v' = 4), NO(B 21-1,v' = 9), NO(D 2 ~ + u' = O)

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Fig. 4. Deconvoluted function of NO(A 2N+, v' = 4) formed when NO at 1.0 Torr was excited by the pulsed 184.9 mn Hg resonance line in the presence of He. The 3"(4,3) fluorescence band was monitored at 209.0 rim. The slopes of the lines are: (a) 2.0 X 107, (b) 3.2 X 107, and (c) 4.5 X 107 S- 1 for He pressures of 35,240, and 500 Torr, respectively.

at the NO pressure around 0.8 Torr. At higher pressures they decrease with the NO pressure in contrast to emission lines due to Q branch excitation. Absorption lines of P and R branches seem so strong that the effective intensity of the excitation light reaching the observation region, about 3 cm inside from the irradiation window, would be reduced to a rather large extent at NO pressures higher than 1 Torr. This effect as well as that by self-quenching decreases intensities of emission lines caused by P and R branch excitation. The quenching half-pressure measured by the steady state experiment is obtained from an apparent intensity variation of the 3(9,4) emission band summed over various rotational lines, and therefore, it overestimates the quenching effect. Direct measurements of the fluorescence lifetime do not suffer such

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Fig. 6. Deconvoluted decay function of (a) NO(D 2 E+, v' = 0) and (b) NO(A 2 N+, v' = 4). NO at 0.3 Torr was excited by pulsed white light and the superposed fluorescence of the e(0, 3) and 3,(4,3) bands was observed at 209.0 nm. The slopes of the lines are: (a) 6.7 X 107 and (b) 9.7 X 106 s-1. experimental difficulties. The fluorescence lifetime of NO(A 2 2;+, o' = 4) was measured at 209.0 nm using mixtures of NO and He excited by the pulsed mercury line at 184.9 nm. This wavelength (209.0 nm) corresponds to the 7(4,3) emission band. The initially formed NO(B 211, v' = 9) is effectively converted to NO(A 2E+, v' --- 4) by collisions with He ~. Fig. 4 shows deconvoluted time profiles of the fluorescence intensity observed for various pressures of He. At the very early portion of each deconvoluted curve, the intensity of the 3,(4,3) emission increases with time as the result of the coUisional intramolecular energy transfer of NO(B 2II, v' = 9) to NO(A 227, v' = 4). The 7(4,3) emission does not decay as a single exponential function. The faster decay appearing in the earlier stage of the curves is estimated to have a decay rate of about 108 s-1 . At present the cause of this fast decay is not clear but may be due to very high rotational levels of NO(A 22;+, v' = 4) formed by intramolecular energy transfer collisions. Rate of predissociation may be faster for higher rotational levels if some rotational perturbations with a dissociation continuum exist. The main part of the fluorescence decay is, however, single exponential. Fig. 5 shows plots of the first order decay rate constant against the pressure of He, keeping the pressure of NO at 1.0 Torr. From the slope and the intercept, the bimolecular quenching :~ The emission spectra observed in the presence of He at pressures of the present experiment consisted mostly of the 3,(4, n) bands and little interference by other emission bands is expected.

T. Hikida et al. / Lifetimes o f NO(A 2 ~+ v' = 4), NO(B 2II, v' = 9), NO(D

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rate constant for He and the decay rate constant of NO at 1.0 Torr are obtained to be (1.0 -+ 0.2) X 108 M-l s -1 and (2.0 -+ 0.2) X 107 s -1 , respectively. Fig. 6 illustrates a typical deconvoluted decay curve of the e (v' = 0) and 3, (v' = 4) fluorescence from NO at 0.3 Torr irradiated by the pulsed white light and observed at 209.0 nm. The profile clearly shows biexponential decay due to NO(D 2y÷, v' = 0) and NO(A 2N+, v' = 4). In figs. 7 and 8, the observed decay rate constants of the fast and slow components such as shown in fig. 6 are respectively plotted against the pressure of NO. In fig. 8 the closed circle indicates the decay rate constant of NO(A 2Z+, v' = 4) obtained from the intercept of the linear plots shown in fig. 5, which corresponds to that for [NO] = 1.0 Torr. The decay rate constants of NO(D 22;+, v' = 0) and NO(A 2E+,

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v' = 4) at zero NO pressure are (5.7 +- 0.6) × 107 and (8.5 -+ 1.2) X 106 s -l , respectively. The self-quenching rate constants are (6.1 -+0.8) × l 0 l l and (1.6 +-0.4) × 10 ll M -I s -1 for NO(D 2E+, v' = 0) and NO(A 21;÷, v' = 4), respectively. Benoist d'Azy et al. [11] excited NO by the dispersed synchrotron radiation at 187.2 nm and observed the decay profile of the undispersed NO fluorescence. They have assigned the fast component as the decay of NO(D 2Z÷, v' = 0) and reported a value of (5.1 + 0.5) × 107 s -1 for the zero pressure' decay rate constant and (4.8 +.0.6) × 1011 M -l s -t for the selfquenching rate constant of NO(D zZ÷, v' = 0), in agreement with values obtained in the present experiment. The quenching half-pressure [12] obtained from static experiments corresponds to a slightly larger quenching rate constant. No lifetime study has been reported for NO(A 2Z+, v' = 4). The lifetimes of NO(A 2N+, v' = 0 - 3 ) , the levels below the dissociation limit of NO, have been studied by various techniques [ 10,I 1,15,18 ]. The lifetime (118 ns) of NO(A 2Z+, u' = 4) obtained in this experiment is slightly shorter than those of the lower vibrational levels ( 1 7 4 - 2 1 6 ns). The slightly short lifetime of NO(A 2~÷, v' = 4) seems to indicate the presence of very weak predissociation. The lifetime (17.5 ns) of NO(D 2~;+, v' = 0) is similar to that of non-predissociating rotational levels of NO(C 211, v' = 0) (15 ns [8], 20 ns [11]). This value seems to be very close to the radiative lifetime of NO(D a2;+, v' = 0) if we compare oscillator strengths [19] and lifetimes of non-predissociating NO(C a11, u' = 0) and NO(D aN+, v' = 0). The total spontaneous emission rate for NO(D aZ.+, v' = 0) determined from the absorption oscillator strength and the rate o f the , cascade radiation to the A state is 5.05 × 107 s -~ (19.8 ns) [13]. Predissociation of NO(D a2;+, u ' = 0) is expected to be slow and probably has no significant effect on the lifetime. Lifetimes of predissociating and of non-predissociating rotational levels of NO(C aII, v' = 0) indicate that the lifetime of predissociation of NO(C aII, u' = 0) is 1.4 [8] or 3.0 ns [11], and the predissociation quantum yield is about 0.9. The lifetime of NO(B 2II, u' = 9) obtained in this experiment is 14 ns and very close to that of predissociation [2,3]. The lifetimes of predissociation of NO(A aN÷, v' = 4) and NO(D aN+, v' = 0) are much longer than those of

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1". ffikida et al. / Lifetimes of NO(A 2Z+, v' = 4), NO(B 211, o' = 9), NO(D 2Z+, o' = 0)

NO(C 21I, v' = 0) and NO(B 211, v' = 9). Electronic excited states of NO, A 2N+, C 211, and D 2E+ are classified as Rydberg states and have their electronic configurations in which the unpaired electron in the antibonding 7r* orbital has been excited into an ns or np Rydberg orbital. The electronic states, a 411 which has been understood as the probable candidate of the predissociation continuum, and B 211 b o t h have the same electronic configuration in which an electron in the 7r bonding orbital has been excited into the 7r* antibonding orbital. These two states are classified as non-Rydberg states and have their potential minima at longer internuclear distances than Rydberg states [20]. Electronic configurations of these Rydberg states are different from that of a 411 by two electron orbitals. First-order s p i n - o r b i t interactions between these Rydberg states and the non-Rydberg state, a 4II, therefore, are expected to be vanishingly small and cannot induce predissociation. The interaction between B 211 and C 211 is the configuration interaction between the states of the same s y m m e t r y caused by the electron repulsion. Since B 211 and a *H have the same electronic configuration, non-vanishing s p i n - o r b i t interaction may be expected between these two states. The electronic coupling between C 2 II and B 2II and the s p i n - o r b i t interaction between B 211 and a 411 thus can induce the electronic interaction between C 211 and a 411. From the potential curves for excited electronic states o f NO [20], F r a n c k - C o n d o n factors seem highly favorable for predissociation of A 2 E+ (v' = 4), C 2II (v' = 0) and D 2E+ (v' = 0) but not for B 211 (v' : 9). F r a n c k - C o n d o n factors between a 4II and B 211 would not be large since the potential energy curves of these electronic states are parallel and do not cross. Thus, C 211 (v' = 0) predissociates with the short lifetime due to the large F r a n c k - C o n d o n factor and the indirect s p i n - o r b i t interaction with a 411. Small F r a n c k - C o n d o n factors of predissociation may be the cause of rather slow predissociation of B 211 (v' = 9). A 2E* (v' = 4) and El 2E* (v' = 0) predissociate very slowly and this is probably due to small electronic interactions with a 4II. A predissociation process has been proposed for higher vibrational levels of NO(B 211) via a dissociation continuum, A' 2 E+ [ 16 ]. This predissociation process, however, seems to be unlikely for B 211,

v' = 9. The present and our earlier results suggest that NO(B 2II, o' = 9) predissociates with a quantum yield of about 0.99. If A' 22;+ is the perturbing dissociation continuum for NO(B 211, v' = 9), only one A component of each rotational level of 2111/2 or 2113/2 can predissociate and another A component does not predissociate by the selection rule [17]. In this case the predissociation quantum yield should be smaller than unity, preferably 0.5. The fluorescence of NO(B 21I, v' = 9) excited with the low pressure mercury lamp was found to decay in accordance with a first order law. This observation also suggests that the E state cannot be an important perturbing state.

References [ 1 ] T. Hikida, N. Washida, S. Nakajima, S. Yagi, T. Ichimura and Y. Mori, J. Chem. Phys. 63 (1975) 5470. [21 T. Hikida, S. Nakajima, T. Ichimura and Y. Mori, J. Chem. Phys. 65 (1976) 1317. [3] T. Hikida and Y. Mori, J. Chem. Phys. 69 (1978) 346. [4] M. Jeunehomme and A.B.F. Duncan, J. Chem. Phys. 41 (1964) 1692. [5] F.F. Marmo, J. Opt. Soc. Am. 43 (1953) 1186. [6] T. Hikida, M. Santoku and Y. Mori, Chem. Phys. Letters 55 (1978) 280. ]7] T. Hikida, M. Santoku and Y. Mori, Rev. Sci. Instr., submitted for publication. [8] S. Yagi, T. Hikida and Y. Mori, Chem. Phys. Letters 56 (1978) 113. [9] J. Hesser, J. Chem. Phys. 4 (1968) 2518. [ 10] J. Brzozowski, N. Elander and P. Erman, Phys. Scripta 9 (1974) 99. I 11 ] o. Benoist d'Azy, R. Ldpez-Delgado and A. Tramer, Chem. Phys. 9 (1975) 327. [12] A.B. Callear, M.J. Pilling and I.W.M. Smith, Trans. Faraday Soc. 64 (1968) 2296. [13] A.B. CaUear and M.J. Pilling, Trans. Faraday Soc. 66 (1970) 1618, and references therein. [14] W.R. Ware, L.J. Doemeny and T.L. Nemzek, J. Phys. Chem. 77 (1975) 2038. [151 J. Brzozowski, P. Erman and M. Lyyra, Phys. Scripta 14 (1976) 290. [16] A. Lagerqvist and E. Miescher, Helv. Phys. Acta 32 (1958) 221. 117 ] G. Herzberg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [181 It. Zacharias, J.B. Halpern and K.H. Welge, Chem. Phys. Letters 43 (1976) 41. [19] H.A. Ory, J. Chem. Phys. 40 (1964) 562. [20] F.R. Gilmore, J. Quant. Spectry. Radiative Transfer 5 (1965) 369.